I am using an Arduino Uno Rev 3. I am making a GSM shield using SIM900 . In the SIM900 Hardware Design Guide it is said that the power supply to the IC should be between 3.2-4.8 and must be able to provide current upto 2 Amperes. What I wanted to know is, if I power the Uno using a 12V adapter which is capable of providing current upto 2 Amperes, can I use the +5V pin to provide power supply to the GSM shield?
No, the +5V output is limited to much less than that (500mA best-case, probably less). You should design around using the Vin pin for powering the GSM shield, not the +5V output. You should also count on using a 12V adapter with a rating of >2A since the cell module needs 2A, but you need current to power everything else too (Uno circuitry), and it's never good to be "on the edge".
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The Gadget Shield: accelerometer, RGB LED, IR transmit/receive, speaker, microphone, light sensor, potentiometer, pushbuttons