Zener Diode as Voltage Regulator: A voltage regulator circuit should keep the load voltage constant in spite of changes in its input voltage or load current and temperature. The series resistance Rs is connected to limit the total current drawn from the unregulated dc supply. The zener diode regulator, as shown in fig.(a), is a shunt type voltage regulator because the control element i.e. zener diode is connected in parallel with the load resistance.
Working of Zener Voltage Regulator: The input voltage Vin is an unregulated dc voltage which is obtained from a rectifier filter combination. Rs is the current limiting resistor and RL is the load resistor. The input voltage Vin should always be higher than the breakdown voltage VZ. The zener diode is reverse biased and operates in the zener region of the reverse characteristics, as shown in fig.(b) If Vin is higher than VZ and if the Zener current IZ is between IZmin and IZmax then the voltage across the Zener will remain constant equal to VZ irrespective of any changes in Vin and IL. As the output voltage is constant and equal to VZ, we get regulated output voltage.
The Zener current IZ should not be higher than IZmax, otherwise excessive power dissipation will damage the Zener diode. The Zener current IZ should not be less than IZmin because the Zener diode then cannot operate in the zener region and cannot maintain constant voltage across it. The regulator keeps the load voltage constant in spite of changes in input voltage and load current.