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Power over Ethernet (PoE) basics and beyond

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257 points voxadam | 2 comments | 16 Oct 25 14:09 UTC | HN request time: 0.6s | source
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rmunn ◴[22 Oct 25 01:24 UTC] No.45663921[source]▶
Practical question for HN: How do you all label your PoE cables so that you don't accidentally plug the powered cable into a socket that wasn't expecting 48 volts on those pins and fry something? (I know the power injector is supposed to only deliver power when it's safe, but if all your devices work exactly as they should all the time, then I'd like to buy that bridge in Manhattan you're selling).

Do you buy Ethernet cables of different colors and say "Yellow is reserved for PoE, all yellow cables should be assumed to have power on them"? Or do you slap a "48V" label on both ends of the cables you're going to use for PoE and the label is what warns you that this cable should only go into the PoE receiver, and not into an unpowered device? Or do you just not label your PoE cables any differently, and trust that the injector will never malfunction at the same time that you plug the PoE cable into the wrong device?

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1. generuso ◴[22 Oct 25 03:13 UTC] No.45664548[source]▶
From what I have seen, Ethernet ports always have a small isolation transformer for each twisted pair, between the connector and the PHY. Usually four of such transformers are combined in one small magnetics package. The insulation in the transformer is specified to withstand over a kilovolt of lightning induced voltage -- that's one of the purposes of such galvanic isolation.

The data travels as the differential voltage in each of the twisted pairs, and is transmitted magnetically by the transformer to the secondary winding. The power is applied between different pairs, and in each pair appears as a common mode voltage. This is all stopped by the transformer, and in devices designed to support PoE, the PoE circuits tap the mid-point of the primary windings to access the supplied voltage.

So at a first glance, it seems that if 48 volts is applied between the twisted pairs to a non-PoE device, this voltage would simply be blocked by the transformer. But since there is a widespread concern about this, there must be more to the story -- maybe somebody who actually worked with these circuits can explain why this is more complicated than it seems at first?

Edit: Found an answer. It seems that at least some of the designs of non-PoE Ethernet jacks terminate the common mode signals to a common ground though 75 Ohm resistors. In this case, if the voltage were applied between the twisted pairs, the resistors would dissipate far too much power and would burn out. So there is definitely a concern with the dumb PoE injectors and at least some non-PoE devices. https://electronics.stackexchange.com/questions/459169/how-c...

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2. sodaclean ◴[22 Oct 25 18:05 UTC] No.45672905[source]▶
Never noticed the 75ohms before- but it'd be 150ohms for passive PoE. (Through one pair and to another, two 75's)

Theres fixes, but passive PoE was a pretty dirty hack- so negotiation got added.