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Vdc and vrms

  1. Oct 14, 2011 #1
    i have read that vrms is the same as equivalent dc voltage but i have also read that vav and vdc are same but vav and vrms is not same vrms of sinosoid is zero but vav of sinousoid is not zero where i am wrong?
  2. jcsd
  3. Oct 14, 2011 #2


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    You have most of that wrong.

    You might like to read the Wikipedia article about this:

    However, the average voltage of a sinewave (over a whole 360 degrees) is zero, but the power isn't zero because positive and negative voltages are equally effective at producing power, assuming a constant resistive load.
    The average over a half wave is 0.637 times the peak value.

    The RMS voltage of a sinewave is 1/√2 times the peak voltage level or 0.707 times the peak voltage.
    Last edited: Oct 14, 2011
  4. Oct 14, 2011 #3
    Vrms = sqrt[Vmeansquare(t)] =sqrt[∫V(t)2dt/∫dt] integrated over a complete cycle.

    Vav = ∫V(t)dt/∫dt.
  5. Oct 14, 2011 #4

    jim hardy

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    RMS mnemonic:
    it's backwards.

    instant by instant:
    Square the instantaneous value
    find Mean (average) of those squared values
    square Root of that mean is RMS

    and clearly that's a lot different from a simple average of the instantaneous values.
    for one thing squaring them discards their signs.

    but i think backwards so it's natural to me...
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