Scope / PSU grounding issue?

Hi,

I have been trying sort out some ringing in a PWM circuit, but I am having trouble believing what I see on my scope.

Basially, the load is a 5R wire wound resistor which has one side connected to the PSU, the other side is is pulsed to ground via a MOSFET. The control circuit is powered via a 12V regulator.

I've noticed that as I turn up the voltage input, the ringing disappears. (or so I thought). But if you look at the attached scope shots, the voltage of the output does not match what I am setting on the PSU. I think the scope is telling me fibs!

I'm measuring with the scope between the common ground, and the load. Anyone know what might be happening?

hi,
The wire wound resistor will have some inductance, unless its a non inductive type.

I know. I am not asking about the source of the ringing. I want to know why my scope might be displaying what it is. It does not seem to be right.

Is it a digital scope?

If so, you could be exceeding the voltage range of the sampling A/D

Yes digital (OWON PDS6062T). Hmm, I think u may be on to something there. If I connect the probe to my PSU, it stops measuring after about 15V. 15V seems ridiculously low for a measurement limit.

What can I do to get around this?

I would expect the oscilloscope to operate over at least the full range of the display (±20V on the 5V scale). If not then you may need to use a 10:1 probe to increase the maximum displayed input voltage by 10 times (50V/division on the 5V scale for example).

Yes, I am surprised it does not cover the full display.

I changed it to 10:1 and I can see now that turning up the voltage does not magically make the ringing disappear as it appeared to before! A fast diode on the output does the trick though.

Thanks!

Sounds like the cut some corners in the design of the oscilloscope.