Blog de AlterMundi

Lab Hacks

DIY PoE-enabled gigabit router

Doing Power-over-Ethernet on a Gigabit port might seem challenging at first sight: since all 4 pairs of UTP are used as data lines, there are no “spare” pairs to use exclusively for sending DC current, as in 100mbit ethernet cables.
However, it’s possible to inject (and extract) DC voltage on the data pairs, using center-tapped transformers, preserving the signal transmission intact and the gigabit speeds of the port.


All ethernet ports are connected to center-tapped transformers, to isolate the noisy cable lines from the very sensitive integrated circuits of the ethernet controller chipset.
The interesting thing is that the center-tapped transformers used in gigabit ports can be used to extract phantom power from the ethernet data lines.

The Ubiquiti AirFiber 24Ghz uses the following standard:

(+) = Pair 1/2 & 4/5 = Orange & Blue pairs
(-) = Pair 3/6 & 7/8 = Green & Brown pairs

Last modification date: Feb. 13, 2013, 9:36 p.m.
There are 3 comments
  • June 15, 2016, 4:44 a.m. - By: chester - (permalink)

    I had the same idea as you and am modifying a TL-SG1008D 8 port gigabit network switch to be a gigabit POE switch.

    It actually has the same exact isolation transformers as your router, the HST-48001S.

    I am a little worried that the transformers designed for POE use are somehow slightly different from the normal non-POE transformers, pulse does list different types..

    Did yours end up working?

    • Aug. 2, 2016, 7:45 p.m. - By: Gui - (permalink)

      Yes, it did work; in fact, in the picture you can see the blue leds lighting up :) This was left running "in production" (at the local community network) for several months, without issues. It finally burned out due to a lightning strike (very frequent in the area). But many other routers were fried as well, so it wasn't related to this modification - it was just "force majeure".

      The only problem, in my experience, with this mod, is that these gigabit transformers (being surface-mounted) have much more delicate legs, and it's very easy to break them during the procedure of lifting them up to desolder them from the board. It actually took me a couple of tries (but no problem, I had 5 chances/ports :P). The broken legs did not hinder the performance of the corresponding data port, it just prevented from successfully completing the PoE mod on that port.

    • Aug. 2, 2016, 8:43 p.m. - By: Gui - (permalink)

      I was just trying to post a comment back on your blog, but couldn't manage to do it :(
      finding no other way to contact you, here you go:

      Hey, Gui from AlterMundi here, pinging back :)
      Regarding this: "My first attempt at using the SG48001G (equivalent to HST-48001S) ended in failure but was most likely caused by one of these other issues: the fact that I first tried to wire ALL the POE pairs to power, not just the two that are required by Ubiquiti gear"

      I'd say it was definitely wiring ALL the POE pairs, since normally the ubiquiti devices have the center-taps of the pairs 1,2 and 3,6 grounded together (since they are not expected to receive useful power, instead are trying to filter out induced currents back to ground), so this effectively short-circuits your + and - .
      Ubiquiti higher power equipment, like the AirFibers, does use all the pairs for power (and, consequently, doesn't ground center-tap in pairs 1,2 / 3,6)

I want to be notified by email when there is a new comment.