Electronic – the function of this choke-like transformer on the PCB side of this RJ45 jack+magnetics connector


I came across this ethernet jack+magnetics connector the other day and was wondering what the transformer on the PCB side was for. I thought it looked like a choke, but there is a centre tap running through the middle and the choke is usually on the network side. Can anyone explain what it is?

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Best Answer

It's a Common Mode Choke integrated in the RJ45 connector.

If there is a voltage spike on an incoming line the common-mode choke will prevent it damaging the silicon by transferring the energy of the spike so that it appears on 10, 11 and 12 together i.e. there isn't a differential spike.