user56989: I am new to arduino and i am making a smart energy meter for my home. I want to run the arduino months so, how to power arduino with a adapter, I have a 12V 1.2A adapter is it ok to power arduino with it or not?. 
Thanks in advance.
As i said i want to run it long time.

user17196: If it is an Arduino Uno Rev3 SMD, the documentation (see Juraj's comment) is of relevance here, click the tab "Tech Specs" to find:

Operating Voltage     5V
Input Voltage (recommended)   7-12V
Input Voltage (limit)     6-20V

While the microcontroller's (ATmega328P) operating voltage is 5 V there is an on-board voltage regulator to make the board work within the input voltage range of 6 to 20 V (max limits) or the recommended range of 7 to 12 V.
Operating this Arduino with a 12 V power adapter is within spec and therefore safe to do. The current rating of the adapter of 1.2 A is also more than sufficient to run the Arduino.

user2967: Yes, you can power an Arduino with 12V into the barrel connector. It will then use it's internal voltage regulator to drop that to the 5V it needs internally. However, that is on the high side of it's recommended range, and will make the Arduino's voltage regulator run hot. A 7.5V supply would be better, and you could probably get more total current out of the 5V regulator without overheating if you fed it 6V or 7.5V instead of 12V. (6V is the lower limit. >= 7V is better.)
(A linear voltage regulator like the one in most Arduino boards essentially uses a transistor as a variable resistor to lower the voltage from the input voltage to 5V, which converts the excess voltage to heat. The higher the input voltage, and the more current you draw from the regulator, the more heat it generates.)

56989: <p>I am new to arduino and i am making a smart energy meter for my home. I want to run the arduino months so, how to power arduino with a adapter, I have a 12V 1.2A adapter is it ok to power arduino with it or not?. 
Thanks in advance.
As i said i want to run it long time.</p>
 17196: <p>If it is an Arduino Uno Rev3 SMD, the <a href="https://store.arduino.cc/arduino-uno-smd-rev3" rel="nofollow noreferrer">documentation</a> (see Juraj's comment) is of relevance here, click the tab &quot;Tech Specs&quot; to find:</p>
<blockquote>
<p>Operating Voltage     5V</p>
<p>Input Voltage (recommended)   7-12V</p>
<p>Input Voltage (limit)     6-20V</p>
</blockquote>
<p>While the microcontroller's (ATmega328P) operating voltage is 5 V there is an on-board voltage regulator to make the board work within the input voltage range of 6 to 20 V (max limits) or the recommended range of 7 to 12 V.</p>
<p>Operating this Arduino with a 12 V power adapter is within spec and therefore safe to do. The current rating of the adapter of 1.2 A is also more than sufficient to run the Arduino.</p>
 2967: <p>Yes, you can power an Arduino with 12V into the barrel connector. It will then use it's internal voltage regulator to drop that to the 5V it needs internally. However, that is on the high side of it's recommended range, and will make the Arduino's voltage regulator run hot. A 7.5V supply would be better, and you could probably get more total current out of the 5V regulator without overheating if you fed it 6V or 7.5V instead of 12V. (6V is the lower limit. >= 7V is better.)</p>

<p>(A linear voltage regulator like the one in most Arduino boards essentially uses a transistor as a variable resistor to lower the voltage from the input voltage to 5V, which converts the excess voltage to heat. The higher the input voltage, and the more current you draw from the regulator, the more heat it generates.)</p>
