I am designing an Arduino shield that requires both 12V power (for relay coils) and 5V power (for the rest of the circuit), each at about 500 mA. For ease of use, I'd like to power everything from a single 12V power supply.
My original thought was to plug in the 12V supply, pull off of the Arduino's Vin pin for the 12V power, and pull off of the regulated 5V line for the rest. However, this of course won't work because the Arduino's linear regulator would have to dissipate 3.5W to supply the required 5V power.
I now plan to put a DC jack on the shield, and supply 5V power with a more efficient switching voltage regulator: http://www.dimensionengineering.com/DE-SW050.htm
However, I'm not sure how to power the Arduino. I can think of two options:
Connect my regulator's 5V line to the 5V pin on the Arduino. The problem I see is if my board is not connected to power, and I plug the Arduino into USB (or accidentally plug a 12V supply into the Arduino rather than my shield), my board will pull 5V from the Arduino and potentially overload the USB circuit or fry the voltage regulator. I could put a diode on the 5V line to ensure that current only flows from my 5V line to the Arduino, and not the other way around, but the forward voltage drop means that the Arduino would get substantially less than 5V.
Connect my board's 12V line to Vin on the Arduino. My regulator would supply 5V to my shield, and the Arduino's regulator would supply 5V to the Arduino. However, I've read that it's tricky to balance the load between multiple regulators working in parallel. I'm not sure if that exactly applies here -- once the boards are powered, their only interaction would be through the Arduino's digital I/O pins. However, I didn't understand the implications well enough to feel comfortable with that design.
Any thoughts on how to solve the problem in #1, or whether #2 would be safe/reliable? Or are there any other approaches for supplying the two voltages to the two boards at the required current?