Resistor Voltage Ratings

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lifepower
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Joined: Sat Feb 28, 2015 8:02 pm

Resistor Voltage Ratings

Post by lifepower »

I have been using your Geiger Counter for quite some time, in fact, own 5 of those that I got from Adafruit, two with the case. This kit is just great!

I'm preparing to use one of them in a weather station running continuously 24/7. Since I have been testing the kit with different tubes, including those that require 10M resistor instead of 4.7M and while looking for that, I've stumbled across required voltage rating.

In all the kits that I've got, it seems that Carbon Film resistors are being used, which really have low voltage rating - most equal or below 250V, with some exceptional cases having 500K ratings. Since the kit has adjustable voltage, which in my measurements can go up to 650V (I measured that using 1G resistor in series), I'm wondering if the currently used resistors have correct voltage ratings.

Specifically, it looks to me that R2 (the one that goes directly to the tube), R4 and R7 (those in voltage divider that connects to the other side of the tube) should be rated at least 1 kV, so they should be Metal Film resistors, most of which are rarely available at 1/4W rating - they usually are 1W or higher (big thick ones).

So my question is, what are the voltage rating used in these resistors and aren't there any issues having over-voltage on them in the long run?

Also, after looking at the schematic, if the voltage at the tube is 500V, after voltage-divider circuit, the resulting voltage on base of Q2 would be 90V, while the transistor seems to be rated at 60V. Won't this damage the transistor in the long run too?

P.S. When experimenting with different tubes and cables that I've soldered to J1/J3 sockets, I've accidentally shorted HV connection with my hand while R2 was unsoldered (I placed it near the pancake tube) - it created spark and didn't fell too good on my finger! Perhaps I could add a bleeder resistor of some sort, maybe 1G or so, would that work?
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mightyohm
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Re: Resistor Voltage Ratings

Post by mightyohm »

Excellent questions.

It's true that many carbon film resistors are rated at around 200V. In practice, these ratings tend to be very conservative (if the manufacturer gives a rating at at all). Keep in mind that this is a pulsed application. R2 only sees high voltage when the GM tube is avalanching - a time measured in milliseconds. The resistors I am using are rated at 250V continuous, 500V max. Worst case, there may be a small amount of resistance drift due to the repeated pulses of high voltage. But this is not a precision instrument - resistance changes of even 10 or 20% will not appreciably affect operation.

The voltage on the base of Q2 is clamped at approx 0.7V, so there is no risk of damaging Q2.

R4 and R7 are never exposed to the full HV supply voltage, since the series combination of the geiger tube and R2 result in a large voltage drop during the avalanche event. R4 gets most of the voltage since R7 is clamped to 0.7V by Q2. The max voltage on any single component will be significantly less than the HV supply voltage due to the high impedance of the supply and the many components in series.

I wouldn't recommend operating the supply significantly above 400V (the intended operating voltage) as you are putting stress on Q1. However, in practice I have never seen Q1 fail.
lifepower
Posts: 6
Joined: Sat Feb 28, 2015 8:02 pm

Re: Resistor Voltage Ratings

Post by lifepower »

Thank you for a quick and detailed response!

I actually do require a bit higher voltages because I'm using the kit with LND 7317 and LND 712, both of which run at 500V or more. The second one also requires 10M resistor instead of 4.7M. Actually, it worked for me with 4.7M, but I've read that's not too good for the tube, so I'd probably be getting 4.7M and 10M, 1kV rated resistors. I'll be soldering directly to the tube - this seems to help increasing the longevity of the tube due to reduced capacitance. I'm also getting a couple of new tubes running at 600V, so will be stress-testing the kit.

Is there anything I could do to make it more tolerant to max settings, or maybe even increasing HV max limit up to 800V or so?
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mightyohm
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Re: Resistor Voltage Ratings

Post by mightyohm »

Q1 and D1 are probably the weak link. Replace both with higher voltage variants.
The ability of the supply to generate high voltage is a function of L1, C3, R9 - at some point these would need to be adjusted as well.
I have not tried to extend the voltage range. The design voltage is 400V.
lifepower
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Re: Resistor Voltage Ratings

Post by lifepower »

I have been doing experiments on increasing the voltage: increased L1 to 15mH, reduced R8 to 4.7 Ohm, increased VR1 to 30 Ohm (to have wider range).

Open-circuit voltage of TP2 was getting to around 800V. However, to get that, I'd have to start with 600V, for instance, and then turn VR1 to get higher voltage. Once TP2 is shorted (simulating tick), HV would drop down to zero and stay there, until I turn VR1 back to lower voltage and HV would start to increase at some VR1 close to 600V checkpoint.

Also, by doing these experiments, I've burned Q3 and 555 chip! At some point, HV would not increase no matter where VR1 was. Initially I thought that Q1 failed, but after replacing few things, it turned out to be Q3. The timer chip seems to also failing - even when plugged to another working kit, instead of 400-500V at TP2, it would produce only half of that. I don't know which of my actions caused this - first I noticed that I forgot to solder C3 back (I unsoldered it earlier trying to put a different cap); curiously, everything worked without C3 for a while! Also, when "simulating" ticks, I've been shorting TP2 each time and Q3/555 burned out right after one of such shorts - I don't know if it could be excess current or some (self)induction of the cables during the avalanche which broke Q3. I suspect 555 got burned out because I forgot to put C3 back.

Anyway, I'll be getting replacements from Mouser, including UF4007 and STX13005. The last two seem to be part of what looks like almost identical circuit at DIYGeigerCounter, which appears to be rated at 1 kV. The only difference that I can see at HV section is that DIYGeigerCounter uses two caps instead of C1 connected in parallel with 1M resistor, but I doubt this has any effect on max voltage.

Also, I wonder, why would the voltage rise to 800V only when starting at lower point like 600V, but not recovering after avalanche?
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mightyohm
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Re: Resistor Voltage Ratings

Post by mightyohm »

Good question - I recommend probing with an oscilloscope (be careful of the high voltage, you may need a 100:1 probe!) or simulating in LTspice.
The STX looks like a good replacement for the FJN3303, but voltage limitations and other performance will be similar. Even better would be a 1kv rated device but I haven't seen one in a TO-92 package - you'll probably need to switch to a TO-220!
lifepower
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Re: Resistor Voltage Ratings

Post by lifepower »

Hi Jeff, thank you for your answer. Speaking of TO-220, do you think BUL-216 would be a good replacement for FJN3303? I can get this on Mouser, but it's pretty expensive, so wanted to be sure. I've never have paid >$2 for a single transistor before.

By the way, previously mentioned components arrived and indeed I could get around 940V from the kit. Last thing that helped me was to replace C1 with MLCC, which is similarly rated but somehow is more stable - I've got mine extracted from other stuff, so had to adapt the leads - the job looks ugly, but I wanted it quick. Also, shorting TP2 with "-" side with 4.7M resistor produces clicks and the voltage is recovered after that (issue where it would not rise again seems to be solved). Still would like to experiment with BUL-216 though. I've attached a photo.
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Geiger Kit running at 943V
Geiger Kit running at 943V
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