Do not power a UNO(, Mega 2560, Leonardo, Nano, Pro Mini) by the "Barrel Jack" or "Vin" if you propose to connect any other devices to it.
If you have a regulated 5 V supply, connect it to the "5V" pin and ground, but disconnect the "5V" pin while you have it plugged into a PC via the USB socket. On a Nano, you do not have to disconnect it.