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!
These may run off of low voltage DC or the AC line but in the latter case convert the AC into DC first and then use a high frequency chopper and small transformer to generate their output.
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? :-)
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:
CAUTION: Do not be tempted to increase the high voltage output of a power transformer by more than 30 percent or so above it rated value (by either driving its primary with a higher than rated voltage or by adding booster windings in series with the primary). Even this may be excessive depending on its design margins. At some point core saturation will result in a dramatic increase in input current, overheating, meltdown, smoke, 6 foot flames, etc.
In addition, the insulation ratings may be inadequate for the increased high voltages now produced by the secondary.
Thus using a 115 V transformer on 230 VAC to obtain double the output is probably not a good idea though I know people who have done this and lived to tell!
For example, using two 380 VRMS transformers in series will result in over 2,000 VDC without playing games and 2,200 to 2,500 V with one of the booster techniques described above.
These are suitable for intermediate size HeNe tubes - 5 to 20 mW.
What this means in the end is that although the open circuit voltage (full wave rectified and filtered) may approach 7 kV (for a 10 kV unit), voltage is down about 30 percent at 6 mA. This is no problem if used on a Variac with current monitoring though but the practical upper limit on operating voltage is only about 4 kV.
Although, as noted, the current limited behavior is not linear, it can be approximated by assuming an internal current limiting resistor and an ideal transformer. For a 10 kVAC RMS, 20 mA transformer, the internal equivalent series resistance would be about 500K ohms across the entire winding or 250K ohms per side (center tapped). When used for the intended application - be it producing an arc to ignite an oil burner flame or powering a neon sign (using a luminous tube transformer which is basically similar), no ballast resistor is needed to protect the load or transformer. Unfortunately, this doesn't help us since the rectifiers and filter capacitors come between the transformer and HeNe tube :-(.
Also, because the open circuit voltage is much higher than the actual operating voltage, there will be a current spike through the HeNe tube at the instant it starts of many times the normal operating current. However, for a typical design, the total energy in this pulse is not very large even at the upper limit of the power supply. For example, with a total filter capacitance of .25 uF, an open circuit voltage of 7 kV, and an operating voltage of 4 kV, the energy delivered to the HeNe tube by this pulse will be only about 2 to 4 w-s (J). I do not know if this will result in significantly shortened tube life in the long run under normal usage (reasonable number of starting cycles).
See the section: Sam's Mid-Size Line Powered HeNe Laser Power Supply (SG-HL2) for a sample design using an oil burner ignition transformer.
The characteristics of these neon sign transformers are similar to those of oil burner ignition transformers (see above) but typically have higher voltage and current ratings, and are thus much larger and heavier. They also may have a more constant current characteristic - delivering nearly their rated current up to a good percentage of their no load output voltage (to handle a variable length of neon tubing). For example, a 12 kV, 30 mA unit may behave like an equivalent 30 kV source in series with a 1M ohm resistor for loads resulting in an output voltage of up to 6 or 8 kV.
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.
_ 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.
CAUTION: This is probably over 700 V with significant current available. Take care. Make a note of your reading and then disconnect power.
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.
Several types are possible:
However, self oscillating designs are generally not as efficient as driven ones (see below) and may be unstable under certain load conditions.
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:
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.
However, with modern implementations of switchmode power supplies utilizing digital control techniques, catastrophic failure due to external faults or user abuse should be a thing of the past. With traditional analog control, pulse width modulator ICs, op-amps, and discrete components are used for the drive of the switching transistors or MOSFETs, and for fault detection. These schemes are often ad-hoc and testing for all possible fault conditions is not really possible. But with digital control, a microprocessor implements the feedback equations in firmware and generates (or at least directly supervises) the switchmode drive signals, and monitors for over-current, over-voltage, arc-faults, and other error conditions on a cycle-by-cycle basis. It can then instantly shut down the supply long before any damage to it or the laser can take place. Explosive decomposition of a potted module due to accdidentally powering up with a shorting strap still attached would be a thing of the past. They would also be able to recognize that a laser wasn't starting and modify the output voltage appropriately. For example, by cycling it between 0 and maximum rapidly to take advantage of the dV/dt of the voltage to help ionize the gas. It could also detect a laser that was having trouble staying lit at the selected current and modify the dynamic ballast resistance as required, or shut down if that didn't help.
By now, the "Smart HeNe Laser Power Supply" should be standard practice in the industry but old habits take time to die. :)
eBay and eBay Stores always have a wide selection of high voltage components, some at attractive prices both individually and in large quantities. And searching is easy and quick. However, keep in mind that many of these parts are imports (China, Russia, and elsewhere) so quality cannot be guaranteed and may vary from batch to batch, and even if there is a warranty, returns may not be worth the hassle and postage.
More on specific types of components below.
These types of devices are generally much more expensive on per-kV basis compared to parts like the 1N4007 as well. ECG, NTE, and SK also have a variety of replacements of various ratings (probably even more pricey). See the section: Mail Order - Electronic Components and the document: Troubleshooting of Consumer Electronic Equipment for contact info.
(Note: As of January 2001, NTE has purchased the assets of ECG so the ECG parts listed below may no longer be available, use the NTE or SK equivalent which usually has the same number with the 'NTE' or 'SK' prefix.)
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!
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:
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.
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:
Current balancing resistors are added to compensate for unequal DC leakage currents.
Matching of the capacitance of the individual parts may be necessary to minimize unequal AC voltage drops.
Note that if the dominant voltage across the series combination is only DC or AC, it may be adequate to only worry about balancing that. For example, a filter capacitor charged from a medium to high impedance source (compared to its reactance) will see mostly DC - the AC will be small. Therefore, only current balancing resistors are required.
A reasonable design approach is as follows:
CAUTION: Make any leakage current measurements in a manner that will prevent damage to your multimeter should the capacitor decide to short out. For example, measure the voltage drop across a high value resistor in series with the capacitor rather then directly with the meter in series.
If locating capacitors with this tolerance is not possible (i.e., your parts supply is limited), additional derating may be necessary - use additional capacitors in the series bank to achieve the needed voltage rating.
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.
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:
However, Michael West (WestMBAKR@aol.com), an applications engineer for Victoreen, LLC, has offered to answer serious resistor related questions via email.
For example:
Note that at the instant the tube starts, there may be much more than the steady state voltage across the resistors. This actual value will depend on how much capacitance there is in the starting circuit and how it is distributed but may be as much as the maximum starting voltage minus the tube voltage. However, everyone ignores this and it doesn't seem to matter in practice. :)
Some types of resistors are better than others in standing up to both the high voltages and high peak currents at startup. The author of the short paper "Wedding Lasers to Power Supplies" [16] who is a cofounder of LaserDrive, Inc. (a major manufacturer of laser power supplies), recommends Hot molded carbon composition type but suggests that some wirewound types also work well. He suggest avoiding non-hot molded carbon composition, highly inductive wirewound (which will increase dropout current, and film types (which may degrade due to the transient current of the starting pulse). On the other hand, the HeNe Laser Manual by Elden Peterson suggests that wirewound types are best. In reality, if you use conservative design, many types will be just fine. Additional ballast resistor info can be found in Power Technology's What is a Ballast Resistor and Why Should I Use One? technical note.
Some additional comments:
(From: Greg Menke
You can get really capable HV resistors, I imagine they can end up being very
expensive however. I looked at a few databooks we have here, and there is a
wide selection, but they appear mostly custom or manufactured on demand type
devices.
Stringing resistors together is a good alternative but can be a little tricky
because you must stay within the dielectric rating for each resistor or it
breaks down, plus you have to manage the dissipation as well. I'm using 2
parallel sets of 30 series resistors as a HV bleeder on my 30 kV capacitor.
At 30 kV, each resistor should be dissipating .4 watts, they are rated for
1/2 W, so I *shouldn't* need an oil bath for them. Actually it was pretty
hard to find resistors to withstand >= 1000 VDC each, regular resistors are
rated for 200 volts or so. Farnell had them, 'metal-glazed', good to 3 kV
each.
(From: Gernot Stoffel (Beamchief@gmx.de).)
In many cases it is recommended to connect mains transformer output of AC HeNe power supplies a well defined way, and do some additional grounding efforts: First, I suggest grounding both the transformer iron core AND the negative (cathode) tube connector, each by 220K resistor or so; or at least couple them by a 470K resistor.
Second, the tranformer pin connected with the *inner* part of secondary coil ALWAYS should be connected next to cathode ground potential of the entire circuitry! (E.g., when using a typical voltage-doubling Greinacher circuit, the two diodes should be connected to the outer end of coil, not the inner one, because this diode potential is oscillating with double transformer peak voltage relative to cathode ground, while the other pin, between those two chains of elcaps, stays just at a stable DC potential, apart from some ripple they work against.)
Reason is that before glow discharge gets ignited, the extended high voltage (up to 10 KV or so) tends to "spray" into the air from any part that's not properly insulated (mostly: ballast resistors). This parasitic discharge circuit gets closed by complement spray discharges - mostly inside the transformer. (Effect might continue even at regular tube working, on a much lower "parasitic" level.) Thus, it's ALWAYS better to connect it the described way, keeping voltages between core and coil as low as ever possible, and to make defined potentials by using the recommended grounding resistors. (Note, the little switching-transformers of DC drivers normally have similar preferences, too.)
Not knowing this, my personal laser GAU was in the early nineties when all of a sudden my fine 10 mW "handmade" NEC was striking, just five minutes before an important presentation on a varnishing in Bonn. I found the big's mains high voltage trafo output coil being completely dead. Removing iron core (fortunately, it was not a :(( welded type), then 5,000+ turns of (too) fine wire (fortunately, it nearly was *not* baked together with PU fluid; just that tiny little bit I had to use a saw against, to get secondary coil finally removed VERY sensitive), I found its innermost filaments highly eroded, in an absolute ridiculous way, if that had not been so bad. Nevertheless, trafo had a good, solid two-chamber insulating frame made of thick glass whisker composite nylon that was not damaged as much one could see! (That's no criterion; for sure.) What had happened???
I use a hand-made case from clear Plexodur (a slightly yellowish derivate of Plexiglas, but without its severe disadvantages, that is brittleness and an outstanding bad chemical sensitivity, e. g., getting into handy pieces by alcohol). It's *absolutely* nice-looking, for that machine is covered with well-done technics and mechanics up to the very top (take you some exciting photo seria as far I've chosen my next digicam, what's a *long* story on its own); and it's handy, too. But naturally, I had to darken its 14-inch laser tube, to prevent discharge from blinding the whole audience when running; for I don't build tricky laser deflectors just for *that*. So, totally coated it with two layers of matt black car spray paint a very careful way and baked it in. (Remember, just the naked tube once made a 1000 $ charge; meaning an entice car load of raw eggs was peanuts against). Tested and calculated before, that this would not effect tube temperature behavior in an intolerably bad way. (Could be proved later-on; while self-adhesive black foil is much worse, by causing heat build-up; by not covering the whole thing; and by highly uncertain long-time stability risks. My beloved Contax all goes into parts just because its glue re-transforms to cosmical dust.) What I did *not* know then, nor expect in any way, was that these black paint sprays *all* are conductive a tiny little bit, by using soot particles for pigment. (Tried nearly ten brands of spray varnish later to avoid this disadvantage; but it's all nearly the same, beside a factor of three or so.) But learned it quite fast, for tube ignition behavior was lousy; so I freed the zones directly around tube-connector passes from paint and extra-insulated them by epoxy-filled, black caps, taken from wonder-glue flasks. (So, this is just another thing best *never* thrown away.) Igniting behavior grew much better, but by far was not perfect; noticed that tube surface still got highly charged during ignition. Hmmm. Fixed the whole problem the "sledgehammer way" then with an extra circuitry, made by a tiny, gas-filled 350 V / 5 kA spark discharger item and a "bullet-proof" 10 nf 630 V capacitor, contacting it by a five-bucks-rhodium-plated-gold-blinking spring contact pin and holder to the tube's central copper collar I made there, fairly under the paint, for proper temperature taking, and ground. During ignition, capacitor now got charged within a second or so, TICK!, discharger ignited with some pretty hundred Amps and little blue spark, and kicked the whole tube discharge a proper way (ask Sam why it did). Truly clever, I thought. Even a touch better than using a piezo lighter module a similar way. Worked fine.
Until the transformer was dead, several years later, the described way. Then it was not clever any more, not at all.
The simple, very obvious and primitive truth is that the metal ends of the tube are connected to the inner voltage(s) as well - lately by the ion discharge! -, especially that one near the anode pin; and so the conducting tube surface was still rather contacted with it. AND the truth was that the transformer output coil by natural means was switched just the opposite way than it would have been able to bear that problem. (As usual, Murphy had done his very best.)
Well - the rest was easy work: Insulated those tube ends, by first another time carefully removing the white silicone caps as I did several times before, e. g. for fine-optimizing the resonator alignment the adequate way it *can* be done at operating NEC tubes if you really know what you're doing, then by using short sections of black shrinking tube covering the removed ring zones of paint, and re-monted the end-caps. Dark as hell. AND I re-assembled the transformer, after professional efforts to improve wiring-cabin (frame) insulation: knowing the primarily-coil number of turns (had been cool enough measuring ten testing turns, manually threaded to that dead transformer before slaughtering, so there was no need to remove/move the core twice, thus putting 75 pieces of thin metal at alternating orientations), by that simply fixed the number of needed secondary coil turns, then calculated optimum wire diameter (0.15 mm) to fill the frame (3.3 times better and twice stable than it was before (0.1 mm)); attended 0.2 kg = 1,220 m of this stuff (needing about 850 m, I knew); and *then*, turn beside turn, layer onto layer, with some additional cross-section insulating efforts I made that new coil manually, 5,375 turns, meaning 21,500 well-guided frame corners, forty layers, just assisted by a little mechanical turn-counter I had put to my wiring "machine" made of 'Fischertechnik' construction kit. Lasted two days or so. Blocked the whole transformer frame with thin, clear epoxy sealing compound at the end, using good vacuum to get any gas away during that procedure, and then re-assembled transformer core. So now it will work properly even at the bottom of Marian trench, if cable is long enough. Since then there is no problem any more.
OK: Cold tube does not ignite very proper (lost function of my fine ignition "sledgehammer", by finally insulating the tube coating from discharge voltages an effective way); but lately *does*.
(Conclusion: HeNe tubes definitely *don't* like conducting black paint, in terms of their ignition behaviour, but just want to show all they got.)
Related items:
Question: Which wire diameter 'd' shall I take to fill a chambered transformer frame cavity quite proper, when 'n' turns are wanted? A needless thin wire type is bad, for increasing resistance, heat-up etc., and often is uncomfortable to practice, too; and too thick wire type does not fit the problem, but is one of the ugliest ways to restart work. And guessing is no method for getting proper results here.
Answer: When the wiring cabin (frame) cavity is 'w' wide and 'h' high (at any side), usable area is A=w*h. Total (!) maximum wire diameter then is d:<=1,075*sqrt(A/n), when coil gets wound turn by turn, layer onto layer. By winding a transformer coil not that exact way (but not extremely crowded!), a safety margin of about -5% in diameter is quite sufficient (because space grows as the square with diameter; so this will yield about 10% additional space: 0.952=0.9025). So you then get d:<=1.02*sqrt(A/n). Or make it easy: d:<=sqrt(A/n). Wire diameter usually gets specified without the insulation coating. A single coating adds about 0.03 mm to the wire diameter, a double coating adds 0.05 mm. Thus, you have to subtract this from your total result, and then take the next available diameter same or below this value. You always will be on safe ground.
(Taking all dimensions in mm, the result is in mm, too.)
[Proof: With a cross-sectional area 'A' and 'n' turns, each one has maximum space 'A/n' to use. Nearly all wires are in a hexagonal grid (if no left-handed gorilla made that coil), where wire diameter is the side-to-side diameter 'd' of such hexagon. A little geometry sketch shows, on just a hundred different ways, that hexagonal area is then sqrt(3)*d2/2. So with A/n>=sqrt(3)*d2/2 you get d:<=sqrt(A/n)*sqrt(sqrt(4/3)). This factor is about 1.075.]
Important: If you wind high-voltage coils, an endless job requiring perfect precision all the time, you may need additional insulating foils between layers. Then substract another 10% from wire diameter before selecting it, and it's best to use double-coated type if you can get it.
Question: Which length of wire do I approximately need with that nearly-optimum diameter?
Answer: When frame chamber kernel is x*y, average length of any turn is (2*(x+y)+h*pi. ('h*pi' is the net turn length around all the four corners.) By this, total wire length is about n*(2*(x+y)+pi*h).
(Taking all dimensions in mm, result is in mm, too.)
Note: This is the absolute maximum length which can occur, beside of some additional cm for connecting.
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:
Keep components adequately spaced from each other, and grounds or the case, where voltages differ substantially. Maintain separation of at least 1 inch per 10 kV though more is better. Or, include additional insulation such as a sheet of Plexiglas or other plastic, or Fiberglass-Epoxy printed circuit board material (without copper or copper on only one side if that side is grounded).
These are both significantly overrated for the laser applications but are also quite robust and resistant to high temperatures and mechanical abuse.
There is special RTV Silicone formulated for high voltage applications. The normal bathtub caulk or window sealer may corrode the circuitry (if it smells like vinegar) or may degrade (decompose, become discolored) after some amount of time resulting in conductive paths. I have used the clear GE stuff apparently without problems on the HeNe laser power supply I routinely use for testing but your mileage may vary.
CAUTION: Know the limits of your power supply. Ironically, being too successful at insulating the starting voltage can overstress marginally specified components which would normally be protected because the voltage never got high enough to exceed their ratings!
WARNING: This resistor does NOT reduce the danger from the supply - it is to protect the equipment. The available current can still be lethal.
There are special resistors designed for use in high voltage circuits and these would be best. However, series strings of lower value resistors should be acceptable with proper construction techniques. See the section: High Voltage Resistors for more information.
You can extend the cable all you want as long as:
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:
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.
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.
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.
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.
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.
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:
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 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.
There are two types of nodes:
In order for no current to flow through the diodes:
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:
In practice the actual output will likely be somewhat less due to stray capacitance and other losses.
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.
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:
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!
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.
Y o----------------+-------+-----------+--------+
| | | T2 |
R1 \ __|__ SCR1 +-+ +-+
1M / _\_/_ TIC106 o )::( o
\ / | )::(
C1 | | | )::(
T o----::----|----+ | )::(
.01uF | | | )::(
2kV | R3 / | )::(
| 1K \ | +-+ ::(
| / | | ::(
| | | C2 | ::(
+----+--+------)|---+ ::(
| | 1uF ::(
R2 / _|_ 600 V ::(
1M \ --- C3 ::(
/ | .01uF ::(
| | 600V ::( CR1
W o----------------+-------+ +---|>|---o HV+
Y o----------------+-------+---------------+--------+
| | | T2 |
| | +-+ +-+
| | o )::(
R1 \ _|_ C2 )::(
1M / --- 1uF )::(
\ | 600V )::(
| | )::(
| | )::(
| | +----+ ::(
| | | ::(
| | SCR1 __|__ ::(
| | TIC106 _\_/_ ::(
| | / | ::(
C1 | | | | ::(
T o----::----|-------|---------+ | ::(
.01uF | | | | ::(
2 kV +-------+----+ R3 / | ::( o CR1
| | | 1K \ | +---|>|---o HV+
R2 / C3 _|_ | / |
1M \ .01 --- | | |
/ uF | +----+--+
| 600 V |
W o----------------+-------+
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
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.
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. 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. :)
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)