explaining about RMS

KlausST said:

Hi,

True RMS means there is a "true" RMS measurement and calculation...with squaring the signal, averaging and taking the square root.

In opposite there are some (how I call them) "fake RMS" calculations.
Often used:
* peak value measurement ... multiplied with 0.707 (for pure sinewave)
* rectified average measurement .... multiplied with 1.11 (for pure sinewave)
* some odd measurement .... multiplied with just a factor to get the expected output value.

You see the "measurements" and calculations are not true RMS. The measurement has nothing to do with RMS, none of them use "square" and "square root". Thus the output is fake.
Only if you have a known, never changing, reliable waveform .... the the results "seem" to be RMS values.

My personal opinion: It's like measuring the size of your feet to find out how tall you are. For many people you may find a good relationship. But for some people the result simply is wrong.

Klaus

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Akanimo said:

It is not dependent on frequency.

Note that 0.707 is only for sine wave. Other waveforms yield different factors.

Actually the factor is the ratio of integrated voltage waveform in one cycle to the total area in one cycle. This is to say the that the factor is the ratio of the area taken up by the voltage waveform over the sum of the area taken up and that not taken up.

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ADD:

Note that the total area is (Vpk * one_period).

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Yes Klaus, thanks.

I just noticed I had made that blunder just before your notification came in.

I had been describing the average voltage all along. RMS is obtained from squaring each sample in a period, summing up all the squared results, dividing by the number of samples and then finding the squareroot of the quotient.

The higher the number of samples, the more accurate the result.

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