The board has a 3.3V regulator, but where does its power come from? Maybe someone else can figure it out.
Okay, that one seems to get power from the ICSP header with an onboard regulator, and looks to have a voltage divider for the Rx in line.Nice little combination of parts.
Ah so. In that case, we're back back to square-1. The XBee has it's own 3.3V regulator, but the powerto it comes from the 5V pin on the ICSP header. And that's the reason why the 5V regulator on theArduino board is overheating in the first place. Back to (12V - 5V) * .25A = 1.75W, which is much toomuch dissipation for smt devices, even DPAK, as noted many posts ago. So, three choices:1. run the XBee at low duty cycle, probably < 20%.2. use the series-R to drop the voltage in the pathway from the 12V battery.3. use a lower battery voltage, like 6 NiMH AA-cells.