I am trying to build a weather station, powered using Power over Ethernet.
Originally I planned to use a Nanode as the microcontroller, but since the Raspberry Pi is so cheap, I decided it was much easier to use that instead.
I bought a TP-Link power over Ethernet splitter, made up some custom cables and after checking the voltages, connected it up to the Pi. All worked fine, until I tried plugging in the HDMI cable to my TV (for debugging). At which point the board kept turning off and then the switch cut out.
At that point I discovered that there was a 40 volt difference between -ve on the PoE output and the ground on the HDMI. I tried a couple of other PoE boards and found there was the same issue on them too.
Is there a solution? Although I don't really care about plugging it into my TV, I am a bit worried about plugging sensors in, that might be grounded and end up putting big voltages across them.
There is a photo here (click for full-size):
Best Answer
TP-link power splitter I managed to find seems to be non-isolated. Bad thing for you is that PoE usually have diode bridges on input (to be polarity-independent and work on straight or X-cables) meaning that if your 48VDC supply is not galvanically isolated and for instance it's minus terminal is grounded, sparks can fly if you miss to crimp ethernet cable right way. Try to swap 4,5 and 7,8 pairs, measure ground potential difference again.
Of course, for careless PoE usage best solution is to use isolated splitter.