This is an example of a cheap, compact, full wave AC to DC rectifier. All of the components were ordered from Radio Shack. The PC board was trimmed with a Dremel cut-off tool after soldering. Shrink tubing or electrical tape can be used to protect it before use. The diode group in the upper right of the second pic is a better theoretical example of a 4 diode rectifier circuit. The array is compacted for the real one.
15vac input equals 12vdc output. All conversions have a loss factor.
Inputs: AC goes to the 2 Yellow leads.
Outputs: Red is DC+ and Black is DC-.
Best results can be achieved by floating the output of an AC coil to the 2 Yellow leads, then feeding the DC output to the battery or other DC components in a closed circuit. Otherwise, it can be used inline ahead of DC components in a mixed AC/DC set-up with a common ground. A Trail Tech or other AC regulator can be used on the AC side to limit max voltage.
A capacitor can be added across the AC input to smooth the AC pulse.
Search "full wave rectifier" online to provide further explanation and orientation of the diodes if you can't see what's going on in the pics. Basically all 4 have the silver stripe toward the DC+ (Red) output.
BTW: Using just a single diode in line for half wave rectification results in a 12vac to 5vdc output. (edited)