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High voltage psu de-potted, need advice
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ChristofferB:
Hey!

A few days ago I posted a teardown of a medical x-ray detector head from an osteoporisis scanner, I got this mainly to re-use the complete scintillation detector unit:

https://www.eevblog.com/forum/testgear/x-ray-densitometer-detector-teardown/

It came with a HV psu and amplification, which I needed to figure out in order to power up the detector. I want a standalone detector that I can use my own amplifiers and variable HV soureces with, you see.

The HV PSU is potted in white silicone, which took an awful amount of time to get rid of (see photo, entire non-soldermasked area was blocked in!).

I've traced out the HV part, a pretty standard single voltage doubler thing, but it has some oddities:


-A high resistance divider leading to a lot of op amp circuitry, presumably a sense output for stabilizing the voltage.

-A very small current limiting resistor: the lid of the PSU sais "1000V  1mA max" yet the series resistor for the PMT is only 100 K


Here's the main question: Is it safe to just use the series 100 Kohm resistor to bias the PMT? Or could this circuit regulate current in some other way?

Thanks for the help!

-Chris
jonpaul:
Hello there I have been doing HV since 1960 s.

A PMT requires a very well regulated voltage between 600-2000V, the tube amplification factors are exponentially related to voltage.   The Load is the dynode voltage divider plus any output current of the last dynode.

The power supply silicone potting is essential to the insulation of the transformer and doubler caps.you will need to repot or run at much lower voltage.   If run unpotted, it will eventually fail, the transformer secondary is most at risk.

I humbly Suggest that you research and read up on PMT and HV supply technology before proceeding.

Good luck and have fun

Just the ramblings of an old retired EE

Jon

ChristofferB:
Oh I'm not going to use the unpotted PSU. I have a rack-mounted variable PMT bias supply I'm going to use instead.

Since the scintillation detector only has one connector I just needed to determine what current limiting resistor was used.

I put R7 and everything after that in a separate box with the LEMO cable out to the PMT, SHV connector for HV IN, and BNC for signal out, to have a separate bias box.

I don't think that part of the circuit needs to be potted, since the capacitors are rated at 6kV each. 
jonpaul:
All fine, thanks for the clarification

The 100K series resistance is to protect against arcs and damage to the power supply and is quite common.

As the PMT current is in the 1-50 uA range it has no effect on the PMT operation.

Suggest a carbon composition 2 W resistor.

Best Regards


Jean-Paul
ChristofferB:
Ah that is good to know. I thought the resistor had to be matched to anodu current, but I see now that 100K is pretty common.

Thanks for the insight!
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