Electronic – Can you identify this component


This may be obvious to some, but not to me. I am an IC design engineer, as such I rarely do any practical work. This morning I had to fix some electronics which were custom made by a third party for my company. On removing the front panel I saw this gray box clipped to a bundle of wires (images below), I have no idea what it is or what it does. After 10 minutes of fruitless googling for "gray box clipped to wires" I find myself here asking this question. The inside looks like some type of metal or ceramic. I thought it might be magnetic but it is not.

So can anyone identify it and tell me what it does?

I fixed the problem, which was not related to the wiring.

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Best Answer

It's a snap-on ferrite core which, when clamped over a number of conductors, becomes part of a common-mode choke. Here is a complete datasheet for a similar product.

The ferrite is lossy so it is better characterized as an impedance at a given frequency. In the case of my example, every wire that goes through the core behaves like it has 241 ohms in series @ 100MHz but only for common mode voltage. If current is going in and out through a pair of conductors the magnetic fields cancel and it acts pretty much like a pair of wires for that current. Unbalanced current (say to ground) will behave like it has the impedance in series.

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It is very important to make sure that all the wire pairs that carry significant current are contained within one core, otherwise the core will saturate and the benefit will be lost.

A typical application of such a core would be for wiring to a noisy source such a VFD (motor drive).

Looping the wires \$n\$ times through the core will increase the impedance by a factor of \$n^2\$. Again, every wire must be looped the same number of times to get the full benefit.

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