DC DC buck Converter

[Ashok]

Hi Everybody,
I am Working on DC DC Buck converter. I need to connect a RFID module which works on 3v/500mah to a battery varies from 10v to 42v/15 Amps. I have browsed the web but i can't get any related documents or circuits, I am totally confused, Please help me in this. How to step down a 600w to 1.5w DC DC buck converter circuit.

[pebe]

I'm not quite sure what you mean by 'working on' and 'can't get any circuits'. Are you trying to built one? You can buy one ready made for less than the cost of the individual components - and know it will work. This one is less than £1 post free:
http://www.ebay.co.uk/itm/1-25V-37V-1pc ... 232f485e04

[Ashok]

Thanks PEBE. I purchased and tested, My RFID device works fine.