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A true rms digital multimeter obtains its reading by

  • A.measuring the peak value and dividing by the square root of two
  • B.counting the zero crossings and inferring the shape of the wave
  • C.squaring, averaging and taking the square root
  • D.rectifying the waveform and multiplying by a fixed factor

Answer: C

Either an analogue computing circuit or a digital calculation from samples performs those three operations, so the reading is correct whatever the waveform. The specification will state a crest factor limit beyond which even a true rms instrument cannot cope.

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