Zener Diode Voltage Regulator
A Zener diode can be used as a D. C. voltage regulator or stabilizer to provide a constant voltage from a source whose voltage varies over a wide range. It is due to the characteristics of the zener diode i.e., if a zener diode is operated in the breakdown voltage, the voltage across it remains constant.
The circuit arrangement is shown in fig. The zener diode of zener voltage regulator is revered biased. It is connected in parallel to a load (RL) across which constant voltage is required. RS is a series resistance to absorb the output voltage fluctuations.
The unregulated D.C. voltage (Vi) is applied across zener diode in reverse bias and a series resistance (RS). The load resistance (RL) is connected in parallel to the reverse biased zener diode and regulated D.C. voltage (Vo) is obtained across the load resistor.
If the input voltage (Vi) increases, then the current through (RS) and consequently, current (Iz) through zener diode increases. This increases the voltage drop across (RS) but there is no change in the voltage across the zener diode. It is on account of the fact that in breakdown region, although current through the zener diode changes but the voltage drop across zener diode remains unchanged at (VZ). Thus, output voltage across load resistor remains constant at Vo = Vz.
Similarly, if the input voltage (Vi) decreases, the current flowing through (Rs) and zener diode also decreases. The voltage drop across (Rs) decreases but there is no change in the voltage across the zener diode. Thus, it is clear that any increase or decrease in the input voltage leads to an increase or decrease of the voltage drop across (Rs) but the voltage across the zener diode remains constant and consequently, the output voltage remains constant. It means that the zener diode is acting as a Voltage Regulator.
A zener diode regulator has the following drawbacks:
It has low efficiency for heavy load current. It is because if the load current is large, there will be considered power loss in the series limiting resistance.
The output voltage slightly changes if load changes.
Therefore, the use of this circuit is limited only to applications where the variations in load current and input voltage are small.