Non Contact Current Measurement
Non Contact Current Measurement
(OP)
One of my fellow retired engineer buddies got to talking about ammeters in vintage vehicles. He mentioned that there were "pass through ammeters" where the meter was not wired directly into the charging circuit, but instead just sensed current from a wire passing through the meter. I asked him how this could work on DC. He stated that the meter is measuring the magnetic field intensity from the current flow in the wire and the meter responds as the magnetic field changes. I was not convinced, but argued that the meter is responding to pulses in the half wave current that a brushed generator puts out. In other words, this pulsed DC current is inducing a current in the meter sensing coil as though it were AC and the meter is reading the induced current, not magnetic field strength of the current flowing in the wire. Which of us is right?





RE: Non Contact Current Measurement
I have used these many times. Refer to the following link;
http://www.crmagnetics.com/ac/dc-hall-effect-curre...
See also:
https://en.wikipedia.org/wiki/Hall_effect
"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)
RE: Non Contact Current Measurement
RE: Non Contact Current Measurement
I have used Hall Effect sensors, but the price never came down to {galvanometer+shunt} range.
I think you can get an indication of {some current|no current} with a magnetic compass held adjacent the wire. ISTR 'starter current meters' that may have worked that way.
Mike Halloran
Pembroke Pines, FL, USA
RE: Non Contact Current Measurement
https://web.archive.org/web/20110727010116/http://...
If the charging circuit isn't completely DC, then even a conventional current transformer could be used. A clamp-on ammeter works on that principle.
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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RE: Non Contact Current Measurement
They could probably make them so coarse it might not matter ~5A+/-??
Keith Cress
kcress - http://www.flaminsystems.com
RE: Non Contact Current Measurement
RE: Non Contact Current Measurement
The culprit.. a major wiring loom went across the full width underhood at the top of the firewall carrying varying currents of course and the generated magnetic field was too close to the compass.. Whatever magnetic shielding by the cowl and hood sheet metal was not enough. Worked great when the ignition was off.
I was always amazed that magnetic compasses could be made to work as well as they do on bridge of a ship. Time to search and read about the huge iron balls located on binnacle on either side of a ship's bridge compass to see how they compensate for all the ferrous material around it.
Today's smartphones have pretty decent magnetometers built in for sniffing around devices emitting magnetic fields.
RE: Non Contact Current Measurement
RE: Non Contact Current Measurement
Remember the high school experiments where a current flowing in a wire would deflect a magnet suspended by a piece of string.
The 50 era Fords used such a meter. There was a brass clip on the back to position the main charging wire close to and parallel to the internal magnet.
Sort of like putting a compass near a DC wiee.
We used current indicators to check charging circuits and starters that worked on the same principle.
Here's one from E-Bay.Link
The third picture shows the clip on the back for positioning the meter on the starter wire.
I had a similar meter but scaled for 60-0-60 Amps.
I had another one with two scales; 60-0-60 Amps and 600-0-600 Amps. The clip for the 600 amp cable was farther away from the magnet.
Here's a dual range model from Amazon:
Link
And:
Link
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Non Contact Current Measurement
RE: Non Contact Current Measurement
It's just not the way we usually think of induction, but Ampere-Maxwell's equation has a static current density component:
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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RE: Non Contact Current Measurement
RE: Non Contact Current Measurement
John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
Siemens PLM:
UG/NX Museum:
The secret of life is not finding someone to live with
It's finding someone you can't live without
RE: Non Contact Current Measurement
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Non Contact Current Measurement
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm
RE: Non Contact Current Measurement
RE: Non Contact Current Measurement
RE: Non Contact Current Measurement
I am thinking that there is a similarity to a brushless DC motor with a low DC voltage applied directly to the windings.
The rotor magnets will align with the magnetic field. Think of it as a small BLDC motor with one turn in the winding and the rotation inhibited by a spring.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Non Contact Current Measurement
While still in the store's parking lot, I decided to test it. I turned on the truck headlights, clamped over the battery cable, and read "0".
Started the engine, and got a small AC signal on the alternator (ripple), switched back to DC, still nothing on any cable.
I went back into the store and asked for my money back. They guy didn't believe me at first, but then he got a clever idea.
He clamped it over his computer's power cord. "Ah yes, it does just read "0"... you must be right". Gave me my money back.
I walked out of the store with a Fluke, repeated the test on the truck, and got very satisfactory results in both DC and AC.
And never once was tempted to correct the guy at the store about the validity of his test.
STF
RE: Non Contact Current Measurement
RE: Non Contact Current Measurement
This is useful for checking low currents, and incidentally a quick demonstration of transformer action and turns ratios.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Non Contact Current Measurement
I don't recall ever seeing induced used for the opposite direction, as in the current through a wire induces a magnetic field around the wire.
RE: Non Contact Current Measurement
RE: Non Contact Current Measurement
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm
RE: Non Contact Current Measurement
RE: Non Contact Current Measurement
https://www.amazon.com/UNI-T-UT210E-CURRENT-METERS...
Another model in the same series will handle 200 A. Don't know the measurement technology, though.
RE: Non Contact Current Measurement
RE: Non Contact Current Measurement
There's a lot of cheap clamp meters out there that combine a basic multimeter (including standard plug-it-in ac and dc current ranges) with a simple ac-only clamp.
Not always a problem, but a potential source of disappointment if you were expecting more than you paid for.
A.
RE: Non Contact Current Measurement
I had one similar but not identical.
Mine was dual range.
The channel on the back was smaller and the range was about 60-0-60 Amps.
As well there was a wider channel similar to the one shown mounted higher on the back of the meter. That was farther away from the internal magnet and had a 600-0-600 Amp scale.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Non Contact Current Measurement
http://i.ebayimg.com/images/a/T2eC16d,!zcE9s4g3hi,...
There was a suit-case with four different inserts with different current ranges.
They worked with moving iron and worked equally well (same scale) with AC and DC.
The magnetic circuit was the same for all ranges but movement sensitivity was different.
I think that it was a British device.
Gunnar Englund
www.gke.org
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