Good day! I am new in this forum and newbie also in the world of Arduino. As a starter with very little knowledge in electronics as well as electricity, I have question about powering up my Arduino Uno.
Here is what I want to do. I would like to power up arduino using 12V 2A power adapter. When I ask a friend he said that I can use 12V 2A but when I ask the customer service of the online store where I will buy the adapter if I can use the said adapter, the answer NO and that I can only use 9V 1A. I am confused who to believed now. Anyone who can shade a light on this matter please. Thank you in advance.
You can power the Arduino with a 12V/2A adapter. There are a few points.
The 12V will be converted to 5V by the onboard regulator and will be available on the 5V pin. You can however not draw much current from the 5V pin as the regulator has basically no cooling. The difference between 12V and 5V (7V) multiplied by the power current consumption of your project will be converted to heat. Therefore a 9V power source will be better because the voltage difference is only 4V (9V - 5V).
The current that the adapter can deliver is irrelevant as long as it can deliver sufficient current for the project. If it can deliver 20A that is fine; only what is needed will be used.
Note
Power consumption is a product of voltage and current. So if your 5V side requires 1A (P = V * I = 5W) and you power it using a 12V adapter, the requirement for the adapter will be I = P / V = 5 / 12 is roughly 0.4A and for a 9V adapter it would be 5/9 A (approx 0.55A)
Wow! Thank you for the enligthenment.
If your project doesn't need 9volt or 12volt, then you could power the Uno with an old cellphone charger with USB socket. Just plug the Uno's USB cable into the charger.
Leo..
Correction. Power consumption is a product of voltage and current... True. But,
(Ignoring, for the moment, power dissipation limitations of the regulator, we'll discuss that in a moment.)
If your Arduino's linear regulator reduces the 12V to 5V and draws 1A, the adapter is still outputting 1A, and will consume 12W. The extra 7W is dissipated as heat in the regulator. There is no transformation of current/voltage in a linear regulation scenario such as is built into the Uno, Nano, Mega, etc. - you have an Uno.
If the regulator were a switching regulator, such as in buck/boost converters found in some more advanced products, then yes,(ignoring efficiency losses), the ratios as stated by @sterretje are correct.
Comments on the Uno specifically. Input voltage is important. Reducing Vin to as close to 7V as practical has the effect of INCREASING the current you can safely draw from the 5V pin of your Uno, because that voltage drop reduction reduces the POWER being dissipated by the regulator. If you use 12V, or above, you should limit your devices powered by your Uno to a few LEDs and other low-demand items. The 9V adapter, on the other hand, will probably allow you to happily add LCD, sensors, and LEDs much more freely, though I would still avoid anything beyond 100 mA. Certainly, motors, servos, relays, and solenoids are still risky, even with suitable protection circuitry, as they typically draw much more current.
Power budgeting, and power problems, are areas we deal with daily on this forum, so be quick to ask when you're developing your Uno's circuits, lest you do damage.
It depends on what you will be connecting to your arduino.
I run my Uno from 12V but only have push buttons, an LED, and a thermistor connected.
I attach components such as I2C LCD with pot and LEDs, 2 pot and 2 servos, pushbuttons and I2C LCD. Basically right now I am playing these basic components since I am new in Arduino.
LCD, servos, pot, LEDs, LDR, IR, touch sensors are so far the ones, I am using.
If you just want to have one power supply, then I would buy a 9V 5A adaptor and a 9V to 5V buck (step-down)regulator.
Power the Arduino and the buck with the 9V and power everything else with the 5V from the buck.
Is there anything that needs 3.3V?
Here is an old post of mine with more info. Remember the Arduino itself will take 10-20 ma that you should subtract from the max current value.
The one that sticks out is 2 servos.
I hope you don't plan to power the servos from the 5volt pin.
Leo..
Unless they are very tiny servos 
.
Yes they are tiny servos (SG90).
The SG-90 has a stall current of ~650mA.
Average current will be lower. How low depends on the load it has to move.
The Uno R3 is a tough cookie. It will warn you with resets (if you're lucky).
Leo..