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I have a wireless bridge that is USB powered in a remote location. I have a standard 802.3af PoE injector on one side of an ethernet run, and PoE splitter on the other side of the run. The POE splitter then provides USB power for the bridge. This has worked fine for ~3 years. Exact parts: injector, splitter.

This has stopped working, and I do not know how to diagnose if it's the splitter or injector that has failed.

I've used a USB meter to try to diagnose the problem and this is what I've found.

  • The bridge works fine when powered by a standard USB power bank. It draws 0.14A @ 5V.
  • When nothing is plugged into the splitter, it reads 5.05V 0A, which is within tolerance.
  • However when I plug the bridge into the splitter, the voltage sags immedietly going as low as 4.20V, and it shows 0A being drawn.

What are the next steps to diagnose this, or can you already tell which device is at fault?

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    It shouldn't be dipping that low. Most power adapters allow for a sag or over voltage. But it shouldn't be more than +.25v or -.25. (-4.75v ~ +5.25v) Jul 11, 2020 at 19:59

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The PoE detection phase uses a voltage of 3 to 10 V - you should be able to measure it between the pairs 1-2 and 3-6.

In order to measure the actual 48 V power supply, the detection and classification phases need to complete successfully, so you'd need another powered device (PD) or a PoE tester. The only other indication would be something getting unusually warm.

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