First time poster and first time user after getting an Arduino Uno for Xmas (been busy deciding what to make it do!)
I'm actually going to intergrate the unit into a solar powered off-grid battery supply system I'm building. It will be using most of the analogue ports to read various voltages, and a few digital inputs too. This will be output over the serial port to a webserver to graph the data over time.
I've no problems with the hardware or software side of things, I think I can get that cracked. I'm just need a bit of advice on the PSU options.
Quick question first: The manual states the PSU supply is selected automatically between the USB or external source, but which does it select if it's got both (since I will be powering the board from 12VDC and using the USB for communication?)
Generally, it isn't a good idea to leave USB connected while powered through the barrel jack/VIN. When the external power is active, so is the on-board 5V regulator. This means you are connected the Arduino's 5V node to the USB's 5V node.
That's not true, if there is voltage on the Vin pin then the auto-voltage selector will turn off the fet that routes usb +5vdc to the arduino +5vdc bus.
if there is voltage on the Vin pin then the auto-voltage selector will turn off the fet that routes usb +5vdc to the arduino +5vdc bus.
Silly me. I just assumed that since there is 5V present on the 5V Pin of the (unconnected) USB connector when a voltage is present on VIN, that it was coming from the on-board regulator. I guess it is just phantom voltage?
Might be some very high resistance current leakage, but if you study the auto-voltage selector circuit in the upper left hand section of the below drawing you will see the T1 fet that connects the usbvcc to the arduino +5vdc bus: