Powering uno from two sources

I have an arduino installed in a system that provides +12V directly to the power jack that is soldered to the board, not using a plug. This works fine as long as no usb plug is installed. As soon as I plug in a USB plug the computer fails to recognize the UNO. If I unplug the soldered 12v connection, then the uno is recognized by a computer.

I am trying to have this UNO power by the system via 12v and then occasionally connect a USB for updates.

I also tried using 5v power directly to the 5v pin on the board but the UNO does not run the sketch that way.

Can I do this in a way that doesn't require me to disconnect the 12v power?

Using the barrel jack with >7V and USB should not be a problem. It switches of USB power that way.

Also

PickyBiker:
I also tried using 5v power directly to the 5v pin on the board but the UNO does not run the sketch that way.

That should just work a charm. So I think it's time to make a drawing (no Fritzing mess) of all connections and what else is connected etc. Because both powering the USB directly from 5V and connecting USB when powering it 12V to Vin / barrel jack does work. Only thing you should not do is power it with 5V (via the 5V / Vcc pin) and connect USB.

Do you have devices connected to the Arduino 5V? These may need a separate power supply when the Arduino is powered by 12V.

Found the problem.

I thought the solder points of the USB connector case were grounds, they are not. Moved all ground connections to the barrel grounds and all is working.

The image shows where I now have the 3 different ground wires attached (GNDs). It also shows were some of the ground wires were soldered when it didn’t work (Not GND).

Thanks for coming back to us :slight_smile: The shield is grounded but via a small inductor. Because it's small that's probably what gave you trouble here :slight_smile:

Hi,
Ops pic;


Thanks.. Tom... :slight_smile: