jferg:
One other thing: Is the available capacity of the 5 volt supply higher when connected to a 12 volt supply than with a the typical USB connection? It seems like it would be, but how to quantify? Also, if my stupid questions might be answered by reading existing text, where should I have looked?thanks for your patience.
john
It is a little hard to quantify. The USB +5vdc power is limited to 500ma by the USB standards and the 500ma thermofuse mounted on the arduino board. The on-board +5vdc regulator is limited by it's own internal current and limited value and the heat dissipation created by the voltage drop between your external power, say +12vdc in your case and +5vdc. You can get more +5vdc current from the regulator is you power your board with say 8vdc then if using 12vdc, but then your shield voltage via the Vin pin won't be +12 as you stated you required.
Lefty