Does a full-wave rectification AC Voltmeter give the same indication as a true rms voltmeter when you measure a 50 Hz sine wave?what happens if the the sine wave is clipped by an inadequate transformer?
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2 Answers
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If it is a pure sine wave, yes. Harmonic distortion figures of 10% on power grids are not uncommon though, so you need to take into account that the reading may be off when you intend to perform a very accurate measurement without a true-RMS meter.
It is definitely going to be off when it's clipped. An analogue meter measures the average value of the rectified AC voltage , which is 63.7% of the peak value in case of a sine wave and calibrates this value to the RMS value (70.7%). So your value will be overrated. With digital meters its more complex, nevertheless they will be off as well.
Maybe not relevant but a full-wave meter has a higher sensitivity (kΩ/V specification) then a half-wave instrument. For low-power circuits that could be a disadvantage.
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Precision rectified AC passed through a low pass filter (averaging method) and then magnified by the appropriate factor is intended to produce exactly the same number/value/voltage as the equivalent RMS for a 50Hz sinewave.
If the sinewave is somehow distorted, the pseudo measurement will likely not be the RMS. If you applied a significantly lower frequency (but still a perfect sinewave), the averaging method/filter would begin to be compromised and again, true RMS would likely not be displayed.
