This circuit is regulator power supply that easily. The heart in the work of the circuit be Zener Diode the connection in base Circuit of transistor. Which be formed appraise output voltage that can want thus. If you want to modify the value voltage output as a result may can by value change Zener diode only. Please remember the some that output voltage, that get be valuable lower Voltage at base of transistor about 0.5V always thus out volt TRUE be valuable lower voltage drop zener diode 0.5V always. such as if , we use Zener diode 12V get voltage output TRUE equal to 11.5V for the current output be valuable equal to value hfe of transistor x base current. Which fixed with , R1 , there. The detail more Read in image.