I have (had?) an Arduino Nano. I had it connected to the USB port and was working a charm.

I also had it connected a 12V battery pack to VIN and GND. I had done this several times without any problems, in fact, it had been running practically all day like this. I read that there's a diode on the board which selects the highest voltage in.

At one point I disconnected the pack and then reconnected. Not too sure what happened next (if LEDs went on or off, etc) since I was not expecting much to happen!

Point is, now the board wont start. Pin 13 LED permanently lights up while connected to USB, reset button does nothing and board appears to be generally dead! When the board is connected to 12V external power only the power LED lights and nothing else seems to work. IDE can see the Arduino in the port but when uploading gives "avrdude: stk500_recv(): programmer is not responding"

I did not change anything in my wiring, just disconnected and then reconnected the 12V external power source. So the 64k question: what happened?

I have ordered a couple more boards since I'm quite certain I fried this unit, but would love to understand what I may have done!

  • 1
    \$\begingroup\$ Maybe you some how erased the bootloader! And now can't upload sketches, that is weird I have had my usb and Vin connected at sametime NO problems! \$\endgroup\$
    – user50186
    Commented Jul 27, 2014 at 7:36

3 Answers 3


This answer has three parts:

  • What you described doing, and how that relates to elements of the Arduino design.
  • What you might have done, to cause the symptoms you describe
  • How you might start to narrow down the fault

What you described doing

I had connected a 12V battery pack to VIN and GND.

That is not a problem, that is the input to the inboard regulator so it's fine

enter image description here

I read that there's a diode on the board which selects the highest voltage in.

Yes, there is a Schottly MBR0520 diode between the USB supply and regulator output, the diode has a Vf of about 0.3v

enter image description here

At one point I disconnected the pack and then reconnected..
Point is, now the board wont start..

I'm not sure what to make of that, I don't see a reason for the board to get damaged

Arduino Nano schematic

What you might have inadvertently done

There's a list of ways to kill an Arduino which consists of

  • Method #1: Shorting I/O Pins to Ground
  • Method #2: Shorting I/O Pins to Each Other
  • Method #3: Apply Overvoltage to I/O Pins
  • Method #4: Apply External Vin Power Backwards
  • Method #5: Apply >5V to the 5V Connector Pin
  • Method #6: Apply >3.3V to the 3.3V Connector Pin
  • Method #7: Short Vin to GND
  • Method #8: Apply 5V External Power with Vin Load
  • Method #9: Apply >13V to Reset Pin
  • Method #10: Exceed Total Microcontroller Current

Number 4 and 7 are interesting candidates because, on the Nano, Vin and GND are adjacent.

If you have a Nano in a solderless breadboard, with wires connecting various IO pins to the breadboard, there is always the possibility of briefly accidentally touching a 12V power lead to some component leads that connect to an IO pin.

What you can do to narrow down the area of the fault

There are many components on the Nano board itself, by disconnecting everything else apart from power to Vin & GND you can use the schematic to work out what voltages or signals should be present at various points and use a multimeter to test whether the expected voltages or signals are present.

For example you can test the input and output pins of the 5V regulator. You'd expect to see 12V at the input and 5V at the output. If you don't get 5V at the output, you might suspect the regulator has failed. You could check it's datasheet and see what kind of protections it has.

You could also check the voltage at the reset pin of the ATmega chip. What happens when you press the reset button?

And so on.

In the end though, it isn't worth spending much time on this. Your aim is not to repair the Nano but to avoid frying another one. Reading about failure modes and taking care with connections is probably the best.

  • \$\begingroup\$ Is it possible the external power source must go in in a certain order? Ie: first the USB then the external source? \$\endgroup\$
    – user36092
    Commented Jan 25, 2014 at 1:37
  • \$\begingroup\$ @user36092 No, there is no order problem. Is it possible that you may have connected (even momentarily) the 12v battery to the 5v input? \$\endgroup\$
    – alexan_e
    Commented Jan 25, 2014 at 2:03
  • 2
    \$\begingroup\$ @user36092 You can check the regulator output to see if you get 5v output (from battery source). You can also check the diode, maybe it has been damaged. \$\endgroup\$
    – alexan_e
    Commented Jan 25, 2014 at 2:14

I have fried the shottky diode on two nanos now. I replaced them with a short.

  • \$\begingroup\$ Not the diode... checked it and its in good working order... The board has power and all pins show a reading... Im baffled! \$\endgroup\$
    – user36092
    Commented Jan 26, 2014 at 17:28
  • \$\begingroup\$ Better to replace the diodes with new diodes given that's what protected the mcu pins from a short. I've also let the smoke out of that diode a couple of times. \$\endgroup\$
    – patthoyts
    Commented Nov 17, 2015 at 12:38

If anything is connected to VIN besides the battery, I believe it becomes a load when the battery is disconnected. Therefore the USB acts like a supply and the voltage regulator on the nano can fry. The schematic I found prevents this if the barrel plug is used but not if soldered directly to the VIN pin. May vary case by case.

  • \$\begingroup\$ Good point! Many 3-terminal regulators are not protected against reverse current flow. Not sure about the 78M05 specifically -- the datasheet I found didn't mention it at all. \$\endgroup\$
    – Dave Tweed
    Commented Nov 27, 2016 at 13:13

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