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Clamp-on Ammeter question

L

larry moe 'n curly

Jan 1, 1970
0
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.
 
M

mike

Jan 1, 1970
0
larry said:
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.

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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B

Bob AZ

Jan 1, 1970
0
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
 
J

John Larkin

Jan 1, 1970
0
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.

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


John
 
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