Quote:
Originally Posted by mpegjohn
I don't NEED to do it, I am just interested how one would go about actually measuring the RMS value.
Yep, I could calculate it from a scope trace.
|
I have two True RMS voltmeters in the lab: an HP 34401A and a Datron 1061A. When switched to their AC ranges but connected to a DC source, they measure zero. Their inputs are AC coupled so they'll only measure the AC component of the input. I think this is how yours is behaving, too.
With your full-wave-rectified-unsmoothed waveform, this will lead to a slightly odd result, because the AC coupling will shift the waveform down to its mean point, and the voltmeter's RMS converter will then full-wave rectify that, leading to a very odd-looking waveform. See attached sketch.
It is this odd-looking waveform that the meter will then calculate the RMS value of. It's fairly spiky so different 'true RMS' meters may manage this with varying degrees of accuracy. However, the sketch shows that what starts as a waveform that's recognisably half of a 12V sine wave, finishes up as something about half as tall once it's been AC coupled and full-wave rectified. Hence the meter's reading of about 6V seems about right. I don't have an unsmoothed DC supply here to test my meters on, otherwise I'd try and reproduce the result.
It would be possible to do the same measurement from first principles by coupling the unsmoothed DC via a (large) capacitor into a load resistor, then using the resistor to heat an object of known heat capacity and measuring the temperature rise.
Chris