Power supply question

Using Arduino Project Guidance
1

Can both the USB and the 9 volt DC be running the board at the same time ?

Thanks

Justin

2

(deleted)

3

Yes - the board will switch between them.

SCNR

4

So - yes and no - both supplies may be connected but if you are supplying 9 V to the "Barrel Jack" or Vin, it will be the one actually powering the board as the board will disconnect the USB supply.

Note that the on-board regulator is unable to power anything other than the Arduino itself and a few LEDs at 20 mA each. If you want to power anything else, do not use the "Barrel Jack" or Vin; use a regulated 5 V supply such as a "phone charger" connected to the USB connector. This is also limited to about 500 mA by the polyswitch on the board.

This refers of course to the UNO. If another board, the rules differ.