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Hi all!

I have an Arduino Mega 1280, and I have read that the power suply is automatically selected, using the external PSU if conected and the usb suply if not.
But, if I have something connected that uses a lot of power, using an external suply, and this PSU breaks, the arduino will be using the usb suply, and I could break my mother board... Am I right?

I will be using just the USB to get data form the PC, and then 3 digital pins to feed some shift registers. Maybe I should leave the arduino being suplied through the usb and suply the other board (the one with the shift registers) through the external PSU, and  then just connect the grounds on the usb and on the external psu?

Or how should I do it?

Thanks in advance!!
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But, if I have something connected that uses a lot of power, using an external suply, and this PSU breaks, the arduino will be using the usb suply, and I could break my mother board... Am I right?
The potential is there.  The Arduino boards do have an on-board polyfuse connected to the USB supply to help in this type of situation.

Maybe I should leave the arduino being suplied through the usb and suply the other board (the one with the shift registers) through the external PSU, and  then just connect the grounds on the usb and on the external psu?
Personally if anything I have connected needs to draw more than 1A, I don't use the Arduino board as a supply.  In that case, it makes more sense to supply the other components directly.  And yes, you'll need to connect the grounds (aka "common".)
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