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555 Geiger counter HV supply troubles

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JulietMikeBravo

Mon Jun 18 2012, 07:02PM

Registered Member #5136 Joined: Sun Jun 03 2012, 03:07PM
Location: Utrecht, Netherlands
Posts: 6

I'm always searching for nice, easy built power supplies for geiger counter use. The internet is full of all kinds of designs. One of the better ones seems to be the one used in the DIY Geiger Counter project. It uses fairly standard parts apart from the HV transistor, has low power use and is easily adjusted to the proper voltage.

So, I built the HV section of the schematic and started measuring. The problem is that the voltage doesn't seem to be stable. I watched the square wave coming from the 555 and the signal at base of the MPSA44 on my scope but when adjusting the voltage, it jumps, suddenly oscillating much faster or completely stopping oscillation.

I'm using a standard 555 instead of the CMOS 555 used in the original schematic but even dropping in a CMOS 555 seems to result in the same behavior. I have used different power sources, decoupled the power supply properly with a 100 nF capacitor but nothing seems to help. I have also cleaned the perfboard PCB to make sure there aren't any (slight) shorts.

What could be the cause of the problem? Apart from the stabililty issue it seems to work as described, I can get up to 580V out of it.

Antonio

Mon Jun 18 2012, 08:35PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

Try to connect pin 5 to ground with a 100 nF capacitor. Also, the high-impedance connection to the reset pin (4) seems not safe. I would prefer a direct connection of pin 4 to the positive power supply (pin 8), and would connect the overcurrent protection transistor Q1 to pin 5 (in parallel with the added 100 nF capacitor).

Sulaiman

Mon Jun 18 2012, 08:42PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

That is a strange hv inverter;
1) MPSA44 is only rated for 400V so at 580V is probably continuously avalanching
2) The only output voltage regulation is the breakdown of the MPSA44
3) if the output voltage is determined only by the current control
then at high count rates the high voltage will decrease.
.... I don't understand this circuit, I must be missing something,

JulietMikeBravo

Tue Jun 19 2012, 01:50PM

Registered Member #5136 Joined: Sun Jun 03 2012, 03:07PM
Location: Utrecht, Netherlands
Posts: 6

Sulaiman wrote ...

That is a strange hv inverter;
1) MPSA44 is only rated for 400V so at 580V is probably continuously avalanching
2) The only output voltage regulation is the breakdown of the MPSA44
3) if the output voltage is determined only by the current control
then at high count rates the high voltage will decrease.
.... I don't understand this circuit, I must be missing something,

Thanks for your feedback. Indeed, the MPSA44 has a max 400V rating. Still, I am rather sure that I can get more than 400V out of it, I clearly measured it. I measure high impedance HV with a DIY 1:10 probe, this is not really accurate but it can't be more than 10-20V off at this voltage.
I wonder, how accurate are those ratings? Can you expect this transistor to start breaking down at 400V or can it be much more in real life?

The circuit is designed to regulate voltage using the voltage from the variable resistor. I understand that it will vary with load but geiger tubes are not a normal, continuous load. When idle they will consume practically no current, only at high count rates there will be a significant current drain.

Anyway, I am a little bit puzzled... Why did the maker of the circuit use a transistor with insufficient rating? I own two of the kits which are very nicely designed and seem to work fine, except from the one on I have on the roof as a weather station which stopped reporting counts, maybe HV failure?
And why am I getting such unpredictable results, is it caused by the transistor, 555 or else?

Proud Mary

Tue Jun 19 2012, 07:33PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

If we imagine a GM tube with anode resistor 5 MÎ© and a 500 V supply, then the maximum current that tube can draw in the case of saturation - ruinous continuous conduction - is 100 Î¼A.

If we consider a count rate of 5000 cps - more than will ever be detected by a GM tube close to the surface of any natural source such as pitchblende - and say that each conduction event lasts 10 Î¼s - then the tube conducts its 100 Î¼A for 0.05 seconds out of every second, so the average current pull at 5000 cps will be 5 Î¼A.

Obviously these figures will vary from case to case, but these are the order of currents we should expect to see, and plan for, in a GM circuit.

I have some power supplies for GM pocket dosimeters, which will provide 500V for 18 months of 24/7 counting from a 3V battery. The power supplies are smaller than a sugar lump.

JulietMikeBravo

Sat Jul 14 2012, 11:32AM

Registered Member #5136 Joined: Sun Jun 03 2012, 03:07PM
Location: Utrecht, Netherlands
Posts: 6

I have ditched this design and built a different circuit:

http://www.julietmikebravo.nl/2012/07/1068/

When idle it consumes very little current. It's one of Charles Wenzel's geiger counter circuits adapted with a high impedance feedback loop.

I have discovered that high voltage / low current HV circuits have to be quite clean. At high voltages stuff which normally isolates very well becomes a conductor. I had to rebuild this new circuit because it wouldn't function properly, drawing too much current and failing to provide the correct voltage. Probably some flux or blackened PCB conducted too much and made the circuit fail.

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