A/C in the output of a Linear DC power supply - Page 1
Forum section: electronics / beginners

author: de.leon
author_rank: Contributor
timestamp: December 11, 2021, 01:39:19 pm
content: Howdy: New to the forum and the hobby. Just bought a used Xantrex xt 20-3 power supply. Noticed an AC voltage between the neutral and earth mains taps. Getting 3v a/c and .1 ma regardless of the settings. Could anyone tell me if this is a normalish situation or does it indicate a flaw, some cap leakage perchance? Xantrex said they spun off this division years back and would not answer me. goes thru the current and voltage tests as per manual ok with a voltage reading that is low by .1v dcThanks for any advice, just want to minimize the chance of blowing up my new scope.

author: Fraser
author_rank: Super Contributor
timestamp: December 11, 2021, 01:50:50 pm
content: I think you will need to provide some more detail here as what you have posted is not clear. You state “Neutral and Earth Taps”. What “taps” are those ?Some comments in case it helps.On the utility mains input side of the power supply you will have Live, Neutral and Earth. The Earth connection is part of a safety system and is not normally connected to the Neutral incoming line within the appliance (at least not in the UK anyway) The two may therefore be at slightly different potentials. This is normal.The output side of the power supply often provides Positive DC, 0V return and an Earth connection. The Earth connection is wired to the utility mains Earth so is part of that safety Earth that may then be connected to the chassis of an equipment under test.Utility mains distribution systems can differ in their topology around the World, but it would not surprise me to see an small ac voltage between Utility Neutral and Safety Earth as such can easily result from inductive and capacitive coupling within the utility cable conductor group.Fraser

author: de.leon
author_rank: Contributor
timestamp: December 11, 2021, 02:24:44 pm
content: Thanks Fraser: Sorry for misusing the "taps" term. A little knowledge is a dangerous thing. I was referring to the output posts. You addressed the situation. I was just concerned that aging ycaps could be leaking or beginning to short. Seems not the case. Its just tough to learn on questionable equipment.

author: Fraser
author_rank: Super Contributor
timestamp: December 11, 2021, 03:05:49 pm
content: No worriesThe output 0V may be tied to the Earth connection by the user ,if desired, but there is a risk of introducing noise onto the 0V rail so I do not do it.As stated, the chassis of the power supply (Earth terminal) can have a small ac voltage on it and you can ignore that as in most cases you will use the 0V and DC output terminals and can ignore the Earth terminalYou are right to want to test a piece of used equipment, especially equipment that is connected to the Utility mains or that will power valuable equipment. You are a wise and sensible personFraser

author: de.leon
author_rank: Contributor
timestamp: December 11, 2021, 03:14:14 pm
content: You are mocking me aren't you...I just seem to be forming an unnatural and protective relationship with my new scope.Thanks for the help.

author: Ice-Tea
author_rank: Super Contributor
timestamp: December 11, 2021, 05:55:06 pm
content: Quote from: de.leon on December 11, 2021, 03:14:14 pmI rather doubt it. People put too much confidence in their testgear. Sooner or later it comes to bite you in the ass. Doubting everything is good practice even though it tends to eat away at your sanity a bit

author: Fraser
author_rank: Super Contributor
timestamp: December 11, 2021, 06:17:11 pm
content: No mocking intended I can assure you !I have received test equipment from eBay through the post that has been damaged before or during transit and was unsafe until repaired. Linear power supplies are especially vulnerable due to the high mass of the transformer and if this moves in transit it can do all manner of harm to the rest of the power supply. I had a large torrid transformer demolish the PCB in a linear power supply that was dropped.I also had a Thandar linear lab power supply destroy a portable digital oscilloscope that it was powering. I foolishly adjusted the voltage pot a little to test the minimum voltage threshold for the battery input and the PSU output voltage jumped from 5V to 15V and fried the scopes DC-DC converterIt turned out that the voltage setting potentiometer had an open circuit dead spot on its track. Test kit can fail, just as with any electronics.Fraser