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Non Contact Current Measurement

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

(OP)
GG--I should have clarified my query a bit. We are talking 1940's and 1950's 6 volt systems. Hall Effect devices are solid state, no?

RE: Non Contact Current Measurement

A galvanometer on a shunt is traditional, repeatable, and thanks to decades of mass production, cheap, so there's no economic incentive for a noncontact ammeter to exist, but that doesn't mean it hasn't been done.

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

The Hall Effect was discovered in 1879, and Edwin Hall was able to measure that effect with instrumentation that was available at the time:
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
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

I'm not sure but I doubt you can use a contactless type ammeter on a car which is DC. The earth's magnetic field is DC too. All the DC Hall Effect sensors I've ever seen need to be zero'd before use to account for the sensor's orientation in the earth's field. Once set they are very accurate. But in a car the orientation to the earth's field will be constantly changing.

They could probably make them so coarse it might not matter ~5A+/-??

Keith Cress
kcress - http://www.flaminsystems.com

RE: Non Contact Current Measurement

(OP)
Thanks, IR. I will need to spend some time reading that link. I was going to play around a bit today using my clamp-on meter to measure the current to run a DC motor . The plan was to measure the current to the motor from a half wave battery charger, then see what kind of result (if any) I would get from repeating the run using a lead acid battery to run the motor. Got out the clamp-on meter and found that the fuse was blown, the battery was dead and there was corrosion to clean up. The experiment will be re-scheduled once the necessary maintenance has been performed.

RE: Non Contact Current Measurement

Adding to itsmoked's post, where a magnetic sensing device is placed in a car could be problematic also due to the numerous sources of magnetic fields in close proximity.. On one vehicle I tried to put in an aviation compass (with adjustable compensation capability) on top of the instrument panel, it would not work very well even with excessive compensation adjusted into it.

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

I have seen just such a meter in a Ford 8N tractor. If I recall correctly, out the back of the meter were 2 metal loops about 2x the diameter of the wire that were fairly close together. I can't recall the scale, but I don't think it was much more than charging vs discharging. I never thought much about how it worked, but I can say it still worked with a more modern 3-phase alternator with a diode rectifier. I'd say that Mike is probably giving the answer. The loop probably just held the wire in ~approximately~ the right spot behind the meter so the magnetic field from the wire could directly cause the movement.

RE: Non Contact Current Measurement

Those old Ammeters were cheap and simple. Imagine a galvanometer. Now replace the coil with a small magnet.
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

(OP)
The blurb from Trojan states that their meter works via induction. This has been the source of my confusion--how can you have induction with DC current unless you have an expanding or collapsing magnetic field, i.e. like with an ignition coil?

RE: Non Contact Current Measurement

The MOVING charges (current) in the cable being tested induces a magnetic field.

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
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

I'm not sure what link you're referring to, but if it's the type of meter being discussed then they're using the wrong word. Nothing is being induced. The magnetic field around the wire is directly causing the meter to move.

RE: Non Contact Current Measurement

Yes, my father used to have one of those 'two-bit' Ammeters with the V-shaped piece of sheet-metal on the back that you held against a section of wire. I should have kept it but it went missing when we had to break-down and clean-out his shop after he passed-away back in 1989 and my mother decided to sell the house.

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

Not induction. Simple magnetism. The word induction is probably unfortunate.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Non Contact Current Measurement

It's not "simple" magnetism, by definition, since it requires electromagnetism. The generation of a magnetic field from moving electrical charges described by Ampere, Faraday, and Maxwell is the foundation of all forms of induction. Without this phenomenon, the ammeter wouldn't work.

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

(OP)
Well, guys, I did a little EE science fair project this afternoon. Wired up a 12v lead acid battery, a 12v motorcycle starter motor and put my clamp on ammeter over one of the wires. Straight DC, no dirty half wave rectified power to obfuscate the results. Hit the switch, the motor ran and the ammeter read about 4 amps.

RE: Non Contact Current Measurement

Looks like magnetic induction to me. The current induces the magnetic field. I don't think that is what comes to mind when engineers usually bring up induction. I think this is a case of physics using a term differently than it is commonly used in engineering.

RE: Non Contact Current Measurement

May I withdraw my comment.
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

A few years ago I bought a clamp-on ammeter, and spent a bundle on it to have a DC current range, not just AC. (Agilent for those who are interested).
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

(OP)
I had never used one of these before, so when I got mine going yesterday, I clamped it over an extension cord and ran an electric drill (yes, you used to have to plug those in). Meter read 0. I figured the 2 ac phases were canceling each other. I split the cord covering to separate one wire and then the ammeter read just fine.

RE: Non Contact Current Measurement

swall; An old trick that you may want to try. If you wrap the cord around the meter prong so that it passes through inside the core twice, it will double your measurement. 5 times around multiplies the reading by 5 times.
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 thought a simple description of induction in this context was producing an electromotive force in a wire due to the wire moving through a magnetic field?

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

my father had one for measuring alternator and starter current. This is the first hit I got from google. just place the wire in the channel and measure current

RE: Non Contact Current Measurement

(OP)
Reminds me of a gauss meter that one uses in magnetic particle inspection.

RE: Non Contact Current Measurement

You can get a digital clamp-on meter measuring AC and DC up to 100 A for around $35
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

DC clamp meters use hall-effect sensors.

RE: Non Contact Current Measurement

Since this is the hobby forum, a word of caution:

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

That brings back memories byrdj.
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

I started my DC career with one of these:
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
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

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