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PoE scheme

Is PoE really that simple as the diagram above describes? I've heard there are some power negotiations, i.e. at first the voltage given isn't 48V but much lower value and then increases gradually to see if everything is OK and whether the device on the other end is adapted to use PoE. Although that seems reasonable, how exactly it works?

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Is PoE really that simple as the diagram above describes

Electrically, yes, it's just that simple. (in theory) The AC component (the ethernet signals) will cross the transformer without issue. The DC component won't affect anything, and can be collected from the taps, minus any resistance of the coil which is normally very low.

(Ron's link covers the finer details of signaling, etc. Wikipedia is a good reference as well.)

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  • Thank you for the answer, but one more thing bugs me now. It turned out I've got some passive PoE injectors (which allows them to work as splitters as well), so there's no negotiation. It's just an electrical thing. But if that doesn't destroy any non PoE devices connected to it, then why PoE negotiations even exist? Only for i.e. powered switches to check how much power PoE powered device needs and how many it should/can allocate? And another one, might be no-brainer actually: could standardized injector power passive splitters? – Gabrijel Šimunović Jan 16 '15 at 16:39
  • passive devices provide low, fixed power output. and there is a minor bit of "negotiation", but it's just simple resistor/capacitor to indicate the device wants to be powered. PLUS, as the diagram shows, any (low) DC component will be filtered/blocked by the coupling transformer. – Ricky Beam Jan 16 '15 at 22:43

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