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    HeNe Laser Power Supply Design

    Sub-Table of Contents



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    AC Line Operated and Inverter Types

    Circuits for Keeping a HeNe Laser Tube Happily Fed

    Many alternatives abound where a HeNe laser power supply must be constructed from scratch. Decisions must be made based on the size or range of sizes of the HeNe tube(s) to be powered, convenience, need for portability (well in a relative sort of way), availability of components, and so forth.

    This chapter deals with the more practical aspects of HeNe laser power supply design including circuits for providing the HeNe tube operating voltage (AC line and inverter types), starters, regulators, and modulators. There are many options for each subsystem and it is often possible to mix and match as desired!

    AC Line Operated Versus Inverter Based Power Supplies

    A variety of techniques can be used to provide the starting and operating voltages for HeNe lasers. It is, after all, just a special type of gas discharge tube so almost any approach that can convince the HeNe tube to pass the proper current will be satisfactory. Both types may include internal current regulation or have their voltage and current adjusted with a variable power input (Variac or variable DC supply as appropriate).

    Modulation inputs may also be provided to permit the transmission of audio or data over the HeNe beam to enable external closed loop control of beam power.

    Some more sophisticated commercial power supply designs provide a variety of 'soft start' and other features to maximize HeNe tube life. Others enable 'instant start' for applications where the HeNe tube must be switched on and off frequently. These sorts of advanced forms of regulation are not really needed for general applications - which is just as well since the circuits tend to be proprietary and not available. Some of it may have been Marketing Departement driven specsmanship - how else to distinguish YOUR HeNe laser power supply from everyone else's virtually identical units? :-)

    Notation used in HeNe Laser Power Supply Diagrams and Schematics

    Throughout this chapter and the next, a quasi-standard method (mine!) is used to identify and label some of the connections between and among parts of the power supply, HeNe tube, and the external world. Here is a summary of the meanings of the notations used in the sample circuit diagrams and complete schematics: Note that in this document and the associated laser power supply schematics, voltages between 110 and 120 VAC Hot to Neutral (220 to 240 VAC between Hots on opposite sides of the line) may be shown for power in the U.S.A. and other parts of North America. Likewise, 220 to 240 VAC may be shown for power in Europe and elsewhere. Where some other voltage is used (such as 100 VAC in parts of Japan), it will be ideentified explicitly.

    AC Line Operated Power Supplies

    These are quite simple consisting of a high voltage transformer, rectifier or doubler, filter capacitor stack, starting circuit, and optional current regulator. Input is from either the 115 or 230 VAC line (or perhaps some slighlty different voltage depending on where you live). For details, see the chapter: Complete HeNe Laser Power Supply Schematics.

    Note: Throughout this document, we use 115 VAC as the nominal line voltage in the U.S. However, the actual measured voltage may range from about 105 to 125 VAC and still be considered to be within acceptable limits by the utility company. For this single-phase system, using both Hot legs of the line will then result in a nominal 230 VAC which may actually range from about 210 to 250 VAC.

    The transformer output generally feeds a half wave rectifier or 2 diode 2 capacitor doubler and filter capacitor stack.

    Either a parasitic voltage multiplier or pulse (trigger) type starting circuit can be used with these designs.

    Compared to inverter type power supplies, line operated units are easier to construct (no custom transformer is needed) and troubleshoot (there are no transistors to blow by the bucketload). Of course, they are not nearly as portable in two ways: the power transformers you are likely to find are usually quite heavy and there is that annoying line cord to drag around!

    However, most of the components are readily available or can be constructed from common parts including the high voltage diodes and capacitors:

    The only problem may be the power transformer which is typically 600 to 1,200 VRMS at 20 mA or so: If you have the option of obtaining a slightly higher voltage transformer than you actually need, go for it. However, the key word here is 'slightly' - not something 10 times too big! Then, if you acquire a higher power tube in the future, you will be all set. For now, it will just require a larger ballast resistor or Variac to run at reduced input voltage.

    It might be worth trying a TV or audio equipment repair shop - they may have spare transformers from old tube sets laying around gathering dust. These are ideal and can probably be had for next to nothing.

    Another option is an electronics surplus supplier - I have seen suitable transformers at some of these in the past but don't know what is currently available.

    A 3 or 4 stage voltage multiplier could be used to boost the output of a lower voltage transformer if a suitable high voltage transformer cannot be located. However, to obtain the needed current, the capacitors would need to be quite large - perhaps 1 uF at 1,000 V or more. Also, you would then probably need to use a pulse type starting circuit as a multiplier type starting circuit may not be able to provide enough output with a reasonable number of stages since the available p-p input voltage will be less with this approach.

    I have recently been using the power transformer from a long dead tube type TV both for testing a higher power commercial HeNe supply board and as the basis for a power supply of my own. See the section: Sam's Small Line Powered HeNe Laser Power Supply (SG-HL1). There was a selector for line voltage adjustment built into the transformer. With this set for lowest line voltage (and thus highest output) and the filament windings connected out of phase, it produces over 900 VRMS at 115 VAC input and over 1,150 VRMS using a Variac that goes up to 140 VAC. This translates into a doubled DC voltage of between 2,500 and 3,000 VDC - more than ample for most HeNe tubes up to 10 mW.

    AC Input Circuitry for HeNe Laser Power Supplies

    For safety and protection, you will need a fuse, line switch, and power-on indicator at a minimum:
    
                      _ F1         S1            T1 or T100
          Hot o------- _---------/ -------+------+        +--------o X
                     1 A        Power     |      |     ||(
                                       R0 /      +---+ ||(
                                      47K \           )||(
                                          /           )||(
                                          |   Primary )||( HV Secondary
                                     IL1 +|+          )||(
                                    NE2H |o|          )||(
                                Power On |o|          )||(
                                         +|+     +---+ ||(
                                          |      |     ||(
      Neutral o---------------------------+------+     |  +--------o T
                                                       |
       Ground o----------------------------------------+------o Tube- (HV Return)
                                                      _|_
                                                       -
    
    
    Note that the fuse is shown as the first component after the line cord. This provides the most protection where the fuse is located on the panel next to the cord entrance. However, it may be more appropriate to put the power switch first if the fuse is located on a circuit board or other distant location. Both arrangements are common in commercial equipment.

    Important: Use a grounded (3 wire) line cord and connect earth ground to the case (if it is made of metal), transformer core, and high voltage return of the tube (Tube- on the schematics below). This will assure that the tube housing is grounded and that no fault (like a short inside the power transformer) will result in any user accessible parts becoming electrically live as long as the line cord is plugged into a properly grounded outlet. The alternative is to double insulate everything but this may be impossible if you are using a commercial laser head where the tube cathode is already connected to its metal shell.

    Boosting the Output of a Transformer with Multiple Secondary Windings

    If you are using a transformer from a tube type TV or amplifier and want to boost the output a bit, determine how to connect the unused filament windings as follows. (This assumes you have a typical pair of filament windings on your transformer):
    1. Temporarily connect the 5 V (yellow) and 6.3 V (green) windings in series. Power the transformer and measure the output across the ends of this 'booster' winding.

      • If it measures around 11.3 V, proceed to (2).

      • If it measures around 1.3 V, interchange the yellow or green wires but not both.

    2. Power the transformer and measure the high voltage winding.

      CAUTION: This is probably over 700 V with significant current available. Take care. Make a note of your reading and then disconnect power.

    3. Temporarily connect the booster wiinding from (1) in series with the primary. Power the transformer and measure the high voltage winding.

      • If the high voltage now reads higher than in (2), you are finished.

      • If it measures less, disconnect power and reverse the connections to the to the booster winding.

    4. Use Wire Nuts(tm) or solder to premanently connect the windings in the desired configuration.
    This approach should result in an increase by a factor of 110/99.7 or about 10 percent. Use a similar procedure to decrease the voltage. Depending on your needs and your particular transformer, adjust appropriately.

    Alternatively, you can probably safely achieve up to a 25 or 30 percent boost using a separate low voltage power transformer to provide your booster winding. (Start with step (2).

    For example, with a 24 V transformer, a 26 percent increase in output voltage will result - this is probably about the limit before you risk core saturation with a typical transformer but your mileage may vary.

    CAUTION: On transformers with dual primary windings (to support 115 or 230 VAC power), it is possibly in principle to use one of these to drive the supply and the other as a booster on the secondary side. I Do not recommend this approach as the insulation between the two primary windings may be inadequate.

    Inverter Type Power Supplies

    Most modern designs are of this type since they can be made small, light weight, and efficient. However, component selection and procurement can be an issue for hobbyists and producing a reliable system can be more of a challenge than for AC line operated supplies.

    Several types are possible:

    1. Power oscillator inverter using a variety of simple configurations. These usually require either a custom wound ferrite transformer or a modified flyback transformer. Aside from the transformer, these are extremely simple requiring typically a half dozen components - all readily available.

      However, self oscillating designs are generally not as efficient as driven ones (see below) and may be unstable under certain load conditions.

    2. Driven inverter using a separate oscillator (e.g., 555 timer) to switch current to a stepup transformer. As with (1) above, the transformer can be custom or a modified flyback. These inverters can be made very flexible with adjustable frequency and duty cycle as well a feedback loop for regulation.

    3. Inverter using controller chips and/or power devices from companies like Linear Technology, Maxim, Motorola, and others. For these, application notes will be available but modifications will usually be needed to adapt their circuits for the required much higher than usual output voltage.

    With any of these, the starting circuit can be separate (a voltage multiplier or pulse type) or built in as part of a high compliance design.

    For DIY projects, it is best to run the inverters from low voltage DC. While it is also possible to build inverters that operate directly from the power line (commercial power supplies often do this) with just rectification and filtering, I DO NOT recommend this as an option here for two reasons:

    1. There is a significant added level of danger when dealing with the testing of line connected circuitry.

    2. It is much easier to blow up mountains of power transistors and other components when running at 150 VDC compared to 12 VDC before a design is perfected!
    See the chapter: Complete HeNe Laser Power Supply Schematics for sample circuits of types (1) and (2).

    For an example of (3), HeNe laser drive circuitry is briefly covered in a Linear Technology Corp. application note: AN-49, p.13. This is a low voltage DC powered circuit using an LT1170 chip for fully automatic starting and feedback control of operating current. It is essentially a constant current supply with a voltage compliance range of 10 kV. HeNe tube power requirements are also discussed. Unfortunately, the special transformer may not be readily obtained.

    Although there is somewhat less of a shock hazard with an inverter running from low voltage DC, grounding the metal case of a laser head and other metal parts is still desirable unless they are totally isolated from user contact (e.g., everything is in a plastic enclosure).

    The main difficulty from the hobbyist's perspective in building an inverter type power supply from scratch may be in obtaining or constructing the required high frequency ferrite transformer since these aren't the sort of thing that can be purchased at Radio Shack. However, I have successfully wound my own transformer (from a bare core and bobbin) to repair a commercial power supply (see the section: HeNe Laser Power Supply from HeNe Laser Pointer (IC-HI3)).

    Another option that may or may not work (I have not tried this) is to use a fluorescent backlight inverter (for/from a laptop or other LCD) or even a battery powered lantern as the basis of a HeNe laser power supply. Since there generate up to 1,000 VAC or more with a few mA available at 10s to 100s of kHz, the addition of a rectifier or doubler, and starting multiplier may be all that is needed. However, some commercial designs are too smart for their own good (at least for this application) and may shut down if the exact conditions they expect are not met.



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    High Voltage Components, Construction Issues

    Standard and Custom HV Rectifiers

    One of the common difficulties in constructing high voltage power supplies for HeNe, CO2, and other lasers is obtaining suitable high voltage rectifiers. However, there are alternatives to ordering special high voltage rectifiers from electronics distributors or surplus sources, or even ripping apart the family microwave or TV to build your laser power supply (though salvage from these sources - after they are certifiably dead and thrown out! - is well worth the effort). Other sources include: electronic air cleaners, igniters, bug zappers, and photocopier and laser printer HV power supplies. Modules from these devices with nice HV components may be available surplus at ridiculously low prices.

    A series string of dirt cheap 1N4007s can be used to construct high voltage rectifiers for line frequency power supplies and seems to be acceptable up to a few kHz for inverter based power supplies. Equalizing components do not appear to be needed - at least where these diodes come from the same lot number. Modern devices are matched closely in terms of leakage current and capacitance. However, the construction DOES need to take into account requirements for high voltage insulation - adequate spacing and the rounding off and possibly coating of exposed wires/connections especially where the very high starting voltages are involved. A variety of mounting techniques can be used including soldering end-to-end installed in a plastic or glass tube, and on or between pieces of perfboard.

    The common 1N4007s cost only a few cents each in quantity (typically between $.01 and $.06, though much higher from Radio Shack). Thus, it is generally possible to construct high voltage rectifiers at significantly lower cost than buying microwave oven, TV, or similar commercial types. And, it is trivial to construct whatever size you need! I recommend derating by 30 to 50 percent just to be on the safe side. So, where you need a 5 kV rectifier, use 7 or 8 1N4007s in series.

    I have used this approach for power supplies that I have built. See the sections: Sam's Small Line Powered HeNe Laser Power Supply (SG-HL1) and Sam's Inverter Driven HeNe Laser Power Supply 1 (SG-HI1). In addition, in order to repair a particular HeNe laser power supply - the one described in the section: Aerotech model PS2A-X HeNe laser power supply (AT-PS2A-X) - after accidentally shorting its output and blowing most of the original HV diodes in the multiplier, I replaced them each with strings of four 1N4007s. There were much much cheaper than the exact replacements and seem to work just as well. However, where really high voltages are involved - like the later stages of the starting voltage multiplier - it is essential to smooth out the additional connections and coat or pot these diode assemblies to minimize the tendency for corona and arcing.

    Note that you still may want to consider microwave oven rectifiers since these are readily available for $2 or 3 in single quantities from service parts suppliers like MCM Electronics. At 12 to 15 kV, the cost is higher than for a string of 1N4007s but the convenience of wiring a single part rather than 15 or 20 may be worth it. However, in their normal application, these is no more than about 5 or 6 kV across the device. Where you are using them close to their PRV ratings, potting would probably be a good idea.

    For high frequency inverters (e.g., 10s of kHz or more), fast or ultrafast recovery type rectifiers must be used since at a frequency equal to 1/(2*Trr), diodes turn into short circuits. For example, a nice 1 kV, 1 A part like the 1N4948 has a reverse recovery time (trr) of 500 ns. Thus, it should have acceptable performance up to at least 200 kHz (my rule of thumb for maximum frequency of a diode: around 20% of 1/trr). Note that putting multiple diodes in series scales the junction capacitance by 1/n (where n is the number of diodes in the string) but does NOT affect Trr.

    (From: Kim Clay (bkc@maco.net).)

    All of my HV diodes have been made using 1N4007s from Dan's Small Parts without any equalizing components. I make my HV diodes in separate assemblies using strips of perfboard cut with an Xacto razor saw to just one hole wide whatever length I need to mount all the 1N4007s. I cut 2 of them for each diode assembly and set the diodes in one strip first (first one anode up, next down, next up, etc...) then slide the other strip over the other ends of the diodes. Then, bend over all the middle interconnecting leads, trim very short, and solder with a nice round ball on each connection to minimize corona. I leave the full length leads on the first anode and the last cathode and now I have a 'custom' diode assembly of whatever voltage I need!

    Standard and Custom HV Capacitors

    Most types and value of capacitors can be obtained from the major electronics distributors or service parts sources (see the chapter: Laser and Parts Sources. Voltage ratings of small ceramic capacitors of 6.3 kV or more (perhaps up to 15 kV), as well as electrolytic types up to 450 or 500 V are available. However, they tend to be somewhat more expensive than low voltage types and are definitely less common than they were in the good old days of vacuum tube technology. :-)

    Microwave ovens include a capacitor of about 1 uF rated for at least 3 kV, and TVs and computer monitors may include 1 or 2 low uF, 1.6 to 2 kV ceramic caps. Higher voltage (but low uF value) caps can be found in equipment like: electronic air cleaners, igniters, bug zappers, and photocopier and laser printer HV power supplies. Modules from these devices with nice HV components may be available surplus at ridiculously low prices.

    In some cases, a stack of regular capacitors will suffice but not always, and using a single capacitor with the proper ratings will be best, especially in areas like the voltage multipliers of HeNe laser starting circuits. See the section: Series Banks of Capacitors.

    For modest values (up to a few thousand pF), homemade capacitors may represent a low cost alternative to hard to locate expensive 'real caps'. However, these will always be larger, bulkier, and more problematic (unless submerged in oil or completely potted) than their commercial counterparts.

    While not very practical for high uF value caps, most coaxial cable can withstand more than 3 kV (at DC). RG58 (50 ohm) is mostly 100 pF per meter while RG59 (75 ohm) tends to be 67 pF meter. Coax using high density polyethylene (rather than foam) may be able to withstand 15 kV or more A similar type is used to attach HeNe laser heads to their power supplies (and it must withstand the starting voltage). So suitable lengths of coaxial cable (all that old network or video cable sitting above the ceiling tiles at work!) might be useful for experimenting. However, wrapping 1,000 meters of old cable inside your HeNe laser power supply to obtain the needed capacitance probably isn't too great an idea! :)

    To achieve high capacitance, you need metal plates separated by as thin an insulator as possible - but one that won't break down under the stress of the maximum voltage applied. Capacitance is proportional to plate area and dielectric constant of the separator material, and inversely proportional to the distance between the plates.

    Common printed circuit board stock - Fiberglass Epoxy - is good for about 1 kV/mil (1 mil = .001 inch) of thickness. Plexiglas acrylic has a puncture voltage of between 450 and 990 V/mil depending on quality. Materials like plate glass, ceramics, and other plastics have similar ratings. In all cases, the quality is extremely important - a single microscopic pinhole, bubble, or other manufacturing defect can render these ratings meaningless!

    The dielectric constants of these materials can vary significantly even within the same product family (see below). Therefore, selection must take this into account as well. Less of a higher dielectric constant material may be needed even if its puncture voltage is lower (and it thus needs to be thicker).

    The Information on Building Capacitors Page (part of the Tesla Coil Mailing List Web Site has information on the dielectric constants and puncture voltages of common materials as well as useful equations for designing home-built high voltage capacitors.

    (Portions from: Dustin Lang (dlang@cln.etc.bc.ca).)

    The basic parallel plate capacitor formula is:

                          A * K * 8.85 pF/m
                     C = ------------------- * (N - 1)
                                  d
    
    Where: Some approxiate values for K: Epoxy - 3.3 to 3.7 (Fiberglass Epoxy PCB material may be higher); Glass - 3.7 to 9.5; Plexiglas - 2.8 to 3.1. (Source: CRC Handbook of Chemistry and Physics).

    As an example: For a 1000 pF, 10 kV capacitor (typical of what might be needed for the output filter capacitor of a small high compliance inverter type HeNe laser power supply), you will need an insulator at least 10 mils thick. Using a piece of 1/8" thick Plexiglas (about 3 mm), we have: d = 0.003 m, C = 1.0E-19 F, assume K = 3.0. Solving for A we get:

                      C * d         1000 pF * 0.003 m
            A = ---------------- = ------------------- = .113 square meters
                 (K * 8.85pF/m)      3.0 * 8.85 pF/m
    
    which means you'll need foil about 34 x 34 cm on a piece of Plexiglas slightly larger to provide a border to prevent air breakdown along the edges (only about 25 V/mil for air!). Or, several smaller capacitors in series (you can share adjacent plates by interleaving connections - e.g., 5 plates (17 x 17 cm) on 4 pieces of slightly larger Plexiglas).

    Using a piece of .010" Fiberglass Epoxy instead (good for 10 kV), the required area would be reduced by more than a factor of 10. This material, which is readily available from PC board manufacturing companies, is excellent for these capacitors and for general high voltage insulation as well. It can be procured bare in which case you add your own aluminum foil plates, or copper clad, in which case the perimeter border needs to be removed. This can be accomplished by etching (messy chemicals) or by scribing along the edge of the desired plate area (taking care not to damage the underlying material) and then pealing away the unwanted copper.

    To prevent accidental contact, cover it on both sides with additional LARGER insulating plates and clamp or tape the entire assembly!

    Connections to copper can be made by soldering, taking care not to damage the substrate (raise a tab if necessary). For aluminum foil, fine wires can be inserted between the foil and insulator which will remain in place once the protective covers are fastened in place.

    There is supposed to be a type of Mylar (from Dupont) that has a dielectric breakdown of 18 kV - over 20 times that of your typical plastic or glass!

    (From: Chris Chagaris (pyro@grolen.com).)

    That is a somewhat higher breakdown rating than is commonly tossed about. This rating would likely be for the specific material that Dupont produces especially for capacitor construction, which may be very difficult to obtain in small quantities. Although, I have seen a similar material available (surplus) in 32 gauge (0.00795" I think) thickness, but only at 3 inches in width. The more commonly available Mylar (polyethylene terphthalate) is usually considered safe to hold off 7,500 volts per mil. How thin you can actually find this material, that would also be commonly available, would be interesting to find out.

    Series Banks of Capacitors

    Electrolytic capacitor voltage ratings are limited to 450 to 500 V but it is often the case that filtering must be provided for multi-kV power supplies. Just connecting electrolytic capacitors in series may result in a bomb :-(.

    Polyester and other non-electrolytic capacitors may be more readily available in lower voltage ratings as well. Failure of these types is also possible (though probably less spectacular).

    (Note that microwave oven high voltage capacitors represent a low hassle alternative to piles of smaller capacitors where modest capacitance (around 1 uF) at 2,500 to 3,500 VDC is required. Of course, these can also be wired in parallel or series to provide increased capacitance or voltage ratings. For series banks, these are treated as non-electrolytic types.)

    When capacitors are connected in series, if one fails shorted, the rest will likely follow in rapid succession. I hope your power supply is fused! This somewhat undesirable behavior is a result of two effects:

    The following assumes the use of electrolytic capacitors rated at 450 V that may have been sitting around for a few years. Reforming can be omitted for fresh ones. However, in general, I would not recommend the use of old electrolytic capacitors if possible since their characteristics will be less stable in the long run even if you can bring them back to some sort of health.

    When electrolytic capacitors sit around unused, they deteriorate and their leakage current increases. Reforming is accomplished by slowing bringing the voltage across the capacitor up to its rated value (and possibly slightly beyond) while limiting the current to a safe value. Initially, there may be high leakage through the capacitor but as it reforms, this will drop down to near zero. If current limiting is not provided, this may also result in a bomb or smoke grenade.

    (From: Charles Mosher (ratranch@svpal.org).)

    In series, each must be shunted by an appropriate equalizing resistor, in order that the variations in their leakage currents will not cause problems with grossly unequal voltage division across them.

    Form up well all the capacitors to 525 Volts if you can, and then check them individually for leakage current at 450 Volts. Choose the shunting resistor value to pass perhaps 10 times this current. Watch out for cooling in the resistors; since little heat sinking will be provided, and in light of the applied voltage and the nominal voltage rating of the resistors, you may want 2 W molded carbon resistors, run at no more than perhaps 1/2 watt dissipation, in order that they be reliable.

    You may have to discard some of the capacitors due to excessive leakage current or complete failure to reform.

    Remember to place insulating sleeves on the cans of the capacitors whose negative sides are not grounded, as there is really no insulation provided between the capacitor negative electrode in the capacitor guts and the can itself.

    Such a capacitor bank may have to be brought up to rated voltage slowly after prolonged non-use because of uneven deforming of the electrolytics. You might have to repeat the whole selection process. I built such a thing about two years ago, and after two years of non-use would not now just bang it on.

    High Voltage Resistors

    The voltage rating of a resistor is not something that you tend to think about until one fails. However, resistors DO have voltage ratings - and they are often less than you might think (or desire). The typical voltage rating for a common resistor - whether it be 1/4 W or 2 W - is often only 300 to 500 V, but possibly as high as 1,000 V. This is grossly inadequate for use in the starting circuitry of HeNe laser power supplies and even for bleeder or equalizing components in some cases.

    Not only can arcing take place across inadequately rated resistors, internal damage can occur which may not show up immediately. Unfortunately, catalogs may not list voltage ratings of resistors.

    There are basically two options to obtain resistors with adequate voltage ratings:

    Note that all of these components must be immaculate - a little dirt or finger grime results in a leakage path that looks like a short circuit compared to a 50 or 100 M ohm resistor!

    Power Supply Construction Considerations

    Since these power supplies generate high voltage, you cannot simply use the same wiring practices as for 5 V TTL computer logic!

    The input side doesn't present too many problems:

    However, there are special concerns for the secondary side. For higher power (higher voltage) HeNe tubes, these are even more critical for both safety and consistent performance:

    Separating Power Supply and HeNe Tube or Laser Head

    For most applications, a short cable between the power supply and HeNe tube or laser head is acceptable but suppose you want to have 20 feet between them?

    You can extend the cable all you want as long as:

    Of course, use cable rated for much more voltage than can be supplied by your starter. Don't just go by the starting voltage rating of the HeNe tube - if it should become disconnected, the full starting voltage will be present, possibly for an extended period of time!



  • Back to HeNe Laser Power Supply Design Sub-Table of Contents.

    Starters - Voltage Multiplier, Pulse, Inverter, Piezo

    Starting Circuits for HeNe Laser Tubes

    The objective is to achieve the required breakdown voltage of the laser tube and then disappear into the woodwork. Almost no energy is required - just voltage. Therefore, any method that can achieve this will be adequate. Conceivably, even walking across a wool carpet on a dry day and touching the anode of the tube would make a suitable starting procedure (perhaps assuming you don't die from the high voltage - this depends on *your* requirements). However, higher tech approaches are usually preferred. :-) In fact, the piezo assembly from a disposable lighter or gas grill can be used as the basis for a starting circuit. See the section: Piezo Impulse Starting Circuits.

    Although usually specified by tube manufacturers to be 7 kV to 12 kV depending on size, the actual starting voltage is often much less, typically only 3 to 5 times the operating voltage. However, to be sure, adhering to the minimum starting voltage specifications is still a good idea unless you intend to only use one tube and test it before constructing your power supply.

    Here are many possibilities for starting HeNe tubes:

    1. Use a 'parasitic' voltage multiplier. This is an n-stage diode-capacitor ladder operating off of the (unfiltered) output of the primary supply. This can be used on both high frequency inverters and line powered systems though component values will differ. Since starting the tube requires essentially no current, the capacitors are small in either case. The voltage multiplier goes essentially in series with the operating supply and is effectively shorted out once the tube fires.

      A voltage multiplier can be constructed from common diodes and capacitors relatively easily. Alternatively, a 3X or 4X type may be purchased as a replacement for the type found in some color TVs and monitors - you may even have one gathering moss in your junk box!

      See the section: Voltage Multiplier Starting Circuits.

    2. Use a trigger transformer such as found in a photographic strobe to pulse the tube. Its secondary (usually in conjunction with some high voltage diodes) would go in series with the positive side of the main supply. A push button can be used to start the tube or a simple automatic circuit which senses that there is no tube current flowing can do it for you. However, a much larger trigger transformer than found in a disposable pocket camera will likely be needed to start it in one shot. A small TV or monitor flyback transformer or automotive ignition coil can also be used. See the section: Starting Circuits Using Pulse or Flyback Transformers".

      The spark generating modules used in electric stove or furnace igniters, and pocket lighters that use a capacitive discharge approach would also work with few additional parts (a pair of HV diodes). See the section: HeNe Starter Using Electronic Ignition (or Other Similar) Modules.

    3. Use an inverter based on the flyback transformer from a small B/W TV, video monitor, or computer terminal. The output goes in series with the main supply but is bypassed by a HV blocking diode. It is activated only when starting the HeNe tube. While the inverter must produce a very high voltage, the required power is negligible, so the drive circuit can be very simple, a one transistor blocking oscillator, for example. However, getting such a circuit to work well is something of an art. An easier, though somewhat more complex design, uses a 555 timer to drive a bipolar power transistor or MOSFET. An even easier, if somewhat inelegant solution, is to rip the horizontal deflection/HV guts out of a computer terminal, monitor, or TV. :-) See the section: Sam's Super-Starter(tm). I guarantee that if the HeNe tube is good, no matter how large, this WILL start it!

      These approaches work particularly well for hard-to-start tubes because up to 16 kV or more is available and mostly enclosed (inside the flyback potting material if it has an internal rectifier). Thus, problems with corona from external wiring are minimized.

      See the section: Starting Circuits Using Pulse or Flyback Transformers, for more information.

    4. Use the piezo assembly from a disposable lighter or gas grill as a source of a high voltage pulse. The only additional parts required are a pair of high voltage rectifiers. This will require pushing a button to start the tube but is by far the simplest approach in terms of the number of required parts. See the section: Piezo Impulse Starting Circuits.

    5. Implement a wide compliant range constant current supply as described with respect to the Linear Technology application note (AN-49, above). Until the tube fires, the supply will attempt to increase the voltage (within its compliance range) until current starts flowing. Also see the section: Sam's Inverter Driven HeNe Laser Power Supplies.

    6. It should be possible to start HeNe tubes having a bare capillary such as those used in large frame external mirror lasers in the same manner as is used to trigger xenon flashlamps - by attaching a pulse starter via a short lead (a few inches at most) to a wire or piece of foil wrapped around the thin bore of the HeNe tube. The capacitive coupling enables the starting pulse to directly ionize the gas in the bore allowing current to flow from the main supply. This approach may work in some cases with small internal mirror HeNe tubes with wide diameter gas reservoirs surrounding the bore as well. However, getting the gas in the central bore of such a tube to ionize in this manner will be a lot more difficult since it is much further away. It may even be possible to just attach it to the anode of the tube (after the ballast resistors) but this is less likely to be reliable if it works at all.

      On some internal mirror HeNe tubes or laser heads, you may find a wire or conductive strip running from the anode or ballast resistor down to a loop around the tube in the vicinity of the cathode. (Or there may be a recommendation for this in a tube spec sheet.) This external wire loop is supposed to aid in starting (probably where a pulse type starter is involved) in a similar manner to what is used for an external capillary (above). However, with an internal mirror HeNe tube, the capillary is usually isolated from the outside envelope. Therefore, I wouldn't expect there to be much, if any, benefit especially when using a modern power supply, but it might help in marginal cases. And, running the high voltage along the body of the tube requires additional insulation and provides more opportunity for bad things to happen (like short circuits) and may represent an additional electric shock hazard.

    For a basic AC line powered supply, I recommend the parasitic multiplier approach (1) as it can be constructed from readily available inexpensive components: 1N4007s (about 5 cents each) and .001 uF, 1,000 V ceramic disc capacitors (about 10 cents each). Depending on the number of stages in the multiplier, between 6 and 28 of each of these components will be required.

    Of course, you can also use higher voltage diodes and capacitors to simplify the construction but they will probably be much more costly. See the section: Sam's Small Line Powered HeNe Laser Power Supply (SG-HL1) for a tested design using inexpensive parts.

    Alternatively, a commercial voltage multiplier block can be used. See the section: Color TV or Monitor Voltage Multiplier.

    Voltage Multiplier Starting Circuits

    These are called 'parasitic multipliers' since they feeds off of the main supply and are only really active during starting when no current is flowing in the HeNe tube.

    A voltage multiplier can be constructed from common diodes and capacitors relatively easily. Alternatively, a 3X or 4X type may be purchased as a replacement for the type found in some color TVs and monitors - you may even have one gathering moss in your junk box! See the section: Color TV or Monitor Voltage Multiplier.

    A typical design with 7 diodes (3-1/2 stages) is shown below. For small tubes, fewer stages can be used. Going much beyond n=7 or 8 (4 stages) is probably not useful as the losses from diode and stray capacitance and leakage will limit output.

    
             R1    C1              C3              C5              C7
      X o---/\/\---||------+-------||------+-------||------+-------||------+
           1M, 1W     D1   |  D2      D3   |  D4      D5   |  D6      D7   |
                   +--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--o HV+
                   |               |               |               |               
      Y o----------+-------||------+-------||------+-------||------+
                           C2              C4              C6
    
    HV- o---------------------------------------------------------------------o HV-
    
    
    Where:

    (Note: all the power supply schematics in this document have these points labeled consistently.)

    With n diodes, HV(peak) is approximately (X(peak) * (n + 1).)+ Y and HV(average) is (X(peak) * n) + Y.

    Note that an even number of diodes may be very slightly better since there is less ripple on the output when the tube is running. However, with a high value R1 (10M) this is so small anyway that it really should not matter and an odd number of diodes saves components but results in nearly the same peak starting voltage.

    For use in HeNe laser starting applications where no real current is required, R1 limits power to the multiplier once the tube fires. Power is then drawn from point Y through the string of diodes.

    Multipliers can be used with both line operated supplies and high frequency inverters but since the capacitors must be larger at the (lower) line frequency.

    The uF value of the caps isn't critical but it needs to be large enough so the impedance of the caps at the operating frequency is low compared to the leakage due to the capacitance of the diodes, stray wiring capacitance, and non-infinite resistance of everything. Aside from these losses, a small uF value will just result in the starting voltage climbing more slowly and thus the HeNe tube will take longer to start. A large uF value will just cost too much.

    The voltage ratings of the diodes and capacitors must be greater than the p-p output of the transformer.

    Because the capacitors used in the multiplier are so small, they cannot really supply much current. Once the tube fires and current starts to flow, the ladder just becomes some series diodes. Then you're back to the basic power supply output (rectifier or doubler and filter capacitors), with a few diodes in series.

    Voltage Multiplier Operation

    Making the assumption that this circuit actually works, it is easiest to understand its operation in the steady state with no load. (Trying to keep track of each of the nodes on each cycle is virtually impossible without a circuit analysis program.) Ideally, in the steady state, no current will be flowing through any of the diodes.

    There are two types of nodes:

    It follows from this that the voltage (DC or average) must increase by a value of V(peak) after each diode. Otherwise, current would be flowing on either the negative or positive peak of the input.

    Assuming a peak-to-peak amplitude of 2 units (just to keep the diagram simple), the voltages at each node will be:

    
           R1    C1   +1(AC)     C3   +3(AC)     C5   +5(AC)     C7   +7(AC)
    X o---/\/\---||------+-------||------+-------||------+-------||------+
                +0  D1   |  D2  +2  D3   |  D4  +4  D5   |  D6  +6  D7   |
                 +--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+---o HV+
                 |               |               |              1|               
    Y o----------+-------||------+-------||------+-------||------+
                         C2              C4              C6
    
    G o----------------------------------------------------------------------o HV-
    
    
    Where: Thus, the nth diode's output has multiplied the peak value by n. The peak value of HV+ will actually be Y + 8 * V(peak) or about 5 times the p-p output of the transformer. (With one more diode and capacitor, this would be a DC output of this same voltage.)

    In practice the actual output will likely be somewhat less due to stray capacitance and other losses.

    Color TV or Monitor Voltage Multiplier

    Some color TVs and computer or video monitors do not generate the high voltage directly from the flyback transformer. Rather, they use a voltage multiplier to step up 6 to 10 kVAC (from the flyback - no rectifier) to the 20 to 30 kVDC or so required by the CRT. If you have a pile of dead TVs or monitors rotting in your garage, at least one may have a suitable multiplier block. If not, they are available for around $10 to $20 from electronics distributors dealing in parts for the service industry (e.g., MCM Electronics, Dalbani, Premium Parts, etc).

    Since these voltage multiplier blocks are designed for horizontal deflection (e.g., ~15.7 kHz), they are ideal for drive from an inverter. I have also tested two different multipliers on my line powered (60 Hz) supply without any problems. In fact, one of those I tried was removed from a TV because it had a short in the focus divider network but this didn't affect HeNe tube starting performance at all!

    These are usually 3X units though some 4X types are also available. Hooking them up is very straightforward and the entire multiplier is well insulated (totally potted in Epoxy) with a high voltage output lead (just remove the CRT suction cup connector). The unneeded terminals (e.g., F, CTL) can be ignored.

    Some of these have a capacitively coupled drive input. If this is NOT the case, the addition of a high voltage capacitor (.001 uF, 3 kV typical) in between the X and IN terminals is required to achieve the full multiplication factor. If you do not know if there is an internal capacitor, include C1 - it will not hurt anything. Since you do not want much current from the voltage multiplier, insert a 1M to 10M series resistor in this path as well.

    
                                            HV Multiplier
                   R1           C1        +----------------+
           X o----/\/\----------||--------| IN          HV |----------o HV+
                10M, 1W     .001uF,3kV    |    ECG535    F |-- NC
           Y o----------------------------| DC         CTL |-- NC
                                          +----------------+
    
         HV- o--------------------------------------------------------o HV-
    
    
    ECG535 is just one typical 3X model. Almost any HV multiplier of this general type will be suitable. However, if you have a choice, obtain a 4X multiplier as this will provide a bit more margin (though a 3X model should be adequate for most HeNe tubes). The IN terminal may be called GND, REF, or COM on some models - or there may be a pair of terminals with two of these names. If so, they should be tied together. F and CTL are Focus and Control respectively. These and any other special terminals can be left unconnected.

    Starting Circuits Using Pulse or Flyback Transformers

    These approaches represent alternatives to the use of a voltage multiplier. With the auto-trigger circuit, they are equally effective (hey, if it starts, it starts!) and may be somewhat easier to construct since fewer parts are involved. Some further simplification is possible if you are willing to forgo the auto-trigger circuit and use a pushbutton switch instead of the SCR.

    Note: These may be called either pulse or trigger starting circuits and I use the terms somewhat interchangeably.

    For some types of specialized or older HeNe (and other gas) lasers where there is a separate gas reservoir and the surface of the capillary is accessible, the output of a true pulse starting circuit (single shot, not a charge pump or inverter since a high dV/dt pulse shape is required) may be applied to an external electrode in a similar manner to that used for triggering a xenon flashlamp. The pulse circuits below may be suitable (but leave out any HV diodes). However, this type of laser construction is not that common and this approach will probably not work for the inexpensive mass produced sealed HeNe tubes with a wide glass envelope surrounding a narrow capillary since it will be difficult to ionize the gas inside the bore in the center of the tube.

    While the current required to start a HeNe tube is negligible, the energy needed to achieve sufficient voltage given the stray capacitance of the wiring, HeNe tube anode, and other connected components is not. With a voltage multiplier type starting circuit, this just means it takes a little time to charge up to the required voltage but as long as the leakage is small (it usually can be ignored), the tube will start eventually. For a pulse (trigger) type staring circuit, the energy must be provided in one shot or a charge pump must be used to accumulate smaller energy packets.

    Thus, pulse starting circuits are more effective if the wire length (and thus the capacitance) between the power supply's final rectifier and HeNe tube anode is minimized.

    Locating a large enough pulse transformer to start the tube in one shot may prove to be a challenge. However, a charge pump (really, just an extra high voltage diode or two) can be added to any of these circuits so that the energy in each pulse can be accumulated. The tiny trigger transformer from a disposable pocket camera will not work very well by itself but should work with a such a charge pump. See the section: Pulse Starting Circuit Using Small Flyback Transformer for the charge pump configuration.

    A flyback transformer from a small B/W TV or computer terminal does work quite nicely with a charge pump. An automotive ignition coil would probably be large enough for single shot operation.

    Replacing the pulse button with a simple transistor oscillator results in an inverter based starting circuit. See the section: Inverter Based Starters for more information on this approach.

    The first three circuits that follow differ only in the type of trigger transformer used but operation is otherwise identical. These have not been tested.

    The final circuit uses a flyback transformer from a long forgotten video display terminal and has been tested using a manual pushbutton.

    Some other options using salvaged parts (or at least salvaged designs) from household devices include:

    Pulse Starting Circuit - Trigger Transformer with Isolated HV Winding

    This is the basic circuit where the primary and secondary of the trigger transformer are isolated with an insulation breakdown rating of at least 12 kV. The major difference between this circuit and those to follow is that it is referenced to the system common (HV-) rather than floating at a high voltage (on one of the filter capacitors).

    Operation is similar to a repeating strobe trigger.

    
     Z o--------------------+-----------+                                  
                            |           |    R5                   o        
                            / R1        +---/\/\--+------+---------+    
                            \ 10M          4M 1W  |      |          ):: +---o Y
                            /      NT1            |    __|__ SCR1   )::(
                            |      NE2            |    _\_/_ TIC106 )::(
                Q1     R8   |      +--+   R3  T   |    / |          )::(
            MPSA43 +--/\/\--+---+--|oo|--/\/\--+--|---'  |   +-----+ ::(
                   |  100K  |   |  +--+   1K   |  |      |   |       ::(
     Tube-  R8   |/ C       /  _|_             /  /      |  _|_ C2   ::(
      o-+--/\/\--|       R2 \  --- C1       R4 \  \ R6   |  --- 1uF  ::(
        |   1K   |\ E    1M /   |  .5uF     1K /  / 600K |   | 600V  ::(   CR1  HV+
        /          |        |   |  200V        |  |      |   |          +--|>|--o
        \ R7       +--------+---+--------------+--+------+---+           o
        / 200      |                                                  T2
        |          |
      o-+----------+
     HV-
    
    
    The voltage divider formed by R1 and R2 charges C1 from the high voltage power supply (Z, a lower voltage tap if possible to reduce the dissipation in R1 and R2). At the same time, C2 charges from R5 and R6 (this time constant is faster than that of the relaxation oscillator). Once the voltage across NT1 (NE2 neon tube) reaches about 90 V, NT1 breaks down dumping C1's charge through the gate of SCR1. This turns on and discharges C2 through T2 generating a 5 to 10 kV pulse in series with the high voltage power supply ionizing the gas in the HeNe tube. CR1 must be a 15 kV or greater high voltage rectifier and prevents reverse voltage from appearing at the tube.

    Should the HeNe tube not fire on the first pulse, the process repeats at about a 2 Hz rate until current starts flowing in the tube. A current of about 3.5 mA through the HeNe tube and R7 results in a voltage drop of .7 V across B-E of Q1 turning it on. This short circuits the relaxation oscillator shutting off the starting circuit.

    Component values can easily be adjusted to accommodate the specifications of your specific power supply voltage and HeNe tube current.

    These are all basically capacitive discharge ignition systems and indeed, an automotive ignition coil may be satisfactory for the trigger transformer where isolation is not needed!

    Isolated Pulse Starting Circuits

    The following two circuits are virtually identical. The only difference is whether the dots on the pulse (trigger) transformer match or oppose. One of these should be used when the trigger transformer shares a common lead between the primary and secondary.

    Operation for both is as follows:

    The trigger capacitor, C2, charges through the voltage divider formed by R1 and R2. When a pulse is input to the gate of the SCR via the high voltage coupling capacitor, C1, it triggers dumping C2 through the primary of the trigger transformer.

    WARNING: The voltage rating on C1 must be adequate - with a safety margin - for your power supply.

    The input, T, comes from the autotrigger circuit which is part of the circuit shown in the section: Pulse Starting Circuit - Trigger Transformer with Isolated HV Winding.

    Pulse Starting Circuit Using Small Flyback Transformer

    This circuit has only been tested with a manual push button. Usually, 3 or 4 presses of the button are needed to build up the needed starting voltage via the charge pump formed by CR1 and CR2. These are needed because a single press of the button may not generate enough charge to achieve the required starting voltage. They enable the charge to be accumulated on the stray capacitance of the wiring, CR1, and HeNe tube anode. The diodes should have a PRV rating greater than the starting voltage requirement of your HeNe tube, 15,000 V should be sufficient. This type of diode is typically used in B/W TVs and monochrome computer monitors. Microwave oven high voltage rectifiers also work but not nearly as well (many more presses of the button are needed) due probably to their higher capacitance and leakage.

    Higher input voltage, more or fewer turns on the flyback, or a different capacitor may improve response - your challenge!

    The primaries on the flyback are ignored and a new one is added - 5 turns of #20 or thicker insulated wire wound anywhere on the core where it will fit. As long as the original primary windings are not shorted, they will not interfere with circuit operation.

    Locate the HV return and guess at the polarity - it will work properly only one way since the output is a huge spike. Reverse the input connections if you cannot get the tube to fire and you think everything else is correctly wired.

    I used a separate 50 VDC power supply to drive this circuit but you can use a tap on the main filter capacitor of the main supply as well.

                         R1
        + o-------------/\/\---+------------+            +----o Y
                         1K    |            |      T2    |
                               |            |         +--+---|>|---+
                               |            |   o  ::(       CR2   |
                               |            +----+ ::(             |
                              _|_ C2              )::(             |
        50 VDC                --- 20uF        5 T )::(             |
                               |  100V        #20 )::(             |
                               |                  )::(             |
                               |            +----+ ::(             |
                               |     S1     |      ::( o           |   CR1
                               |     _|_    |         +------------+---|>|---o HV+ 
        - o--------------------+-----o o----+             
                                    Start
    
    

    HeNe Starter Using Electronic Ignition (or Other Similar) Modules

    No, not the ignition module from your 1985 Chevy - though that would probably work at least as well. :-)

    Many modern gas stoves, ovens, furnaces, and other similar appliances use an electronic ignition rather than a continuously burning pilot flame to ignite the fuel. Well, guess what is in one of these modules? That's right - a high voltage pulse generator which is ideal for starting HeNe tubes!

    The typical design is remarkably similar to that of the circuits described in the previous sections starting with: Starting Circuits Using Pulse or Flyback Transformers. It is powered either from 115 VAC or 24 VAC and generates of a series of sparks at a rate of perhaps 1 or 2 per second. These modules should be readily available as replacements at your local appliance repair shop or parts supplier.

    Internally, a typical 115 VAC unit consists of a simple rectifier/filter or doubler, neon bulb relaxation oscillator triggering an SCR which dumps a capacitor's charge into a pulse transformer - very similar to trigger circuit of a repeating strobe.

    All that is needed to convert one into a HeNe starter is a suitable power source and a pair of HV diodes to form a charge pump (as described above). Depending on design, it may be possible to run these from a DC power supply or one derived from your HeNe operating voltage.

    The Harper-Wyman Model 6520 Kool Lite(tm) module is typical of those found in Jenne-Aire and similar cook-tops. Input is 115 VAC, 4 mA, 50/60 Hz AC. C1 and D1 form a half wave doubler resulting in 60 Hz pulses with a peak of about 300 V and at point A and charges C2 to about 300 V through D2. R2, C3, and DL1 form a relaxation oscillator triggering SCR1 to dump the charge built up on C2 into T1 with a repetition rate of about 2 Hz.

    Based on the 10 kV or so output of this module, I estimate that T1 has a step-up turns ratio of about 35:1 but have no easy way of determining the exact number of turns since it is potted in clear plastic. Fortunately, the primary side of the circuit was accessible after prying off the bottom cover plate. My totally wild guess is that the primary and secondary are about 25 and 900 turns respectively.

    
               C1                A       D1                     T1 o
        H o----||----------------+-------|>|-------+-------+       +-----o HVP+
              .1uF     D2 1N4007 |     1N4007      |       |  o ::( 
              250V   +----|>|----+                 |       +--+ ::(
                     |           |                 |           )::(
                     +---/\/\----+                 |       #20 )::( 1:35
                     |  R1 1M    |             C2 _|_          )::(
                     |        R2 /            1uF ---      +--+ ::(
                     |       18M \    DL1    400V  |     __|__  ::(
                     |           /    NE2H         |     _\_/_     +-----o HVP-
                     |           |    +--+         |     / |
                     |           +----|oo|----+----|----'  | SCR1
                     |       C3  |    +--+    |    |       | S316A
                     |   .047uF _|_        R3 /    |       | 400V
                     |     250V ---       180 \    |       | 1A
                     |           |            /    |       |
             R4 2.7K |           |            |    |       |
        N o---/\/\---+-----------+------------+----+-------+
    
    
    For use on DC, remove C1 (and D1, D2, and R2 if you like - they won't affect anything) and provide about 300 V from your HeNe operating supply (typically from point A of the schematics shown in the chapter Complete HeNe Laser Power Supply Schematics. Just add a pair of HV rectifiers (D3 and D4) to form a charge pump and current bypass, and an optional auto-shutoff circuit (not shown).

    The resulting HeNe starting circuit is shown below:

    
                                                      T1 o             D3
       Z o-------------+-----------------+-------+       +----------+--|>|--o HV+
                       |                 |       |  o ::(           | 15kV
                       |                 |       +--+ ::(           |
                       |                 |           )::(           |
                       |                 |       #20 )::( 1:35      |
                       |             C2 _|_          )::(           |
                    R2 /            1uF ---      +--+ ::(           |
                   18M \    DL1    400V  |     __|__  ::(      D4   |
                       /    NE2H         |     _\_/_     +--+--|>|--+
                       |    +--+         |     / |          | 15kV
                       +----|oo|----+----|----'  | SCR1     o
                   C3  |    +--+    |    |       | S316A    Y
               .047uF _|_        R3 /    |       | 400V
                 250V ---       180 \    |       | 1A
                       |            /    |       |
            R4 2.7K    |            |    |       |
       N o---/\/\------+------------+----+-------+
    
    
    Some types of lighters generate a spark in a very similar manner. The guts from these may be pressed into service as HeNe starters (a much more noble cause than their original application, I might add!) with the addition of a couple of HV diodes. One type uses a 12 V battery, 330 uF, 16 V capacitor, trigger switch, and tiny pulse transformer (less than 0.5" x 0.5" x 0.4"). There are about 12 turns on the primary and several thousand turns on the secondary producing an output of more than 5 kV. The circuit is basically similar to that shown in the section: Pulse Starting Circuit Using Small Flyback Transformer.

    The trigger portion of an electronic photoflash or strobe (not the main energy storage/discharge components and xenon tube) may also be used to start HeNe tubes. These run on either batteries or the AC line (sound familiar?) and produce HV pulses of between 4 to 10 kV. You may already have a dead pocket or disposable camera laying around (or your expensive, neglected Nikon) which has such a module. See the document: Notes on the Troubleshooting and Repair of Electronic Flash Units and Strobe Lights for detailed sample circuits as well as schematics from some common pocket and disposable cameras and separate flash units.

    HeNe Starter Using Electronic Air Cleaner HV Module

    Electronic air cleaners include a high voltage low current power supply and oppositely charged grids in the air flow. A typical unit might produce 7.5 to 10 kV at a few uA - perfect for a HeNe starter!

    Here is a circuit which I assume is for an electronic air cleaner or something similar. I received this thing in the mail (no markings). I did check to make sure it wasn't a bomb before applying power. :-) A photo is shown in: Air Cleaner HV Module or go to EIO's 99 Cent Specials Page. This one is nice because all parts are accessible so modifications are easy to make. Or, just strip the HV diodes and caps from a couple of these modules and build your own voltage multiplier type starter. Complete air cleaners may use a potted circuit that looks like a mini-HeNe power supply brick but isn't. These can still be used for HeNe laser starters but your options are more limited.

    The AC line powered driver and HV multiplier are shown in the two diagrams, below:

    
                       D1                                           T1  o
      H o--------------|>|----+---+--------------------+               +-----o A
                     1N4007   |   |        Sidac     __|__ SCR1     ::(
                              |   |   R3  D2 100 V   _\_/_ T106B2   ::(
      AC                  C1  |   +--/\/\---|>|      / |   200V     ::(
     Line      Power   .15uF _|_     1.5K   |<|--+--'  |   4A     o ::( 350 ohms
              IL1 LED   250V ---                _|_    |  +-------+ ::(
            +--|<|---+        |              C2 ---    |  |        )::(
            |   R1   |   R2   |         .0047uF  |     |  | .1 ohm )::(
      N o---+--/\/\--+--/\/\--+                  +-----+--+        )::(
               470      3.9K  |                                +--+    +--+--o B
                1W       2W   |                                |    R4    |
                              +--------------------------------+---/\/\---+
                                                               2.2M (Remove,
                                                                see below)
    
    
    The AC input is rectified by D1 and as it builds up past the threshold of the sidac (D2, 100 V), SCR1 is triggered dumping a small energy storage capacitor (C1) through the primary of the HV transformer, T1. This generates a HV pulse in the secondary. In about .5 ms, the current drops low enough such that the SCR turns off. As long as the instantaneous input voltage remains above about 100 V, this sequence of events repeats producing a burst of 5 or 6 discharges per cycle of the 60 Hz AC input separated by approximately 13 ms of dead time.

    The LED (IL1) is a power-on indicator. :-)

    The transformer was totally potted so I could not easily determine anything about its construction other than its winding resistances and turns ratio of about 1:100.

    
                                                 A o
                                          C3       |
                                   +------||-------+
      (-5 kV)  R5     R6      D3   |   D4     D5   |  D6     R7       R8   (+5 kV)
        Y o---/\/\---/\/\--+--|>|--+--|>|--+--|>|--+--|>|---/\/\--+--/\/\---o HV+
              10M    10M   |      C4       |                220K  |  10M
                           +------||-------+                      |
            D3-D6: 10 kV, 5 mA            _|_                    _|_
            C3,C4: 200 pF, 10 kV          --- C5                 --- C6
            C5,C6: 200 pF, 5 kV            |                      |
                                      B o--+----------------------+
    
    
    The secondary side consists of a voltage tripler for the negative output (Y) and a simple rectifier for the positive output (HV+). I assume this asymmetry is due to the unidirectional drive to the transformer primary.

    From my measurements, this circuit produces a total of around 10 kV between HV+ and Y, at up to 5 uA. The two output voltages are roughly equal plus and minus when referenced to point B.

    The only modification required for our needs is to remove R4 to isolate the HV secondary from the AC line and earth ground. Once this is done, it may be necessary to add insulation and/or reroute some of the wiring on the PCB to prevent arcing as the HV attempts to find its way to ground via the AC since the negative of the main HV power supply (i.e., Tube-) should be grounded.

    Then, connect the high voltage outputs up like that of the other inverter based starters using a HV bypass diode (at least 10 kV) to isolate the HeNe tube anode circuit from the main power supply. A 1 nF, 10 kV capacitor across the diode may be needed to reliably start tubes on short cables (which don't have much capacitance). Using a pushbutton switch to activate the circuit is probably easiest though leaving it on all the time will probably do no harm since its output current is so small. For the purist, an opto-triac or similar device can be added to disable the pulse discharge circuit once current flows in the HeNe laser tube.

    Where there is no access to R4 (e.g., the unit is totally potted), it probably won't be possible to obtain the full output voltage from the positive or negative leads to ground (only between them). This is because tying one of the outputs to ground (as would be required if the cathode of the HeNe tube is earth-grounded as it should be unless totally enclosed and insulated) will partially short the internal voltage multiplier via R4 to the AC line (the Neutral wire is grounded at the service panel). In this case, leave the negative output of the air cleaner module floating. This may still result in enough starting voltage for smaller HeNe tubes (up to perhaps 2 mW). Don't even think about attempting to isolate the HeNe tube or starter from ground unless as noted, it is totally enclosed!

    A quick check to see if one of these modules has enough output voltage to start your HeNe tube is to connect it up as follows:

    
                   +-------------+ +    100K   +-------------+
           H o-----|             |------/\/\---|-          |-|-----+
         AC Line   | Air Cleaner |             +-------------+     |
         115 VAC   | HV Module   | -              HeNe Tube        |
           N o-----|             |-----+                           |
                   +-------------+     |<-- (Connect only if       |
                                       |     R4 is removed)        |
           G o-------------------------+---------------------------+
    
    

    The HeNe tube will glow faintly (actually pulsing at the AC line frequency) if there is enough voltage to start. Assuming the tube is good, there may even be an indication of lasing, but with very low (and possibly erratic) power - maybe as much as 1 microWatt. :) So, one of these modules can be used to perform a quick check of a HeNe tube as well.

    Alternatively, with a bit more rewiring, the multiplier can be used in-line with the HeNe laser operating voltage supply. Or, the high voltage diodes and capacitors (probably from a pair of these units) could be removed and used for a parasitic multiplier driven from the AC line or inverter based power supply.

    I tested this starting approach with the inverter, doubler, and filter capacitor portion of SG-HI3 (with no starting multiplier, see the section: Sam's Inverter Driven HeNe Laser Power Supply 3 (SG-HI3).) It worked quite well starting instantly every time. The electrostatic air cleaner unit used for this experiment, while similar to the one described above, was totally potted and therefore I had to leave its negative lead floating. But it was still more than adequate to start a 1.5 mW HeNe laser tube.

    I intend to use one of these modules in conjunction with a partially broken Aerotech brick power supply for HeNe lasers in the 5 mW class - model LSS5(L)6.5, 2,500 +/-300 V at 6.5 mA. This particular unit has lost its ability to generate sufficient starting voltage, peaking at only 4 kV (it should be greater than 10 kV), cause unknown. However, it runs just fine. The external starter will enable an otherwise useless brick to be salvaged for about 99 cents plus the cost of a HV diode, HV cap, and pushbutton switch (or opto-triac if I feel more ambitious, or maybe have both manual and automatic start, switch selectable). I have already tested the setup but still need to mount everything in a nice box. The biggest problem was rerouting one of the traces on the air cleaner PCB to prevent arcing once it was isolated with its outputs floating on the HV+ side of the Aerotech brick. :)

    Inverter Based Starters

    An alternative to the pushbutton switch (above, or the SCR circuits in the previous designs) is to construct a high voltage inverter using a flyback transformer that is only active when starting.

    The HV output is then placed in series with the HV+ of the main supply as above (Y to HV+) through high value isolation resistors (10M or greater rated for at least 15 kV). These are required for safety and protection: so that the output of the main supply cannot appear on the inverter with any significant current and so that the inverter is not overloaded (partially short circuited) by the tube once it starts. In addition, the resistors prevent any significant ripple on the inverter output from turning the HeNe tube *off* once it starts. While it takes 1000s of volts to start a HeNe tube, it only takes a few volts to shut it off!

    A high voltage blocking diode (e.g., one or more 15 kV PRV microwave oven HV rectifiers) bypasses operating current around the inverter once the tube starts. A stack of 1 kV diodes (as many as needed for the maximum starting voltage) can also be used. General purpose (non-fast recovery) types like 1N4007s are fine. Make sure they are well insulated - inside a thick plastic tube, for example. Since 1N4007s are only a few cents each in modest quantity (not from Radio Shack!), this may be much cheaper. It IS more flexible as a rectifier of nearly any desired voltage rating can be custom made. I have replaced a fried (from overvoltage) microwave oven rectifier (in a HeNe laser power supply, NOT a microwave oven!) with a stack of 20 1N4007s to create a (hopefully) more robust 20 kV unit. However, it would probably be better to derate them by 25 to 50 percent to be sure of the minimum PRV rating.

    The 1nF capacitor may only be needed when driving HeNe laser tubes or heads attached by short cables or open wiring. Normally, the capacitance of the cable to a laser head will provide enough energy storage to keep the discharge going once it strikes. But where this is inadequate, starting may be erratic. If the output of the power supply always goes though a few feet of coax, it probably isn't needed.

    
           Y o-------+-----|>|-----+-------o HV+ (to ballast resistor)
                     |    15kV     |
                     +-----||------+
                     |    1nF      |
                     /             /
                  R1 \          R2 \
                     /             /
                     \             \
                     |             |
                     o - Starter + o
    
    

    As shown above, the inverter is configured to add its output to the operating voltage of the main supply. For this scheme to work, the negative terminal of the source must float at the voltage at 'Y'. This may be possible when using a flyback transformer with your own windings. However, it is almost never possible when using existing circuitry. In that case, the negative can be tied to ground (R1 is omitted) but then the benefit of having the sum of the operating and starting voltages is not available. The added voltage may not matter unless the HeNe tube needs all the help it can get!

    To make these fully automatic - to disable the starter once the HeNe tube fires - a very simple circuit needs to be added to the inverter controller. For example, where drive is provided by a 555 timer chip in astable mode, the following circuit will work:

    
                                    Vcc (555)
                                       o
                                       |
                                       /
                                    R4 \
                                    1K /
                                       \
                                       |          _
                                       +--------o R - Reset input to 555 (low)
                                       |
                                     |/ C Q1
         Tube- o-------+-----+-------|    2N3904
                       |     |       |\ E
                       /     |         |
                    R3 \    _|_ C1     |       This assumes the cathode of the
                   270 /    --- .1uF   |       HeNe tube was grounded.  Current
                       \     |         |       sense resistor and 'Beam On' LED
                       |     |         |       note shown.
                       +-----+---------+
                                      _|_
                                       -
    
    

    Once the HeNe tube starts, current of more than about 3 mA through R3 switches the transistor ON which disables the inverter drive by forcing the 555 to the reset state. C1 prevents the inverter from being cutoff if the tube pulses momentarily but doesn't stay on.

    For examples of inverter based starters in actual HeNe laser power supplies, see the sections: Sam's Mid-Size Line Powered HeNe Laser Power Supply (SG-HL2), and Sam's Ultrabeam(tm) Line Powered HeNe Laser Power Supply (SG-HL3).

    Inverter Based Starter Examples

    Most of the following approaches really only make sense when dealing with larger (i.e., over 5 mW) or hard-to-start HeNe tubes. For your .5 mW barcode scanner laser, they would be gross overkill - somewhat like using a 100 A power supply for a single LED!

    Since it is relatively straightforward to generate voltages up to 15 kV using small readily available flyback transformers (even more using those from color TVs or monitors), and this gets ADDED to the operating voltage, such an approach is quite effective for large and hard-to-start HeNe tubes. In addition, because the inverter does not depend on high voltage AC or DC from the main supply, it is also easier and safer to construct and troubleshoot, and this also provides added design flexibility.

    Here are some possibilities:

    Sam's Super-Starter(tm)

    This would be called a kludge by some, a Rube Goldberg by others. But, hey, as still others would say: "If it works, use it!".

    The entire horizontal deflection and high voltage sections of a long obsolete and lonely ASCII video display terminal were pressed into service for starting larger HeNe tubes. A source of about 12 VDC at 1.5 A is needed for power and a 555 timer based oscillator is needed to provide the fake horizontal sync:

    I guarantee that "Sam's super-starter(tm)" - or its big brother, "Sam's hyper-starter(tm)" using parts from a color TV or monitor - will start ANY HeNe tube that can possibly be started! These also make nice self contained HV sources for other experiments. :-)

    CAUTION: Since these power supplies were designed for a specific purpose under specific operating conditions, their behavior when confronted with overloads or short circuits on the output will depend on their design. It may not be pretty - as in they may blow up! Take care to avoid such events and/or add suitable protection in the form of fast acting fuses and current limiting to the switching transistor.

    Piezo Impulse Starting Circuits

    These circuits use a manual pushbutton but are about as simple as you can get.

    They are based on the spark generating assembly from a disposable butane lighter that uses a piezo element to generate the spark (as opposed to old fashioned flint and wheel) or the starter assembly from a gas grill. These devices are based on the piezo-electric effect which results when certain materials are twisted or otherwise deformed. Even these dirt-cheap lighter or gas grill starters are able to produce more than 10,000 V mechanically with the press of a button.

    A simple charge pump using a pair of high voltage diodes completes the starting circuit. These are needed because a single press of the button may not generate enough charge to achieve the required starting voltage. This is true at least of the type from disposable lighters. Perhaps, one from a gas would work by itself. The charge pump enables the charge generated by each button press to be accumulated on the stray capacitance of the wiring, CR2, PE1, and the HeNe tube anode. The diodes should have a PRV rating greater than the starting voltage requirement of your HeNe tube, 15,000 V should be sufficient. These type of diodes are typically used in B/W TVs or monochrome computer monitors. Microwave oven high voltage rectifiers also work but not nearly as well (many more presses of the button are needed) due probably to their higher capacitance and leakage.

    Both of the circuits below work but the second one may have an edge since the capacitance being charged is slightly smaller since it doesn't include PE1 and its wiring:

      
                                         HV+
                                          o
                       CR1         CR2    |     Rb
              Y o------|>|----+----|>|----+----/\/\-------o Tube+
                              |           |
                              |   -   +   |
                              +----|X|----+
                                 |--^  PE1
    
    
                                           HV+
                                            o
                         CR1         CR2    |     Rb
            Y o-----+----|>|----+----|>|----+----/\/\-------o Tube+
                    |           |
                    |   -   +   |
                    +----|X|----+
                       |--^  PE1
    
    
    Complicated, huh?

    CAUTION: Since the piezo assembly/push button is connected to the positive terminal of the high voltage power supply, make sure it is well insulated or the only thing being triggered may not be the HeNe tube!

    Make sure the wiring is short and well insulated - these high voltage pulses tend to go wherever they want unless properly trained. :-)

    I tested this with the piezo assembly removed from a Scripto lighter found in the park. (You will have to agree that this is classic McGyver!) The required part is self contained and pops off with minimal disassembly (just remove the sheet metal flame guard). With this particular device, the pointy electrode is negative. I do not know if this is always the case. If the HeNe tube does not start after a reasonable number of button presses, try interchanging the connections to the piezo assembly.

    Replacement push button starters for gas grills can be purchased at most home centers and may even work better because they are likely to produce a higher voltage higher energy spark. I will not be responsible if your dad cannot barbecue the dogs and burgers because you 'borrowed' the starter. :-)

    Long-reach butane "multipurpose lighters" are also readily available almost everywhere. I paid 77 cents for one on a special. As with the Scripto, the piezo mechanism is a self contained module but keeping the entire lighter intact is most convenient since it includes a large trigger button and nicely insulated high voltage wires. The butane tank can't easily be removed since part of it is the support structure for the piezo module and trigger, but it could be drained via the fill valve thing at the bottom outdoors and away from open flames if you can't wait through the required number of cookouts. In any case, I'm sure you'll find a way to use up the butane so that only the much more useful spark remains! ;-)

    As with transformer based pulse starting circuits, these are more effective if the wire length (and thus the capacitance) between the first high voltage rectifier (CR1) and the HeNe tube anode is minimized.

    For my bare 1 mW HeNe tube, two presses of the button were required. For a laser head with a 3 foot long high voltage cable, 4 or 5 presses were needed but it always started eventually.

    RF or Microwave Starters

    The following isn't really a serious suggestion - at least not as part of a little HeNe laser. However, it could possibly be a way of getting a really hard-to-start tube to start for testing at least: Since anything at all that gets the gas ionized in the bore should work as a starter, it may be possible to use a source of RF (e.g., from that long forgotten Ham rig) for this purpose. A small Tesla coil might also be suitable its voltage isn't high enough to puncture the glass. :( How to couple the RF energy to the tube is left as an exercise for the student - it could be via a wire wrapped around the tube, placing the tube near the output coil or antenna, or by attaching the anode directly via a small high voltage capacitor.

    And, here's another one to be taken even less seriously: Cut a pair of holes in the front and back of the a microwave oven just large enough for some of those longer HeNe tubes to fit! Starting cycle: 3 seconds on HIGH (with a cup of water to act as a load)! Yes, I know, it would look kind of silly to have your HeNe laser tube sticking out of the family microwave! I did say I wasn't really serious, didn't I? :) WARNING: Please don't attempt this stunt - both dangerous levels of microwave leakage and HeNe tube frying arcs are quite possible!

    OK, I think that's enough on HeNe laser starters! ;-)



  • Back to HeNe Laser Power Supply Design Sub-Table of Contents.

    Current Regulators

    Regulation Requirements for HeNe Laser Power Supplies

    Since for optimal performance, the HeNe tube expects a specific operating current, a regulator is desirable but not essential. Current is not nearly as critical with HeNe lasers as with laser diodes. A few percent on either side of the recommended value won't matter.

    Supply voltage fluctuations do affect operating current proportionately more than would be expected since the voltage drop across the tube is fairly constant. Therefore, nearly the entire change appears across the ballast resistor. The smaller the ballast resistor, the larger the current change for a given voltage change. This may mean that a 2 percent change in line voltage produces a 10 percent change in tube current - still not a big deal for most applications. For experimental use, it is perfectly acceptable to use ower supply without regulation and a Variac to control the input as a way of adjusting output current. The setup I routinely use for testing HeNe laser tubes is a combination of a Variac, an unregulated HeNe laser power supply, and a ballast resistor selector and meter box. See the section: Ballast Resistor Selector and Meter Box. Some commercial HeNe lasers have even used a power rheostat in series with the input of the main HV transformer for this purpose!

    However, where stability is required (both short term and long term due to component and tube aging) or where it is desired to be able to switch tubes without monitoring tube current and adjusting the input voltage, a regulator is essential. This also provides an easy way of adding a current adjustment control to accomodate a variety of HeNe tubes.

    In addition to the circuits below, see the chapter: Complete HeNe Laser Power Supply Schematics for other possibilities.

    Low-Side Post-Regulator

    This circuit (similar to the one described in the section: Spectra-Physics Model 155 HeNe Laser Power Supply (SP-155) can be used to stabilize the output of an unregulated supply. Within its compliance range, the current will be approximately: 10.3 * V/R2.
    
                                             +------------+---o Tube-
                                             |            |
                             R1            |/ C Q1        |
               Z o----------/\/\------+----|    MJE3439  _|_, VT2
                             5M       |    |\ E         '/_\  150V
                             1W       |      |            |
                                     _|_,    /            |
                                VR1 '/_\  +->\ R2        _|_, VR3
                            1N5241B   |   |v / 5K       '/_\  150V
                                11V   |   |  \            |
                                      |   |  |            |
                                      +---+--+------------+---o HV-
    
    
    CAUTION: If your tube is mounted with its cathode connected to a metal case, earth ground must be tied to Tube- for safety as Tube- may be at a potential of several hundred volts with respect to HV-.

    The compliance range is about 300 V so a ballast resistor still needs to be selected to enable the regulation to be effective. Use the procedure described in the sections starting with: Selecting the Ballast Resistor to determine an appropriate ballast resistor value for the desired tube current and a power supply voltage 150 V less than that of your (unregulated) supply. This circuit should then maintain that current constant and permit some adjustment (or modulation if desired) of the current on either side of the set-point.

    For greater compliance, a cascade of high voltage transistors can be used as outlined in the section: High Compliance Cascade Regulator. This shows a low-side approach. Also see the section: Spectra-Physics Model 247 HeNe Laser Power Supply (SP-247) which is a high-side version of a similar circuit.

    High Compliance Cascade Regulator

    The circuit below is suitable for providing current regulation for high power HeNe laser power supplies. As drawn it should be capable of about 1,000 to 1,200 V compliance at up to 10 mA. Additional high voltage NPN transistors and base resistors may be added to extend its range still further.

    
              Rb   Tube+ +------------+ Tube-   Rs   IL2 LED         G
     HV+ o---/\/\--------|-         |-|----+---/\/\----|>|------+----+
                         +------------+   _|_   1K  Beam On     |    |
                              LT1          -                 R1 /    |
                                                           120K \    |
                                                             2W /    |
                                                                |  |/ C Q1
                                                                +--|    MJE2360T
                                                                |  |\ E
                                                             R2 /    |
                                                           120K \    |
                                                             2W /    |
                                                                |  |/ C Q2
                                                                +--|    MJE2360T
                                                                |  |\ E
                                                             R3 /    |
                                                           120K \    |
                                                             2W /    |
                                                       R4       |  |/ C Q3
                                                +-----/\/\------+--|    MJE2360T
                                                |     120K         |\ E
                                                |      2W            |
                                                |                    |
                                                |                    |
                             Rx                 |      R5          |/ C Q4
             Z o------------/\/\----------+-----+-----/\/\------+--|    MJE2360T
                            470K          |     |     10K       |  |\ E
                                          |  R6 /             |/ E   |
                                          |100K \<------------|      |
                                    CR1  _|_,   / Adjust   Q5 |\ C   |
                                 1N4744 '/_\    |       2N3906  |    |
                                    15V   |     |       (PNP)   +----+
                                          |     |   +--+             |
                                          |     |   |  v R7    R8    |
                                  HV- o---+-----+---+-/\/\----/\/\---+
                                                  Range  5K   1.5K 
    
    

    The PNP transistor (Q5) buffers the reference voltage so that the very low current source (point Z through Rx tapped off of the main filter capacitor string) can drive the base of the cascade. The way to understand its operation is to realize that Q5 implements an emitter follower with respect to the tap on R6. Without it, the cascade would be driven directly from the 15 V reference point; Q5 drags down Q5's base to the value (+0.7 V) set by R6. I think, I hope. :) If the reference can supply a couple of a few mA (say, from a 50 V source through 10K (Rx), R6 can be reduced in value and the circuit could be simplified as shown below:

    
                                                |                    |
                             Rx                 |            R5    |/ C Q4
             Z o------------/\/\----------+-----+       +---/\/\---|    MJE2360T
                            10K           |     |       |   10K    |\ E
                                          |  R6 /       |            |
                                          |  5K \<------+            |
                                    CR1  _|_,   / Adjust             |
                                 1N4744 '/_\    |                    |
                                    15V   |     |   +--+             |
                                          |     |   |  v R7    R8    |
                                  HV- o---+-----+---+-/\/\----/\/\---+
                                                  Range  5K   1.5K 
    
    

    The base resistors, R1 through R4 (more or less) equally distribute the voltage between G and HV-. The respective transistors act as emitter followers and maintain approximately the same voltages across their C-E terminals. Within the compliance range, the voltage across R7+R8 will be equal to the voltage on the wiper of R6 (give or take a diode drop depending on whether Q5 is present).

    CAUTION: Make sure that the open circuit voltage of your power supply minus the laser tube and ballast resistor voltages cannot exceed the total breakdown voltage of the transistor cascade (4 * Vceo in this case as bad things may happen)! As drawn, it is certainly suitable for a 2,200 V power supply with almost any tube. However, if you are using this with a 3,000 V supply and a .5 mW tube requiring 1,100 V at 3 mA, the compliance range will be exceeded and the transistors may blow - pop-pop-pop-pop! Then, you get maximum current through the ballast resistor and laser tube which will make them extremely unhappy :-(. A bag of 200 V zeners in series could be used to limit the voltage across the transistor cascade. Or, see the section: Spectra-Physics Model 255 Exciter (SP-255) for an example of a simple overvoltage protection circuit.

    R7 sets the overall current range. R6 adjusts the current from a minimum determined by the maximum voltage across R1 through R4 to the limit set by R7. However, regulation should be better when R6 is set near its maximum since small changes in voltage/current will have correspondingly less effect than if you are trying to regulate with just a few mV difference to play with! The adjustable R6 can be omitted entirely if desired (tap off the top of CR1) and R7 used as the current adjust. The only thing undesirable about this is that this control is somewhat non-linear so that the high current portion is all squashed at one end.

    Rs is a current sense resistor. Monitoring across it with a volt meter is a convenient way of setting tube current. Sensitivity is 1 V/mA. A current meter can also be put across it and will read tube current directly. The LED provides a 'Beam On' indication and rough measure of tube current as well.

    Increasing Linear Regulator Current

    While the regulators described above will be perfectly happy with the current required by most internal mirror HeNe tubes (3.3 to 8 mA), there are larger (or different types of) lasers and unusual configurations where more current is needed.

    The basic linear cascade regulator should be capable of higher current as long as the transistors are mounted on adequate heatsinks can care is taken to ensure they don't operate anywhere near the limits of their Safe Operating Area (SOA).

    Devices like the MJE3439T are often used without any heatsinks in commercial designs so adding one will go a long way to preserving their lives. What may need to be done in addition is to reduce the value of the base resistors so that there is adequate drive based on the transistor's Hfe specifications. To be safe, increasing the number of stages in the cascade to compensate for the additional current and keep the power dissipation of each transistor may be prudent.

    For dual (or multiple) tube applications or dual discharge lasers like the Spectra-Physics model 125 (SP-125), a pair of regulators could also be used. (Note that separate ballast resistors will be required for these.) For the SP-125 in particular, a common cathode at the center of the tube is fed from separate anodes at each end. Where a power supply is to be built (or the original one repaired or rebuilt), the total discharge current of 35 mA can easily be split into two currents of 17.5 mA each - easily handled with very minor modifications to the basic design in the section: High Compliance Cascade Regulator. See the section: Spectra-Physics 120, 124, and 125, HeNe Laser Specifications if you are curious about the SP-125.

    Another alternative is to parallel multiple cascades and just combine their outputs. However, with proper heatsinking, this shouldn't be necessary for HeNe lasers. But carbon dioxide, helium-cadmium, or other lasers requiring greater discharge current, this and/or the use of higher power transistors may be required. Or better yet, the use of a more efficient high frequency inverter instead!

    Simple Shunt Regulator

    For some types of power supplies, a shunt regulator can be added to the output prior to the starter. This, in effect, acts like a huge high power zener diode across which bypasses enough current through it to keep the voltage below a selected value.

    A simple approach like this works quite well for supplies using high-droop transformers (e.g., oil burner ignition or luminous tube/neon sign types). The no load voltage on these can easily exceed 1.5 times the operating voltage and the excess energy in the filter capacitors would normally be dumped into the HeNe tube when it starts - which is hard on the tube!

    A shunt regulator consisting of a stack of zener diodes (or gas tubes, remember those?) prior to the bypass HV diode could be added to reduce this difference by providing a substitute current path - loading down the transformer - until the HeNe tube starts.

    In this case, since the regulator is only passing significant current before the tube starts, with careful design, the zeners or whatever don't need to dissipate high power for too long. That is, unless the tube fails to start or becomes disconnected!

    For high droop transformers, a dropping resistor is probably not needed at all. However, for normal power transformers, a dropping resistor (Rd in the diagram below) should be used to prevent the transformer from being overloaded (as well as the regulator being smoked. These are able to source much greater than their rated current (at least for a short time) with little reduction in output voltage.

    
                                  Rd
                           Y o---/\/\---+----o Y' To Starter
                                        |
                                       _|_,
                                  ZD1 '/_\ 
                                       _|_,
                                  ZD2 '/_\
                                   .    |
                                   .    |
                                   .   _|_,
                                  ZDn '/_\
                                        |
                                        o
                                       HV-
    
    
    When contemplating using this sort of approach for regulation of the operating voltage itself, both the continuous power dissipation of the regulator components AND power supply transformer (or inverter) must be taken into consideration. For example, to regulate at 2 kV with a maximum current of 8 mA using a power supply with a no-load output at Y of 3 kV, you can use 20, 100 V 1 W zeners. Rd should be about 120 K, 10 W.

    We won't even talk about the efficiency of the power supply when running continuously with a shunt regulator but you wouldn't be playing with HeNe lasers at all if overall electrical->optical output efficiency were a prime consideration!

    Kim (see the sections: Kim's Mid-Size Line Powered HeNe Laser Power Supply (KC-HL1) and Kim's Flyback Based HeNe Laser Power Supply (KC-HI1)) first suggested this after he somehow acquired a pile of 36 V, 400 mW zener diodes and was trying to figure out a good use for them! (From: Kim Clay (bkc@maco.net).) I did try the zener diode shunt regulator a few different times and it worked quite well. I just have to set it up for each tube that I'm using AND those little zeners can put out lots of heat when there are so many in a small area! have a small muffin fan in the 5 kV supply & I made sure there was airflow over them.

    Inverter Switchmode Regulator Controller

    Inverter based HeNe (and CO2) laser power supplies generally use pulse width modulation (PWM) to implement the regulation of operating current. This can be accomplished with a simple 555 timer but most modern designs use an SMPS PWM IC like the SG3524 or UC3842. These are inexpensive, robust, and provide startup and protection features that may be used if desired.

    The major internal functional blocks of these IC are the Sawtooth Oscillator (ramp generator) and Voltage Comparator. Timing components (Rt, Ct) set the (constant) oscillator frequency. The Voltage Comparator, subtracts the error signal (Ve) from the instantaneous value of the sawtooth waveform. Its output is high only if Verr is greater than Vosc. The pulse width is therefore a linear function of error voltage over a fairly wide range.

    The output may need to be buffered to drive the main switchmode transistor (bipolar or MOSFET). Until the HeNe tub starts, some means must be provided to assure that the duty cycle is limited to prevent damage due to transformer core saturation and excess current.

    
                      Rt       Ct              Voltage  
                  +--/\/\--+---||---+        Comparator    Buffer
                  |        |        |   Vosc            
                +---------------------+ |/|/|/ |\          |\       | |
                | Sawtooth Oscillator |--------|- \  _|_|_ |  \    _|_|_
                +---------------------+   Verr |    >------| A  >--------o Drive
                                           +---|+ /        |  /
                                 +------+  |   |/          |/
                             +---| F(s) |--+
                             |   +------+  |
                             |    |\       |       Vcs = V(Current Sense)
        Vcs (+) o-----/\/\---+----|- \     |       Vcl = V(Current Level)
                             |    |    >---+ 
        Vcl (-) o---+-/\/\---+ +--|+ /            Typical PWM Drive (Expanded) 
                    |   | Rcl _|_ |/                     _             _ 
                    +---+      -         Verr Low   ____| |___________| |______
                    Current       Error                  ______        ______
                     Limit         Amp   Verr Med.  ____|      |______|      |_
                                                         ___________   ________
                                         Verr High  ____|           |_|        
    
    
    The Error Amp may actually be part of the PWM controller IC. External sense circuitry may consist of nothing more than a resistor in the HeNe tube return to provide a voltage proportional to current, and F(s) may consist simply of a RC network to control loop response.

    Typical oscillator frequency is 200 kHz. To analyze this circuit precisely would require digital signal processing (DSP) techniques. However, where the loop response is limited to much less than the switching frequency, analog techniques can be used. Consult application notes for the PWM controller ICs for details.

    See the section: HeNe Inverter Power Supply Using PWM Controller IC (IC-HI1) for an example of a design using this approach.



  • Back to HeNe Laser Power Supply Design Sub-Table of Contents.

    Modulation, Demodulation, Pulse Drive

    Modulation of HeNe Lasers

    Impressing an audio or digital signal on the light beam from a HeNe laser can be accomplished in a number of ways:

    Electronic modulation techniques are covered in the remainder of this chapter. They are easy to implement and inexpensive.

    However, mechanical and electro-optic approaches do have their advantages including a potentially much wider modulation intensity range (from full off to full on, which, as noted, is not really possible with electronic techniques controlling tube current). They also do not require any modifications to the HeNe laser power supply and are thus necessary if this is not possible or desirable.

    Electronic Modulation of HeNe Lasers

    The basic techniques are all similar and modulate optical output power by controlling tube current. These are often combined with the current regulator or provided as an addition to it or used on a power supply without regulation. (They cannot be used in addition to regulation as the modulator and regulator will fight one-another.)

    Several suggestions for implementing power supply output side modulation are outlined below. However, the following should be kept in mind:

    Where wide range (approaching 100 percent) modulation is required, external electro optical or electro mechanical modulation may be a more effective solution.

    Depending on the type of information you would like to transmit over the beam and its bandwidth or data rate, dual tone or some kind of frequency or phase modulation may be better than simple amplitude modulation.

    Note: These circuits have not been fully tested. Some tweaking may be necessary.

    Modulation Using an Audio or Power Transformer

    Add a small audio transformer or low voltage power transformer (in reverse) with its primary (high impedance/high voltage side) in the return (cathode end) of the tube and drive its primary from an audio amp or whatever you like. You probably want a couple hundred volts at full amplitude. Just don't drive it to the point of the tube turning off.
    
                                             HeNe tube
                             Rb     Tube+ +------------+ Tube-
        Laser PS+ o---------/\/\----------|-         |-|-------+----+
                                          +------------+       |   _|_ (see note
                                                               |    -   below.)
                                                           +---+
                                                        ||(
                                            o---------+ ||(
                                                       )||(
                                       Input from   LV )||( HV
                                        audio amp      )||( 
                                                       )||( Small audio or
                                            o---------+ ||( power transformer
                                                        ||(
                                                           +---+
                                                               |
        Laser PS- o--------------------------------------------+
    
    
    CAUTION: If your tube is mounted with its cathode connected to a metal case as is the case with typical commercial laser heads, earth ground must be tied to Tube- for safety as Tube- may be at a potential of several hundred volts with respect to HV-. If the tube is enclosed and insulated from the user, this is not necessary.

    Modulation Using a High Voltage NPN Transistor

    Put a high voltage transistor (e.g., MPSW42) in the return (cathode end) with a pair of 120 V zener diodes in series across it to prevent the voltage from going above 240 V. Bias the transistor to obtain linear control of tube current. Then, drive the transistor from your audio signal. This is basically a voltage controlled current source.

    The circuit below is designed for a 4 mA operating current. With R3 set at 200 ohms, a 1 V p-p modulation input will vary the tube current from 1.5 to 6.5 mA. However, the range of your tube may be much more restricted than this depending on your tube, power supply voltage, and ballast resistor value.

    
                                             HeNe tube
                             Rb     Tube+ +------------+ Tube-
        Laser PS+ o---------/\/\----------|-         |-|-------+
                                          +------------+       |
                                                         R1    |
                                                    +---/\/\---+-------+
                                                    |   100K   |       |
                                           C1 10uF  |        |/ C      |
                                     + o------)|----+--------|    Q1  _|_,
                                             -  +   |        |\ E    '/_\  ZD1
                                  Line level        |  MPSW42  |       |  120V
                                    audio           /          / ^    _|_,
                                                 R2 \       R3 \<-+  '/_\  ZD2
                                     - o       1.5K /      500 /  |    |  120V
                                       |            |          |  |    |
        Laser PS- o--------------------+------------+----------+--+----+
    
    
    CAUTION: Do not use this circuit with a tube whose metal housing is not totally insulated from the user since the cathode will have a potential of several hundred volts PS- (and the signal return). For such tubes, see the alternative circuit below and the section: Enhancements to AT-PS1 for some possibilities.

    The use of a transistor like an MJE3439 or MJE2360 would permit a wider range of control but 200 V is likely to be sufficient. One of these would also be needed for higher current tubes since the MPSW42 is only rated at about 1 W (100 V * 4 mA is .4 W). A vacuum tube triode or high voltage FET in a similar configuration could even be used. The tube would be kind of quaint. :-) See the section: Modulation Using a Vacuum Tube for details.

    The Vceo rating for such a transistor may need to be several hundred volts depending on your HeNe tube and power supply, ballast resistor value, and how much modulation is desired. The voltage divider formed by R1 and R2 and emitter resistor R3 set the operating current and voltage across the transistor with no signal.

    A small HeNe tube will have perhaps 1,200 V across it and 300 V across the ballast resistor at the recommended operating current. The voltage across the tube (a negative resistance discharge device), will be more or less constant around the operating current (but may increase as the current is reduced). It is the 300 V that you have to play with. If the transistor drops 150 V, the ballast resistor will only drop 150 V and the current will be cut in half.

    The compliance range for the circuit shown is about 200 V so a ballast resistor still needs to be selected to enable the regulation to be effective from 0 to maximum modulation.

    Use the procedure described in the sections starting with: Selecting the Ballast Resistor to determine an appropriate ballast resistor value for the desired tube current and a power supply voltage 100 V less than that of your (unregulated) supply. This circuit should then maintain that current constant with no audio input and permit modulation of the current on either side of the set-point up to a change of approximately +/-100/Rb mA.

    Modulation Using a High Voltage PNP Transistor

    The following circuit permits the cathode to be at earth and signal ground potential so that metal cased laser heads can be modulated without fear of electrical shock:
    
                                             HeNe tube
                             Rb     Tube+ +------------+ Tube-
        Laser PS+ o---------/\/\----------|-         |-|-------+----+
                                          +------------+       |   _|_
                                                               |    -
                                     + o------------+----------+--+----+
                                                    |          |  |    |
                                                    /          /  |    |
                                  Line level     R2 \       R3 \<-+   _|_,
                                    audio      1.5K /      500 / v   '/_\  ZD1
                                                    |          |       |  120V
                                           C1 10uF  |        |/ E      |
                                     - o------|(----+--------|    Q1  _|_,
                                             +  -   |        |\ C    '/_\  ZD2
                                                 R1 /  MPSW92  |       |  120V
                                               100K \   (PNP)  |       |
                                                    /          |       |
                                                    |          |       |
        Laser PS- o---------------------------------+----------+-------+
    
    
    This circuit uses a PNP transistor to permit the signal input to be near the laser tube cathode and earth ground. Otherwise, it operates identically to the previous one.

    Modulation Using a Vacuum Tube

    Transistors like the MJE3439, MJE2360T, or even the MPSW92 or MPSW42 are generally adequate for these tasks. A high voltage MOSFET can also be used. However, if you long for the warm quaint glow of a vacuum tube (valve, for those of you on the other side of the pond!), a similar approach is easily implemented with common (well they used to be common) miniature receiving tubes. In the tube's defense, it may be more difficult to fry tubes than semiconductors!

    If I recall anything about tubes (which was a long time ago) select a cathode resistor to bias the grid negative to set the laser tube operating current (less than its maximum) and then drive the grid from a voltage source like the speaker output of an audio amplifier. The voltage on the plate will then vary resulting in an incremental current change of roughly: delta(Vin)/TC where TC is the transconductance of the vacuum tube at its operating point.

    For example, using a 12BH7A medium mu twin triode (you can use the other half for a fabulous audio preamp!), the grid voltage will need to be in the range -0 to -30 V for a compliance range of 400 V. At a plate voltage of 250 V, TC is 3,100 umhos resulting in an overall sensitivity of about 3.1 mA/V. Thus, this circuit would work quite nicely with line level audio signals since a total variation of only 1 to 3 mA should be needed to modulate a typical HeNe laser tube.

    In the circuit below, R2 and R3 set the bias point depending on the current requirements of your HeNe tube, power supply output voltage, and ballast resistor. C2 is the cathode bypass capacitor (low frequency roll off of around 20 Hz at the minimum (higher current) setting of R3).

    
                                     HeNe tube
                       Rb   Tube+ +------------+ Tube-
      Laser PS+ o-----/\/\--------|-         |-|------+----+
                                  +------------+      |   _|_
                                                      |P   -
                       C1                            ---
      Signal in o------||------+------------------- - - - G  ET1
                      .1uF     |                    .---. K  1/2 12BH7A
                      500V     |          +------+--+ ^   
                               |          |      |   | |
                            R1 /          |   R2 /   F F
                            1M \          |  470 \   (6.3 VAC or 12.6 VAC
                               /     C2 +_|_     /     depending on connections)
                               |   10uF  ---     |
                               |   100V - |   R3 /
                               |          |  15K \<-+
                               |          |      /  |
                               |          |      |  |
      Laser PS- o--------------+----------+------+--+
    
    
    CAUTION: If your tube is mounted with its cathode connected to a metal case as is the case with typical commercial laser heads, earth ground must be tied to Tube- for safety as Tube- may be at a potential of several hundred volts with respect to HV-. If the tube is enclosed and insulated from the user, this is not necessary.

    Where to find a 12BH7A? Actually finding a new one may be difficult and expensive (I have heard of a retail price of around $50 - wow! In its day, that tube probably went for $2 - and most of the difference isn't due to inflation.) I just selected the 12BH7A (out of the RCA Receiving Tube Manual, 1970 edition) as a triode with a suitable maximum plate voltage rating. Many many other tube types (likely cheaper also - the 12BH7A seems to be even more expensive than other similar tubes for some reason - will work as well. Or, just go to a hamfest or electronics flea market - people are always unloading old tubes. Or, check out a few garage or yard sales - you can probably pick up several old (but not yet antique) tube type radios for less than the cost of a single new vacuum tube!

    Of course, if you really want to impress your friends, use a transmitting tube like a 6146, 1625, 811, or even an Eimac 3-1000Z - that's good for a kW!

    Simple AM Demodulation of a HeNe Beam

    A reverse biased photodiode will produce a current proportional to beam intensity. This signal can be input to the summing junction of an op-amp or just a single transistor circuit to change it to a voltage. Then, output that to an audio amp. See the section: Sam's Super Cheap and Dirty Laser Power Meter for a starting point. The maximum modulation index is probably 15 percent or less so you will need to null out the DC offset or AC couple the output (which the audio amp will probably do anyhow).

    Pulsing a HeNe Laser for High Power?

    Unlike solid state (e.g., ruby, Nd:YAG) lasers which produce extremely intense short pulses of light, HeNe lasers really are continuous wave devices. It is not possible to obtain high peak power from a HeNe laser by driving it harder, Q-switching, or other exotic techniques.

    A high voltage power supply that produces short pulses exceeding the tube's starting requirements will result in output pulses of light. With appropriate design, pulse duration and repetition rate can be controlled. Depending on needs, a flyback transformer or automotive ignition coil may be suitable. The stability of the beam on such short times scales may be questionable and high repetition rate pulsing may result in shortened tube life as well. See the section: Pulse Type Drive and Modulation of HeNe Tubes.

    As with modulation, mechanical or electro-optic techniques are also possible. A simple chopper wheel in the beam path may be perfectly adequate for your needs and a lot less complex and costly than high tech alternatives!

    Pulse Type Drive and Modulation of HeNe Tubes

    While you can amplitude modulate the tube directly, the index of modulation that can be achieved in this manner is not very high. For a typical power supply and tube combo, you will be lucky to get a 5 or 6 percent modulation index before the tube wants to go out. By increasing the ballast resistor and using a higher power supply voltage, this may be increased to perhaps 15 or 20 percent which is still pretty pathetic. You won't talk very far using this approach as most of the carrier power is wasted. It is possible to modulate on the high side of optimal current (increasing current decrease the optical output) and achieve a high modulation index but this will reduce tube life (it takes 2 to 3 times the optimal current to reduce optical output to zero).

    HeNe tubes can be pulsed - there is no law that says DC must be used.

    However, to achieve rated (or even just reasonable) tube life and output power, discharge current must be controlled within specified limits. This may not be quite as trivial as suggested below - especially for larger HeNe tubes!

    WARNING: The comments below apply to the low power (0.5 to 1 mW) HeNe lasers found in bar code (UPC) scanners and similar products. If you attempt this with a 35 mW HeNe laser, you may blow the tube, burn out the power supply, fry yourself (or all of the above) in the process.

    (Portions of the following from: K. Sorter (w1_k_sorter@qsl.net).)

    Modulation methods are a primary discussion amongst our group although many are now converting to IR (invisible) lasers and IR sensitive PM tubes and APDs for long range work (Mia and Rayleigh scattering/adsorption effects are much less severe in the atmosphere at IR).

    High index of amplitude modulation is not possible by smoothing varying the input to a HeNe tube. As soon as the the supply voltage/current is reduced slightly, the tube winks out. However, it was found that the HeNe tubes could be pulsed at about 30 khz by simply removing the filter capacitors in the small switching power supplies used with low power HeNe lasers (yes, they were not potted!) Presto - the units had a 100 percent index of modulation! These supplies and tubes are common and cheap in rejected scanner units that are upgraded to diode lasers.

    Therefore, to get the highest index of modulation (and therefore the most miles per watt), you will need to switch the tube on and off at 20 to 50 kHz. This means running the tube on AC or pulsating DC. Unfortunately, there are a couple of issues that may prevent the successful modification of surplus HeNe power supplies:

    One user vividly recounted purchasing a bag full of decommissioned 12 VDC low power HeNe tubes and PS units at a swap meet. These were replaced with newer laser diode types, so the old stuff was junk, and was sold cheap. These were from old style supermarket scanning units!

    This was followed by a brief discussion in which the question of switching the laser on and off (a destructive process) was discussed, and it was determined that there was a big difference between keeping the plasma lit while pulsing the laser on and off AND turning the tube completely on and off and restriking the plasma (as in a cold start).

    Note that the tube continues to have internal plasma during the 'off periods', but that the tubes didn't lase until the pulse came along and the input voltage got high enough. A photodiode placed against the tube envelope showed constant output all the time with higher levels when the tube lased. The output of the tube however would go completely on and off (at the output mirror of the tube).

    IMPORTANT: In this type of operation, the starting pulse is only applied initially to start the process-if the starting pulse was applied 30 or 40 thousand times per second, the tube would be destroyed in short order.

    Some tube manufacturers say that pulses are OK, others suggest raw AC with appropriate current limiting resistors. At the prices of these things (as in: dirt cheap), experimentation is encouraged!

    You can also use the laser in relaxation oscillator mode by simply putting a capacitor across it. It will oscillate at a fairly constant frequency, although modifying that frequency (for voice/data transmission) is not so easy. It is useful if you need a constant monotone output. However, whether this will lead to significantly reduced tube life will depend on the actual frequency of the relaxation oscillator as well as the type of starting circuit used in the power supply. Above a frequency of perhaps several kHz, the discharge will restrike without requiring the normal high starting voltage (and the power supply's starting circuit will not be activated). If the time constant is too long (too low a frequency), the tube would need to cold start each time. Repetitive application of a high energy starting pulse may result in rapid degradation of the tube characteristics.

    I (K. Sorter) used a .05 uF disk capacitor and ran my 2 mW HeNe tube in that mode for 2 weeks constantly. The starter pulse in the power supply was activated by a relay, so each time I would turn on the supply, it would click. There would be no clicking while running in relaxation oscillator mode. Thus, the power supply thought the tube was running normally. The purist might believe this would lead to degraded life expectancy of the tube, but it hasn't been proven in actual real life.

    You then FM modulate the 20 kHz frequency with your desired audio. There are various ways of doing this by injecting the audio signal into the frequency determining circuitry of the pulse width regulator chip. This specifics will, of course, depend on the design of your particular power supplies.

    The receiver is tuned to the carrier frequency and demodulates the FM signal. This can be easily accomplished with a NE556 chip and photodiode/low noise op-amp front-end.

    The best deal in optical front ends these days is the Burr-Brown OPA210/OPA211 op amp/photodiode in a chip combo at 8 bucks! You add an appropriate external feedback resistor (depending on the desired bandwidth), and you have the whole front-end in a simple chip!

    It should be noted that FM is a poor performer if your goal is long distance communications! But it certainly can have near broadcast quality audio which is a big advantage if that's your goal.

    We have many users that are using this strategy in communications everyday.

    For more information and discussions on amateur laser communications, join the Laser Reflector. See the section: Amateur Laser Communications for details.



  • Back to HeNe Laser Power Supply Design Sub-Table of Contents.

    Powering Multple HeNe Tubes or Discharges, Remote Operation

    Driving Multiple HeNe Tubes (or Discharges) from a Single Power Source

    In general, attempting to design a single power supply to drive multiple HeNe tubes in series or parallel is more trouble than it is worth: However, when driving specialized, older, or very large HeNe tubes, there are cases where this approach may make sense. Since you may need to construct a custom power supply to meet the non-standard tube requirements anyhow, this could result in a significant cost savings:

    Driving Multiple HeNe Tubes or One with a Center-Tapped Discharge Tube

    Where there is a center-tapped discharge tube, two similar power feeds are needed (in effect, there are two HeNe tubes in parallel probably sharing a common cathode). Only a single operating voltage supply with its associated AC line circuitry is needed. Since the characteristics of the two discharge paths are likely very similar, further simplifications may be possible for current balancing and/or regulation.

    A typical circuit is shown below where the characteristics of the tubes are sufficiently similar that separate current regulators are not required.

    
                     Ri1    S1
      Start+ o---+---/\/\---+                    LT1             o + Test1 - o
                 |          |   Rb1   Tube1+ +----------+ Tube1- |   Rs1     | 
      HV+ o---+--|---|>|----+---/\/\---------|-       |-|--------+---/\/\----+--+ 
              |  |   CR2                     +----------+             1K        | 
              |  |                                                              |
              |  |   Ri2    S2                                                  |
              |  +---/\/\---+                    LT2             o + Test2 - o  |
              |             |   Rb2   Tube2+ +----------+ Tube2- |   Rs2     |  |
              +------|>|----+---/\/\---------|-       |-|--------+---/\/\----+--+
                     CR2                     +----------+             1K        |
                                                                                |
                              +----------------------------+                    |
      HV- o-------------------| Optional Current Regulator |--------------------+
                              +----------------------------+                   _|_
                                                                                -
    
    (Note that separate cathode feeds probably will not exist on a center-tapped discharge tube.)

    Rb1 and Rb2 are the ballast resistors. Where no separate regulators are used, one or both of these can include a suitable variable resistor and/or trimming resistors can be added to balance the currents. Just be sure to use a well insulated tool or adjust with power off and capacitors confirmed to be discharged!

    This approach can easily be extended to more than two HeNe tubes.

    The starter can be a voltage multiplier or inverter type. Note that since the HeNe tubes will not start at exactly the same time, Ri1 and Ri2 (the HV isolating resistors) must have a high enough resistance so that the starter supply is not loaded down excessively even when all but the last HeNe tube has started. It may also be possible to use a large pulse type starter applied to points S1 and S2 via HV coupling capacitors.

    It is desirable to connect the return (-) of the starter supply to HV+ to take advantage of the additional boost provided by the operating voltage supply. Where the return cannot be isolated, it will have to be connected to ground. CR1 and CR2 are HV diodes to bypass operating current around the starters once the discharge is initiated.

    Driving Multiple HeNe Tubes with an Exposed Capillary

    Where the capillary is accessible (the gas reservoir is separate, not surrounding the capillary as with common mass produced internal mirror HeNe tubes), starting can be accomplished using the output of a pulse transformer but applied to an electrode external to the tube (rather than in series with the anode). This is similar to triggering a xenon flash lamp. See the section: Starting Circuits Using Pulse or Flyback Transformers and the document: Sam's Strobe FAQ for circuit ideas.
        
                                          +-----------------+-----o Start Pulse
                                  LT1 ____|____             |
                     Rb1   Tube1+ +---------------+ Tube1-  |
          HV+ o--+---/\/\---------|-            |-|---------|---+ 
                 |                +---------------+         |   | 
                 |                                          |   |
                 |                        +-----------------+   |
                 |                LT2 ____|____                 |
                 |                +---------------+             | 
                 +---/\/\---------|-            |-|-------------+
                     Rb2   Tube2+ +---------------+ Tube2-      |
                                                                |
                                                  o - Test + o  |
                 +----------------------------+   |    Rs    |  |
         HV- o---| Optional Current Regulator |---+---/\/\---+--+
                 +----------------------------+        1K      _|_
                                                                -
    
    
    Rb1 and Rb2 are the ballast resistors. Where no separate regulators are used, one or both of these can include a suitable variable resistor and/or trimming resistors can be added to balance the currents. Just be sure to use a well insulated tool or adjust with power off and capacitors confirmed to be discharged!

    If the tube cathodes can be isolated from ground, current monitoring can be accomplished by adding sense resistors (Rs1 and Rs2) and measuring voltage at Test1 and Test2 (1 mA/V) or with a 10 or 20 mA full scale current meter to these. If this is not possible, one alternative is to use a single sense resistor (Rs) and run only one HeNe tube at a time while adjusting current making sure that the input voltage is identical in each case.



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