I made myself a little shield that has an ACmains-to-12V power supply. It all sits in a neat little project box w/ a switch for the mains, and the arduino communicates w/ the computer via USB. Problem is, when I switch off the AC mains, it switches over to USB power but the USB power isn't enough to drive some other components on the shield and my main output from the shield doesn't stay off.
Is there any way to permanently disable the USB power connection? From the schematic, it looks like I could remove the FDN340P, but before I start desoldering, I thought I'd check to see if that is the easiest way or not.
Yes, I think you've hit upon the easiest way to achieve what you want. Removing the FDN304V transistor will prevent the USB port from powering the Arduino.
The other option (removing the 500mA PTC USB fuse) is not good because it removes USBVCC power to the ATmega8U2 controller and will interfere with USB communication.
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The Rugged Motor Driver: two H-bridges, more power than an L298, fully protected
Or take a cheap spare USB cable and cut into and break the +5vdc power wire going to the connector. The USB comm should still work with the USB ground and two data lines attached. Might be worth a shot Vs surgery on the board?