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RE: update

"Reduce the sweep speed so you can see low frequencies, and the 120Hz modulation envelope will be clear."

I hadn't realised that these switchers were operating on unfiltered rectified AC. That could perhaps explain why the scope trace that vinnie got was so shaky.

But more significantly, it could also mean that the true rms output voltage may be substantially less than might at first appear. If that 35V peak-to-peak is only what is achieved at the peak of the mains voltage cycle, and it is then dropping down to essentially zero during the part of the mains cycle where the switching transistors are receiving only a small voltage from the rectified mains, then the true rms output voltage from the transformer, with the averaging now being done over the entire cycle of the mains frequency, will be much less than simply the rms calculated under the previous assumption of a continuous somewhat distorted 35V peak-to-peak square wave.

Maybe this could indeed bring the true rms voltage down close to the advertised 12V. I can imagine the relation between the instantaneous rectified mains voltage and the output square wave voltage on the transformer could be a rather non-linear one, and so estimating how to do the further rms averaging over the 120Hz cycle might be tricky.

It would be very interesting indeed to see a scope trace of the modulated output voltage with the timebase synced to the mains frequency. This could allow a fairly useful estimate of the true rms output voltage to be made.

Chris


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