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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).

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It won't do any harm to try it at 18V for a while. Most voltage regulators have thermal protection and just cut out if they get a little too hot. If it works reliably, stay with it.

EDIT: Their Uno page warns against overheating the board, so I would test it for a short while and make sure there is no overheating or strong "plastic" smell coming from the board. If there's enough heat to worry, then it's best not to run like that for extended periods.

If 18V is too high, you could get a power resistor (I'd guess say a 10 watt resistor around 24 ohms) to waste approximately half an amp from the power supply. Connect this to the supply, in parallel with the Arduino. It's wasteful (unless your room needs heating anyway) but will lower the voltage (that reaches the Arduino) a little.

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    Thank you. I should have mentioned I mean 24/7. Probably a power resistor is the way to go. And no, the room is in no need of heat; we run A/C almost 9 months of the year! So I figure it wastes about double, since it burdens the A/C as well. Partially related (because I like this topic): While the power isn't wasted if heating is desirable, there are more efficient heating methods of heating. Thus, we can't fully shrug it off if we're using a more efficient heating system to heat the room. – Nateowami Jun 1 '16 at 13:51
  • By the way if the most efficient performance is required, I would actually recommend getting a 12V supply that is better regulated. You then won't need to waste heat. Or build/buy a switching type converter to bring the current 12-18V down to 5V. (But both of these are going off the topic.) – Andy Jun 1 '16 at 14:26
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    Fortunately, power efficiency isn't much of an issue. However, wouldn't a regulated power supply simply waste the power itself (assuming it wasn't a switching regulator)? I'll revisit this answer when I settle on a solution. If you know an advertised power dissipation for the Arduino's regulator, that would be nice (I couldn't find it, though probably there would be a way to look at the Arduino's hardware design and find out what regulator they used). – Nateowami Jun 1 '16 at 14:57
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    I'd measure the voltage while attached to the Arduino board. A small load could already bring the voltage down enough to be acceptable. – Gerben Jun 1 '16 at 18:04
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    Well... I think my multimeter lied.Or the power adapter decided to behave. More likely the multimeter; the ohm meter is broken but the volt meter seems to work (usually). It registers 12.34v when the Arduino is running, and only slightly more with no load (the other day it was showing 18v, or for a few moments 20v). Had it been 18v, I think I would have used two 1/4Watt 220 ohm resistors in parallel, to drop the voltage by 5.5v. Then if the voltage drooped to 12v, it would still get 6.5v. Running at 0.05A that shouldn't be much of a problem. Thanks for the help, and sorry for the mistake. – Nateowami Jun 2 '16 at 8:14

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