Cheap passive PoE injectors probably use pairs (4,5) and (7,8) exclusively for power, and disconnect them from data, so probably will only work at 100 Mbps.
I bought some cheap PoE injector/splitters from ebay. They claim gigabit speed, and a quick test shows they negotiate at that.
Pins 1,2,3,6 are directly connected through. Pins 4,5,7,8 seems to be connected with a serial DC blocking cap of what my multimeter says is ~93nF.
Cheap component testers says:
It seems to be a bias T! I was expecting transformers with central tap, as is used on the ends normally.
I thought 12V would be enough for low power stuff / short wires, but with the Pi in a box even 10 meters is pushing it at 12V.
The Pi crashed unpleasantly, causing file system corruption, when running with 12V into a high resistance cable of 2.5R per wire.
I will install box 0 with a 20V supply I found.
pi draws up to 3a in normal non PD mode at 5V - 15W. Heltec board up to about 2.5W. Other bits maybe max 0.5W. 18W needed. 18W @ 12V about 1.5A + conversion loss, say 1.7A. 12V supply is 3A rated. Converter is fine at 3A (tested). CAT5e I have is 24 AWG. 24AWG copper: 88 Ohm / km For 10m: 0.88 Ohm For a pair: 0.44 Ohm / 10m. For a pair including return: 0.88 Ohm. It's more though, because of twisting, copper is longer than cable length. Say 1 Ohm per 10 meters. It might be more actually. Max R (including both sides) 1.75 Ohm. 20m is therefore too long for this cable / voltage. With 20V, in max acceptable R is about 4.48R. So then 40m is ok. Apparently Cat 6 is usually AWG 23.
Four pins, each connects to centre tap of each pair's transformer.
Only needs two pairs for 100 Mbps and power.