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Clamp-on Ammeter question

Discussion in 'Electronic Basics' started by larry moe 'n curly, Jun 13, 2005.

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  1. If a clamp-on ammeter probe is clamped around both the power wires and
    the ground wires, shouldn't it read zero amps because the magnetic
    fields should cancel?

    I recently measured the amps that my computer motherboard drew by
    clamping the AC/DC probe around the two yellow +12V wires of the
    auxillary 4-pin power connector. The meter read 4.7A, but when I tried
    also clamping the two black ground wires on the same cable, the reading
    dropped to 2.6A. I taped the wires together and even tried arranging
    them to alternate yellow-black-yellow-black, but the reading stayed at
  2. mike

    mike Guest

    Did you also try removing ALL the other ground return paths?

    There are technologies available that claim to be able to measure the
    current in a two-wire cable. Something about slight differences in the
    field due to slightly different wire distances. Read it somewhere on
    the web so it must be true...

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  3. Bob  AZ

    Bob AZ Guest

    Seems that I remember these are switching power supplies so the "Clamp
    On" would have to be sensitive to other than 60 HZ that most of them
    are designed for. The the ground currents in the ground wire wold have
    to be identified as to their source and the currents in the subject
    wire identified also. After all this you could only determine the sum
    and not the particular parts.
    It might be an exercise in futility unless you could contact the
    designer. Who is probably long gone somewhere.
    Bob AZ
  4. Oops. That probably explains it.
  5. John  Larkin

    John Larkin Guest

    Probably some of the +5 or whatever current is sharing the same ground
    wires and messing up the balance.

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