5V power from USB and direct connect at the same time ok?

is it safe to power a UNO or NANO from 5V power supply at the same time USB is connected? I need to provide power to my NANO because its driving 5 servos and USB power isn't enough but i'm worried if i connect a 5V power supply while the USB is plugged in something bad might happen to the arduino or worse yet, my PC

any advice please, is there a way to disconnect 5V supply from the USB?


The best thing to do is just connect the external +5V to the servos and not the Arduino. That way the Arduino is powered via USB and the Servos only draw from the external +5V supply. You will still need to connect the Arduino ground to the ground of the external supply.

problem then is when i want to run the arduino unconnected from the PC i have to rewire the power supply then when i need to go back to PC connected rewire again

i found this thread

i guess a possible answer is cut the 5v wire in the USB cable

I've just checked and the +5V from the USB is isolated from VCC via a diode (D1 on the schematic).

I've just checked and the +5V from the USB is isolated from VCC via a diode (D1 on the schematic).

is that on a UNO, nano, both?

does this mean it should be safe to put 5v into Vin and still connect the USB 5v at the same time?

i've cut the 5V line in my USB cable to get my system working but that's not a great solution when i start selling products. i'd really like to know if its safe to have 5V power and USB at the same time.

i see nano should have it too
but my board has the CH340G

ok, it looks like the chinese ones have it too http://tefatronix.g6.cz/images/nanoclone_schem.png


I'm facing a similar / the same problem and decided to use a small boost converter to convert 5V to 9V and feed that into the Arduino's Vin (Vraw) / barrel.

You can not feed 5V into Vin (Vraw) / barrel as the voltage drop over the regulator will lower the Vcc for the boards electronics to a level that s putside its specs.

I'm using a Mega, but the above applies to all boards.

Nano has a diode, so is safe.

Uno/Mega/Leo should not have external 5v applied to 5v pin when powered by usb. I would cut up a usb cable and use that to supply power from the external 5v, do its not possible to accidentally leave ext 5v on ehen connecting to usb