On the barrel connector I accidentally reversed the input voltage to an Arduino Mega. The center post received ground while the outer connection received +12DVC, basically I have the Arduino -12VDC. I have since fixed this problem. When I power the Arduino with the USB I am able to program the Arduino without a problem. When I power the Arduino with the +12VDC the program still runs (confirmed by programming the blink sketch). If I have the +12VDC connected and try to program the Arduino then I get the timeout communication error.

Do I just need to re-burn the bootloader? The Arduino's symptoms match those listed here when both cables are connected: Arduino Mega timeout communication with programmer error

Is there anything else I should verify? Or is the Arduino always going to be damaged?

Also as a note I did the same thing to an Arduino Uno (same rail going to both) and the Uno has no problems when both USB and power are connected.

3 Answers 3


Don't reprogram the bootloader.

If you can upload sketches that is unnecessary.

It will probably be fiddly to identify and fix what has failed, but since you can reprogram it via USB it is still 99% working.

I suggest you get one of those USB charging wall-warts that provide 5 V, and just plug that into the USB port if you want to run it away from the PC. You know that the USB power part works.


Chances are you have blown up the circuitry that performs the switching between barrel jack and USB power sources. As a result the USB interface chip isn't getting power when it should.

In short: "It's dead, Jim!"

  • It's still functional as long as I unplug the power connector before programming. It seems like it should be a small fix if I can identify the issue. Commented Aug 25, 2015 at 21:05
  • There's an op-amp and a P-channel MOSFET. You may need to replace one or both of them. First though diagnose the problem with a DMM.
    – Majenko
    Commented Aug 25, 2015 at 21:13
  • Agreed. Trace the power with the schematic in one hand and a multimeter in the other. There's a reverse-polarity protection diode (D1) as well. 12 V shouldn't have blown that. Still, worth checking out.
    – Nick Gammon
    Commented Aug 25, 2015 at 21:16

The Mega has a reverse polarity protection diode (D1) on the board, so you plugging in the wrong polarity barrel didn't damage anything.

So your current problem in unrelated to this.

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