Explain with circuit diagram operation of zener diode as a voltage regulator.
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Diagram of zener diode as voltage regulator:

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Working:  Zener Diodes are widely used as Shunt Voltage Regulators to regulate voltage across small loads. Zener Diodes have a sharp reverse breakdown voltage and breakdown voltage will be constant for a wide range of currents. Thus we will connect the zener diode parallel to the load such that the applied voltage will reverse bias it. Thus if the reverse bias voltage across the zener diode exceeds the knee voltage, the voltage across the load will be constant. 


Characteristics

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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.

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Operating Principle 

For proper operation, the input voltage Vi must be greater than the Zener voltage Vz. This ensures that the Zener diode operates in the reverse breakdown condition. The unregulated input voltage Vi is applied to the Zener diode. Suppose this input voltage exceeds the Zener voltage. This voltage operates the Zener diode in reverse breakdown region and maintains a constant voltage, i.e. Vz = Vo across the load inspite of input AC voltage fluctuations or load current variations. The input current is given by, image


We know that the input current IS is the sum of Zener current Iz and load current IL 


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As the load current increase, the Zener current decreases so that the input current remains constant. 

According to Kirchhoff’s voltage law, the output voltage is given by, 

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As the input current is constant, the output voltage remains constant (i.e. unaltered or unchanged). The reverse would be true, if theload current decreases. This circuit is also correct for the changes in input voltage. As the input voltage increases, more Zener current will flow through the Zener diode. This increases the input voltage Is, and also the voltage drop across the resistor Rs, but the load voltage Vo would remain constant. The reverse would be true, if the decrease in input voltage is not below Zener voltage. Thus, a Zener diode acts as a voltage regulator and the fixed voltage is maintained across the load resistor RL

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Zener diode as voltage regulator:

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Zener Diodes can be used to produce a stabilised voltage output with low ripple under varying load current conditions. By passing a small current through the diode from a voltage source, via a suitable current limiting resistor (RS), the zener diode will conduct sufficient current to maintain a voltage drop of Vout. 


The resistor, RS is connected in series with the zener diode to limit the current flow through the diode with the voltage source, VS being connected across the combination. The stabilised output voltage Vout is taken from across the zener diode. The zener diode is connected with its cathode terminal connected to the positive rail of the DC supply so it is reverse biased and will be operating in its breakdown condition. Resistor RS is selected so to limit the maximum current flowing in the circuit.


With no load connected to the circuit, the load current will be zero, ( IL = 0 ), and all the circuit current passes through the zener diode which in turn dissipates its maximum power. The load is connected in parallel with the zener diode, so the voltage across RL is always the same as the zener voltage, ( VR = VZ ). The supply voltage VS must be greater than VZ.


A zener diode is always operated in its reverse biased condition. A voltage regulator circuit can be designed using a zener diode to maintain a constant DC output voltage across the load in spite of variations in the input voltage or changes in the load current. 

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