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RE: ok, what is happening here????

One needs to be a bit careful because of the shape of the waveform. If it were a precise square wave, then 12V rms would require 24V peak-to-peak. If your rather shaky looking scope trace is accurate, then the sides that should be vertical in a true square wave actually have quite a slow rise and fall time. This means the rms voltage is somewhat less than simply half the peak-to-peak voltage.

That is why I was saying that your 35V peak-to-peak is maybe corresponding to something of order 15V rms, rather than the 17.5V rms that it would have been for a pure square wave.

(It would have been about 12.3V rms if it were a pure sinewave with 35V peak-to-peak. But the waveform you showed in that photo of the scope trace looks as if it would have a bigger area under the square of the curve than for a sinewave, but certainly less than for a pure square wave.)

It might help if you could get a steadier scope trace by fiddling with the sync level, as TK suggested. One could then more easily estimate the rms voltage.

If I understand correctly, your "square wave" is from a switching power supply operating at about 140KHz or so; is that right? (I'm guessing your statement of the horizontal axis being 1ys> per division was meant to be saying 1 microsecond per division?)

A typical cheap "true rms" meter might very well give a more or less meaningless reading at such a frequency. (I saw some discussion of this issue higher up the thread.)

Chris


Edits: 12/03/16 12/03/16 12/03/16
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