Recently [The Parallel Port] was asked to take a look at repairing the PoE++ feature on a Ubiquiti switch that otherwise worked fine. This is a pretty nice 24-port rackmounted switch with 2.5 Gbit-capable RJ-45 ports and two 10 Gbit SFP+ ports, so by itself it’s pretty useful, but having the 400 Watt PoE feature just go AWOL still stings, especially if you bought it new for $800.

With power applied the switch starts up as normal, including its 1.3″ touch screen that provides direct port information as well as a fancy screensaver cum QR code for the AR feature.

After logging into the switch’s BusyBox console it showed that all ports reported bad for the PwrGood status, and there were PoE power status request errors in the log, but this could be just the consequence of something else. Using a PoE splitter it was confirmed that the PoE functionality was indeed dead.

After disassembly and some testing with a multimeter and thermal camera, a short and related hot spot on the PCB that the PoE power board connects to was identified, with the short persisting after removing this PCB from the switch. Since the switch had suffered a bit of an electrical event the TVS diode was checked, but it turned out to be fine.

Next to it, marked as fuses, were protective thyristor surge protection devices (Trisil), functioning as a crowbar device. These aren’t supposed to be shorted to ground until a surge event occurs, but these were indeed both shorted when measured directly. Clearly they had taken the brunt of the electrical event and sacrificed themselves in the process.

One replacement later of these devices the switch now happily reports a good PoE status and even was able to power a DC fan via the PoE splitter. Even if it wasn’t a particularly hard fix, this is definitely one of those cases where knowing how the protective circuit works can save a lot of time in diagnosing and fixing a fault.