# Sensing current in a conductor

If I have 120 vac power cord and I wrap a coil around one of the conductors, will a current/voltage be produced in the coil? If so what are the equations that could be used to characterize this arrangement? I.e., how many coils, what size wire, etc. To produce a certain voltage in the coil?

berkeman
Mentor
If I have 120 vac power cord and I wrap a coil around one of the conductors, will a current/voltage be produced in the coil? If so what are the equations that could be used to characterize this arrangement? I.e., how many coils, what size wire, etc. To produce a certain voltage in the coil?
Welcome to the PF.

Try Googling "current transformer". Let us know if you have questions about that reading.

Welcome to the PF.

Try Googling "current transformer". Let us know if you have questions about that reading.
Looked there but didn't find exactly what I was looking for. Want to wrap the coil directly around the conductor without using an iron or steel ring. This config is what I'm interested in. Will it produce a voltage without an iron ring?

berkeman
Mentor
Want to wrap the coil directly around the conductor
In that case, the magnetic field from the current in the conductor will be at a right angle from what you need to induce a voltage in the pickup coil. Not going to work that way, sorry.

I'll try to find some good reading for you to learn about how transformer action works. Basically, the primary current needs to generate a magnetic flux, and that changing magnetic flux needs to be flowing through the area of a pickup coil. The magnetic flux generated by a current in a conductor flows around that conductor, so if you wind the pickup coils around the conductor, they are parallel to the magnetic field instead of orthogonal to it, and will not pick up any induced voltage...

If you cannot use a current transformer, your next choice is to use a Hall effect pickup for measuring the current (and its magnetic field). Have you looked into Hall effect sensors yet?

berkeman
Mentor
First figure shows the magnetic B-field generated by a current in a wire. Second figure shows how a changing B-field needs to pierce the area of a conducting loop to induce a changing voltage in that conductive loop...

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/imgmag/magcur2.gif

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Ok. First question answered. Thank you.
Next question:
Is a metal core "ring" required to surround the conductor or can the secondary coil be a simple "air" coil positioned circumfrentially (perpendicular) to the conductor?
Side newbie question; How do you include a picture or sketch in a post?

berkeman
berkeman
Mentor
Is a metal core "ring" required to surround the conductor or can the secondary coil be a simple "air" coil positioned circumfrentially (perpendicular) to the conductor?
Running the wire through the core of a toroid and wrapping many turns of a sensing coil around the toroid gives you the best coupling and the highest signal-to-noise, which gives you a better and more accurate measurement of the AC current in the wire. You can just put an air-core coil next to the wire, oriented in the same plane as the wire so that the B-field shown in the first figure goes through the plane of the coil. You need the changing B-field piercing the surface of the circle that is inside the coil in order to induce the AC voltage in your pickup coil.

BTW, this technique is used in high frequency EM Interference debugging, where you use a "sniffer probe" which is a 1-turn air-core coil that you connect to a preamp and spectrum analyzer. You "trace" the high frequency currents through wires and metal chassis parts as you try to figure out why your product is radiating certain EM frequencies that can be a problem in passing FCC testing.

https://www.langer-emv.de/fileadmin/near field probes sniffer probes current correction magnetic field correction EMC RF Langer EMV-Technik 02.jpg

Side newbie question; How do you include a picture or sketch in a post?
To upload a PDF or JPEG image, click the UPLOAD button in the lower right of the Edit window. To link to a picture on the web, click the little picture icon (between the smileys icon and the 35mm film icon on the toolbar), and paste in the URL.

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ZapperZ
Staff Emeritus
Ok. First question answered. Thank you.
Next question:
Is a metal core "ring" required to surround the conductor or can the secondary coil be a simple "air" coil positioned circumfrentially (perpendicular) to the conductor?
Side newbie question; How do you include a picture or sketch in a post?

Even after all this, I don't believe that you've understood the explanation, or the physics, that has been given to you. I think Faraday's law has hardly been mentioned directly.

Secondly, I am not quite sure what this is leading to. You obviously have something in mind that you wish to do, but you're not telling us.

Finally, you ARE aware that there are commercially-available clip-on probe that attaches to a multimeter, aren't you? This is simply a clip that forms a closed loop that you can clamp onto a wire to mainly check if there is current flow and the magnitude, even in AC mode.

Zz.

berkeman
Tom.G