So I'm aware the Arduino Uno can take 6-20 volts, with a recommended voltage of 7-12. But I have an unregulated 12v power adapter, which puts out about 18v with no load.
Most of the time the Arduino won't be doing much of anything but powering a 16x2 LCD, which should take only a couple mA.* I have a 10k ohm resistor on the back light, so it should only take 0.5mA.
*I don't have a datasheet, but from what I've read 1-2mA is reasonable
The Arduino won't be doing anything else that would draw any meaningful amount of current (except looping to check for input, etc). Online sources generally indicate the power draw when not doing anything is a little under 50mA, so I'm guessing that's about what the current draw will be. Using the formula in this answer, I calculate the power dissipation by the voltage regulator to be (18v-5v)*0.05A=0.65 Watts.
- Is this correct?
- Is the voltage regulator likely to overheat?
I'm not really concerned about power usage, because it will be plugged into the wall all the time. Also, the Arduino will be out of sight, but will have plenty of air around it to disipate heat (though not access to a breeze).