I am using Arduino mega 2560 and have connected 3 sensors of 5volts input and 1 GSM sim900a 5v module and 1 20x4 LCD module. I have connected 12v power supply external dc power supply adapter. The Arduino board is getting heated up very badly. Is it normal for the board to get heated up a lot or is there any troubleshooting I should take care off.
You are feeding in 12v and your onboard regulator is dropping it to 5v. This means 7v at up to .5A. This is 3.5 watts.
You need to reduce your 12v supply to 7.5 to 9v or use an external switch mode regulator for the 5v supply to the ancillary circuits.
The problem if I use lesser than 12v power supply the GSM module itself doesn't switch on. If I increase the voltage to 12v then it powers on or else if I remove led module and 1 of the sensors then GSM module works. With everything connected and with 5v power supply the GSM and few other sensors won't work efficiently.
How to go ahead on this ?
What is the amperage output on your 5 Volt PSU ?
Just a suggestion but if its on the low side go for a higher output capacity and slightly increased voltage. There is quite a bit of "wiggle room" here between 5 and 12 Volts.
If you have an adjustable supply then ramp up slowly.
Also have you considered giving some of your sensors thier own supply thereby reducing the load on the Arduino itself ?
Only the ground is common to the sensor and Arduino.