I have fried three Pixhawk Mini's and I still have no clue why.

I am using the autopilot on a 3D foamy fixed-wing. The wiring setup is like this, except that:

- the 8 channel PWM board is also powering the aileron, elevator, and rudder servos (the rail is powered by the motor's ESC)
- I have a 5.6V Zener Diode attached to the PWM board
- the power distribution board is also powering an ODROID XU4 (first going through a voltage regulator)
- there is no telemetry dongle, instead, the TELEM port is used for communication between the ODROID and Pixhawk Mini. The communication wiring I use is described here. I am only using the TX/RX/GND wires and the communication works.

The problem never happens in flight, it is always while the aircraft is on the ground, armed. The first time I encountered a problem, the control surface servos started flicking back and forth and then going full deflection. One of their circuits burned and started smoking. I keep encountering a similar problem: the control surfaces started flicking, I quickly unplug the battery, and then the Pixhawk Mini no longer functions properly. On one of them, I can see that the TI 6C 2115 part (a voltage regulator, I think?) is burnt (see image where the component is circled in red). The other two Pixhawk Minis are also dysfunctional. I can't see signs of burning but I smelled it at the time; I quickly unplugged the battery when the control surfaces started going wacky. I have tried replacing the power distribution board, Pixhawk Mini, ESC, and GPS. I have put an oscilloscope to basically every wire going in and out of the Mini and have yet to detect any issues.

Anyone have any idea why this is happening?

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That's a weird one. I'm guessing you've got a power loop going from the Odroid to the Pixhawk Mini, possibly because you've left the V+ cable connected on the serial cable connecting them? It's always a risk when you've got two systems with two separate power supplies and you connect more than their grounds. 

That was one of my first thoughts, but no, I definitely am not using the VCC wire, just the GND, TX and RX connections. 

To be clear, btw, the two systems have the same power supply, the LiPo battery (11.1V, 3S). The Pixhawk and the ODROID are both connected to it via the power distribution board that came with the Pixhawk Mini (the ODROID through the Q-BEC voltage regulator).

If you have any other ideas I'm all ears. Thanks :)

Chris Anderson said:

That's a weird one. I'm guessing you've got a power loop going from the Odroid to the Pixhawk Mini, possibly because you've left the V+ cable connected on the serial cable connecting them? It's always a risk when you've got two systems with two separate power supplies and you connect more than their grounds. 

The fact that it burned up a servo first is a big clue, since the power for them is just coming from your ESC. Have you measured the voltage coming off that BEC?

Yes, I've measured it with an oscilloscope and not seen any irregularities. I have also added a Zener diode to the servo rail to restrict the voltage to less than 5.6V. And I've used two different ESC's (same model, though) and had the same failure.

What's also curious to me is that this issue doesn't happen immediately upon power up. I have always flown multiple times before, all of a sudden, the servos are wagging and the Pixhawk (or servo) is beginning to burn.

Chris Anderson said:

The fact that it burned up a servo first is a big clue, since the power for them is just coming from your ESC. Have you measured the voltage coming off that BEC?

thank you, i like your 3D foamy fixed-wing Airplane

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