Custom board for Nano


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

I see that on Nano board, 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 if needed.

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 stepper 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 the rest of my board iuf needed), 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. Do I even need to drive pin 27 with +5 V?

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

Thanks for responding!

It would be better if you'd provide a schematic, but from what I understand it could be enough to have a ferrite bead before the pin, between the cable and the pin, or stick a ferrite bead on the cable itself; or use a cable that has already a ferrite bead on it (the ones that have a thicker node at one of the ends or both).

Hope it helps

Attached is a schematic of my board. Nothing special here. I have 3 motor/stepper driver sockets for A4988 type motor driver board, connection pins for A0-A7 signals which have Vcc and ground along with the signal (for Servo type cables), and room for 3 different type of connectors for the motors/steppers (Screw type, as well as socket & pin connectors). The only "Interesting part of the design is that I have a bridge rectifier in-line with the +12V input. This allows for a barrel connector with the center connector as either +12V or Ground. Either will allow the power to drive the board with no problems. I also have jumpers for connecting end-stop and other inputs similar to a CNC shield for an Uno board…

Don’t think a cable with a Ferrite bead will help with the power issue I am experiencing. I believe it has to do with my on-board +5V 7805 regulator and the +5V regulator on the Nano board both connected to pin 27 of the Nano board when I have my power connected as well as the USB cable (which is driving the Nano on-board regulator)

Arduino Nano-Steppers.pdf (57.5 KB)