I generally run my projects at 12v as I have other cmoduless and that's convenient for them. A quick read of this suggests 12v is OK: Arduino Playground - WhatAdapter but the regulator is getting pretty hot. I suppose that is what it does but is it happy like that all day long or should I really consider using a 9v supply or even a direct 5v for better life/reliability?
"pretty hot" is usually fine - most regulators are speced to run to 125C or more (which is extremely hot).
External regulation (which proper heatsink) is the way to get temperatures down and allows the
option of a switched-mode converter which is less wasteful...
Lower voltage = cooler regulator = longer life, in general.
I use 7.5V when I can when a board needs to run from an external supply.
http://www.dipmicro.com/store/DCA-07510
Last night I was using a USB/wallwart to bring 5V in thru the USB connector.
Mine is single port with "enercell" on it, my wife picked it up somewhere.
Dude, we've just asked exactly the same question 
See my thread at Nano gets very hot! - General Electronics - Arduino Forum
Essentially the advice I received boiled down to: yes getting too hot does matter, you can avoid this by using an external regulator.
In my case, I'm using a Nano, which has a tiny regulator and am going to add an external LM317 to drop the 12V to 7V before feeding it into the Nano. That way the external regulator can dump the heat better (as it's much larger), and avoid frying the Nano.
However, it's hard to make an objective assessment without knowing the temperature. In my case, I defined "hot" as being too hot to hold my finger on for an extended period. That could be much less that 120C, I think it's probably about 70C or so in my case. So maybe I am OK after all!
Whether or not you can run the Arduino from 12V without the regulator getting too hot depends on which Arduino it is and what additional loads you are driving from the 5V pin, 3.3V pin and output pins. As already mentioned, the Nano has a puny regulator that gets hot even when you use 12V input with no additional load. The Uno has a better regulator and can handle 12v provided you don't have much connected to those pins.
One hint it that if you are using an LCD display, don't power the backlight from 5V. Instead, power it via a series resistor from the unregulated supply.
My general "rule of thumb" is that if it gets too hot to keep my finger on it, it is too hot.
I'd never consider running a voltage regulator hotter than boiling water. Heat kills, and it especially kills any electrolytic caps near it.
I use one of those cheap adjustable (in my case 12V to 7.5V) step down modules to power my Arduino
Good for up to 2A (maybe 3A if you add heatsink).
CrossRoads:
Last night I was using a USB/wallwart to bring 5V in thru the USB connector.
Amazon.com. Spend less. Smile more.
Mine is single port with "enercell" on it, my wife picked it up somewhere.
I've read those things are killing people!
I've read those things are killing people!
What, are they choking on them? Keep them out of your mouth!
Because of small size, high and low voltages are very close (sometimes less than 1mm) and if you touch the metal of the usb connector you may be electrocuted
This is where those cheapy arduino clones come in handy...
for $15 i get a "compatible" uno, i'd unsolder the linear regulator and hook it up to a switching regulator...
guix:
Because of small size, high and low voltages are very close (sometimes less than 1mm) and if you touch the metal of the usb connector you may be electrocuted
What?... if that was true then how come any device connected to it does not go poof
The 5v USB has to be regulated, what you saying, a capacitor and 5v1 zener diode to provide 5v out? I seriously hope not, something's not ringing true maybe a shorted lead? but if you can get electrocuted what happens to the device connected?
This problem comes up so often, I wrote a page about it. I think it was actually looked at, at least once. See Table 2, look for the red blobs, that's your v.reg melting.
http://www.ot-hobbies.com/resource/heat-diss.htm
LarryD:
Try a Heat Sink
Aluminum SMT Heat Sink - 0.27x0.27 square : ID 1043 : Adafruit Industries, Unique & fun DIY electronics and kits
I would be interested to see how well that thing cools a winkiedinkie smt v.reg. in actual practice.