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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?

enter image description here

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  • \$\begingroup\$ It's a Common Mode Choke, isn't it? \$\endgroup\$ – Jesus Castane Jun 6 '13 at 8:45
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    \$\begingroup\$ 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 \$\endgroup\$ – Andy aka Jun 6 '13 at 9:01
  • \$\begingroup\$ @JesúsCastañé Correct, it is indeed a CM choke. \$\endgroup\$ – Adam Lawrence Jun 6 '13 at 11:33
  • \$\begingroup\$ I create a answer with these comments. Is this ok? \$\endgroup\$ – Jesus Castane Jun 7 '13 at 9:04
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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.

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