Zener Diode as Voltage Regulator

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Zener Diode as Voltage Regulator

The major application of a zener diode in the electronic circuit is a voltage regulator. One of the most important uses of zener diode is in making the constant voltage power supply. Its working is based on the fact that in reverse breakdown (zener) region, a very small change in voltage across the zener diode produces a very large change in current through the circuit but the voltage across the zener remains constant.It provides a constant voltage to the load from a source whose voltage may vary over sufficient range.The diagram shows the circuit arrangement.Zener Diode as Voltage Regulator

The zener diode of zener voltage\({ V }_{ z }\) is reversely connected across the load \({ R }_{ l }\) across which constant voltage \({ V }_{ 0 }={ V}_{z }\) is desired. A resistor R is connected in series with the circuit which absorbs the output voltage fluctuations so as to maintain constant voltage \({ V }_{ 0 }={ V}_{z }\) across the load.

Let a varable voltage \({ V }_{ in }\) be applied across the load \({ R }_{ l }\). When the value of \({ V }_{in}\) is less than zener voltage \({ V }_{ z }\) of the zener diode, no current flows through it and the same voltage appears across the load. When the input voltage \({ V }_{ in }\) is more than \({ V }_{ z }\), this will cause the zener diode to conduct a large current \({ L }_{ z }\).

Consequently, more current flows through series resistor R which increases the voltage drop across it. Thus, the input voltage excess of \({ V }_{ z }\) (i.e \({ V }_{ in } – { V }_{ z }\)) is absorbed by the resistor. Hence, a constant voltage \({ V }_{ 0}\) ( =\({ V }_{ z }\) is maintained across the load \({ R}_{ l }\)

In the above discussion it has been seen that when a zener diode of zener voltage \({ V }_{ z }\) is connected in reverse direction parallel to the load, it maintains a constant voltage across the load equal to \({ V }_{ z }\), and hence stabilise the output voltage.

Key points to keep in mind

Working of Zener Diode as Voltage Regulator: When the D.C voltage across zener diode increases beyond a certain limit (i.e. zener breakdown voltage), the current through the circuit rises sharply, causing a sufficient increase in the voltage drop across the dropping resistor R, As a result of it, the voltage across the zener diode remains constant and hence the output voltage lowers back to normal value.

When the D.C voltage across zener diode decreases, the current through the circuit goes down sharply causing sufficient decrease in the voltage across the dropping resistor. As a result of it, the voltage across the zener diode remains constant and hence the output voltage is raised to normal.  Hence the output voltage remains constant.

Important: In order to have a good load regulations, the value of the series resistance R should be such that the current through the zener diode is much larger than the load current. For this, we select the zener current five times the load current.

 

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