Hi. I wanted to know what components should i use and how, so that i can make a variable lab bench supply from a 24vdc ac adapter with 3A as current rating. Can i use lm 317? Also if i choose a adapter with 2A and 1A, what limitations would i get?
That's like saying "I want to build a house with 4 windows and a door"...
Still a million options with pro's and con's. Just look up some designs and try to figure out what those pro's and con's are and what you like.
For example, a LM317 is just 1,5A. Also, it's a linear regulator so will produce a lot of heat. Even with a large heat sink it might be to much.
And you can't just combine a 2A and a 1A supply to make a single 3A supply if that's what you try to say.
If you have a spare ATX power supply around you can convert it to a lab power supply to give voltages from 3-24volts.
Attached is a pic of the one I made with some 3D printed parts and parts I had on hand. Works perfectly for powering prototypes and testing stuff.
iClint:
[...]you can convert it to alabbench power supply to give voltages from 3-24volts.
Corrected it 
Okay. Thanks for replying. If not not lm317 then what? And i meant to say what limitation do i get when i use a 1amp or 2 amp supply instead of 3amps?
If you use a supply of 1A for a bench/lab supply of 3A the current limitation of the bench/lab suppy does nothing and you kill the 1A supply.
Like I said, just look up different designed and there pro's/con's.
If not not lm317 then what?
Some kind of switching regulator...
And i meant to say what limitation do i get when i use a 1amp or 2 amp supply instead of 3amps?
Less available current.
How much current do you want/need?
The thing about a linear regulator is - Heat is generated by the combination* of current and the voltage dropped across the regulator. If you are feeding-in 24V and getting 5V out, that's 19V across the regulator. With 1.5A or 1A the regulator may overheat and shut-down, even with a heatsink, and even though it's rated for 1.5A. (Most of these regulator chips won't "fry" and die... They'll usually shut-down or partially shut-down and oscillate when overheated, then they are OK after you let them cool down.)
...Several years ago I made a little hobby bench supply with linear regulators. It has fixed +5 and +/-12V. I never tested it's current limits. It's just for "light duty" use.
It wasn't built from a "DC power supply", it uses a regular-old AC transformer, bridge rectifier, and capacitors.
I don't have a good idea about current drawn by components. Can you give me some idea about the current required by some electronic components. Like what components cannot be used with 1amps and 2 amps respectively, so that i can decide my requirements.
What components are you talking about?
Motors can draw from mA to thousands of amps....
Transmitters can draw from 5 mA to 100's of amps...
Most small indicator LED's pull from 10mA to huge outdoor flood lights that pull 5 amps.
Having no clue what you are building and what parts you are using it's impossible to tell you how many amps you need.
I will say almost all of my smaller projects never exceed 1 to 2 amps...
Go grab an Instructable on bench top power supply and follow their guide.
I'm getting the feeling you don't have enough grasp of the concepts to create one from scratch.
And that's not a bad thing! We all started somewhere, so I feel you will enjoy much more success by starting with something that stretches your skills a tad but does not try to stretch them so far to make a bridge...
Small steps 
Build a few supplies with instructions so get get the concepts and understand them, then start to branch out with your own designs.
Thanks mate. I work with arduino mostly. I will need the bench supply for prototyping smaller projects only, so i think 1.2 amps should be suffiecient for me as i have a wall wart of 12 v and 1.2 amps. I dont want to mess with atx power supply as i know its dangerous. I will use a buck boost converter to get my desired volatge.
ATX power supplies are VERY safe.
They have short protection, overload protection, they have an outer case keeping the high voltage contained so you can't touch it, and they only output 12VDC at the most (Well technically you can get it to do 24 if you know what you are doing.)
But still nothing that is going to hurt you.
Honestly I would say an ATX build would be the SAFEST project to start with!!
Just ground the green wire on the "motherboard" connector to any ground (Black wires) and this will turn the power supply on without a computer or motherboard "commanding it to come on" Great way to add on off of switch to your supply this way too.
There were some recent favorable reviews of the Switching Lab Power Supply modules from "RD Tech."
ATX power supplies are VERY safe. They have short protection, overload protection...
It probably won't electrocute you. But an ATX supply that has "overload" protection somewhere near 20A is far from "safe" if you're talking about Arduino-class circuitry. The 22g wire typically used in breadboards/etc is only rated for about 5A max, and 20A will happy convert your finer PCB traces and such to vapor in a fraction of a second.
A power supply without a variable USER-SETTABLE current limit is NOT a "lab bench power supply" IMO.
(and a reasonable quality lab power supply (20-30V, 2-3A CCCV) is a very useful piece of equipment, even if you spend over $100 for one.)
An LM317 has a current limit of about 1A, and will have power dissipation issues at much lower currents if you're feeding in 24V. I once had in mind a project that would use multiple 317s in various places (switching pre-regulator, negative bias CC sink to get to 0V, multiple parallel 317s for more than 1A, etc) But with the prices of those RD Tech Modules, it's really not worth it.
I saw in YouTube that the atx capacitors hold very large and i not quite sure about how to discharge them as I havent done anything like that before. However i have an atx that was switched on probably 5 months before, so will it have any charge in it?
Anvay:
i have an atx that was switched on probably 5 months before, so will it have any charge in it?
Yes they would be very dead and discharged.
While yes they do have 20 amps they react VERY quickly to overages (Most likely going to be a dead short)
So it would be very tough to overload one in such a fashion to use just enough high amperage and not go WAY over and instantly trip its protection...