Hi, I would like to use passive PoE (Power Over Ethernet) on non data wires (4,5 and 7,8).
I look at Arduino YUN which support active PoE (IEEE 802.3af). They use PoE Module which extract power from data wires (802.3af).
I need only passive PoE on unused wires. Is possible to connect wires 4,5 and 7,8 directly to 7805 (or any other) voltage regulator?
Is it safe to connect +5V from USB to ATmega32u4?
Yes, if you're careful. For carefully controlled home projects, it's no biggie. If you're going to deploy this anywhere, you need to be far more careful.
You'll want to spend some time thinking about fault protection either way though. If you transmit +12-48v on pins 4/5, and Gnd on 7/8, what happens if 4/5 short to 7/8? What happens if the cable mixes up the pairs so 4/7 are +12v and 5/8 are Gnd? What about if the 4/5 is 0v and 7/8 is +12v? (Don't underestimate how many patch cables and patch panels are improperly wired out there...)
Count on voltage drop, and don't pull too much current. The PoE spec calls for something like 10-15W. That's not much, use it wisely. You might want to use a switching regulator so your line voltage can be up to 24 or 48v. Higher voltage = lower current.
This might be obvious, but the schematic you attached won't work for this.
You could do a 10BaseT Ethernet connection, and then use the other four wires for your own power setup. I'd probably run 12v on the power wires and a 7805 regulator chip at the board to supply the board directly with 5v.
