Power Over Ethernet (PoE) Introduces It’s Own Problems.
To run data and power down a network cable you need to use a PoE injector. Often implemented when you need to power wireless routers throughout a building but don’t want the cost of putting a 13amp socket next to each one as well. At the CDC Weather Station we hit a problem with distance. Running a DC current down a 90 meter cable using a PoE injector will get some voltage to the end, but the drop off due to resistants in the cable is significant. Just like watering the garden; the longer the hose the more the water trickles out at the other end.
Using the voltage drop calculator below we determined we’d need to send 24 volts down the cable to guarantee a decent output at the other end. We were potentially facing a 50% drop in voltage. The Raspberry Pi only needs 5v consistently, no computer likes a varying current. As other factors such as kinks in the cable and change in temperature could play a part we’ve given ourselves some headroom so 24v should get us to around 12v, which in reality is probably going to be more like 7v or 8v.