I need a power supply design to power up 12V, 3W LED string from 220 VAC supply to illuminate stairs. It should be tiny. How can I do this?

 

Switch mode psu is smallest, ready made one can be bought cheaper than making one.

 

Definitely go with an agency approved switchmode supply to provide isolation and some protection against fire and shock.

 

specs on the LED string..
does it already have current limiting?
Is 12V the Vf of the LED or the operating voltage of the strip.

More than likely the LED strip already has current limiting so any regular 12V "wallwart" could be used provided it has more than a 500mA output capacity.

 

You can't do it cheaper, better, or safer than a simple switchmode power supply. Even I can't!
Take the easy way out on this one. Buy the well designed and tested product.

 

specs on the LED string..
does it already have current limiting?
Is 12V the Vf of the LED or the operating voltage of the strip.

More than likely the LED strip already has current limiting so any regular 12V "wallwart" could be used provided it has more than a 500mA output capacity.

All of your questions' answer is YES.
What is "wallwart"?

 

Wall-wart is american slang for DC psu adapter...


https://en.wikipedia.org/wiki/AC_adapter

 

SMPS is much complecated to me... actually I am not very well in power electronics... I thought that there may be another simple way to acomplish this kind of job. Once I saw a tiny 12V power supply that converts 220V mains to 12 VDC @ 1amp. that was a professional tool and used few components... doide, transistor and a small roomed transformer as far i remember.

 

if it small its definitely smps, a linear transformer is too large,

smps design is outside the realm of most hobbyists, easier to buy a ready made psu .

 

SMPS is much complecated to me... actually I am not very well in power electronics... I thought that there may be another simple way to acomplish this kind of job. Once I saw a tiny 12V power supply that converts 220V mains to 12 VDC @ 1amp. that was a professional tool and used few components... doide, transistor and a small roomed transformer as far i remember.

You just need to buy one of these if you don't already have one laying around your house..

http://www.amazon.com/Wall-Adapter-Power-Supply-12VDC/dp/B006GEPUYA
BUT with the proper plug for your country..
Plugs right into the wall outlet and will output 12VDC to feed directly into your LED strip..