USB power or External Power - not both?


trying to figure before I blow my Mega.

Description says that I can connect USB power or external power. Can I connect both at the same time?

Essentially I want the arduino to run by itself, but from time to time connect via pc to read data through the serial usb port. Normally the PC is shut off. Hence I need to have the USB connected also and when I turn it on both power supplies would be active.

Looking at the schematic, I’m not sure what the ops at the bottom right do, but the usbvcc seems to switch on the NDT. I guess the 2 supplies would work together but would like to confirm. In case they work together can I pull more than 500ma, basically adding the usb and external amps up?

Thanks a lot for your answers.

Yes, you can run the board with both USB plugged in and an external DC supply hooked up. However they don’t work together, what happens is there is a automatic switch over to the the external power if it is at a high enough voltage, and a automatic switch back to USB power if the external power is disconnected or it’s voltage value drops too much. The op amp circuit you mentioned is where the voltage level sensing/switching is done.

You can draw a little more current from the external supply possibly double the MA that you can draw from the USB port, but the regulator current spec (and it’s heat dissipation spec) of 800ma is the ultimate limit but even that may be optimistic. To power things like servos or large number of LEDs one should look at using independent external DC power supply, just be sure to tie the ground together if you do that.


What RetroLefty said. It’s one or the other, with external taking precedence.

You can use things like a transistor if you want to use the Arduino to control the flow of more power, so you don’t have to run motors and whatnot from current drawn thru the board.