Active vs Passive PoE: What Is the Difference?

Active vs passive PoE is the difference between Power over Ethernet that asks before it powers and PoE that does not. Active PoE is the IEEE-standard kind, defined in 802.3af, 802.3at and 802.3bt. Before any real power flows, the source and the device carry out a handshake, so a device only ever receives power it has asked for and can handle. Passive PoE skips the handshake and applies a fixed voltage to the cable as soon as it is switched on, whatever is plugged in at the other end.

Both put DC power on the same twisted-pair cable as the data, and both can run cameras, radios and access points. The difference is safety and interoperability. Active PoE works across vendors and will not harm a laptop or printer plugged into a powered port. Passive PoE works only when both ends are designed for the same voltage and wiring. Our guide to what PoE is covers the basics both share.

An active source, called the power sourcing equipment (PSE), runs a short sequence on each port before power is applied.

Passive PoE is a simple DC supply wired onto spare conductors. On 10/100 links, which use only two pairs for data, it usually puts positive on pins 4 and 5 and negative on pins 7 and 8. Gigabit passive gear has to share the data pairs instead. A passive injector is often just a power supply and a pair of connectors with no electronics to check the load.

It became common on outdoor wireless equipment because it is cheap and simple, and because many radios were designed to accept it. The common voltages are 24 V and 48 V, with some 12 V and 18 V devices. Lower voltages lose more power over long cable runs, so 24 V passive equipment often specifies shorter maximum distances than the 100 m of standard PoE.

Mixing the two is where problems start. Passive power into a device that expects active PoE, or into a device with no PoE at all, can overheat or destroy its Ethernet interface. A 48 V passive supply into a 24 V passive radio usually damages the radio. The reverse case is safer but still fails: an active switch will not power a passive-only device, because the device has no signature resistor, so it simply stays off.

Some equipment accepts both, and adapters exist. An active-to-passive converter takes 802.3af or at power and outputs a fixed passive voltage for older radios. A PoE splitter does the job at the device end, taking active PoE and delivering 5, 12 or 24 V DC to a camera or sensor that has no PoE input, which is a common way to bring older equipment onto a PoE network. Label every passive injector and port clearly, because the RJ45 connector looks the same either way.

For interoperable, protected deployments, specify active, standards-compliant PoE end to end. Passive PoE still has a place in closed, known systems such as a matched pair of wireless bridges, but it should never be mixed with standard PoE equipment without care. When an existing site already has passive radios, the cleaner route is usually a standards-based switch plus a converter for each passive device, so every new camera, access point or computer added later can plug straight into a protected port. Our industrial PoE switches and PoE computers use IEEE 802.3af, at or bt ports, and our guide to industrial Power over Ethernet covers active and passive PoE in more depth.

We supply PoE switches, PoE computers and powered devices matched to one standard and tested together as standard. Send us the devices you need to power and we will specify an active PoE set-up to suit.