Zener Diode as Voltage Regulator
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.
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| 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 Vin is an unrelated DC voltage which is obtained from a rectifier filter combination. RS is the current limiting resistor and RL is the load register. The input voltage Vin should always be higher than the breakdown VZ. The circuit diagram shows state the zener diode is Reverse biased and operates in the zener region of the reverse characteristics.
if Vin is higher than VZ is the Zener current IZ is between IZmin and IZmax then voltage across the zener will remain constant equals to VZ irrespective of any changes in Vin and IL. As the output voltage is constant equal to VZ, we get a regulated output voltage.
Zener current IZ should not be higher than IZmax, otherwise excessive power dissipation will damage the zener diode. IZ should not be less than IZmin 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 :
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Then I increases
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But IL is constant as IL=VZ/RL
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Hence IZ increases as IZ=I-IL
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If IZ<IZmax then the zener operates in the zener region and output voltage remains constant.
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Then I decreases
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But IL is constant
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Hence IZ decreases
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If IZ>IZmax then the zener operates in the zener region and output voltage remains constant.
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