Bad PSU? Floating ground? Or something worse? - Page 1
Forum section: electronics / projects

author: cprobertson1
author_rank: Frequent Contributor
timestamp: April 06, 2017, 10:01:05 am
content: Technically this isalreadyrepaired - but there's something worrying me and I figured I'd better consult the expertsI have a DVD/TV combo here - PSU had dead rectifier diodes, and stuff just wasn't working properly - replaced the diodes and everything seemed fine - as a double check that I'd done everything correctly, I hooked up the multimeter ground lead to the chassis (I had confirmed ground continuity earlier) and probed around the main board making sure the voltage rails were where they are supposed to me.They were spot-on: good; so I went to disconnect the ground probe... and got a tingle..."Huh - that's strange!" I thought to myself, as I touched the +ve probe with my finger (um, might not have been the best idea in retrospect- and sure enough, ~50V between the chassis and me.Further investigation led me to conclude that the chassis was at ~50V relative to mains earth"Aw Jeez - I've got a floating ground in my house" I thought - so I unplugged the unit and measured between earth and neutral... and... huh, 0V.I tentatively hooked up the TV again - and probed between earth and the chassis - 75V this time...Turned it off, and watched the voltage decay back to 0 ish.So I turned it back on, and tested between Neutral and Earth: there it was again - this time between neutral and earth on the mains ring.WELL - at a loss of how to fix the problem I closed up the TV again and contacted the client to explain that it was tentatively fixed - but that there may be a safety issue - I'll get back to him once it's resolved (have a no-fix/no-fee policy, so if it's unsafe I'll declare it unfixed and he won't get charged)I'm pretty sure the house has TT earthing - which makes me wonder if the earth is no longer connected properly (I don't have an Earth tester) - on the other hand, it makes me wonder if the neutral is just floating because I'm too far from the transformer (which doesn't seem very safe the more I think about it...)NOTE: I never checked neutral-->Earth resistance at the socket - I'll probably do that when I get home!Any ideas?I've already confirmed the following (but will reconfirm tonight just to be certain!):-Low resistance/continuity between chassis and mains neutral-High resistance between chassis and mains earth (no mains earth reference)-Chassis 50-80V above mains earth (DC and AC measurements: AC was usually a higher reading)-No voltage difference when unit turned off-TV still works (and is double insulated so it's probably fine)

author: capt bullshot
author_rank: Super Contributor
timestamp: April 06, 2017, 10:16:02 am
content: Thats pretty much normal for any switch mode power supply with floating (non-earthed) output.Reason is the EMC capacitors installed from primary to secondary side of the PSU. They're safety rated for this purpose, so there's no reason to worry.

author: langwadt
author_rank: Super Contributor
timestamp: April 06, 2017, 10:17:25 am
content: it is common that if the chassis isn't connected to real ground it'll be floating at 1/2 mains via the capacitive divider of the y capacitors in the input filter

author: Nusa
author_rank: Super Contributor
timestamp: April 06, 2017, 12:05:35 pm
content: Remember when you're measuring earth to mains ground, that you're also collectively measuring everything plugged into that circuit.

author: cprobertson1
author_rank: Frequent Contributor
timestamp: April 07, 2017, 08:07:51 am
content: Awesome! Thanks for the replies folks - that makes perfect sense now: I was forgetting (or more accurately, ignoring) the filter caps on the non-isolated side as I didn't suspect anything was wrong with them (and then kept ignoring them because I'd already discounted them!)On the other hand, I don't generally compare the isolated side to the non-isolated side - or the earth to the chassis for that matter - which was why I was throwing a blank on what could be causing it! In other words, I knew the caps were there but never occurred to me that it was the causative factor behind it!So - put simply, I was measuring the leakage from the PSU the whole time? Ha! I may have facepalmed a little when I read your posts and realised what I've been looking at this whole time xDIn case anybody is wondering the leakage current is 136uAUm, I think the way I measured it means that would technically be the Touch Current (TC) - I didn't have a clamp meter handy to measure the... oh I've forgotten the name for it? The current going through the earth wire: Earth Current? Ground Current? I'm sure it was a 3-letter acronym... Earth Conductor Current? Ground Conductor Current? I'll need to refresh myself later on the terminology!Anyway, that's the TV sent back - repair involved replacing 5x diodes and made me a nice 3333% profit: not bad for three hours work (plus faffing around trying to figure out why the chassis appeared to have a voltage on it which I didn't charge for)Thanks again folks!

author: Zero999
author_rank: Super Contributor
timestamp: April 07, 2017, 07:22:01 pm
content: Does the TV have an earth lead?To illustrate what others have said: it's sounds like it's the leakage across the Y capacitor.