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What is this, and what does it do?


Super Moderator
Hi Craig,

Welcome to ETO.

Any numbers on the unseen side or markings?


Well-Known Member
Most Helpful Member
Looks like a transformer to me. What does the bottom look like Each wire should have a partner (other end) that you should be able to determine with an ohm meter There may be four different wires (up to eight connections from photo)


Well-Known Member
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Most likely an inductor of some sort, from the shape.

At a guess, a common-mode choke in line with a power input connection - though there are various other possibilities, without seeing how it is interconnected with the rest of the circuit.


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Most Helpful Member
It looks like it was a common mode choke, a bifilar wound one. The tracking on the circuit board should confirm that, two connections in and two out, with one side connecting to a power input.

They are typically used for interference suppression, to reduce electrical noise passed back through power cables, that could otherwise act something like antennas.

That one has a toroidal core inside the casing. Without the case it would look rather like the one at the left of this photo:

More about them, in general.

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