UK How a PoE injector works
A PoE injector is a single-port power source for Power over Ethernet. It has two RJ45 sockets, usually marked Data In (or LAN) and PoE Out, plus a power input from a mains adapter or a DC supply.
A patch lead runs from the switch or router into Data In. The injector passes the data straight through to PoE Out and adds DC power to the cable there. A standard injector behaves exactly like a PoE switch port: it checks for a valid powered device, reads its class and only then switches on around 48 V DC. The camera or access point at the far end sees one cable carrying both.
Injectors are common in offices and small sites, where one new camera or access point needs adding and the existing switch has no PoE ports. They also suit testing, because an engineer can power a device on the bench without a PoE switch.
In IEEE terms, an injector is a midspan: power sourcing equipment that sits in the middle of the link, between the switch and the device, rather than at the end of it like a PoE switch port.
Types of PoE injector
Injectors look alike, but the standard and the build matter. Pick one that matches the device it will power.
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802.3af and 802.3at injectors. Up to about 15 W or 30 W, enough for most fixed IP cameras, phones and access points.
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802.3bt injectors. Up to 60 W or 90 W over all four pairs, for PTZ cameras with heaters, larger access points and PoE powered displays.
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Multi-gigabit injectors. Pass 2.5, 5 or 10 Gbit/s data through, needed for Wi-Fi 6 and Wi-Fi 7 access points. A gigabit injector caps the link at 1 Gbit/s.
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Multi-port midspans. Rack or panel midspans with several injector channels, used to add PoE to an existing switch.
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Industrial injectors. DIN rail mounting, wide temperature range and a 12 to 48 V DC input from the cabinet supply.
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Passive injectors. Apply a fixed voltage with no detection. Our active vs passive PoE FAQ explains why they should only power devices built for them.
How to connect a PoE injector
Connecting an injector takes three cables and a minute, but the order matters.
First, plug the injector into power. Second, connect a patch lead from a normal switch or router port to Data In. Third, run the long cable from PoE Out to the powered device, up to 100 m away. Never plug PoE Out into a computer or switch port, and never connect Data In to another PoE source.
If the device does not power up, check three things. The injector may supply a lower class than the device needs, which often shows as a device that starts and then reboots. The cable may have a fault on one pair, which data can sometimes survive but power cannot. Or the Data In and PoE Out leads may be swapped, which is the most common mistake on site.
If the device has no PoE input, add a PoE splitter at the far end. The splitter takes the PoE cable and outputs data plus a DC supply, often 5, 12 or 24 V, to suit the device.
PoE injector vs PoE switch
An injector and a PoE switch port do the same job electrically. The difference is scale, tidiness and control.
| Point | PoE injector | PoE switch |
|---|---|---|
| Devices powered | One per injector | Many, one per PoE port |
| Power supplies | One adapter per injector | One supply for the whole switch |
| Budget | Full class power to one device | Shared budget across all ports |
| Remote control | None | Managed models can monitor and restart each port |
| Best fit | One or two extra devices on an existing network | Several cameras, access points or sensors |
Once there are more than two or three devices, a row of injectors and adapters becomes untidy and hard to back up. Industrial PoE switches or PoE computers power several devices from one supply, and one UPS can back up all of them.
Choosing a PoE injector
Check these points against the device datasheet before buying.
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Standard and power. The injector must supply at least the class the device asks for. When unsure, choose the higher standard; a compliant PoE++ injector safely powers a PoE device.
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Data speed. Match the device’s network speed, especially for multi-gigabit access points.
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Input power. Mains for a comms room, 12 to 48 V DC for a control cabinet or vehicle.
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Environment. Outdoor or cabinet installs need wide-temperature, DIN rail or sealed injectors.
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Pairs used. Most devices accept power on either pair set. Our PoE Mode A vs Mode B FAQ covers the detail.
Quick answers on PoE injectors
PoE injectors: common questions
Does a PoE injector reduce speed?
Not if it matches the link. A gigabit injector passes gigabit data unchanged. It only limits speed when a faster device, such as a 2.5 Gbit/s access point, sits behind a gigabit injector.
Can a PoE injector damage a device?
A standard one will not. It only powers devices that present a valid PoE signature. A passive injector can damage a device that is not designed for its voltage.
Do I need a PoE splitter as well?
Only for non-PoE devices. A PoE camera or access point takes power straight from the cable. A splitter is needed only when the device has a DC jack and no PoE input.
Can I use an injector with a PoE switch?
Yes, on a non-PoE port. Feed the injector from any switch port. A non-PoE port is the cleaner choice, because it keeps one power source per link and makes the installation easier to maintain.
How far can a PoE injector send power?
Up to 100 m. The standard Ethernet limit applies to the run from the injector to the device.
We build PoE computers as standard, with the PoE ports and power budget specified for the cameras and devices they run, so injectors are rarely needed in the systems we supply.