I was testing my foldable quad. It was powered by the battery, but also connected (via USB) to my computer.
I noticed that my computer was rebooting - and smoke was coming from my flight controller!
I shut everything off and detected that horrible "something is terribly wrong" burning smell coming from the flight controller.
I disconnected all wires and did a short check. The battery (+) lead (16V) was shorted to the 5V power feed to the controller! The little "buck" regulator on the power module - which measures battery voltage and current and also provides 5V to the controller, had failed and given the system 16V instead of 5.
The controller is supposed to be protected against such things, but after inspecting the controller carefully, I found that the protection diode had not even been installed! Note to self - do not buy cheap Chinese-made boards. But wait.... I HAVE to buy cheap Chinese made boards, since that is the only place they are made. But next time, I'll inspect the PCB before I install the replacement. If there is no protection diode (Zener), I'll add one.
So, I inspected the new flight controller (and added the Zener) and installed it. It seemed to work, but the compass and GPS did not. I checked the GPS/compass module and - sure enough, it was "blown" as well. After I took off the cover, I found a black spot on the board where the voltage regulator normally sits.
So, I replaced the GPS/compass unit and re-applied power. But the flight controller STILL did not recognize the compass!. After doing some testing, I found out that when the original GPS/compass unit was destroyed by the over-voltage, it shorted and fed a solid 5V to the data lines going back to the controller. When I put in the new controller, the old GPS/compass unit had fed power to the data lines of the NEW controller, destroying it!
Next note to self: Check all components in the system individually after a failure. Don't just replace the part you feel is defective.
So, I replaced the flight controller again (this was getting expensive), hooked up the new GPS/compass and that all worked. Great!
But the camera gimbal didn't After checking, I found that the 16V applied to the 5V control line of the camera gimbal controller had destroyed that!
What a mess! Fortunately, I had a few other devices on-board that were powered separately (GoPro, Raspberry pi, 4G LTE dongle).
Final note to self:
If you run redundant power to your controller (which I always do), diode OR them with Schottky's.
Use hefty 5.6V Zener or 5V TransZorbs on every 5V powered device. Flight controller, GPS/compass, radios./gimbal controller.
Add 3A fuses in line with the flight controller and gimbal.