←back to thread

257 points voxadam | 2 comments | | HN request time: 0.418s | source
1. Animats ◴[] No.45664741[source]
Not clear on how multiple sources on the same cable work. Is that allowed? Is there a power break at midspan, or does power flow through? How are the regulators coordinated?
replies(1): >>45665449 #
2. eqvinox ◴[] No.45665449[source]
There are never multiple sources; a midspan PoE injector breaks the power flow and injects only its own power supply.

It's done through the center tap on the Ethernet transformers. Midspans have another set of those transformers and inject the power on the PoE PD facing side. Whole pair(s) carry common-mode DC current, so basically your green pair could be ±48V and the orange pair 0V. If any, the upstream switch's injected power would just end at the other coil of the Ethernet transformer in the midspan. However, the midspan also doesn't pass through PoE negotiation, so the switch won't turn on power to begin with.