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?
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.