Power supply :
12V 20Amp Power Supply by IC 7912 + 2N3055

Here is circuit power supply 12V 20A , USE IC 7912 and 2N3055 .
It easy circuit , Low cost. see detail more in image.

This power supply is the result of the use of components at hand (junk box). The core of the project is a number of 2N3055 transistors and a transformer with a 16 volt 18 amp secondary winding.

As it is constructed using the parts I had available at the time it may not be the most cost effective design if all parts are purchased new.
Circuit

Rather than use a more conventional arrangement with a positive supply it was easier to use a negative supply and make use of a 7912 negative regulator as the voltage control. Then by using available NPN transistors to boost output, an output up to 20 amps can be obtained at 12 volts.