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.
