While going through my electronic project folders, I came across one of my past project, the gigabit power over ethernet (PoE) injector that started some time in May 2012. The motivation for this project was when I wanted to add a second WiFi access point in order to expand wireless coverage for the whole house.
There were limitations or requirements for the task, firstly was the location only had an RJ45 termination nearby and the nearest power socket was at least 5 meters away. Secondly the access point that I would be using is an 802.11n rated 450 mbps transfer speed. In order to fully utilized it, the uplink would have to be gigabit ethernet.
I did a proof of concept that any mikroTik device with passive PoE capabilities could be modified into an 802.3af standard active PoE.
I converted my 250GS managed gigabit switch that I had lying around. The current passive PoE design basically have all the necessary parts to make this possible.
1) Center tap magnetics, bridge rectifiers on all 4 pairs.
2) Reverse polarity protection diode on DC input. Prevents back-feed to the DC input when both DC and PoE is present, provided that the DC input voltage was lesser than the PoE.
For the proof of concept setup, the mikroTik switch will act as the power sourcing equipment (PSE) using the Linear Technology LTC4263 PSE controller. I'd tested numerous powered device (PD) controller and found that LTC4257-1 were the easiest to implement.
PoE negotiation and powering up works properly. The switch detected the link at gigabit rate and throughput test showed no problems.
Seeing how everything went well, I'd decided to modified the mikroTik switch into a permanent PoE PD. The PD controller and rectification portion was built on a strip board since it was a fairly simple design and the DC-DC buck converter was built on a custom etched PCB.
Casing and PCB were built for the PoE injector and splitter.
The completed pair.
Power efficiency and bill of materials cost just for reference.
Links and reference:
PD & DC-DC Schematic
PD & DC-DC Board