10.1 Zener diode as a voltage regulator

  Semi-conductor Devices and Electronics

    1.0 Introduction
    2.0 Types of semiconductor
    3.0 Mass action law
    4.0 Electrical conductivity in semiconductor
    5.0 $p-n$ junction
    6.0 Breakdown voltage
    7.0 $I-V$ characteristics of a $p-n$ junction
    8.0 Rectifier
    9.0 Light emitting diode (LED)
    10.0 Zener diode
    11.0 Transistor
    12.0 Boolean identities
    13.0 Logic gates
    14.0 De Morgan's theorem

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10.1 Zener diode as a voltage regulator

When the AC input voltage of a rectifier fluctuates, its rectified DC output also fluctuates. To get a constant DC voltage from the DC unregulated output of a rectifier, we use a Zener diode.

The circuit diagram of a voltage regular using a Zener diode is as shown in the figure.

If the input voltage increases, the current through $R_S$ and Zener diode also increases. This increases the voltage drop across $R_S$ without any change in the voltage across Zener diode. This is because in the breakdown region, Zener diode remains constant even though the current through the Zener diode changes.

Similarly, if the input voltage decreases, the current through $R_S$ and Zener diode also decreases. The voltage drop across $R_S$ decreases without any change in the voltage across the Zener diode. Thus any increase/decrease in the input voltage results in increase/decrease of the voltage drop across $R_S$ without any change in voltage across the Zener diode. Thus the Zener diode acts as a voltage regulator.

We have to select the Zener diode according to the required output voltage and accordingly the series resistance $R_S$.