There are several variable zener diodes available in IC market, but in Sri Lanka it is difficult to obtain variable zener diodes. (For example LM431) So I build this circuit to get the functionality of Variable Zener diode. This circuit is based on two transistors and for Q1 you can use any general purpose NPN transistor and for Q2 use any general purpose PNP transistor.
Using this circuit it is possible to get 3V to 25V (400mW) output. I use this circuit for one of my SMPS project (as a replacement for LM431) and it produces an excellent results.