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An average responding digital multimeter is calibrated to read the root mean square value of

  • A.the fundamental component alone
  • B.a sinusoid
  • C.any waveform whatever
  • D.a square wave

Answer: B

The rectified mean of a sinusoid is 0.9 of its rms value, and the scale factor built in assumes exactly that. On a distorted waveform the assumption fails, and the error can be tens of per cent on a peaky current such as that drawn by a switched-mode supply.

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