I have an adapter with an output of 12 V and 2 A. I understand the Uno should operate on 7 to 12 volts. Will a constant input of 12 V be too much over the long run?
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Thank you in advance for your help.
Some people say that 12V can make the regulator run quite hot. So 9 or 10V DC appears to be nicer on it.
It will be fine. The 5V regulator may get pretty warm if you are driving a lot of LEDs at one time, as an example of a heavy load.
I prefer to use a 7.5V or 9V supply myself when not using USB.
As long as the output of your power supply is regulated 12v you should be ok.
However, it depends on the current being drawn through the on board 5v regulator.
Let us say you were drawing 1/2 amp.
The regulator would get hot at ~(12-5)*0.5=3.5Watts
At 7 volts this would be ~(7-5)*0.5= 1 watt.
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I've had an Arduino running in a car (12V+) for a couple of years. I'm just driving some LEDs & solid state relays.
However, the more stuff you "power" from the Arduino, the higher the voltage you feed-in, and the hotter the environment, the hotter the voltage regulator will get. (And it's heat that will (potentially) kill it.
It depends on the current. How much current are you asking the 5V and 3.3V pins to supply when running on that 12V input? It is pretty easy to overheat the regulator with just one LCD backlight.
If you want to dig deeper, have a look at the following link which gives a very comprehensive overview about powering Arduinos and the different issues going along with the different methods:
Hi,
If you get that power supply, pictured,
Use a DMM to check its output voltage with no load first.
Tom.... 
If a power supply can take 100-240volt AC, then it should be a regulated switching supply.
Ok to use with an Uno...if you don't draw a lot from the 5volt or 3.3volt pins.
So only low power sensors and a few LEDs.
Feel the temp of the part between the DC jack and the USB jack.
If it sizzles, and you can see the shape of the part on your finger, then it's too hot.
Leo..
use external 7805 regulator and power arduino with it.using this the internal regulator has nothing to regulate and it want get warm.
sperera:
use external 7805 regulator and power arduino with it.using this the internal regulator has nothing to regulate and it want get warm.
So buy an Arduino with onboard regulator, and then you don't use it.
12volt on the DC socket is not a problem, unless you draw too much current from the 3.3volt or 5volt pins.
Leo..
Wawa:
So buy an Arduino with onboard regulator, and then you don't use it.12volt on the DC socket is not a problem, unless you draw too much current from the 3.3volt or 5volt pins.
Leo..
So then you have to have an external regulator. It is usually a bad idea to have two 5V power supplies, particularly if you have any analogReads. So you have an Arduino with an onboard regulator which you don't use.