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):