Arduino "ON" LED

Hi everyone !! I recently decided to hook up my Arduino Uno to 12V that is coming from a car's power distribution module. I measured the voltage and made sure that the car is outputting 12V. So the Uno turned on a few times that I connected it to the 12V power jack. I could tell it was on because the green LED near where it says "ON", turned on. However, after a few times it turned off, never to turn on again via the 12V power from the jack. Curiously enough, when I Immediately afterwards hooked it to my computer using USB, It turned on. But it wouldn't turn on from via the 12V form the jack. Two interesting pieces of information:

  1. I also noticed that the green LED light was slightly flickering when it was hooked to the jack, as opposed to when it is just on, when I hook it up via USB to my PC.

  2. After Uno wouldn't turn on using the jack, my PC wouldn't recognize the board's COM port either and I couldn't select it in the Arduino IDE. I tried restarting my PC, but that didn't fix it so I ended up uninstalling the COM driver from the device manager and reinstalling it. This ended up working.

I would appreciate any hints or ideas about what is happening.

It could be that the 5V voltage regulator burned out. You would still be able to power the board from the USB cable but not from the power jack or Vin pin.

Cheers!! :) Is there any way I could tell for sure that the fault is from the regulator?? Also, any idea what usually causes something like this?

DVM.

With a more stable source, measure the 5V pin on the power header. If you're not getting 5V, the regulator is likely bad, or the reverse polarity diode has been damaged. Both can be repaired with some careful soldering. What causes it is likely high voltage spikes on the 12V line.