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Measuring AC current on DC meter

Discussion in 'Electronic Basics' started by Suraj, Jan 12, 2004.

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  1. Suraj

    Suraj Guest

    I was wondering if I could measure AC current drawn by a device on my
    digital multimeter which has DC current measurement (10A Max). We have AC 50
    Hz @220 V here. As far as I understand, the current wil change direction 50
    times in a second, therefore the DC meter should positive and negative
    readings alteringly (If I can see it and if the sampling rate of the digital
    meter is high enough?)

    Should I try it? How are AC ampere meters different? do they calculate the
    integral of the Sine Wave?

  2. Passing AC through a DC ammeter will not hurt the meter, as long as
    you do not exceed its rating, but it will not give a useful result.
    The DC meter displays the result of an average over some time interval
    (perhaps as long as a quarter second), so the average of alternating
    half cycles will be close to zero. AC meters rectify the signal into
    a unidirectional magnitude and average that. Then the result is
    scaled to produce the effective current (the DC current that would
    produce the same heat in a resistor if the current were DC, with the
    assumption that the current is sinusoidal). Really good AC current
    meters square the instantaneous current, average the current squared
    over some time interval, and take the square root of that average to
    get an effective current that is accurate even if the waveform is not
    a sine wave.

    One fairly accurate way to read AC current with a DC meter is to pass
    the current through the window in a current transformer, put a bridge
    rectifier on the transformer output, with a scaling resistor on the
    output of the rectifier. With the right choice of scaling resistor,
    your DC meter will read a fractional multiple of the AC current, based
    on the number of turns on the current transformer. THis approach has
    the added feature that it isolates the metering from any voltage on
    the current carrying conductor.
  3. JeffM

    JeffM Guest

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