Confused - AC from power supply - Page 1
Forum section: electronics / beginners

author: ianxtreem
author_rank: Contributor
timestamp: April 11, 2016, 05:24:31 pm
content: I finally bought a power supply! Years and years I have waited. So I hooked the output of my power supply to my oscilloscope and I see a beautiful 60 Hz sine wave coming out. 2V PP. Undoubtedly this has something to do with picking up wall power but 2V ? Really? Any idea's on how I should approach figuring out why?Thanks, Ian

author: Rager
author_rank: Contributor
timestamp: April 11, 2016, 05:28:33 pm
content: That looks weird indeed. How are the other channels doing (Osci and Power supply)?Is the scopes GND on the - and the Probe on + ?

author: Blaffetuur
author_rank: Regular Contributor
timestamp: April 11, 2016, 05:29:41 pm
content: Is the scope's ground hooked up properly ?

author: ianxtreem
author_rank: Contributor
timestamp: April 11, 2016, 06:15:19 pm
content: Thanks for the replies guys. I did some more poking around and nothing and couldn't figure anything out, then your comment about ground caused me to go get my outlet tester and plug it in..... and it indicated no ground! Moved it to another outlet and it works just fine.

author: Rager
author_rank: Contributor
timestamp: April 11, 2016, 06:20:26 pm
content: Just to make sure, could you explain or better draw/picture how you connected to scope to the PSU. Maybe it is something simple you just overlooked ^^

author: Andy Watson
author_rank: Super Contributor
timestamp: April 11, 2016, 07:23:20 pm
content: Quote from: ianxtreem on April 11, 2016, 06:15:19 pmYou should not be relying on the the mains ground to provide signal integrity.

author: RobertPS
author_rank: Contributor
timestamp: April 11, 2016, 08:02:19 pm
content: To what is the wire on the green connector connected?

author: ianxtreem
author_rank: Contributor
timestamp: April 11, 2016, 08:54:56 pm
content: Quote from: Andy Watson on April 11, 2016, 07:23:20 pmHmmm, I am not sure I understand this completely. The ground lead on the probe was connected to common on the power supply. Are you saying that I need another ground?

author: ianxtreem
author_rank: Contributor
timestamp: April 11, 2016, 08:56:27 pm
content: Quote from: RobertPS on April 11, 2016, 08:02:19 pmIt is connected to the anti-static mat.

author: ianxtreem
author_rank: Contributor
timestamp: April 11, 2016, 08:59:56 pm
content: Quote from: Rager on April 11, 2016, 06:20:26 pmIt is really quite simple. I don't have banna plug leads yet so I took power and ground over to a breadboard via the red and black 22gauge wires. The oscilloscope probe and ground clip attached to posts stuck in the breadboard.

author: Andy Watson
author_rank: Super Contributor
timestamp: April 11, 2016, 09:07:59 pm
content: Quote from: ianxtreem on April 11, 2016, 08:54:56 pmI believe (could be wrong) that the green terminal on the power supply is a mains ground. The actual power supply outputs are AFIK floating/isolated from the mains ground. So, yes, you do need to add a ground to the power supply output.I had originally assumed that you were using the mains ground(s) as your common/system ground.Edit: May be talking at cross purposes here. Can you clarify what you are calling ground and common, and where these are connected on the power supply?

author: wraper
author_rank: Unknown
timestamp: April 11, 2016, 09:13:05 pm
content: You should connect the earth clip of the oscilloscope to the black terminal of particular PSU output

author: ianxtreem
author_rank: Contributor
timestamp: April 11, 2016, 10:31:25 pm
content: QuoteYes, i think I use the wrong terms at times, I will be more careful. I do some boat power distribution work at times and all the terms are overloaded. so I looked at the manual, and the green terminal is connected to the power supply chassis which is connected to mains ground.What I was calling "common" is the black negative terminal, so I think I had it right. Thanks much.

author: Andy Watson
author_rank: Super Contributor
timestamp: April 11, 2016, 11:02:05 pm
content: Quote from: ianxtreem on April 11, 2016, 10:31:25 pmAh, no. My mistake. I had misinterpreted what you were doing with thegroundgreen terminal on the power supply.