Is it possible to disable/avoid the Arduino Nano from powering by USB?
My USB ports still provide the Arduino with power when my PC is turned off (enabled in bios to charge my phone etc.).
Is there some way to avoid the Arduino from powering while the external source is not giving any power?
Thanks in advance
You could just cut the trace coming from the USB connector.
BillO:
You could just cut the trace coming from the USB connector.
Thanks, never thought it would have been that easy. I thought the Arduino needed the power from USB to power up the FTDI chip (:
Works like a charm!
Now while I'm testing it I notice that it doesn't work that well at all.
The Arduino shuts down as soon as I turn off my PC, however, when I boot my PC up again Arduino does not get regonized on my computer until I unplug the "VIN" pin and plug it back in.
Any idea how I could work around this issue? The Arduino will be plugged into my PC, so I can't disconnect the VIN pin and plug it back in after every boot.
Where is VIN coming from?
Perhaps you could provide a more detailed description of your application/project?
I'll be using this Arduino to control a led strip which I built into my case.
The VIN/Power is coming from my PSU, 12V which also powers the LED strip.
The reason why I want the Arduino to turn off is basically because of power consumption.
This is because my motherboard always provides the USB ports with a voltage (whether the PC is turned on or not) 
What you want is to have the built in FTDI chip powered only from the USB +5vdc and the reset of the board to be powered from the output of the built in +5vdc regulator. That board is pretty small so I don't know how easily you could cut the required traces and add jumper wires as need to accomplish the modification above. Possible you could cut pins 20 and 4 on the FTDI chip, bend them up a little and solder a wire from them to USB +5vdc. The standard Seeeduino 328p 'clone board' uses that set-up (FTDI chip is usb powered only) while rest of the board uses the standard auto voltage selection of either external or usb as available and external if both are available.
Seeduino schematic: http://www.seeedstudio.com/wiki/images/7/7b/Seeeduino_v3.0.pdf
Arduino Nano schematic: http://arduino.cc/en/uploads/Main/ArduinoNano30Schematic.pdf
However the concept of your USB ports having power on for external charging use even when the PC is powered off may be a fatal flaw for any solution to this problem. Do other USB devices work ok if left plugged in and the PC is then powered off then on?
Lefty