I've built many 12v power supplies before... Just started on another.

The transformer is a piece of crap I made by re-winding a microwave oven transformer... Dual 12v secondaries outputting 10.8vdc, each individually rectified, then fed into a massive 1 farad 25v smoothing capacitor. Open circuit voltage is 14.2 volts. Under a 20 amp load, it drops to 13.1v. Haven't tested it higher than that.

This transformer is VERY inefficient, due to the fundamental design flaws (read: cheapness) of a microwave transformer, and the poorly wound secondaries. The primary and core are paired with the intent of a constant, unvarying load, with a fairly low duty cycle. It draws 8 amps @ 120v just sitting there.

I plan to source a better transformer for this build, but this one was free, so I figured I'd roll with it for now. The whole thing will be properly assembled in a custom cabinet, with fused protection on both sides of the transformer, a voltage meter, cooling fan, etc.