I'm trying to find a suitable PoE MagJack for use with my Phy, KSZ8051MNL. The problem is that I can't find one with the same suggested magnetics topology. The closest one I can find has one extra inductor.

Suggested magnetics on the left. Available magnetics on the right. PoE Ethernet magnetics options

The available jacks with integrated PoE magnetics don't have the centre tap on the network side of the transformer. Instead they have the centre tap on an extra coil which is obviously for the common mode power. However, I really don't need that as I am going to use the two spare pairs for power.

Is this extra coil likely to be a problem?

Additional information: I do not intend to implement IEEE 802.3af standard, as this application is not intended to be compatible with any other PoE device. It will be running over cables no longer than 2m.

Added: Here's a few of the Jacks with integrated PoE magnetics I can find:

These are just a representative sample. I can find quite a few, but all of them seem to have some difference to the suggested ones.

  • \$\begingroup\$ Can you give us links to the datasheet of the "available magnetics"? \$\endgroup\$
    – user3624
    Apr 27 '12 at 3:11
  • \$\begingroup\$ The suggested one on left does not support POE. WHy? \$\endgroup\$ Apr 27 '12 at 3:26
  • \$\begingroup\$ @Tony - The one on the left is one of the example magnetics from the datasheet. \$\endgroup\$ Apr 27 '12 at 9:38

TLA-6T127LF(-T) supports POE in stock.

But the extra coil can be terminated to ground. But no problem if open as it is common mode without much different load.


PoE Integrated Magnetic without the extra inductor.

Brand: B-TOP

Part #: B50(27-11)FP4-151-A622-B12


POE can be up to 44V at 350mA. Pins 4 and 5 are plus, pins 7 and 8 are minus. There are 2 x 75 ohm resistors between the V+ and V- in series. V=IR gives us 293mA. I^2*R gives us 6.5W of power through each resistor.

I doubt the resistors can handle that much wattage without issue. Typical resistors of this size would be rated at 1W maximum. One resistor will fail, break the line, and pull up one of the center taps rendering Ethernet useless.

  • 3
    \$\begingroup\$ You overlooked the fact that there are 1nF capacitors in series with those resistors. Thus for DC on pins 4,5 & 7,8, there should be no direct current flowing through those resistors. I think the RC circuit is just to filter out any transients. \$\endgroup\$
    – CyberFonic
    Sep 26 '14 at 7:18

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.