Adding an optional external power supply to Arduino Shield
Backfeeding 5V to the Arduino is OK - so long as it is well enough regulated. The reference UNO schematic has a comparator that is used to shut off the USB supply when the voltage at the power jack is over 6.6V, but when the USB connection alone is used, it is just connected to the output of the 5V regulator. You could use a similar scheme to isolate the supply.



My cheap Chinese copies of the UNO, however, don't have this, and the USB output is connected direct to the regulator output - so if a higher voltage is applied, or the regulator fails, this would then be fed back to the USB ports on my PC. Not good...