High voltage psu de-potted, need advice - Page 1
Forum section: electronics / projects

author: ChristofferB
author_rank: Frequent Contributor
timestamp: February 29, 2020, 08:24:00 pm
content: 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 KHere'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

author: jonpaul
author_rank: Super Contributor
timestamp: March 01, 2020, 11:28:02 am
content: 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 funJust the ramblings of an old retired EEJon

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 01, 2020, 12:18:42 pm
content: 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.

author: jonpaul
author_rank: Super Contributor
timestamp: March 01, 2020, 03:09:34 pm
content: All fine, thanks for the clarificationThe 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 RegardsJean-Paul

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 01, 2020, 03:42:09 pm
content: 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!

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 01, 2020, 05:38:11 pm
content: Sorry, thought I added the picture earlier. I've moved the highlighted parts to a separate box, and tested it. It works great! Pulses start appearing at 800V bias and from 900V to 1100V I get a constant ish background count rate!Thanks again for the advice.

author: jonpaul
author_rank: Super Contributor
timestamp: March 01, 2020, 10:33:30 pm
content: Christopher very glad that I have been able to assist youEnd use of PMT for research, medical, astro or nuclear research? Pulse energy analysis?CiaoJ

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 01, 2020, 11:32:47 pm
content: Most of the above, actually!I'm planning to use this specific probe for gamma spectroscopy and related experiments, but I'm also setting up for a small water Cherenkov detector to monitor cosmic radiation.All of it as hobby, though.--Chris