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| SiliconWizard:
Would some of you guys happen to know how much power can be drawn from the port of a typical Ethernet switch (not meant for PoE)? I'm typically looking to draw under 100mW, and not requiring any additional PS with the possibility of plugging to any switch would be great. I haven't really studied PoE in details yet, so maybe the obvious answer is: no problem. Or maybe: forget about it. Any thoughts? |
| xani:
From electric point of view: Ethernet is +/- 2.5V into 100 ohm load, but isolated via transformer so no DC without PoE, and to get AC you'd have to have something to send traffic to that port. And there is a good chance switch will just interpret it as link down, or as error and dont do anything on that port. From system administrator point of view: If you connect a POS that tries to do that in MY network, I will find you, and I will kill you. :-DD |
| MagicSmoker:
You can't draw power from the active data pairs because that will upset the impedance and you can't inject DC onto them, either, because the transformer at either end will just short it out. I suppose you might be able to use the Tx pair for (+) and the Rx pair for (-) but that sounds really sketchy and, again, you have to be really careful not to upset the termination impedance at either end. It's best to use one or both of the normally unused pairs in a CAT5/6 cable but then you are looking at a proper PoE solution... |
| Someone:
--- Quote from: MagicSmoker on October 11, 2019, 02:31:03 pm ---It's best to use one or both of the normally unused pairs in a CAT5/6 cable but then you are looking at a proper PoE solution... --- End quote --- Except the base PoE standards have a huge hole, they require a minimum power draw which eliminates lots of interesting low power "edge" nodes. Not sure if that was addressed in the 802.3bt update. |
| NiHaoMike:
--- Quote from: Someone on October 11, 2019, 10:30:18 pm ---Except the base PoE standards have a huge hole, they require a minimum power draw which eliminates lots of interesting low power "edge" nodes. Not sure if that was addressed in the 802.3bt update. --- End quote --- The cheap PoE adapters solve that by adding a load resistor, the somewhat better ones automatically disconnect the resistor if the load draws enough power by itself. I suppose a supercap could be a way to make a lossless solution. |
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