Zener Diode as Voltage Regulator – Diagram, Working Principle & Operation

Zener Diode as Voltage Regulator 

   

What is Zener Diode?

A zener diode is a special type of semiconductor diode whose construction is similar to that conventional p-n junction diode. However in constructing zener diodes, the reverse breakdown voltage is adjusted between 3V to 200V.
Its application is based on this principle; hence zener diode is also called as breakdown diode.

How Zener Diode works?

To adjust the precise value of reverse breakdown voltage, the doping level of impurity is controlled during zener diode manufacture. The zener diode can be forward biased or reverse biased. In forward biased mode, zener diode works same as P-N junction diode but the operation in the reverse biased mode is substantially different.

Let’s see how zener diode operates in when applied electrical voltage.

In forward biasing mode, the anode is connected to the positive terminal of DC source and cathode to the negative terminal of DC source, thus the zener is said to be forward biased.

In this mode, zener behaves identical to a forward biased diode. The zener diode is not generally used in forward biased condition. 
When the cathode is connected to a  positive terminal and anode is connected to a negative terminal of a DC Source, then the zener diode is said to reverse biased.
In reverse biased mode, The zener Diode act as voltage Regulator.

Zener diode as a voltage regulator

What is voltage regulator?

A voltage regulator is device which maintains fixed output voltage irrespective of variation in input voltage or load conditions. It provides constant voltage to the output and generally used in computers, battery chargers and several electronic circuits.

Working principle & Operation

A zener diode can provide a constant voltage from a source whose voltage may fluctuate over a specified range by acting as a voltage stabilizer or 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.

Zener diode voltage regulator circuit diagram

The zener diode regulator is a shunt type voltage regulator because; the zener diode is connected in parallel with load resistance. the input voltage   V_in is an unrelated DC voltage   which  is obtained from a rectifier filter combination.    R_S is the current limiting resistor and R_L   is the load register.  The input voltage V_in should always be higher than the breakdown V_Z.  The circuit diagram shows state the  zener diode  is Reverse biased and operates  in the zener   region of the  reverse characteristics.

 if V_in is higher than V_Z  is the Zener current I_Z is between I_Zmin and I_Zmax then voltage across the zener will remain constant equals to V_Z irrespective of any changes in V_in and I_L. As the output voltage is constant equal to V_Z, we get a regulated output voltage.   

 Zener current I_Z should not be higher than I_Zmax,  otherwise excessive power dissipation will damage the zener diode.  I_Z should not be less than I_Zmin  either because then the zener diode cannot operate in the Zener region and cannot maintain constant voltage across it.  The regulator should keep the load  voltage constant in spite of changes in the input voltage and load current.

 Regulating action with a varying input voltage :

• If V_in increases 

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    Then I increases

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  But I_L is constant as I_L=V_Z/R_L

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 Hence I_Z increases as I_Z=I-I_L

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  If I_Z<I_Zmax then the zener operates in the zener region and output voltage remains constant.

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• If V_in decreases 

  ⬇

    Then I decreases

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  But I_L is constant 

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 Hence I_Z decreases 

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  If I_Z>I_Zmax then the zener operates in the zener region and output voltage remains constant.

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