Unlike laser diodes, the HeNe drive is not nearly as critical to performance and tube life :-). Therefore, even if you do not have a datasheet for your tube, you can probably guess fairly closely as to its requirements and then optimize performance based on optical output alone.
With these, at most you will have to add a ballast resistor and power line or battery connections. Models are available that run off either low voltage DC (regulated is desirable) or 110 or 220 VAC.
See the section: Examples of the Use of Commercial Power Supply Bricks.
See the section: Laser Safety with respect to the optical hazards associated with lasers. While not in the metal cutting class, careless use of especially higher power HeNe lasers can result in permanent damage to vision. Many are only Class II or Class IIIa but some are well into the Class IIIb range.
HeNe laser power supplies utilize high voltage at low current. The few mA needed to operate the HeNe tube or even a 10 KV starting pulse tube can certainly give you an unpleasant shock but will not likely be enough to do lasting damage. However, there may be much more current actually available and there is always the possibility of collateral damage:
Here are six options for providing the operating voltage. These examples assume an output of at least 1,800 VDC:
You will probably need to modify these mostly by increasing the number of turns on the output windings of the inverter transformer - which in itself may be a challenge to maintain low capacitance and high voltage insulation.
See the section: Simple Inverter Type Power Supply for HeNe Laser for one modified for driving a HeNe laser tube.
See the sections starting with: AC Line Operated Versus Inverter Based Power Supplies for more discussion of features, basic operation, and design issues.
+----------+ +------------------+ HV+ Ballast Resistor o------| |-----| Starting Circuit |--------------/\/\----+ Input, | Main | +------------------+ Rb | AC line | Power | | or DC | Supply | +-----------+ Tube- +-----------+ Tube+ | o------| |-----| Regulator |----+----|-| -|-------+ +----------+ HV- +-----------+ _|_ +-----------+ - HeNe tubeSee the Typical HeNe Laser Tube Structure and Connections for the basic hookup diagram.
The ballast resistor (Rb) may sound really boring but it is essential to provide stability and limit the current to the value specified for your particular tube. The most common value is 75K but slightly smaller and much larger values may be used to match a particular HeNe tube to a power supply.
A higher voltage supply and larger ballast resistor will be more stable if there is no built-in regulation. This results from the fact that the voltage drop across the tube is relatively independent of tube current so it subtracts out from the supply voltage. What is left is across the ballast resistor and changes by a proportionally greater amount when the line voltage varies. The smaller the ballast resistor, the more this will affect tube current.
You really do want the current to be close to that recommended for your tube both to maximize the life of the tube and because maximum optical power output is produced at the optimal current.
This can be accomplished by adjusting either the input voltage to the power supply or the output current and/or by selecting the value of the ballast resistor. Excessive current is bad for the tube and will actually result in decreased optical output. If the current is too high by a factor of 2 or 3, there will be no output at all. It is not possible to pulse a HeNe laser for higher power (though pulse drive is possible for modulation - see the section: Pulse Type Drive and Modulation of HeNe Tubes. Without any regulation, the value of the ballast resistor is more critical and power line fluctuations will significantly affect tube current though such variations may not matter for many applications.
Note that since the tube itself provides a relatively constant voltage drop around its nominal current, a small change in line voltage will affect the tube current to a much greater degree than would be expected by the percent of the actual voltage change. The incremental change in tube current will be closer to: delta(I) = delta(V)/Rb where Rb is the ballast resistor. This will be on the order of 10 to 20 times more sensitive to input voltage changes than with a straight resistive load. Thus, a regulator is often desirable.
The large (can) electrode of the HeNe tube is the cathode (-) end. It may run if connected backwards but the small anode (supposed to be positive but now incorrectly connected as negative) will get very hot since much of the power dissipation is at the negative electrode due to positive ion bombardment. The resulting sputtering may damage the mirror at that end and tube life will likely be shortened.
Design considerations and a variety of sample circuits are provided in the chapters: HeNe Laser Power Supply Design and Complete HeNe Laser Power Supply Schematics.
For detailed plans, see the books listed in the section: References on Laser Principles, Technology, Construction, and Applications.
However, if you have trouble changing a light bulb, never fear - packaged solutions are readily available at attractive prices. You don't need to build your own HeNe power supply unless you have some special requirements, a really strange HeNe tube, or just for the shear joy of creation!
Cost may be anywhere from $25 to $100 or more depending on power capability, whether new or surplus, and other factors. Such power supplies (and HeNe tubes to go with them) may even be offered on the Internet, possibly at much more attractive prices. Of course, the quality and reliability (of both the equipment as well as the seller) may be unknown.
Power supplies may also be packaged along with a small HeNe tube, ballast resistor, and wiring as a complete laser optics assembly. These have become available at very attractive prices as products like UPC scanners and laser disc players have switched to over to the use of laser diodes. Since nearly everything but a wall plug is likely included in such a package, this approach will result in a working laser with minimal effort.
The following sections lists the wiring color codes for some typical 'brick' type HeNe power supplies and describe the required connections and additional circuitry that I used to make complete lasers using two types of HeNe tubes and power supplies that were available from Herbach and Rademan.
Note: color1/color2 means color1 with a color2 stripe.
The CDRH (Center For Devices And Radiologic Health of the Food and Drug Administration) delay mentioned with respect to some of these power supplies prevents the beam from coming on for 3 to 5 seconds after power is applied (i.e., should the ON switch be hit accidentally) and may be needed to meet certain regulatory requirements.
If the power supply is a name brand unit or was pulled from a device like a bar code scanner, it is safe to assume that the connections (the output in particular) are correctly labeled. However, some power supplies sold to hobbyists (e.g., from places like Herbach & Rademan or HSC Electronic Supply) have been known to have incorrectly marked high voltage connections. (The models I have seen this on are a low voltage inverter type with red and black wire leads for the 9 or 12 V input and spade terminals for the high voltage marked with hand printed or typed stickers for VA and VC - apparently at random.) If your power supply looks like it was put together in someone's basement, this should confirmed to prevent damage to the HeNe tube from reverse polarity.
At the same time, the HeNe tube current can be checked.
Put a 1K, 1/4 W resistor in series with the cathode return and measure across it with a voltmeter for the correct polarity and current. The end of the resistor attached to the HeNe tube cathode should be positive and the current will be 1 mA/V of your reading. Also see the section: Making Measurements on HeNe Laser Power Supplies.
This uses a short (140 mm) HeNe tube and power supply running off of 9 VDC. I built these into a case which was from a 1/8" cartridge tape backup system in its former life. In order to obtain regulated 9 VDC, an LM317 IC regulator on a heat sink was added along with a power switch, power-on LED, and the required ballast resistor of 150K. The ballast resistor was determined by monitoring the HeNe tube current and selecting values until the current was correct. The HeNe tube was mounted on standoffs using a pair of nylon cable clamps and aimed through a hole drilled in the plastic case.
S1 +-------+ In+ +--------+ HV+ Vin+ o--/ --+---| LM317 |---+-------+-------+-------| |-------+ Power | +-------+ | | | | | | | | A / | / R3 | | / Rb | | \ R1 | \ 500 | 9 VDC | \ 150K | | / 240 | / | HeNe | / _|_ C1 | | +_|_ C2 | | Laser | | Tube+ --- .1 +-------+ --- 10 | | Power | .-|-. | uF | - | uF __|__ IL1 | Supply | | | | \ R2 | _\_/_ LED | Brick | | | LT1 | / 1.5K | | | | | | | \ | | | | ||_|| | | | | Gnd | | HV- '-|-' Vin- o------+-------+---------------+-------+-------| |-------+ Tube- +--------+It may be easier to locate a 12 VAC, 1 A wall transformer since these were commonly used on with older obsolete modems. In this case, add a bridge rectifier and a 5,000 to 10,000 uF, 25 V filter capacitor to the input.
HeNe tube: Siemans LGR7631A with attached ballast resistor and Alden HV connector. Its specifications are similar to the 1-2 mW tube listed in the section: Typical HeNe Tube Specifications. The H&R catalog no longer lists this tube but their TM92LSR1954 or any other type requiring up to 1,500 V at 5 mA could be used.
This was a power supply and HeNe tube combination. This brick can be wired for either 110 VAC or 220 VAC operation and includes an 'enable' input which must be pulled to around +5 V to turn on the tube. Rather than using a separate power supply just for this, I provided a battery holder with 4 AA cells. Even old tired decrepit ones work fine in this application! The only other parts I added were the line cord, power switch, fuse, light, and enable switch.
The ballast resistor was already built into the tube mounting so that this was truly a 'plug-and-play' assembly.
S1 _ F1 white +----------+ HV+ H o---/ ---- _----+-----------------+-------| |--------<<---+ Power .5 A | | white | | red | / +-------| | / Rb R1 \ | 110/220 | \ 75K 47K / S2 Enable yel/wht | VAC HeNe | / | +-----/ -------------| Laser | | Tube+ +|+ | | Power | .-|-. IL1 |o| | | | | | yel/blk | Supply | | | NE2H |o| +---||||||||---------| Brick | | | LT1 Power On +|+ +| | | | - | | | | | B1 5-6 V | | ||_|| | yellow | | black '-|-' N o---------------+-------------------------| |--------<<---+ Tube- +----------+ HV- green | G o-----------------------------------------------+Notes:
The purpose of the ballast resistor is twofold:
Note: Although a single resistance value may be specified below for Rb, it is better to construct the actual ballast resistor from several lower value resistors in series to achieve the required voltage and power ratings - see the section: Power Supply Construction Considerations for more info.
Some of the procedures below may require measurement of voltage or current in the HeNe tube circuit. Before proceeding, for your own safety and the continued good health of your test equipment, see the section: Making Measurements on HeNe Laser Power Supplies.
While the theory is complex, the result is simple: It turns out that the value of the *negative* resistance of a typical HeNe tube around its optimal operating point is usually approximately -50K. The reason is that larger HeNe tubes tend to have longer but wider bores and the effects of these tend to cancel out. For a stable discharge, the value of the total effective resistance must be positive - else you get a relaxation oscillator. Using a 75K ballast resistor generally provides adequate margin without dissipating more power than needed.
Rb = (Vout - Vo) / Io;
to estimate the ballast resistor value. If you do not know Vo and Vout, start with a value of 75K.
If there is no input voltage at which the tube starts and the discharge is stable, 75K may already be too large and it is likely that the power supply is inadequate for your HeNe tube.
Here are a couple of approaches to selecting the ballast resistor for a small HeNe tube and simple unregulated power supply like the one described in the section: Edmund Scientific HeNe Power Supply.
(From: Steve Nosko (firstname.lastname@example.org)).
The ballast resistor, the actual voltage at the doubler output and the tube running voltage determine the tube current. Look at the tube ratings for the running voltage and tube current - 1,150 V at 4 mA for the tube I was using.
One can approach the design from two directions:
Where the voltage drop across your HeNe tube is known (from its specifications or having been measured), a dummy load will also permit a ballast resistor value to be selected such that the regulator operates near the middle of its compliance range.
CAUTION: Determining the limits of some power supplies is like testing a new jet fighter; pushing the envelope entails some risk as the power supply can be damaged or destroyed if run outside its design specifications. This is especially true with power supplies that are current regulated. Damage could result if the design is not adequately protected. Many commercial designs are quite robust but don't push your luck! If your power supply is current regulated, see the section: Determining the Compliance Range of a Power supply.
The discussion below assumes a power supply which does NOT include a current regulator. If yours does, see the section: Determining the Compliance Range of a Power Supply if you want to determine more about your power supply's characteristics.
Use a voltage divider constructed from a 500K potentiometer (actually four 100K (5 W) resistors and five 20K, (1 W) resistors to get high enough wattage - not an actual pot since such things probably do not exist), 75K (5 W) resistor and 1K (sense) resistor.
This load will short out (and thus disable) voltage multiplier type starting circuits or fool any auto start circuit into thinking the tube is running.
These resistor values should work for most tubes rated between 1100 V and 2,400, and currents between 4 mA and 8 mA with typical power supplies but yours may be the exception. Therefore, the resistor values may need to be adjusted if you cannot obtain meaningful results.
Connect the components as follows:
+----+ +--o Measure o--+ | | | | R1 v | R2 v Rs v I -> PS+ o-------/\/\--+-----/\/\------+------/\/\-----+------o PS- 500K 25 W 75K 5 W 1KMeasure the voltage across Rs to determine current. The sensitivity will be 1 V/mA. Alternatively, simply put a 10 or 20 mA current meter across or in place of Rs. (Then, Rs is effectively 0 for the calculations, below.)
Rb = R1 + 75,000 + Rs - (Vo/Io);where Vo is the operating voltage from the tube specifications.
By varying the load using the test circuit (above), and taking measurements of both voltage and current, the limits of such a regulated power supply can be explored. Over the useful compliance range of a power supply, output voltage will automatically vary to maintain the output current nearly constant independent of input and load variations and ripple from the power supply itself.
However, since the HeNe tube introduces some negative resistance into the equation, actual performance may not be quite the same as with the dummy load. Thus, although the power supply may be able to maintain a specified current over a certain range of dummy load resistance values, some portions of this range (usually at either end) may not result in stable behavior with actual HeNe tubes.
CAUTION: There is always some risk of damage if the specifications are unknown and the designers didn't provide adequate protection. Keep this in mind if you decide to test your power supply in this manner.
If the load resistance is too high, these types of supplies simply do not regulate properly and the output current will be reduced. Where the voltage drop across a (large) HeNe tube is too great, it will not operate properly. However, damage to the power supply itself is unlikely unless left in this condition for a long time.
For power supplies with linear regulators, the safe limits are between 0 and Vsafe volts across the regulator components where Vsafe is less than the specified breakdown voltage of the pass transistor(s). With better designs, Vsafe will be restricted in other ways for protection but the output current may increase once Vsafe is exceeded. The actual output of the power supply will have a compliance range between (Vmax-0) volts and (Vmax-Vsafe) volts where Vmax is the output voltage under the specified load but with the regulator short circuited.
Without knowing the design in detail, there is really no risk-free way of determining this on the high end though better designs will limit the drive to a safe value before parts blow up. As the load resistance is increased, a point will be reached where regulation is no longer possible.
Problems are most likely when driving HeNe tubes that are too large. The supply will be operating at or beyond its maximum specified output voltage which may result in a dead supply if left that way for too long. There may be no warning, or the HeNe tube may blink or flash, behave erratically, or not start at all - these conditions are stressful for both the power supply and tube.
What happens with too small a load resistance (or too small a HeNe tube) is more difficult to predict but could there could also be problems. At the low end, the current will start to rise if the drive pulse width cannot be reduced sufficiently. This is analogous in some ways to the minimum load requirements of a typical switchmode power supply (which is essentially what these are).
CAUTION: For regulated power supplies (inverters in particular), only leave power applied long enough to take your readings if the current is not being maintained at the specified value as this may be stressful to some designs. Where the specifications include a voltage rating (e.g., 2,500 V), this is generally the maximum that should be output continuously. In this case, the upper limit is already known - going beyond this is asking for trouble.
CAUTION: For power supplies with linear regulators, it is best to use a variable input source and bring up the supply slowly while also monitoring the voltage across the pass transistor(s) to assure it doesn't begin to approach their breakdown voltage. If this happens, R1 is too small (beyond the lower limit of compliance).
Current can be measured as described above. Vout can be measured if you have a voltmeter with a high enough range or calculated as:
Vout = (R1 + 75,000 + Rs) * I;
Kill power and confirm that any capacitors have discharged before touching anything. Modify R1 as appropriate and goto step 2. You are finished if the entire range over which current is nearly constant has been explored.
(R1(u) * Io) > Vo > (R1(l) * Io);where R1(u) is the value of R1 at the upper end and R1(l) is the value of R1 at the lower end of the range where the current (I) is constant.
For a given HeNe tube with specified Vo and Io, the Rb which has the power supply in the center of its compliance range is given by:
Rb = (((R1(u) + R1(l)) / 2) + 75,000 + Rs - (Vo / Io);
An alternative is to perform the compliance tests with a HeNe tube installed. The problem with this is that starting may make measurements more difficult and result only applies directly to the HeNe tube that is used. However, this IS a more accurate procedure. The series resistance is varied in the same manner as described above except that it will much smaller since much of the voltage drop is taken up by the HeNe tube. The modified setup is shown below:
+----+ +--o Measure o--+ | | LT1 | | R1 v | R2 tube+ +-------+ tube- v Rs v I -> PS+ o----/\/\--+----/\/\--------|- |-|-------+------/\/\-----+------o PS- 200K 15 W 50K 5 W +-------+ 1KThe procedure is otherwise similar but take care when making measurements since the starting circuit is no longer disabled and use an initial R1 of 25K instead of 250K. As noted above, if you had previously determined the limits of your power supply using the dummy load, with the HeNe tube installed, the discharge may become unstable before these limits are reached. Details are left as an exercise for the student :-).
CAUTION: These power supplies may attempt to maintain the set HeNe tube current even when the operating voltage is beyond their specifications as might be the case with a high power HeNe tube on a small power supply or a ballast resistor that is too large. The power supply may overheat or be overstressed under these conditions and fail without warning. See the section: Typical HeNe Tube Specifications to determine if your HeNe tube is likely to be within the capabilities of your power supply brick.
Where the current rating of your HeNe tube does not match the power supply (and it cannot be adjusted), your options are limited. There is no easy way to increase or decrease current with external circuitry. However, if it is within +/- 10 percent of the optimum current, don't worry about it. If the current is within +/- 20 percent of optimum, it still isn't the end of the world (though a different power supply would be the best solution):
There are two types of problems. You need to determine if the discharge is being initiated at all. If the starting voltage is adequate, there will be momentary flashes that may be extremely short and weak and only visible in a darkened room. As you approach a stable condition, these will become brighter and longer. At the hairy edge, you may get a nice flashing laser.
WARNING: If your HeNe tube doesn't start after a reasonable length of time (like a minute), don't leave the power supply on overnight in a futile attempt to get it going. Starting is a stressful time for power supply components, especially some wide compliance designs, and may result in total failure. If the laser is flashing, this may be ultimately bad for the tube as well. Step back and try to determine what is wrong.
No action at all generally means the starting voltage is inadequate for the tube, there are other circuit problems, or the tube is bad. Tubes with longer and narrower bores (capillaries) will generally require greater starting voltage.
In the case of an enclosed laser head with an Alden connector, HV cable, and internal (potted) ballast resistor, there may be a breakdown in one of these components and it may only show up when starting voltage is applied (not with an ohmmeter). Here are two ways of testing for this situation:
If the tube now starts, one of the original components was faulty (most likely the potted ballast resistor assembly if the negative connection runs through it) and this will need to be replaced.
Assuming the power supply and wiring check out and the tube is good, the only solution is to boost the starting voltage or use a different type of starting circuit (inverter instead of voltage multiplier, for example).
One test for this is to try the tube with reverse polarity on its input. Connect the positive output of the power supply to the ballast resistor (don't omit this!) and then to the cathode (can electrode) end of the HeNe tube. Connect the negative of the power supply to the anode of the tube. You are only doing this for testing! Do not be tempted to leave the tube wired this way permanently should it actually start.
Based on tubes I have tested, the starting voltage is much lower with the anode and cathode connections interchanged. However, the voltage drop across the tube when running with reverse polarity is much higher than with correct polarity. Thus, the tube may not run within the normal operating voltage range of your power supply even if the discharge is initiated - it may just pulse.
Nonetheless, even if it just pulses, at least you know the tube is not totally dead. If the tube is otherwise undamaged, there should also be an indication of (at least weak) laser output from the business end of the tube. Perhaps, all you need is a power supply with higher starting and/or operating voltage. An inverter type starter using a flyback transformer appears to be particularly good for hard-to-start tubes. Unfortunately, I do not know of any reliable way of determining the likelihood of success without actually trying it.
I have one 5 mW HeNe tube that requires (depending on its mood) as much as 15 to 20 KV to start (it should be less than about 10 KV). However, once started, it runs with a normal operating voltage of about 1,800 VDC.
WARNING: Do not let the HeNe tube run for any length of time with reverse polarity as damage may occur due to heating and sputtering at the anode end of the tube.
To produce a stable discharge, the following must be satisfied:
These factors are not independent. Since the negative resistance and sustaining voltage of the tube are not normally specified and depend on current, some amount of trial and error may be required to achieve consistent stable operation but in most cases it really is very easy.
Cycling behavior can be due to several factors:
If the transformer or inverter drops too much under load, the tube voltage may fall below the minimum for the tube/ballast combination as soon as it starts. This cycle will repeat continuously or it occasionally may catch.
Use a higher voltage and larger ballast resistor, and/or increase the uF value of the main filter capacitor (and/or the one in the DC supply to an inverter type supply as well if it isn't regulated).
Minimum capacitor values for less than 5 percent voltage ripple (typical voltage and current requirements):
Actual ripple in the current to the tube may be several times greater than this since it depends on the change in voltage with respect to the total effective resistance of the PS+tube+ballast resistor combination). However, the resulting ripple in the optical output power will be 2 to 10 times lower than the ripple in the current depending on operating point. The lowest will occur around the tube's optimal current specification.
For an unregulated power supply, increase the operating voltage and/or decrease the ballast resistance.
For a regulated power supply, decrease the ballast resistance so that the voltage for the desired operating current falls within its compliance range.
Shorten the wiring - minimize the distance between the power supply and ballast resistor, the ballast resistor, and tube anode, and don't use long runs of high voltage coax (which may have higher capacitance). Increasing the energy of the starting circuit slightly may help as well.
It is difficult to measure the output voltage of a HeNe laser power supply with a multimeter even if it is supposedly within your meter's range. Connecting the meter across the tube while it is on will likely extinguish the arc due to the capacitance of the probe inducing a momentary dip in the voltage. Even if the tube remains lit or restarts, there may actually be oscillation resulting in an erroneous reading. Leaving a multimeter connected during starting may prevent the tube from firing due to its additional capacitance and reduced resistance. And, the meter may be damaged due to arcing from any high voltage starting pulses. A VOM or DMM with a suitable high voltage probe can be left connected to a wide compliance type power supply and possibly on one using a voltage multiplier (though it may load it excessively) but should probably not be used with a pulse (trigger) type starting circuit.
Commercial supplies may not provide access to any convenient voltage test point. With a suitable high voltage probe for you multimeter, it may be possible to measure between the power supply side of the ballast resistor and the HV return ONCE THE TUBE IS RUNNING. However, this is risky - if the tube goes out, the starting voltage will appear at this point and may fry your meter or find a convenient path to ground through YOU.
Measuring the starting voltage can be difficult depending on the type of circuit used in your power supply. See the section: Testing a HeNe Power Supply for more information.
Where you are interested in AC voltage (ripple, for example), couple the test point through a HV coupling capacitor to block DC. Its voltage rating must be adequate to hold off the maximum possible output of the power supply and its uF value must be large enough to minimally affect the measurement accuracy. At 60 Hz, for example, a .01 uF capacitor is large enough to to produce less than a 5 percent error on a 10 M ohm input impedance DMM. CAUTION: Provide some sort of excess voltage or surge protection (e.g., an NE2 neon bulb) across the inputs to the multimeter or scope so that multi-KV transients don't find their way into your test equipment!
Obtain a 10 mA panel meter (I use a surplus Triplet - it probably dates from the 1950s). Mount it in a well insulated or grounded case, and add a set of well insulated color coded (red and black for + and - respectively) high voltage leads with banana plugs on the ends. Then, any power supply should include a 1K resistor in series with the cathode return connected to jacks on the case. This 'current sense' resistor (Rs) will have no affect on power supply performance but will prevent any significant voltage from appearing at the test jacks if the meter is not present. The 10 mA meter will effectively short out Rs so essentially all the currennt flows through it. A voltmeter can be used instead of a current meter across Rs. The sensitivity will then be 1 V/mA. Either type of meter can be left in place permanently if desired.
Some commercial laser power supplies already have a built-in sense resistor in an easily accessible relatively safe location for current monitoring or you can easily add one.
Where you are building your own power supply, make sure it has its negative output earth grounded (3 prong line cord or separate wire screwed to a suitable ground) if at all possible. This will assure that the cathode end of the HeNe tube, metal parts of the laser head, and current measurement test points are all at or very near ground potential - and thus less of a hazard should you touch any of them (though I am not recommending this!).
For commercial units, test to see if this is already the case or is possible (it almost always is but there are no doubt exceptions). Such precautions will greatly reduce the chances of shocking experiences since the only part of the laser head at a high potential will be the ballast resistor and tube anode.
See the chapter: Complete HeNe Laser Power Supply Schematics for more information and sample circuits.
If there is a continuous glow from the inside of the HeNe tube, the power supply is probably working properly though the current could be incorrect. However, this would result in reduced output power or excessive heating - but not a totally dead laser unless the current was more than 2 or 3 times too high. (In this extreme case, if the tube is good, there would likely be at least a flash of laser light from its output at power-on and/or power-off as the tube current passes through the normal operating range).
Where there is no sign of a discharge - even momentarily - the problem could be with the tube, wiring, or power supply.
In either case, it would probably be a good idea to see the section: How Can I Tell if My Tube is Good before getting into heavy troubleshooting of the power supply.
To determine if the power supply is working requires testing it for both the starting and operating voltage (and proper current if it includes an internal regulator). Of course, the easiest approach is to substitute a known working HeNe tube, but this is not always a viable option.
Before proceeding, for your own safety and the continued health of your test equipment, see the section: Making Measurements on HeNe Laser Power Supplies.
This load will short out (and thus disable) a voltage multiplier or fool any other type of starting circuit into thinking the tube is running.
Add a 1K ohm resistor in series with this load to use for measuring the current (and from this, the total voltage from the power supply).
+--o Measure o--+ | | Rl v Rs v PS+ o-------/\/\-----+-----/\/\------+------o PS- 1KAs an example, for a typical 1 mW HeNe tube requiring 3.5 mA at 1.4 KV, the load resistor (Rl) should be 400K ohms. The power dissipation at this operating point would be about 5 W so use a 10 W resistor for Rl.
Measure the voltage across Rs to determine current. The sensitivity will be 1 V/mA. Alternatively, simply put a 10 or 20 mA current meter across or in place of Rs. Power supply output voltage is then Vo = (Rl / Rs) * V(R2).
If the power supply has a current adjustment, see if this behaves as expected. A control that does nothing could indicate that the load resistor is sized incorrectly and the compliance range of the power supply is being exceeded (low or high).
In all cases, a defective regulator or control circuit could result in these faults as well.
The NE2s can also be used in place of the large resistor described above:
+--o Measure o--+--o Measure o--+ IL1 IL2 | IL20 | | Rb +--+ +--+ v +--+ v Rs v PS+ o---/\/\----+|oo|--|oo|--//--+------|oo|-----+------/\/\-----+---o PS- 75K, 5 W +--+ +--+ . . . . . +--+ 1K NE2 NE2 NE2Note: This example uses 20 NE2s - adjust this number for your particular expected power supply output.
Measure the voltage across Rs to determine current. The sensitivity will be 1 V/mA. Also measure the voltage across the right-most NE2 (IL20 in this circuit). Operating voltage is then: (75 * V(Rs)) + (20 * V(IL20)).
This approach can also be used instead of the that using a variable resistor in the section: Selecting the Ballast Resistor Using a Dummy Load. Simply insert or remove individual NE2s as a means of evaluating the power supply's characteristics. However, see the caution in that section with respect to possible damage to the power supply.
Alternatively, knowing the input impedance of the voltmeter, a high value resistor can be added to extend its range. See the document: Simple High Voltage Probe Design.
Note: Some inverter type power supplies (especially power supply bricks) use a combination of a voltage multiplier AND medium compliance design so testing with an instrument that does not have a high impedance input (i.e., greater than 250 M ohms) may be misleading (the portion of the starting voltage produced by the multiplier will be effectively shorted out leading you to condemn a power supply that is actually good).
CAUTION: Don't omit the load resistor to limit current - otherwise, the internal filter capacitor(s) of the supply will discharge rapidly if a spark jumps the gap and this may be bad for the supply.
WARNING: Use a well insulated tool (like a plastic stick) to adjust the spark gap if necessary. There may be 10 KV or more present if your power supply is working properly and that can bite (especially since if it jumps to YOU, the charge on the main internal filter capacitor of the supply won't be far behind!
For dry air, breakdown voltage is about 25 KV per inch. However, many factors affect this including the shape of the contacts (pointed or smooth), temperature, humidity, etc. so this will not be a precise measurement. Setting the gap at about 1/4" will result in a breakdown voltage of about 5 to 7 KV. There should be sparks periodically (one every few seconds to several per second depending on the power supply - if the starting voltage can jump the gap. If there is no evidence of sparks even when the gap is very small, there is a problem with the starting circuitry.
WARNING: Just because these are often compact units doesn't mean they cannot be lethal. See the document: Safety Guidelines for High Voltage and/or Line Powered Equipment before working on any type of equipment which uses line voltage or produces high voltage.
CAUTION: Make sure you discharge both the output of the power supply AND the HeNe tube itself with a high value power resistor (e.g., 100K, 2 W) before touching anything. Else, you may be in for a surprise!
There really isn't any way I know of to open these non-destructively for analysis or repair. However, make sure your wiring is correct and that you are attempting to operate the supply on the proper input voltage (e.g., 12 VDC, 110/220 VAC - though it may be too late if you guessed wrong and connected 220 VAC to a 12 VDC unit) and that your HeNe tube and ballast resistor are appropriate for the power supply's ratings - start with a small HeNe tube or even just a load resistor and multimeter. See the sections starting with: Selecting the Ballast Resistor.
I have a couple of supplies with these types of faults.
These types may also develop excessive leakage and reduced capacity (called 'deforming') from long periods of non-use. This is the same thing that happens to the large energy storage capacitors of photoflash or laser flashlamp power supplies when neglected for years. A simple test is to measure the time constant of the capacitor(s) with a high value resistor. Usually, you can do this without removing them from the power supply and just using the normal bleeder resistors that should already be present. If the discharge time is much shorter than expected, excessive leakage is the likely cause. It may be possible to revive these by running the power supply from a Variac, first starting at low input voltage and slowly increasing it as the capacitors 'reform'.