Power over Ethernet, PoE

It is nice to be able to power a device over the same connection that is used for signalling.  We take this for granted with USB, however with Ethernet devices we typically need a separate power connector.  PoE enables the sending of power down the unused wires in the cable or even on the used wires.

10Mb and 100Mb Ethernet only uses the middle four wires of the eight wires in the connector.  The centre two wires – pins 4 & 5 are used for one signal and the next two wires, ie pins 3 & 6 are used for the other signal.  In fact you can actually use the other two pairs in the cable for a second Ethernet connection in the same cable – pins 1 &2 and 7 & 8 are unused.  So these wires could be used for power.  Typically about 50V is used and the current kept low to reduce the losses / voltage drop in the cable.  The maximum current for 21AWG conductors is 0.36A giving a maximum power that can be drawn of about 36W.

For gigabit Ethernet, all eight wires are used.  however power can still be sent between two pairs of signals with the actual signal sitting on top of the power.

for more information, a good paper on the subject can be found at

http://www.bb-elec.com/Learning-Center/All-White-Papers/Ethernet/Power-over-Ethernet-PoE.aspx

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