Hi everyone, I´m aware of the issues of powering a Nano with an external PS while connected to USB.
I was trying to power a SIM800l GSM module with a 4V external power supply while the Nano was connected to the PC usb port to send AT commands. When I connected the 4V supply the CH340G chip somehow ran out of white smoke ;-)...
The Nano and SIM800 grounds are connected together, see attachment. Everything was working "fine" while I was trying to power the module from the nano 5V pin (except brownouts on transmission burst, thus the external supply).
I´m using a DIY bench supply made from an old ATX PSU, with a LM317T as a variable voltage source...Could the PSU somehow be the culprit behind the smoke, because I dont se what else could fry the chip..
Did you connect the external PSU while 5V output from Nano still was connected to VCC of SIM800L?
If so, you connected 5V from USB to 4V from external PSU which killed the 5V source whatever it is.
Of course SIM800 cannot be supported by the 5V output of Nano because the SIM800 chip needs 2A pulses which cannot be provided by the Nano. If the external 4V are connected like in your circuit diagram there should be no problems.
However....stupid me. Just remembered that back when I was modding the ATX into a PSU, I took the -12V and +12V rails to make the 0-24V adjustable voltage source...so when I connected the ground from the PSU (-12V) and arduino ground (0V) it, of course, went up in smoke.