Board design to include a Nano


I have designed a PC board to have a socket for a Nano and have a question of design.

I see that on pin 27 (+5V) is listed as an input/output pin & my design has a 12V input for running stand-alone (with an external +12 power brick) and an on-board 7805 +5V regulator for powering the board and all components. The +12V is also used for motor control as well.

Now the problem. When I have my board populated with the Nano and connected to my computer to verify functionality, I didn’t have the power brick plugged in since the USB connector was powering the Nano and board, but after initial testing, I wanted to test motor control and plugged in the +12V power as well. Didn’t take long for the +5V regulator on the Nano to get hot and smoke!! Nano still works OK as long as it is just powered by the USB cable, but won’t work stand-alone with the +12V brick.

It seems that while pin 27 is listed as input/output, it can’t have a computer USB cable plugged at the same time as +5V feeding pin 27. Do I have this correct?

I guess I need to modify my design with a 3 position jumper to make pin 27 an output (when the +12V power from my board is not connected (allowing the Nano to drive the +5V to my board), and then change the jumper to allow my on-board regulator to drive pin 27 as an input when the Nano is not connected to the USB cable.

Anyone see a better solution? What do other designs that have a socket for a Nano implement?

Thanks for responding!

Are you feeding the 12V to Vin on the Nano AND the +5V from your 7805 regulator to pin +5V at the same time?

That would likely produce the magic smoke!

You can have both the USB and a 5V supply and the Nano will select the higher (the USB is usually about 4.7V) using a Schottky diode.

On a design of mine I feed +12V to Vin and take 5V from the Nano to power the rest of my board. I've not had any problems with having the 12V and USB connected at the same time.

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