Power Supply Grounding Query - Page 1
Forum section: electronics / beginners

author: Greyhound
author_rank: Newbie
timestamp: September 02, 2023, 08:19:41 pm
content: Hi All,This is my first post, so apologies if I inadvertably don't follow any rules etc.My question is: I need a power supply for my first audio project which requires +12VDC, -12VDC and 0v. Because eventually I may need to connect external equipment (maybe speakers / line out etc) a 0v matching ground would be ideal.I looked around at different options, and bought a cheap power supply from ebay (for now I want something cheap that I won't be upset breaking). The model number given is: PS15 W1V12. Initially I assumed the ground would be on the DC side, and that a AC side would consist of just Live/Neutral (my plan was to manually wire an earth to the chassis for safety). However, the unit came today and I immediately spotted the 'ground' terminal already has a trace off to the chassis, so I am assuming the DC side now is likely ground referenced?As a way forward I wondered if a good option might be to voltage divide between the +12 and -12, to get a 0v back, and would this mean I still have an isolated DC side (one I can freely probe around with my oscilliscope?).I hope I have explained this ok, and I'd really appreciate any advice.Thanks,James

author: TimFox
author_rank: Super Contributor
timestamp: September 02, 2023, 08:33:38 pm
content: Is there a third (PE) terminal on the AC input?There may or may not be a connection between the third prong of the mains connector (PE) and the common of the DC output.Before connecting the power to the power supply, use a simple ohmmeter to check for continuity between the third prong, the chassis, and the DC common."Voltage division" of the +/-12 V is sometimes done (only if the two currents are closely balanced), but the DC rectifiers probably already have a common connection for a split output.

author: Greyhound
author_rank: Newbie
timestamp: September 02, 2023, 08:44:41 pm
content: Sorry I should have said in my original post!The terminals are:230V AC L (I'm in the UK with mains being 230ish V)230V AC NGround (no more information than this I'm afraid!)+12V DC-12V DCThanksJames

author: TimFox
author_rank: Super Contributor
timestamp: September 02, 2023, 09:04:44 pm
content: Are you sure this is a split supply, designed for +12 V, com, and -12 V?It could be a single 12 V supply, with + and - terminals on the output.If it is really a split supply, then I would have labelled the third DC output terminal "com", not "ground", unless it really connects to the chassis."L" and "N" are obviously the mains input.

author: abeyer
author_rank: Frequent Contributor
timestamp: September 02, 2023, 09:21:38 pm
content: Quote from: TimFox on September 02, 2023, 09:04:44 pmYeah, pretty sure this is the case -- looks like maybe this modelhttps://www.germanyledlight.com/index.php/ps15-w1v12-sanpu-power-supply-15w-12v-smps-small-led-driver.html

author: TimFox
author_rank: Super Contributor
timestamp: September 02, 2023, 09:54:09 pm
content: In that model, the L and N are the AC input, the ground symbol is the metal chassis, and a DC voltage of 12 V exists between the + and - terminals (neither of which is internally connected to ground).

author: donlisms
author_rank: Frequent Contributor
timestamp: September 02, 2023, 11:49:56 pm
content: In other words, you only have one of the two supplies you will need - one from 0 to +12, and another from 0 to -12. You will make the 0 by connecting the -12 terminal in the first supply to the +12 terminal on the second supply... between the two in series.

author: Greyhound
author_rank: Newbie
timestamp: September 03, 2023, 05:59:47 am
content: Thanks everyone for your help, it's really appreciated!So am I right in guessing the supply is probably ground referenced? So even if I don't use the ground on the DC side, I still shouldn't be probing around without a lot of thought (and the ground lead always attached to ground) ?

author: TimFox
author_rank: Super Contributor
timestamp: September 03, 2023, 02:01:27 pm
content: Assuming it looks like the one cited above, the "ground" screw is merely the metal chassis, and the single 12 V supply on two screws (+ and -) is not connected to the chassis nor the ground screw.

author: sparkydog
author_rank: Regular Contributor
timestamp: September 03, 2023, 03:19:26 pm
content: Okay, hold on y'all...I would strongly sosupect on a suspect a PSU like that, that "+12V" and "-12V' mean "there is 12V across these terminals; this one is + and this one is -".NOTthat one is +12V relative to the metal case and one is -12V relative to the metal case, and you are expected to use the case as COM. There are similar PSUs with only four inputs/outputs that also label the DC side "+V" and "-V"; thosedefinitelyonly put out the stated voltage between "+V" and "-V".Of course, this is easy to check with a DMM; probe the voltage across +12V and -12V and see if you get ~12V or ~24V.

author: TimFox
author_rank: Super Contributor
timestamp: September 03, 2023, 03:44:05 pm
content: Yes, this is the same as the familiar 9V alkaline battery, which has the two snaps labeled + and -, and puts out 9V between the snaps.

author: donlisms
author_rank: Frequent Contributor
timestamp: September 03, 2023, 11:53:08 pm
content: I remember being a little mixed up about these things when I started.There are "dual" power supplies for sale on the auction site; they have + - and ground terminals, and two big meters... so it's not hard to see how the seller might get confused when he makes the listing. It's a single supply, with a convenient ground terminal if you choose to anchor either of the voltage terminals, and V and A meters.