Can a hall current sensor measure the current flowing in a shielded conductor? - Page 1
Forum section: electronics / general

author: onemilimeter
author_rank: Frequent Contributor
timestamp: June 26, 2011, 12:40:07 am
content: If the conductor shown in the figure below is shielded (e.g. the shield is grounded), can the hall current sensor detect the current flowing in the conductor?Cheers

author: johnmx
author_rank: Frequent Contributor
timestamp: June 26, 2011, 12:54:32 am
content: I can't see any image, but if the cable is like a coaxial cable then the answer is no.It won’t work even if the cable is made of two parallel wires, because the magnetic field of one wire will cancel the field of the other. Electric current has opposite directions on both wires.You need to isolate one wire to measure its magnetic field.

author: onemilimeter
author_rank: Frequent Contributor
timestamp: June 26, 2011, 01:26:49 am
content: Quote from: johnmx on June 26, 2011, 12:54:32 amThanks... and now the image has been uploaded.

author: Psi
author_rank: Super Contributor
timestamp: June 26, 2011, 01:34:50 am
content: They're magnetic, so if you have two conductors with equal current flowing in opposite directions the magnetic fields cancel out and you will read zero.But there is no problem measuring current on a cable that is covered in plastic, or a cable that contains other conductors that aren't connected to anything.You can even measure the total current of two separate circuits by feeding through the sensor either, the positive from both circuits, or the negative from both circuits. If you happened to feed the negative from one circuit and the positive from the other circuit through you would read the difference in the current of the two circuits.Make sense?One side note: Hall sensors that read AC current don't do any processing with the data, on the output you will get an AC waveform that represents the current, not a steady voltage. You need to do your own RMS calculations if you want to measure AC current.For DC current through the sensor though, you get a DC voltage that you can read directly to get the DC current on the conductor.Quite often the output is like 2.5V at zero Amps and moves up to 5V or down the 0V depending on the direction of the current.

author: onemilimeter
author_rank: Frequent Contributor
timestamp: June 26, 2011, 02:33:27 am
content: Is there any magnetic field surrounding a current carrying shielded conductor?

author: Jimmy
author_rank: Regular Contributor
timestamp: June 26, 2011, 03:02:19 am
content: it would measure the current flowing through the shield

author: Psi
author_rank: Super Contributor
timestamp: June 26, 2011, 04:31:28 am
content: Unless the cable shielding is made of mu metal, (they're not) i don't see it causing much of a problem.The shielding in cables is electrical shielding, magnetic shielding is quite different and requires a material that blocks magnetic flux, like Mu-metal.But if you are pushing current down the center of a coax cable and returning it on the shield (or vice versa) then that's totally different and won't work. There would be an equal magnetic field passing through the sensor in both(opposite) orientations and the sum of that is zero.Note: i can't say i've ever tested a hall sensors on a cable that's electrically shielded but i'd be very surprised if it didn't read the current. There is likely to be a small effect of magnetic flux shielding provided by the metal of the coax shield, but i wouldn't expect it to be that noticeable. Most metals are pretty poor at magnetic shielding and hall sensors aren't super accurate at current sensing.If you wanted to be sure you could always use a "open" hall sensor loop (one that has an opening an not a closed hole).Then cut the coax plastic down the length for 3cm or so and, without breaking the cable, split the shielding from the center core then put only the center core into the hall sensor. (assuming you don't want to cut the coax cable and rejoin)