We have one Eval board made for Atmega328P. But we are using nano board directly for processor section. Attached is the schematic. We had first version working correctly. But this time, as soon as 12V power is connected with the nano board connected to PC, nano board is going bad. Smoke is coming from nano board. We lost 3 boards now. I am not sure what could be the reason (Recently I got 20 nano boards CH340 based from ebay and also got 20 main boards assembled). So not sure if entire lot of nano is having issue or somethng else)
Note that in the board, U8 (AMS1117-5) and P1 (PSM) are not mounted. JP1 is shorted so that nano board will supply 5V to the main board through nano connector. So 12V power through TB1 or J4 is mainly for relay. Otherwise entire board derive 5V from nano boards (either through USB or through Vin getting again from main board).
When I connect only nano board to USB port and measure the voltage at Vin Pin of nano, I see around 5V . Is this could be causing any problem when we connect 12V to Vin with USB connected? What happens if we give 5V input to a regulator's output pin? Do we get 5V at the input? Even I checked this in Arduino Uno and see that Vin pin gives 5V when Vin is not cnnected.
Or is this could be the problem associated with Cheap CH340 product?