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I'm looking to make a high voltage supply for some geiger-mueller tubes (the ubiquitous sbm-20 russian surplus tubes that everyone uses). these don't require much current at all, so creating high voltage shouldn't be too tricky.

I'm using a 12 volt source, and I'd really like there to be some way to fine tune the voltage (either by setting a duty cycle, or a current set resistor, or something similar). I'd also like it to be as cheap as possible (meaning probably a preference for discrete components over ICs) while being reasonably sized (say, the size of a clamshell phone).

It seems that a reasonable way to approach this would be to create a two-stage converter: a boost converter to get me up to, say, 48VDC, followed by a series of voltage doublers until I get to 500 - 700 VDC.

This two-phase approach allows me to fine tune the voltage relatively easily, and I should be able to get away with using cheap discrete components.

My questions are:

  1. Is this a reasonable approach? Are there simpler ways to create a similar, charge-pump style voltage supply? I'd really like to have the simplest circuit I can while maintaining some semblance of tunability and stability.

  2. If so, how would I couple the boost converter to a voltage doubler? Do I need a transformer? Can I just use the diode and capacitor as the first diode/capacitor pair in the ladder? Do I need a second FET to switch the boost converter's load capacitor into the voltage ladder?

Any schematics or similar would be extremely helpful.

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Just addressing the 'cheap' aspect, have you considered re-purposing the circuit from a relatively cheap COTS electric flyswatter.

Not exactly a good way to learn how it was designed, but it might get you there quick and dirty. You might even need to use a voltage divider to bring the voltage back down

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I guess Maxim are a trusted source of designs and this probably does what you want: -

enter image description here

The design was found here

Below is another common design I've seen: -

enter image description here

It came from here

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A flyback came to mind as something to consider. Something like the following:

Dr. David Prutchi's hacked flyback
Dr. David Prutchi's flyback driver

Wind a smaller, more suitable flyback transformer by hand and use a long series chain of NE-2's for regulation on the secondary. It's been many years, but this is close to what I did for my hand-held, battery-powered Geiger counter then.

The above links are actually used to feed a 250kV system used for physics experiments discussed at:

Dr. David Prutchi's 250kV Cockcroft-Walton

But you don't need that.

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