DIY Power over Ethernet (PoE) Injectors



Well, for a long time I have wanted to set up an AP to be on my roof. The AP in the house works fine, but if I am outside in the garden (or in front of my place in one of the cafes) during hot summer months, I can't get a wifi signal. As such I decided to place one of my AP's on the roof, just for those occasions.

Well, my "AP" as such is an old laptop running linux and hostap. Yes I know that it's not the most efficient thing to use, but it was free. Anyway, PoE is actually an official standard, namely IEEE 802.3af. We will be using the same cabling as the standard, but different voltages. This means that our PoE setup will not be compatible with official PoE equipment, but as this is just a single point-to-point link, it's not a big deal. What we will do is create two injectors, so that is looks a bit like this:

| Network |-------.  
 ---------        | 
 ---------      --------------               --------------
|  PSU    |----| PoE Injector |====[20m]====| PoE Injector |
 ---------      --------------               --------------
                                               |        |
                                               |        |
                                            | Access Point |

The Network signals and the PSU power gets merged into a single CAT5e cable, via a PoE injector (what we will be making here) and this is sent along the cable for a certain length (I said 20 metres, but that was just an example, I don't know how much I will need yet). Once it reaches the AP, another PoE Injector will split the power and network signals, and send them to the AP as seperate lines.

The Injectors

For the injectors, I'm going to modify normal ethernet extenders, which are two female ethernet sockets wired together, like so:


First step is to dissasemble the extender. Upon doing this I find out that they were nice enough to number the lines for me. We shall be using pins 4-5 for positive and 7-8 as negative (as specified in the standard). 


Here are the two extenders after modification. In addition to connecting power to pins 4/5 & 7/8, I cut the same lines on the other side,  to make sure no power gets into the switch and network card. For the power I decided on a thinkpad adapter (because I have loads of them kicking around from laptops I fixed/scrapped). It provides 19V@4A  for a total of 76 Watts of power.

Because I cut the lines on one side of the extenders, I will have to label them so as to know which side provides the PoE output and which is plain ethernet. I reassembled the extenders and labeled each side accordingly. The final result can be seen here:

Now one of my worries were that of transmission losses. Cat5 cable is not very thick, which means that is has quite a high resistance. As power dissipated = I2R, the larger the current the more power is lost during transmission per metre, mostly as heat. In order to minimise losses the standard specifies a higher voltage (~50V) at low current (350mA). Unfortunately our hardware doesn't support 50V, so we are stuck with the 19V line with a higher current.

The laptop says it will draw 3.19A of power, but manufacturers always specify maximum power draw. This means that the laptop will probably only ever draw 3.19A of power when it's running at full tilt and charging the battery. In it's current role it doesn't have a battery (as it is shot) and it's load is always low, so it will probably never draw this much power.

Nether the less I'm keeping watch on how hot the cable gets (scientific I know, but my multimeter needs a new battery) and so far it only got a little warm during inital bootup. I'm going to continue to keep watch. I am using a 10m cable to reach up to my roof. Here is the laptop running via PoE and testing the network:


 So far so good! The network works and the laptop gets power with no issues. Next job is to waterproof it and stick it on the roof.