This circuit is the basic 12V Regulated Power Supply circuit diagram:
The above circuit applied a zener diode 13 volts, D2 which is used as voltage regulator. Aprroximately 0.7 volts are dropped across the transistors Basic-Emitter junction, leaving more of a current supply output 12.3 volts. This circuit can supply loads up to 500 mA. This circuit is also known as amplified zener circuit.
12V Regulated Power Supply circuit source: http://www.zen22142.zen.co.uk/Circuits/Power/psu.html