Hello, I recently upgraded RB951G to hAP AC. But unpleasant problem showed up - when I connect some devices to USB - in my case LTE modem ZTE MF823, the hAP AC reboots. I have 2 units, both do the same. The same thing happens when I do USB power reset from terminal. I investigated more and it looks like the +5V power supply for whole RB is overloaded by charging some capacitors inside USB device (in my case the modem). And also I don't know the topology inside, but it looks like there is not much voltage tolerance for the +5V. I tried testing it more and I tried just 100uF ceramic capacitor (had it laying around, probably the issue will express even with smaller one) with 1k resistor (parallel to discharge) connected to USB will reboot whole RB! And it stays in boot-loop - because when it reboots it disconnects USB power, capacitor discharges and connects it after booting. And it causes it to reboot and again, again, again...With the modem insted my test-setup it just reboots, not end up in boot-loop like with my test setup. Probably because the modem will not discharge the internal capacitors down to 0V so it is softer on the power supply on next connection.
After connecting the USB load, the voltage on main 5V capacitor dropped about 0.4V and then some watchdog shut down the power supply and then restarted it again.
See attached image. USB connection caused the small drop right when the oscilloscope triggered, the rest is some shutdown watchdog followed by some off-delay and then startup.
Personally for me it was quite problem because I sometimes need to reset the LTE modem from script and doing this restarts whole RB!
I figured solution - I connected 2200uF capacitor (probably unnecessary big one, but I had it in my drawer ) parallel to +5V rail and now it works fine. Maybe your HW guys should look into it because the older RB951 worked fine.