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I want to build a 12V DC 0.20A power source

 with 230v ac as I/PI know I have to build a rectifier a full wave, but how to get a particular 0.20 A current plz help....?

The thing you are powering will DRAW 0.2A, unless its something like an LED or Laser. ust provide a 12V supply that can deliver at least 0.2 A

Steve
Steve is a wise man
+1
+1.
I think the people answering this question are confused, because it is not clear exactly what kind of power supply you seek.

Do you want a 12 V constant-voltage supply, capable of supplying 0.2 A depending on the impedance of the load?

Or do you want 0.2 A constant-current supply, capable of of supplying  up to 12 V depending on the impedance of the load?

The first one, a constant-voltage supply, is more common. A battery is essentially a constant-voltage supply.   The ubiquitous "AC adapter" that you find anywhere, and very cheaply on the used/thrift market, these are constant-voltage supplies.

The second one, a constant-current
http://en.wikipedia.org/wiki/Current_source
supply, these are used for powering certain kinds of loads that require constant current, e.g. high power LEDs, laser diodes, electrolysis setups, and probably other applications. 

A search for instructables on "constant current":
https://www.instructables.com/pages/search/search.jsp?&q=constant+current