I have a CRS125-24G-1S that was previously powered fine via PoE from various PoE providers and DC input jacks over the years. Yesterday I was switching out the cable for the PoE and it powered up briefly and then immediately powered down.
I thought that I had killed the device, but then confirmed that it does power up if I use my PoE-to-DC barrel jack adapter cable through the back DC w/a UniFi PoE injector. I used a testing tool to see what the PoE looked like from the UniFi PoE device and it was a 45/78 at approx 23.5V. When I used the same testing tool on my PoE switch, it appeared to be ~25.5V, on the same pins.
Why would the CRS power up with <24V but not with >24V? The PoE and DC barrel jack inputs on the CRS are confirmed to work at ~23.5V. When feeding it PoE power with exactly the same pinout and slightly higher voltage, there’s no life in the CRS at all (no blue LED).
Does this suggest that part of the power regulation circuit is busted?
What supplied the PoE for the device? The CRS125 is a passive PoE in device so it might be that the device on the other end is active and didnt like being connected to the passive device (dont think this is the case since the barrel adaptor worked) or the PoE out lines might not be right? Are you able to check and see if it is offering power in mode B configuration output on pins 4-5 and return on 7-8?