Various aspects of home brew inverters

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Joined: 07/11/2007
Location: Australia
Posts: 1904
Posted: 11:47am 08 Jul 2018      

  Warpspeed said   Klaus,

With a multimeter you don't get the same visual clues of waveform distortion that there will be with an oscilloscope.

The signal generator may be tuned to 75Hz o/k, but there might be a visible peak at one of the signal generator harmonics which might lead you astray, and its fairly easy to get caught that way. That 1uF does sound rather small which is a bit of a concern.

One way to be more certain is to tune the signal generator over a much wider frequency range, especially much higher, to make sure you settle on the largest real peak there is. Then use the multimeter for finer final measurement.

Tony, I'm sure your CRO is way better than my 20+ year old dual trace BWD job. Watching for tiny amplitude changes of a none to stable trace (can't slow the timebase too much or I get intermittent trace bursts) is not easy at all.

I did wind up the frequency to over 200Hz and back down several times, the only amplitude increase was at 75Hz with the selected capacitor.
Any bigger caps gave that peak at much lower frequencies.

But I will test for your harmonics suggestion tomorrow. Might write it down as a list for each different cap sizes and test to over 200Hz for each.

BTW, I did not notice any "distortion" of the sine wave at the resonance peak, just an amplitude change. I do remember from trade school way back the very distinct peak of a parallel resonance but that was at a very much higher frequency and smaller coils.

You could easily try my multimeter trial at your place, I cannot try your precision oscilloscope method due lack of good enough equipment. Lets see what you find out.