When I connected an external power supply to my Arduino Uno R3 (it was connected to PC via USB cable too), I heard Windows disconnected device sound and the device isn't visible since then (nothing happens when I plugin the USB). Cable is ok, because it works with the second Arduino. TX/RX LEDs don't blink anymore.

Following this guide, I checked two things:

  • I was able to flash ATMEGA328 with LED blinking program using external ICSP programmer and it worked
  • I was able to flash ATMEGA16U2 with the official firmware and it completed without any error

Is there anything else I can check? I'm not really desperate to fix it - I just want to know which component is broken.

  • Is it possible that the voltage on your external power supply exceeded the limits of the Uno's input? The range is 6-20V DC with (+) at the center. If you exceeded 20V or if the polarity was reversed the Uno may be damaged.
    – jwh20
    Dec 9, 2020 at 16:59
  • Does the power LED come on?
    – Majenko
    Dec 9, 2020 at 17:00
  • Power supply is 12V. Yes, the power LED is on (both with USB or power supply). The board itself works just fine. The only problem is with missing USB signal. Dec 9, 2020 at 17:22
  • trace usb lines on arduino board. could be dead usb-tty chip.
    – Abel
    May 4 at 11:21

1 Answer 1


Is it possible you connected the 12V to the 5 volt pin. This would probably blow the port on windows and destroy the arduino but it appears you got lucky with the arduino. Try something else not related to the Arduino stuff after powering down and restarting the windows computer. If that does not work the odds are high the Windows port is blown. If it now is working try again you may be all set.

  • 1
    USB port in PC is ok. I checked it with my 2nd Arduino. Dec 11, 2020 at 17:52
  • Good Job, I am glad the PC is OK, I am a Linux person so about the only thing I know about windows is I look through them. At this point without the board and proper tools I cannot be of any more help, good luck.
    – Gil
    Dec 12, 2020 at 0:17

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