how does the Raspberry Pi PoE shield work?

Question

I would like to do PoE for some Pies, but I dont like the real-estate the PoE hat takes up. what exactly is required for the PoE to work? what pins must I use on the raspberry PI, and what does the PoE hat do? An electronic schematic would be nice. I saw the mechanical drawing of the PoE hat, its pretty vague.

my guess is that PoE comes in at ~48VDC, and that gets converted to +5VDC, with maybe some noise filtering. But is that all?

what are the 4 POE pins on the raspberry PI?

Answer 1

Here's a schematic you could use as a reference:

X2 is wired in so-called passive mode which uses the pinout of 802.3af mode B - DC plus on pins 4 and 5 and DC minus on 7 and 8. These are available on RPi PoE header as TR2_TAP and TR3_TAP respectively:

The supply is expected to provide power without any negotiation. The project page provides an example of such a power injector if you wonder how to build one:

Make sure you read and understand all the warnings regarding passive PoE injectors. In short, don't plug your PoE cable into sockets which don't need to be powered, as that may result in damage.

Also note that the MC34063A chip used in the schematic is rated at 1.5A output current, so you should use a beefier one (NCV891330?) if your target is to power an RPi 4.

Answer 2

what are the 4 POE pins on the raspberry PI?

They are the four cable side center taps off the Ethernet magjack.

what does the PoE hat do?

Broadly speaking it

1. Rectifies the incoming power from the four cable-side center taps with an extended bridge rectifier (8 diodes, probably Schottky rectifier types)
2. Presents the correct impedances during the PoE negotiation phase to turn the power source on.
3. Provides isolated conversion from the 48V PoE supply to the 5V needed for the raspberry pi.
4. Feeds that back into the Pi through the 5V pins on the GPIO header.