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I noticed, that there is no clear notification about PoE voltage. For example, I bought TP-Link WA901ND WiFi AP, which is entitled as supporting PoE. There is no clear statement, on which voltage this PoE should be. Simultaneously, this AP is packaged with 12V adapter and PoE adapter, which allows to feed power into the wire.

It works, but what will happen, if I plug this AP into switch also supportin PoE like D-Link DGS-1008P? There is also no clear statement, which voltage it passes to the line.

In wikipedia I see it can be more than 40V!

What will happen if I plug WA901ND into DGS-1008P? It will just burn?

How to avoid this?

Or PoE can adapt voltage automatically somehow?

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    PoE is typically 48v, and it is a "negotiated" service. When a PoE device connects to a switch, it does handshaking with the switch on it's initial connection to start the PoE power. In normal PoE equipment there is no danger in connecting a non-PoE device to any PoE port. Wikipedia has a some nice info on how PoE works en.wikipedia.org/wiki/Power_over_Ethernet Basically, if your device supports standard PoE, it will tell the injector/switch what power it needs and the switch/injector will provide it to the device. There is no danger to your device.
    – acejavelin
    Jul 27, 2016 at 11:23
  • I think it's important to distinguish active Poe and passive Poe, active poe corresponds to the 802.3* standard and is usually expensive equipment that adapts itself between 12v and 48v by negotiation. Passive is the cheap Chinese version which just injects 12v into a pair directly meaning slower speeds and can damage equipment
    – Tofandel
    Mar 30 at 21:40

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POE can be anything up to 48 volts. While there are some standards (802.3af), there are many systems which run lower voltages - 12 volts is common.

802.3af can push out about 15 watts - which is quite a lot. Generally the higher the voltage the higher the amount of watts which can be pushed through a cable. (802.3at - which I know little about can push about 25 watts). A lot of systems will simply inject 12 volts on the unused pairs, and use something to break it out the other side - this is still considered POE - even though it does not meet the 802.3aX standard.

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POE treats each pair as a single conductor, and can use either the two data pairs or the two spare pairs to carry electrical current. Power over Ethernet is injected onto the cable at a voltage between 44 and 57 volts DC, and typically 48 volts is used.

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The amount of power contained in PoE is the amount of power injected by the PoE power supply.

A quick look at the manual for your device shows that it comes with a "power injector" which is how the voltage from your AP's own power supply gets into the Ethernet cable.

Another way to understand, in the case of slower Ethernet (under gigabit speeds), cables use only 2 of the 4 pairs of wires in the cable. The extra 2 pairs can be used by POE enabled devices like your AP. Ethernet itself doesn't provide that voltage tho, the PoE power injector does, it's simply using unused pairs in the cable. The power supply plugged into the injector is what determines the voltage your AP is receiving.

In the case of gigabit and faster, the method becomes more technical because the data and power do travel over the same pairs in the cable. Again however Ethernet is not supplying the power, it is still injected by a power injector or PoE power supply. For further explanation related to gigabit Ethernet, I would suggest this thread from Cisco.

The short answer to your question tho, is voltage is determined by what's injected for a specific device, its not determined by an Ethernet spec.

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  • What about PoE over Gigabit Ethernet?
    – u1686_grawity
    Jul 26, 2016 at 13:15
  • @grawity answer edited
    – Tyson
    Jul 26, 2016 at 15:58
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Electric voltages can be adapted, reduced and increased. That's how the 12V DC adapter turns 230V AC or 110V AC into 12V DC.

Similarly, this is how PoE-compatible devices (like your AP) will reduce the 37~57V of PoE to 12V, 5V or 3.3V, depending on the voltages they need for their internal components (some ICs can't bear voltages above 1.8V, yet you feed 230V or 110V in your PC everyday).

You AP being labeled PoE-compatible, it should be equipped with an adequate voltage-adapting system. If it's not and your AP burns up, your vendor has been false-advertising.

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    That's not always true... just writing "PoE" doesn't tell one whether it uses 802.3af or 802.3at (negotiates ~50V automatically) or nonstandard "passive PoE" (always receives 15V or 24V).
    – u1686_grawity
    Jul 26, 2016 at 13:14

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