Generating High Voltage from Very Low Voltage

One of the things I want to do with these Geiger tubes, is to make an ultra-portable 'pocket' device. This is planned for the SI-19BG miniature α tube, which is only about 20mm long!

At present, I'm working on this circuit -

Where possible I've kept the values as stated, but as I don't have any of the specified 2N series transistors, I'm using whatever I have in stock, namely a BC212L and a 2N3904. The 1N914 is replaced by a series string of 47V Zeners and a couple of miniature neon bulbs! The transformer is a miniature audio transformer. The transistors have lower specs than those stated, and this might well affect the results. The closest I have in stock to the specs of the 2N4401/2N4403 are a couple of BC327/BC337 pairs. These have total power dissipation of 625mW same as the specified devices, but slightly lower collector-emitter voltages, however the collector current is higher at 800mA against 600mA. I might try these instead of the quickly-grabbed BC212L and 2N3904.

It is working on the bread-board, but due to loading effects of my HV resistor chain (remember this is only 100MΩ) the voltage reading is poor with the 3V supply (2xAA). With a 9V supply (PP3), the loading is much less of a problem, and I can get the circuit to produce a reading of about 380V with two neons and four Zeners, oh and the neons glow quite nicely! At 3V the neons glow is quite dim, and extinguishes when the voltage is measured.

I quite like the idea of having at least one neon in the feedback circuit - its glow is a good safety check!

I'm not entirely sure what controls the available power of this circuit yet, which may be critical to getting it to work at 1.2V or lower, which is my ultimate goal, so the pocket unit can run on a single AA NiMH cell. This might prove too difficult a voltage to start from, so I may end up using a 3V supply and finding a way to miniaturize the battery! Using higher voltage Zeners will also drastically lower the component count and physical size of the built circuit - as would using a single inductor in place of the transformer.

The 6.3mm fuse clips for the tubes have been delivered. I expect the 1GΩ 2kV resistor to arrive tomorrow. That will massively assist in getting accurate voltage readings!

There is a variation of this circuit that uses the feedback transistor to control the base bias of the oscillator transistor, which is said to give lower current drain, so I might try this out. I'll try the circuit at just 1.5V as well from a single AA cell, and see if it runs!