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Clamp-on Ammeter question

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

2.6A.
Reply to
larry moe 'n curly
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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... mike

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Reply to
mike

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

Reply to
Bob AZ

Oops. That probably explains it.

Reply to
larry moe 'n curly

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

John

Reply to
John Larkin

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