I had my Arduino Uno connected to an external 12 V supply for 2-3 hours as I was working on other parts of my project. Suddenly the voltage from pin 9 which I was monitoring in the oscillator dropped down to 0 approximately.

I went on removing the external supply. The board was very very hot. I then connected the USB to my laptop and I got an error 43 (Windows did not allow the device to connect because it was reporting problems). I can't load any program of course. The 5 V pin works but it outputs a voltage lower than usual (at about 3.9 V). When I connect the external supply the 5 V pin outputs full voltage.

Any idea? Any fix?

  • Have you tried rebooting your laptop?
    – PhillyNJ
    Oct 16, 2014 at 19:50
  • yes of course...and reseting arduino as well...
    – Controller
    Oct 16, 2014 at 19:52
  • What is pin 9 of the oscillator? Oct 16, 2014 at 21:35
  • pin 9 is from arduino
    – Controller
    Oct 16, 2014 at 21:42
  • Is this a knock-off Arduino or Genuine Arduino?
    – PhillyNJ
    Oct 17, 2014 at 16:20

2 Answers 2


Something is shorted out.

When you connect via USB, the USB port cannot supply enough current because of the short. The output drops to 3.9V and the USB port on your computer reports an error, probably due to over current.

When you connect the external power supply, it has enough power to supply the short. The output remains at 5V.

You can think of power supplies as having a very small internal resistance (R_p), and the board as being a resistive load (R_b). This forms a voltage divider, but because R_b >> R_p the voltage seen by the board is basically the same as that of the power supply. With a short, R_b is often < R_p, and thus the voltage seen by the board is much lower than the power supply voltage.

The fix will depend on which component is getting hot. Plug in the external power supply and check which one it is. Note: The component will get hot enough to burn your finger after a few seconds. One way is to check one component at a time, disconnecting the power between each check. Start with the big ICs.

If either the Atmega328P or the smaller ATmega8u2 (?) are hot, they will have to be replaced.

Finally, boards don't short out by themselves. Something is wrong with your circuit or you may have placed the board down on some conductive surface. If you post you circuit we might be able to spot the problem.


I've been dealing with the same issue, and have resolved it in a manner similar to random approach described above.

Drivers will get corrupted as we all know. But if you want to get past that Code 43 error, you need to have the arduino unplugged from the USB when reinstalling. Otherwise windows will bind the USB.inf generic driver to the device, which prevents the install of the CH341SER driver.

So unplug USB, delete the unknown device via Device mgr, run the CH341SER, *then plug in the USB cable. So I think that random approach worked not because the 9v reset was done unplugged, but because that was first chance (unplugged) that the CH341SER driver had to install without resources bound to generic USB.inf driver.

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