High voltage potentiometers - Page 1
Forum section: electronics / projects

author: b_force
author_rank: Super Contributor
timestamp: April 28, 2022, 01:19:30 pm
content: So I have a question regarding a potentiometer in an high voltage circuit (around 350Vdc).In this case the voltage across the potentiometer itself is only a couple of volts (and just a few mili amps), so there are no issue there.But the voltage from the potentiometer referenced to ground, as well as the chassis, is much higher. (around the 300-350Vdc)I have been searching and looking into datasheets for days.Even contacted multiple manufactures, but I can't get a satisfying answer at all. (most manufactures even don't seem to answer the question)Most datasheets only give some very vaguely defined value for the insulation resistance at around 500V (around 100 mega ohms @ 500V )In my opinion, as long as the chassis is grounded well and/or an insulated potentiometer (like plastic casing and wiper), there aren't any issues?oh, btw, this circuit is not connected to mains, so after secondary side, CAT I environment.

author: TimFox
author_rank: Super Contributor
timestamp: April 28, 2022, 01:36:32 pm
content: Older oscilloscopes that needed to put a pot at a high common-mode voltage used insulated shaft couplings.I remember units that used a phenolic tube, threaded at the pot end to accept the bushing, with an insulated shaft inside the tube.

author: CaptDon
author_rank: Super Contributor
timestamp: April 28, 2022, 01:39:33 pm
content: In most of the old oscilloscopes using 5CP1, 5UP1, 5DEP31 and so forth the intensity and focus controls were at a very high negative voltage referenced to chassis ground. The pots themselves only had maybe 100 volts terminal to terminal. They were often mounted on a bakelite or FR-4 substrate with plastic shafts extending through the front panel.

author: b_force
author_rank: Super Contributor
timestamp: April 28, 2022, 01:43:23 pm
content: Quote from: edavid on April 28, 2022, 01:32:11 pmThat's the voltage across the potentiometer itself, which isn't the same as the voltage from the potentiometer to chassis.(aka, creapage/clearance distance)Second to that, is that this value only often gives an AC value, never a DC value.

author: b_force
author_rank: Super Contributor
timestamp: April 28, 2022, 01:44:55 pm
content: Quote from: TimFox on April 28, 2022, 01:36:32 pmYeah, I am familiar with those old scopes and similar equipmentI am wondering what the correct way if for nowadays standards?

author: tggzzz
author_rank: Super Contributor
timestamp: April 28, 2022, 01:57:58 pm
content: Might it be worth investigating these focus and intensity pots? Unsurprisingly, a lot of the service manual is detailing how it should be sealed

author: Gyro
author_rank: Super Contributor
timestamp: April 28, 2022, 04:37:58 pm
content: 350V DC to chassis doesn't sound that dissimilar to knob operated 240V AC mains dimmer switches. These often have (had?) the Aluminiun heatsink plate for the triac sandwiched behind the plastic front plate, retained by the potentiometer bush. They use plastic bushed potentiometers with plastic shaft (and a plastic knob to be on the safe side).Your track dissipation sounds negligable, so an ordinary plastic cased, plastic shaft pot ought to be fine.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: April 28, 2022, 05:09:38 pm
content: Here's what the multi kV focus and intensity pots look like in my scope, they're on some kind of spacer shaft and shrouded in a nice plastic cover. It probably also helps prevent too much dust getting attracted there too. Although the scope is flooded with filtered air when powered...