Scoped power supply confusion - Page 1
Forum section: electronics / beginners

author: AldenFernsod
author_rank: Newbie
timestamp: December 30, 2023, 10:14:43 pm
content: Hi Folks,Wondering if someone could help me understand what I'm seeing when connecting my scope to a cheap 24V wall-wart power supply.The power supply does not have an earth terminal on the mains plug, and using a DMM, I've double checked that neither of the power supply outputs are connected to the live or neutral.I expect to that probing the positive terminal while connecting the ground lead and PS ground, should show a flat 24V signal, aside from whatever noise is present.Instead, what I see is the huge 150V peak-peak, 60 Hz signal (see simple.jpg), with a bunch of odd harmonics.Why is this happening?Other notes:I also did a differential measurement (one channel connected to PS ground and the other to +) and got the results shown in Differential.jpgThe scope is a Rigol DS1054, which has scope ground connected to mains earth, though I don't think that should matter in this case.Thanks in advance!Alden

author: coppercone2
author_rank: Super Contributor
timestamp: December 30, 2023, 10:21:33 pm
content: are you in ac coupling

author: AldenFernsod
author_rank: Newbie
timestamp: December 30, 2023, 10:28:24 pm
content: Nope, DC coupled.

author: Benta
author_rank: Super Contributor
timestamp: December 30, 2023, 10:34:20 pm
content: What exactly is "PS ground"?What does your scope show when you connect the two probe leads (probe ground lead and point) to the two output terminals?

author: coppercone2
author_rank: Super Contributor
timestamp: December 30, 2023, 10:42:30 pm
content: connect a resistor that gets warm to the power supply and connect the oscilloscope across the resistor

author: AldenFernsod
author_rank: Newbie
timestamp: December 30, 2023, 11:05:09 pm
content: Hi Folks,I think I found the issue. Embarrassing as these things often are...It seems that the jack through which I was measuring the power supply voltage wasn't reliably connecting the ground lines, so most of the time the result was the same as just connecting the tip of the scope probe to the power supply + and leaving the ground clip unconnected. When I force the ground to connect, I see the flat 24V signal I expect.I suppose then it's not so surprising the large, 60hz AC signal without the grounds connected, right?And thanks very much coppercone and Benta for the help.Alden

author: AldenFernsod
author_rank: Newbie
timestamp: December 30, 2023, 11:20:16 pm
content: Further update -- arggh!I cut the connector off the power supply output and am now probing the power supply output + and - leads directly, and I'm back to seeing the big 60 Hz signal.I see the exact same result if I disconnect the probe ground clip, leaving just the tip connected -- big 60 Hz signal.I thought perhaps there's an open circuit in the - wire of the PS output, but if I I connect a 200 Ohm power resistor across the power supply outputs it gets hot, so obviously there's current flowing. Coppercone -- the signal looks the same with the resistor connected.??

author: themadhippy
author_rank: Super Contributor
timestamp: December 30, 2023, 11:49:46 pm
content: Sure its not an AC wall wart??

author: Benta
author_rank: Super Contributor
timestamp: December 30, 2023, 11:54:29 pm
content: Your scope ground lead is dodgy.

author: coppercone2
author_rank: Super Contributor
timestamp: December 31, 2023, 01:07:48 am
content: connect it to the scope with wire that you wrap around the bnc connector and stick into middle post carefullyor alligator clip on the ground post and contact the middle by touching it wiht a wire.it sounds like your oscilloscope or porbe might be messed upyou can connect a aligator clip to the ground of your scope calibrator/compensator output and touch the center receptacle with a wire

author: coppercone2
author_rank: Super Contributor
timestamp: December 31, 2023, 01:16:01 am
content: actually leave the probe connected but don't use the probe ground use the scope calibrator ground lug with a alligator clip to make the ground connectionthen you can see if it work when you have a good chassis ground connection. if it still dont work then try the wire instead of the probe. if it still don't work then you have some bad problem

author: AldenFernsod
author_rank: Newbie
timestamp: December 31, 2023, 01:50:08 am
content: Coppercone, Benta -- well done. I think you both got it.If I jiggled/bent the ground lead wire I could get the scope to show the 24 V DC signal I expect. And, connecting an alligator clip to the scope's calibrator ground made the problem go away.Bizarrely, the intermittent was present onbothof the two channels I tried! Of course, this made everything much more confusing. The two remaining channels seem to have working ground leads.I removed the shrink tubing from the end of the ground lead, and saw this...Must have been metal fatigue leading to failure, and it makes sense because I used those two channel (#1 and #2) most frequently.Thanks again!