Building a Power-over-Ethernet adapter

I received an Avaya VoIP phone for testing. The problem with it was that had no power adapter – it was exclusively PoE. I have no PoE switch, so I ran a search for whatever adapter I could find in the vicinity of the place where I live. The units could only be ordered, and hot-tempered as I am I decided to see if it would not be too hard to build one.

A few Internet sources (among them a Wikipedia article on PoE) suggested that connecting a 48V DC across two normally unused Ethernet pairs should do the trick. But where can one get 48 Volts? It’s not an easy thing to get around, as many parts don’t fly that high. For example the popular LM317 voltage regulator is limited to 37 Volts max. Luckily enough, I found a ready made power plug adapter that supplied 48V on a standard DC plug, in an electronics store nearby.

I also had a spare dual Ethernet wall-mount faceplate. I found a nice plastic case that should be able to hold all the stuff inside. Now what was left was to put it all together.

Parts for the PoE adapter

I figured out a fuse never hurts, since I had no idea how the phone will comply with my assumptions of how PoE was integrated into it. The black thing with red cables is the fuse case. I bore holes in the white plastic case for the DC jack, fuse holder and the Ethernet jacks and screwed on the faceplate onto it.

Plastic case for the adapter

I wired the jacks according to 802.3af Standard B, the contents of which I found on the Wikipedia page on PoE (nice table). I connected the data pairs (pins 1-2,3-6) straight-through, but left the “spare” (4-5,7-8) pairs unconnected on the “switch” jack. On the “phone” one I put the “+” from the DC jack (through the fuse) on 4-5 and “-” on 7-8.

Inside the PoE adapter

I made my last wish, closed the case and connected the phone and the power supply.

VoIP phone powered by my PoE adapter

It works!