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.

enter image description here

enter image description here

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    \$\begingroup\$ There is a lot of things on your images,do you mean the clip on ferrite? \$\endgroup\$
    – PlasmaHH
    Commented Sep 9, 2016 at 13:13
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    \$\begingroup\$ You work with IC design but never had to suffer EMC tests? Otherwise you'd probably recognize ferrites. It is basically something you desperately add to your wires in the last minute, after finding out far too late that your design sucks. That whole picture looks like "we had some severe EMI problems here". The "lab-junk" chassis wire looks like it was added last minute too. \$\endgroup\$
    – Lundin
    Commented Sep 9, 2016 at 13:48
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    \$\begingroup\$ Thanks for the answers, I had a feeling it was some sort of EMC related device. The ICs I design are prototypes and will never leave the lab, I rarely put anything else on chip beyond some ESD protection for the pins. All the EMC concerns are handled on the test pcbs, or just thrown in a shielded box with co-ax's. \$\endgroup\$
    – IC_Eng
    Commented Sep 9, 2016 at 14:44
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    \$\begingroup\$ Noooo it's a ferrite bead I missed my golden opportunity noooo!! \$\endgroup\$
    – Bradman175
    Commented Sep 9, 2016 at 14:50
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    \$\begingroup\$ On the bright side, this post is now the first Google result for "gray box clipped to wires". \$\endgroup\$
    – Jason C
    Commented Sep 9, 2016 at 16:08

3 Answers 3


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.

enter image description here

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.

  • \$\begingroup\$ The magnetic field doesn’t completely cancel even for balanced currents. Magnetic field from a pair of conductors can stray into this choke as well, especially if any of them touches the ferrite with its insulation. \$\endgroup\$ Commented Sep 10, 2016 at 21:54

It's a ferrite choke. It is a passive electric component that suppresses high frequency noise in electronic circuits.

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    \$\begingroup\$ They're very commonly seen where lines that are intended to only have low frequency signals exit a shielded case that contains components that emit high frequency signals. Likely an unshielded wire will be connected to that connector, and it would become a noisy antenna if the high frequency signals inside the case leaked into those wires. \$\endgroup\$ Commented Sep 9, 2016 at 18:29
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    \$\begingroup\$ The clever thing about those ferrite blocks is that because they are clamped round the whole cable they only suppress common mode not differential mode. So interference is suppressed while legitimate high frequency signals pass. \$\endgroup\$ Commented Sep 9, 2016 at 18:38
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    \$\begingroup\$ Also called a "ferrite bead". I hadn't seen a removable one before. en.wikipedia.org/wiki/Ferrite_bead \$\endgroup\$
    – AshleyZ
    Commented Sep 9, 2016 at 20:50

Rarely you see the snap on type. They are very common in round ring type ferrite cores where the set of wires are looped once. I see them mostly inside all type power supplies.

enter image description here

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    \$\begingroup\$ Snap on ferrites are actually quite common on a variety of cables. \$\endgroup\$
    – Wesley Lee
    Commented Sep 29, 2016 at 8:37
  • \$\begingroup\$ Wesley is correct - the bead type is used typically for on-board inductors. For example. \$\endgroup\$
    – Jay M
    Commented Feb 1, 2017 at 5:49

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