How is DC Current Measured by a Clamp Meter?

A dc current flowing in a wire will have a static magnetic field surrounding it, so with a clamp on toroidal core around the wire a static magnetic field is set up in the core, if you use a linear hall effect device in the core's path you can measure the strength of this fiels and measure it...

A more accurate method would be to have a winding on the toroidal core (clamp on bit) and introduce a dc current to oppose the magnetic field until the hall effect sensor indicates zero again and then the current and turns on the core give a more precise reading of the current in the wire.

Remember, that for a toroid the number of turns is the number of times the wire passes through the centre of the toroid.

John.

The previous item in the thread was Earth resistance measurements. The two are not the same. Telecom, power utility and radio tower installers have a need to know the exact resistance to ground from a specified "grounding point" in their system. The Clamp-on meters now commonly used for these measurements injects a calibrated pulse into the non conducting and grounded cable under test. I used one when working for a power utility company, but the supplier declined to reveal any real details of the inner workings. I would hazard a guess it works somewhat like a reflected power meter. For field calibration several lenghts of "known" resistance stiff wires came with the kit. One was 0.5 ohms one was 1.0 ohms and one was 3.0 ohms

The latter being considered a reject or bad ground reading.

Elnav

I fail to see how this answers the question how does a dc clampmeter work?

John.

Hall effect devices are quite delicate so you don't want to overstress them. I used to repair Tektronix DC current probes and they were easy to pop. Hall effect transducers are also used in automotive distributors and shaft rotation indicators. They break quite frequently in those applications.

BTW, any semi-conductor will detect magnetic lines of force. More lines, more conduction.

Hall effect devices are remarkably robust semiconductor devices with regards to overloading them...

They can be physically weak due to incorrect mechanical stresses on the package, especially when they have a flux concentrator built in, but that is a mechanical weakness, they can be used in magnetic fields considerably more than their effective range without any damage.

John.

I agree.

Every brushless DC motor I've ever disassembled (dozens, mostly fans and floppy or hard drive motors) has had one or more Hall effect sensors. I have removed them and used them for other purposes without ever having one fail. I have never heard of a motor failing due to its hall sensor(s).

Thanks to all feedbacks and best wishes for 2007.

Does it mean that the Hall effect transducer is situated inside the meter to sense the magnetic field of the clamp caused by the passing DC current.

Yes Horace, they are normally mounted in-line with the toroidal core so as to measure the magnetic field in the core..

That is the open end of the jaw allows entry of the cable and at the jointed, hinged side of the toroid the hall effect device is mounted, usually inside the instrument.

John.

Thanks Electroman and others.

I have bought a clamp meter (on eBay) to measure DC current (up to 400 amps). It arrived yesterday and I have read through the instructions and only on one page is there a single reference to 'Hall components.' No other info in this respect is given at all.

So it looks like Hall effect is used by this meter, and hence 'ten out of ten' for those who said so.

I will have to wait and see how durable it is.

Delicate component? we had them mounted on all of our Robinson R-22 Helicopters in our flight school, on the main shaft gearbox flex plate, as rotor RPM sensor. This is super critical application where sensor's failure will most probably mean a crash if it is a low hours student pilot. and a big problem for any pilot. Do you believe that the FAA or Robinson helicopter will approve such a hazard?. Thousands of flight hours, never even a single failure.

Wangito.

Thank you dkwarner and wangito....

I just knew that by posting the fact that these devices are robust I would have a load of replies saying ..."But they fail in my application..." etc...

John.

hi yes it can be I have one

It uses a moving iron movement it's bulky but it did work it is very old

aussepom

I don't believe that the use of Hall effect devices is (or at least was) the only method used. Some DC clamp meters used the saturation in the toroid: they superimposed an AC signal and adjusted a compensating current so that the distortion became symmetrical (i.e. no second harmonic was generated). The possible disadvantage with these was that they were intrinsically sensitive to additional asymmetrical AC signals

Fyz

Hi well it is true

the moving ion instument was also used for AC in panel meters as well this clamp meter is a moving ion is is well over 40years, I did this in tafe and I had one given to me 30 yrs ago.

aussepom

Try a search for a company called LEM they show clear diagrams of open loop and closed loop hall effect transducers and explain why to use closed loop. It is just like electroman says with clear diagrams.