High Voltage Generator for Geiger Tubes
These circuits generate high voltages and can cause dangerous shocks! Do not build these devices unless you are experienced and qualified to work on high voltage devices.
The following two circuits are an improvement over the older circuits below. The circuits are shown generating 500 volts but they may be modified to supply a couple of hundred to nearly 1000 volts by changing the zener diodes.
The difference is subtle; the feedback signal increases the voltage on the base of the 2N4403 to stop the oscillator instead of stealing current from the capacitor on the emitter. The result is much lower power dissipation when there is little or no load on the high voltage. The new circuit draws less than 1/2 mA when operating at 9 volts without a load using a 1:1 600 ohm audio isolation transformer. The 3 volt circuit may be modified in the same way but make sure to switch to a MPSA18 (or a similar very high gain transistor). The 120 volt zeners are also an improvement over trying to grade ordinary diodes; grading is just too much trouble! A 1N5273A is a typical type to try. Remember, this circuit can only supply a few microamperes so an ordinary 10 megohm voltmeter will load the output too much. (500 volts/10 megohms = 50 uA.)
500 Volt Geiger Counter Power Supply
Note: With some transformers and zeners, the circuit will work better if the 10 megohm resistor is moved up to be in series with the diodes (see next schematic). It is a good idea to add a resistor in series with the diodes anyway, perhaps 100 k, to prevent damage when probing around. When operating properly, the current should drop down to below 1/2 mA with no load. The series 10 megohm resistor will make gas discharge devices work well in place of the zeners, too (neon bulbs, for example). Also try a .1 uF capacitor from base to emitter of the MPSA18. This capacitor modification combined with the series 10 megohm allowed a single Lumex gas discharge tube to regulate the output voltage of the circuit at 600 volts while drawing only 300 uA, unloaded.
The transformer in the prototype is a small isolation transformer with opposite ends of the primary and secondary connected together to boost the output voltage. Other transformers will also work, including tiny audio interstage transformers, as long as the impedance is relatively high on both windings. If you don’t get a high voltage, try reversing one of the winding connections. If the current doesn’t cut back with no load, try the techniques mentioned in the note above. The circuit will work without the secondary connection simply by connecting the collector of the MPSA42 directly to the first .02 uF cap. and diode and leaving the secondary winding disconnected. Using the two winding voltage boost is recommended when attempting to run the circuit on a lower supply voltage.
Here is my favorite 9 volt version using only a molded choke (the ones I have in quantity):
Other combinations of zener diodes and gas tubes may be used to get the desired output voltage but the upper limit is about 1000 volts.
The 0.1uF capacitor on the base of the MPSA18 will reduce the current consumption when using Lumex or some other types of gas tubes and some zeners. It is unnecessary in most cases but the right value or no cap at all can push the no-load current really low, depending on the characteristics of the selected breakdown devices. This circuit will operate on lower supply voltages but, at some point, it will not "cut back" and will draw several mA all the time. The transformer version can more easily generate higher voltages when operating from lower battery voltage. Keep in mind that the current consumption of this circuit is very low when operating properly and a 9 volt rectangular battery will last a long time, giving perhaps a month of continuous duty.
The output voltage can vary by up to 20 volts from no load to a light load due to the measurement of the voltage before the voltage doubler. The regulation is much better once a little current is flowing. For much more precise regulation, connect the zeners or gas tubes and resistor from the base of the MPSA18 directly to the output. You will need twice the breakdown voltage, however. I connected four neon lamps, type WL-1MH (similar to NE-38), in series with the 10 megohm from output to base and the resulting voltage was 540 volts and only changed 7 volts when loaded by 300 meghoms. Switching over to 4, 120 volt zeners gave an output voltage of 520 volts that changed less than a volt when loaded! The current drain seemed to improve in both cases, dropping to 200 uA with no load. If you don’t mind using twice the number of zeners, this is a nice modification. For fun, I replaced one of the high voltage zeners with several lower voltage zeners until I hit exactly 500 volts. It took 6 zeners to do the job and I suspect it’s overkill for most projects. But if you want a precise voltage and have a sufficient pile of parts, why not!