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Author Topic: 500VDC power supply circuit?  (Read 1996 times)
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Hello,

I recently was given a Geiger tube for free. I have already built the detector, but the tube I have requires 500VDC to operate at full potential. I have yet to find a pre-made power supply, or a schematic that will provide that voltage. Does anyone know of any schematics they have found/designed? Thank you.

-Flam
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http://uk.farnell.com/hitek-power/gma500p/converter-dc-dc-1-5w-500v/dp/1187172

Google is always worth a try !
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There are many possibilities:

You could use a transformer (like the transformer from a flash light) to get the 500V at once.
You could use a more normal transformer with a diode cascade.
Some use only an inductance with a diode cascade.
Some use a feedback with zener diodes.
You could read the voltage with the Arduino, but the voltage divider will draw some current, even if the resistor for high voltage is 2 * 22M.
You coulde use the Arduino to generate a signal for the transformer.

Take a look at the schematic of this one : https://www.sparkfun.com/products/10742
It uses a normal audio transformer and a diode cascade.
These projects are helpful : http://hackaday.com/tag/geiger/
This one is also with full schematic (click on design files) : http://mightyohm.com/blog/products/geiger-counter/

I had some trouble to reach a high voltage for my Geiger tube. So I tried to find the resonance frequency of the transformer (flash light transformer). It was between 40kHz and 50kHz, and suddenly I could reach 900V. I read the voltage with the Arduino via 2 * 22M, and I adjust the frequency of the transformer driver (using BC547/BC557). But my circuit is specific for that transformer.

Which Geiger tube do you have ? I hope not the CI-3BG (also called SI3GB, SI38G, etc), they are not sensitive.
« Last Edit: August 27, 2012, 07:24:13 pm by Krodal » Logged

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Thank you. I do not yet posess the equipment to find the resonant frequecy of anything, although I might try to make a CRT osscilloscope in the future. The tube I got is from LND, inc. It is model LND-712, but I was originally going to use the CI-3BG until I heard of its terrible sensitivity. Is there any way I can find the resonant frequency without the osscilloscope? Personally, I have never seen transformers in flashlights, but I can't say I've taken many apart.

-Flam
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I ment a flash, the one for a camera : http://en.wikipedia.org/wiki/Flash_%28photography%29
A transformer is used to make a few hundreds of volts from 3V batteries.

Since I used the Arduino with TIMER1 to drive the transformer (not directly, but with transistors), I let the Arduino make a frequency sweep and I measured the high voltage after a rectifier. Later I added a series capacitor to the primary side of the transformer. A capacitor of 100nF worked best for the transformer I have.

Working with high voltages could be dangerous. If you solder something badly, the high voltage could turn up everywhere in your circuit. I wrote my own code for the TIMER1, since I don't know if the tone() function is able to get to 50kHz.

As you can see in the schematic, I used a lot 1N4148 diodes for protection. The Arduino is also used for other things, and I don't trust my circuit that well with very high voltages. I also use a full bridge rectifier, just because I liked to do that. Reading the analog voltage was very noisy, so I added a capacitor of 1nF (C10), and I use the average a few samples in my sketch. The 2*22M draw current from the high voltage, and I had to find the new resonance frequency.


* geiger with tranformer resonance.png (9.6 KB, 2104x625 - viewed 75 times.)
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OH! I actually removed the flash from a disposeable camera for the purpose of making the power supply. Do you think I could use a voltage multiplier to step up voltage to 500v? And what is a diode cascade? Is it similar to a voltage multiplier? (CW)

-Flam
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Yes, stepping up is normal if the transformer can't get there at once.
I called it a "diode cascade", but "voltage multiplier" seems to be the more common name.

You could try a frequency of roughly 1kHz to 100kHz with that transformer. It would be fun to see what the effect is.

There is one thing I forget to mention: you can't measure the high DC voltage. Well, you can if you know what is going on. The impedance of 10M of a good multimeter might drop the high voltage with 50 or 100 Volts. A cheap multimeter with 1M impedance can't be used at all.
I calibrated the voltage measurement like this: I connected both the 2*22M for the Arduino and my multimeter. I fixed my calculation in the sketch so the values were the same. Then I removed the multimeter and the Arduino could read the big increase of the DC high voltage because the multimeter didn't draw current any more.
« Last Edit: August 29, 2012, 04:49:39 pm by Krodal » Logged

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Interesting. I will try that out. I guess with a lot of tinkering I could get it pretty close to what I want.

-Flam
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Or you could simply use two sets of camera flash modules and run them in series with each other.  Since there may be direct feedback from output to the battery side you may need to have independent battery sets as well.
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Should I put a potentiometer on the end and tinker until I reach target voltage?

-Flam
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I don't know where you want to place that potmeter, but you can't use a potmeter with 500 Volts.

I use the frequency to tune to a certain voltage.
After finding the resonance frequency, I use that for the maximum frequency and a lower frequency is a lower voltage.
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Okay. Would a non-digital multimeter work to measure voltage?

-Flam
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No, the impedance of an analog multimeter is even lower that the digital multimeters.
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One could use the CCFL inverter of an old TFT/LCD display. I'm not sure if it will give you exactly 500V, but at leas something in that region. Pro: available for free from a defective display + runs on 12 ~ 24V
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Okay, thank you.

-Flam
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