I am currently working a robot project that is using an Arduino mega2560 in conjunction with a raspberry pi. The Arduino is connected to the raspberry through the USB. I am powering the whole system with a 14.4V LiPo battery. I had the 14.4V going on the Arduino Vin pin and had the raspberry powered on the a 5V wall adapter. However the raspberry apparently is back-powered through the USB, that means that even with the wall adapter unplugged the raspberry is still on, drawing its power from the Arduino USB. The first time I turned this setup on, the Arduino's 5V regulator overheated and had a thermal shutdown. After this I have not been able to detect the Arduino's on my computer.
Currently this is the state of the Arduino: The On LED still lights up. 5V and 3.3V Still have the correct voltages when powered trough the USB. I am 95% sure that the program on the arduino was the Analog Write Serial Example, but neither Rx nor Tx LEDs light up when the arduino is on.
I believe that the FTDI chip or something concerning the arduino's USB was damaged, howerver I am not sure why. Is it safe to back power through the arduinos USB? I also had the raspberry plugged on the 5V wall adapter, could that have caused a short? Should I be safe repeating the setup on another arduino but not having the raspbery plugged on the wall, and trusting the arduino back-power as the only source for the raspberry?