The previous reply makes no sense to me.
If you have 12V supply and some 12V circuitry, just power the arduino Vraw and the external circuitry from the 12v.
Don't even think about somehow routing the 12V via the arduino board at all, you can only get 5V out of the Arduino regulator and its not rated for 0.5A at 7V, since 3.5W will overheat it.