Normal voltage spikes from computer PSU ?

[lordvader88]

I'm just checking a 750W PSU that might or might not have problems. It powers up fine, and starts a little dual core PC. Looking at the outputs tho IDK if it's normal or not.

It's switching at ~140kHz
So the 12V rail, I get the inductive ringing of 2.0Vpp max on the 1st ring, 1.25Vpp by the 2nd ring, and by the 6/7 ring it's decayed to next to nothing. Those rings in total span ~600ns, the period of the switching is 7us.

The 5V rail looks the same shape/time wise, with the 1st ring max of 1.84Vpp, 1.33Vpp on the 2nd, and drops off by 6-7 rings.

The ripple looks fine. I'm just wondering if those inductive spikes sound about normal at 2Vpp ?

I could pull the output caps and check ESR. I don't have any proper way to load test these PSUs, this test rig is under well under 300W.

[David Hess]

How did you make the measurement?

It is very easy for common mode switching noise from the power supply to corrupt a single ended measurement where a common ground is used.  The ATX specifications require noise measurements to be done differentially.

You may be able to verify a problem with common mode noise by connecting the oscilloscope probe tip and ground lead to the spot where the ground lead is connected for the measurement.

I would expect 10s to low 100s of millivolts of peak-to-peak noise maximum.

[lordvader88]

I ordered diff. probes, they're in the mail.

I'm going to probe my main PSU, see what it looks like.

[T3sl4co1l]

That's not going to affect anything attached to it, and as alluded to, it's more likely common mode.

I strongly suspect the poor bugger can't pass emissions (up the power cord) though.

Tim