Passive Power Over Ethernet

Power Over Ethernet (POE) provides for devices such as IP phones, WiFi Access Points etc. to receive operating power through their data cable.  Standards based POE (802.3af, 802.3at) require that the provided voltage is around 48v and that the Power Sourcing Equipment (PSE) will only switch on the 48v if the Powered Device (PD) negotiates that it is 802.3af / at compliant.  This protects, and allows for non POE devices to be connected to a PSE (e.g. switch) that provides POE capability over all its ports, without risk of damage.

Non POE compliant devices may be powered over the data cable by ‘injecting’ operating voltage into the data cable and ‘splitting’ it out at the cable end to feed the DC input to the device.  This is commonly known as “Passive POE”.  Some devices may accept passive POE directly via their data connection, and therefore the “splitter” component is not necessary.  As the operating voltage is ‘hardwired’ on to the data cable, there is no protection for any device that may inadvertently be connected to the cable.

Passive POE Injector / Splitter

Main powered Passive Injector

Passive POE PDs typically require a much lower operating voltage (typically between 5v and 24v depending on the device), so the injected voltage has to correspond.  With lower voltages used for POE, losses in the cabling become more significant.  This usually results in a shorter cable run that can be used reliably.  For 5v powered devices, 5m may be the maximum whereas 12v to 24v devices can usually be operated with 20~30m of cable.  If longer distances are required, it is possible to use 48v on the data cable and use a voltage converter to provide the required low voltage at the PD end.

8 Port Midspan Injector

Some multiport passive injectors may have isolating diodes as part of the design; these will add to the voltage loss.  Sometimes it is possible to compensate for losses by injecting a higher voltage than the PD requires: e.g. injecting 15v into a long cable to cover the cable losses for a 12v PD.

POE for 10/100Mb vs Gigabit Data

Cat5 / 5e / 6 data cable has four pairs of wires (1+2, 3+6, 4+5, 7+8) given by the connector pin number.

For 10/100 Mbps links, two pairs are used for data (1+2, 3+6), and the remaining two pairs (4+5, 7+8) are typically used to carry the POE voltage

For Gigabit (1000 MBps) links, all four pairs are used to carry data, so special transformers need to be used at both the PSE and PD ends to allow POE voltage on to pairs that are carrying data.  So passive POE adaptors should indicate whether they support 10/100/1000 Mbps speeds.   In the case of Gigabit POE, either of the two pairs or all four pairs could carry the POE voltage.  Standards based POE requires the PD to accept voltage on either of the two pairs.  Passive POE is not quite so well defined, although for Gigabit speeds, the POE voltage is typically found on pairs 4+5 and 7+8.