Hi & a question about CPM

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2 years 11 months ago - 2 years 11 months ago #4608 by Simomax
Replied by Simomax on topic Hi & a question about CPM
If you have already fitted the jumper and need to take it off then make sure you restart the counter after doing this, or any time you do change the jumper. It says on their website this: "To switch 400V/500V settings of PWM set Jumper-1 and restart the power of the kit. With jumper installed it has 500V output, with jumper removed it has 400V output."

Oh, also make sure the polarity for your tube is correct. It looks like + and - are marked on the board and you should see a + at one end of the SBM-20.
Last edit: 2 years 11 months ago by Simomax.
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2 years 11 months ago #4609 by Simomax
Replied by Simomax on topic Hi & a question about CPM
I have just had a look at the PDF manual for the counter and this jumped out at me: "HV output is set with jumper to select 450V or 500V output."
The SBM-20 tube is nominal 400v so pushing 450v into it may cause the tube to avalanche. I also think 10M ohm anode resistor is a bit much for the SBM-20 but a higher resistance will only lower the counts. The recommended anode resistor for the SBM-20 is 5.1M ohm. Full specs of the SBM-20 here: www.gstube.com/data/2398

I'm also surprised there is no circuit diagram with the kit. IMHO there should be!
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2 years 11 months ago #4617 by docsquee
Replied by docsquee on topic Hi & a question about CPM
Thanks, Alpha01. I've check the voltage and it's sat around 400V (380V +). I'll compare using shorter wiring to the tube from the PCB and see if that helps steady counts in the future.
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2 years 11 months ago #4618 by docsquee
Replied by docsquee on topic Hi & a question about CPM

What everyone else said. There is a small chance it could be interference from something electrical. Cheap nasty Chinese switch mode power supplies can often emit noise. Make sure you have a decent PSU running your Pi and counter. As an example, running a plasma ball near a Geiger counter will make it go crazy!

As mw0uzo mentions, leave it running for a few days to see if there is a pattern of it counting higher at nights. Also, do you any kind of check source such as Uranium glass, fiestaware pottery (Uranium glaze) or something else you could use? If so then place it right at the tube to get the highest count then slowly move it away from the tube. If you see the counts going down steadily as it gets further away the tube voltage is probably set OK. If the counts stay high then drop in sort of chunks then the voltage may be too high causing the tube to avalanche. Don't rely on the CPM display though as this takes ~30 seconds to settle. Instead listen to the clicks it makes.

I Have just had a look at the kit. Make sure the jumper is removed for correct 400v. With the jumper fitted it will be set to 500v which is too much for the SBM-20 tube. Also if this was a kit you have built then check that the correct 10M ohm resistor is correct and not mixed up with another value resistor.

If all seems well then jet it run a few days and we can have a look at the data. Times/counts etc. There is a possibility this was an event, though at 70CPM it's nothing to worry about if it has come and gone. I have had a couple of times when it has rained hard and my counts have gone up due to the rain trapping radioactive particles in the air and bringing them down to ground level.

Don't worry about it though. There are a lot of clever people on this site so we'll get you up and running properly :)


Well there goes my 80s retro night! :)

Very interesting phenomena after rain, a lot of what people have mentioned is new to me, much reading to be done.

I've double checked the jumper and it's set to 400V.

Thanks for your reply.

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2 years 11 months ago #4619 by docsquee
Replied by docsquee on topic Hi & a question about CPM


I'm also surprised there is no circuit diagram with the kit. IMHO there should be!


RH sent a printed copy of the circuit diagram!

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2 years 11 months ago - 2 years 11 months ago #4620 by Alpha01
Replied by Alpha01 on topic Hi & a question about CPM

Thanks, Alpha01. I've check the voltage and it's sat around 400V (380V +).

Should be OK if you got the measurement with a high Z probe.
Little correction to my previous comment, the theoretical anode resistor is 5.1M instead of 5.6M for the SBM20, but that will not produce a big change...

I'll compare using shorter wiring to the tube from the PCB and see if that helps steady counts in the future.

I forgot also to said that too long connection wires could act as antennas and render the circuit sensitive to strong or near EM fields (too close mobile phone, WiFi router, etc.)
Reducing the connection length to the tube is anyway a good practice (with good isolation quality cable on the anode side)...
Last edit: 2 years 11 months ago by Alpha01. Reason: Adding some more explanations and typo corrections.
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