The purpose of such a variable power over Ethernet supply would be to test network equipment that are receiving power over PoE under condition of conditions of low voltage or current and determining the behavior or the equipment.

I know that TIA/EIA-568 mode B uses DC+ on blue and DC- on brown wires - the property I would like to use for my testing.


  • Is the TIA 568 mode B a property of the network equipment? If so - how do I find out if my device is mode B or mode A?
  • Is connecting a VPS to the spares of mode B wire a possible solution to the problem? Any remarks would be helpful.
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer.
    – Ron Maupin
    Commented Dec 25, 2018 at 9:31

1 Answer 1


The PSE decides on which pairs power is supplied. A compliant PD needs to be able to use either pairs.

Connecting a power supply to the spare pairs (only possible with 10BASE-T and 100BASE-TX) is not compatible with IEEE PoE. While most devices would probably work, some won't, and some may even break.

My advice is to use a proper PoE injector and modify it so you can lower the output voltage. While you're at it, get a 1000BASE-T compatible one.

The easiest way would be to disconnect one of the middle contacts from the PD-side magnetics and reconnect with a variable resistor in between (50 Ohm or so) - make sure the poti can handle at least 5 W, better 10 W. You'll likely have to turn the resistor all the way down at first to make the device MDI signature work.

(Active) PoE requires the magnetics/transformers to have center contacts where you can inject or extract power without messing with the data signal which is essentially floating. If you add an in-line resistor on the center contact, you divide the PSE voltage between the PD (plus cable) and the resistor. Put another way, the resistor limits the current which causes the PD voltage to drop.

Check out Figure 1.2 from here where you can see the center tap on the transformers where VA1 and VA2 are connected. enter image description here

For better voltage control, you can try using a programmable low-drop voltage regulator instead of the poti - the injector might likely fail to detect the signature this way, and the setup won't work.

Details to 802.3 PoE ("DTE Power via MDI") can be found in IEEE 802.3 Clause 33.

  • Thanks a lot, it would be great if I could make something like this, because it would be somewhat reusable. I check for PoE injector teardown pictures to better understand the part where you say: "disconnect one of the middle contacts from the PD-side magnetics and reconnect with a variable resistor" - could you, a person with much more experience with electronics, add a small visual example (screenshot or picture) of the part you think I should modify? Obviously I will do much more reading and some practice before attempting this - but it just gets more and more interesting :) Commented Sep 17, 2018 at 6:48
  • I've added details to the answer.
    – Zac67
    Commented Sep 17, 2018 at 9:33

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