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How is Power Transferred Through the Cable
A standard CAT5 Ethernet cable has four twisted pairs, but only two of these are
used for 10BASE-T and 100BASE-T. The specification allows two options for using
these cables for power, shown in Figure 2 and Figure 3:
The spare pairs are used. Figure 2 shows the pair on pins 4 and 5 connected
together and forming the positive supply, and the pair on pins 7 and 8 connected
and forming the negative supply. (In fact, a late change to the spec allows either
polarity to be used).
Figure 1 - Power Supplied over the Spare Pins
The data pairs are used. Since Ethernet pairs are transformer coupled at each
end, it is possible to apply DC power to the center tap of the isolation transformer
without upsetting the data transfer. In this mode of operation the pair on pins 3
and 6 and the pair on pins 1 and 2 can be of either polarity.
Figure 2 - Power Supplied over the Data Pins
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