It has to be admitted that we missed the reintroduction of the Pi's PoE hat. It was withdrawn due to an interesting design problem but now its back and it raises the question - how do you power your Pi?

If you are running a Pi 3B hanging off a wired network cable then you might consider powering it using Power over Ethernet. The basic idea is that it allows up to 13W of power to be delivered over the twisted pairs in an Ethernet cable without interfering with the transmission of data. The Pi has a suitable socket but it doesn't have the necessary hardware to do the power conversion from 48V to 5V. The problem is that it makes use of a flyback transformer to do the job. Put simply the conversion circuit was putting current spikes into the system which was upsetting the Pi. The product was withdrawn and a filter was added to smooth the supply. 

So all you have to do now is install the PoE hat and note it only works on the Pi 3 B+ because the connector has some extra pins it make use of. The hat costs $20 but you will also need a switch that supports PoE or you will need an injector to send the power along the cable and these vary in price from around $10 to $20.  If you don't want the ease of use of a PoE hat it is worth mentioning that there are PoE splitters that can be used instead and these cost around $10. An injector puts the power onto the cable and a splitter takes it off, usually at a number of different voltages, and you can use the 5V option to power the Pi.

More Information

https://www.raspberrypi.org/products/poe-hat/