Programming Arduino Nano while plugged in via 5V pin

Ok so basically, I am working on some sort of LED box that should light up, the project uses three potentiometers for colour control, and a button to change animations and stuff. Now, I am using about 25-30 neopixels (strip format), so powering via USB is a no-go. As a result, I have decided to use an RPi compatible supply, regulated, outputs roughly 5.1V, ran my 9 LED + arduino test perfectly, the board is being supplied from the same power source, in parallel, via the 5V pin.

Now the question is, would I be able to plug in the whole circuit (so the board recieves power through the 5V pin) and also plug in the USB to program the Nano?

Yes, that is safe on a Nano.

Maybe not on a UNO.

Oh, intriguing, I assume the Nano has a reverse current diode on the usb port that the Uno lacks?

Oh, intriguing, I assume the Nano has a reverse current diode on the USB port that the UNO lacks?

Indeed it does.

This means that it is a bad idea to supply projects from the USB port on the Nano and take power from the “5V” pin since you lose voltage in the diode and its current rating is limited.

The UNO includes a FET switch between the USB port and the 5 V pin and circuitry which is controlled by an op amp to switch off when power is supplied to the (rather useless - negligible heatsink) on-board regulator. There is also a “polyswitch” rated at 500 mA in series. The result is that you generally do not lose voltage in the connection but are limited by the polyswitch, and of some concern, if you supply 5 V to the “5V” pin at a greater voltage than the USB, you can back-feed current into the PC’s USB hardware which may disable or even permanently damage it.

I generally suggest that this cannot be too great a risk as the overwhelming majority of “powered” USB hubs do precisely that, however contributors here have in the past described such problems. :astonished:

Very interesting, thanks a lot!

it's such a relief that i can have the board powered externally while also conencted to the USB without any problems, the box i am building is rather small, in the case of a bug or something..taking out the board would not be that...optimal...