Powering Arduino with regulated 5V source over 5V pin and using USB simultaneously

Hello community. Can I use USB for purpose of reading debug messages over Serial, while Arduino Nano is powered over 5V pin with 5V regulated power source?

Thanks in advance!

Why waist power into +5 when USB carries 5 volt?
I might not be good to do like You say.

Because system is not regularly supplied over USB, I want to use USB only for debugging purposes. I am using external 5V regulator which suits for my needs and I am using it to power Arduino over its 5V pin. But, I need to know is it safe to connect the USB while system is working without damaging Arduino.

The only risk is driving power into the USB port. It's probably not a problem but if you want to be absolutely sure, take an old USB cable, slit it open, and cut the red wire. That will disconnect the 5V line from the USB port.

Absolutely no problem with the Nano.

The problem is with the UNO/ Mega 3560/ Leonardo where is is possible to feed the 5 V supply back into the USB port of the PC and cause that to malfunction.

That said, I must mention that the majority of "powered" USB hubs do precisely that so I suspect the potential problem exists, but is uncommon. :grimacing:

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I do it all the time, with a 5.2V powered hub, no problems so far. Be very careful with UNO and Mega though.

Thank you guys! May I ask why is it problem on other boards and Nano is the exception?

The Nano has a diode. Because of that it runs at 4.5V when USB powered, but the 5V pin can not carry current into the computer.