Voltage regulator circuit using zener Diode and Transistor

Transistors are very useful component in electronics.  In this article, I will explain how can we design a voltage regulator using zener diode and transistor. Here both zener diode and transistor will work as the main component. I think, you are familiar the common emitter or emitter follower configuration of transistor. The circuit is designed based on this configuration. Let's start designing....

See the figures below,

Fig:(a)

Fig:(b)

In the circuit in figure(a), a positive voltage regulator circuit is shown using NPN transistor and in figure(b) negative voltage regulator circuit is shown using PNP transistor.

For designing this circuits, the things you should remember,

1. Resistor value R1 should be according to your maximum and minimum current for zener diode. Current through the zener diode will increase by decreasing the resistor value and current will decrease by increasing the resistor value. If the current through R1 (also through the zener diode as they are in series) is greater than the maximum current that zener diode  can pass through itself, the zener diode may damage. If the current through R1 is less than minimum current of zener diode at which it can operate, the diode will not acquire the deserved voltage. i.e voltage regulation will not continue. 
2. Choose the zener diode voltage according to your output voltage (The voltage that you wish to have after regulating)  and higher wattage. 

*Use heat sink for transistor.
The above configuration is suitable for getting  higher output current. If you wish to have reasonable lower current, see the circuit below,

Fig(c)

The process of choosing  resistor and zener diode value  is same as above. For getting the negative voltage just change the polarity of zener diode and capacitor.