Is this kind of voltage supposed to be there? - Page 1
Forum section: electronics / general

author: objekt406
author_rank: Newbie
timestamp: October 13, 2021, 07:46:00 pm
content: Hi,a few days ago i got shocked by touching the pins on the mains cable going to my lab psu, while moving it. Now im curious if this is a major fail or if its normal to have this high a voltage on the input to the psu. I tested with a PC psu and it didnt even measure above 2V.

author: objekt406
author_rank: Newbie
timestamp: October 13, 2021, 07:50:29 pm
content: I have to note, that the main power switch was off while measuring

author: capt bullshot
author_rank: Super Contributor
timestamp: October 13, 2021, 07:51:51 pm
content: Your description is unclear.My crystal ball says: This could be the residual charge of the EMC (X class) capacitor across the PSU input.Yes, it's normal for older units, modern stuff must discharge this capacitor within a few seconds (by applicable safety regulations), most probably because people had experiences like you.

author: objekt406
author_rank: Newbie
timestamp: October 13, 2021, 07:53:35 pm
content: How can i describe it better for you, so its clearer?

author: TimFox
author_rank: Super Contributor
timestamp: October 13, 2021, 07:59:12 pm
content: For example, I read your post and his answer to indicate the following description:1. You shut off the PSU at its power switch.2. You unplugged the PSU's power cable from the outlet.3. You touched a contact on the power plug and felt a shock.4. You did the same with another PSU and measured a very small voltage at the plug.Before I retired, I found that people (both co-workers and customers) were afraid to over-specify a situation when reporting a problem.

author: capt bullshot
author_rank: Super Contributor
timestamp: October 13, 2021, 07:59:45 pm
content: I want to move my lab supply: Turn it off, pull the mains plug, accidentally touch the pins of the mains plug, get shocked.Repeat with voltmeter measuring the DC voltage between the pins instead of touching them, voltage decays while measuring.This is what I understood, and would be the residual charge effect.

author: objekt406
author_rank: Newbie
timestamp: October 13, 2021, 08:02:48 pm
content: Yea this is maybe how i should have described itI was just thinking that it wouldnt be possible to get shocked because there is some kind of bleeding resistor to get rid of the high voltage

author: capt bullshot
author_rank: Super Contributor
timestamp: October 13, 2021, 08:05:39 pm
content: Yes, modern mains powered devices must have this bleeding resistor (required by safety standards). But there's equipment out there that doesn't.

author: objekt406
author_rank: Newbie
timestamp: October 13, 2021, 08:09:23 pm
content: Hmm, then i just have to accept that my psu for some reason doesnt have this kind of protection i guess. Its not a big problem honestly, its not like im touching it every day in the morning to wake up or stuff like thatAppreciate your answers, was thinking in the same direction of some kind of cap that held some charge, but just wasnt sure if it could be true

author: shakalnokturn
author_rank: Super Contributor
timestamp: October 14, 2021, 09:23:34 am
content: It's good when bleeder resistors are there (although adding a little power wastage) always assume they've failed open though...I'd be curious to know how many deaths have been caused by assuming bleeder resistors were fail-proof in microwave oven capacitors.