Describe, with the help of a circuit diagram, the working of Zener diode as a voltage regulator.
How Zener diode used as a voltage regulator?
Zener diode as a voltage regulator:
To get a constant d.c. the voltage from the d.c. the unregulated output of a rectifier, we use a Zener diode. The circuit diagram of a voltage regulator using a Zener diode is shown in the figure below.
The unregulated d.c. voltage is connected to the Zener diode through a series resistance Rs such that the Zener diode is reverse biased. If the input voltage increases, the current through Rs and the Zener diode also increases. This increases the voltage drop across Rs without any change in the voltage across the Zener diode. This is because the Zener voltage remains constant in the breakdown region even though the current through it changes.
Similarly, if the input voltage decreases, the current through Rs and the Zener diode also decreases. The voltage drop across Rs decreases without any change in the voltage across the Zener diode. Thus, any increase or decrease in the input voltage results in an increase or decrease of the voltage drop across Rs without any change in the voltage across the Zener diode. Hence, the Zener diode acts as a voltage regulator.
I-V characteristic of a zener diode is as shown in the figure.
In breakdown region, the voltage across Zener diode remains constant even when the current through it changes by the large amount.
In a Zener diode, when reverse bias reaches a particular value, the current increases suddenly. This voltage is called Zener voltage or Zener breakdown voltage.
Zener voltage ranging from 2V to 200 V can be manufactured by controlling doping levels.
Zener diode as voltage regulator:
When the voltage is applied to the circuit current I flow through it. I is divided into IZ and IL, where, IZ = current flowing Zener diode and IL = current flowing through load resistance.
From the figure, I = IZ + IZ
∴ `V_i = IR_S + V_Z` [∵ V = IR]
But `I = I_Z + I_L`
∴ `V_i = (I_Z + I_L)R_S + V_Z`
∴ `V_i = (I_Z + I_L)R_S + V_O` [`:.V_O = V_Z`]
If input voltage `V_i` is increased beyond zener voltage, I increases such that current IZ through zener diode
increases but current IL through load resistance remains same.
Therefore, output voltage VO across load resistance remains the same.
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