Author Topic: GM tube hv transformer  (Read 1155 times)

Offline AlexanderHun

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GM tube hv transformer
« on: January 24, 2020, 06:24:25 PM »
Hello!

I managed to get two pieces of Geiger Müller tubes, museum pieces, unfortunately they are operating at a high enough voltage. I need a switch with adjustable output voltage of 1200-1500 volts to test them. Anyone have any ideas?








Offline haversin

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Re: GM tube hv transformer
« Reply #1 on: January 24, 2020, 10:29:14 PM »
I use three parts from a microwave oven  (The Hv transformer, Hv diode and Hv capacitor) and power it with a variac to control the voltage. Most DMM's can measure the output voltage directly. Don't forget a 10 meg anode resistor (some tubes use less) on the GM tube.   

Offline klugesmith

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Re: GM tube hv transformer
« Reply #2 on: January 24, 2020, 10:53:42 PM »
To get 2000 V for detector bias,
my first try was a small variac + tiny NST + diode, capacitor, permanently connected kV meter, and a weakening resistor between variac and NST.

Too much ripple voltage ( a volt at 60 Hz?  Some mV at 120 Hz with bridge rectifier & bigger capacitor ) for the Charge Sensitive Preamplifier and my proportional counter tube.  When checking ripple voltage with your oscilloscope in AC-coupled mode, mind the kilovolts.

Then I went out and bought an old NIM Bin, and a 0 to 5000 volt bias generator module to plug into it.
The 1960's or 1970's design has an 8 or 10 stage voltage multiplier, following a small transformer running at 25 kHz, driven by a multivibrator made from discrete transistors.  The size of capacitors in voltage multiplier makes the output impedance about 5 megohms.

That's how I would go for a scratch-built bias supply.  As found online in many GM counter projects.
See also the HV power circuits in battery-powered civil defense instruments.

If ripple voltage is not a big concern, you might try a mains-frequency voltage multiplier, optionally following a step-up transformer.  Haversin's MOT approach is probably easier & better.  Small MOT might not need active cooling if operated at 1/3 or 1/2 of nominal voltage.

« Last Edit: January 24, 2020, 11:35:35 PM by klugesmith »

Offline haversin

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Re: GM tube hv transformer
« Reply #3 on: January 25, 2020, 01:15:21 AM »
Too much ripple voltage ( a volt at 60 Hz?  Some mV at 120 Hz with bridge rectifier & bigger capacitor ) for the Charge Sensitive Preamplifier and my proportional counter tube.

GM tubes are generally operated in the middle of the Geiger-Muller region where there is a plateau in the counting rate verses voltage response curve. Voltage regulation within a few volts is perfectly acceptable in this region. Operation in the proportional region is at a lower voltage and close voltage regulation is required.
« Last Edit: January 25, 2020, 01:28:47 AM by haversin »

Offline futurist

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Re: GM tube hv transformer
« Reply #4 on: January 25, 2020, 11:01:24 AM »
I built a geiger counter for high school project using Wenzel's ferrite choke HV supply. It uses 555 monostable for pulse detection and electrolytic capacitor for "counting" pulses per minute
http://www.techlib.com/electronics/graphics/geiger2ind.gif

Power consumption is amazingly low, less than 1 mA, my counter still has original 2011 batteries
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« Last Edit: January 25, 2020, 11:14:08 AM by futurist »

Offline shrad

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Re: GM tube hv transformer
« Reply #5 on: January 28, 2020, 12:32:39 PM »
CFL inverters make good small power supplies for GM tubes

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Re: GM tube hv transformer
« Reply #5 on: January 28, 2020, 12:32:39 PM »

 


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