Zener Diode forms an important topic from Gate point of view. A question is always expected from this topic . Learn how to solve questions on Zener Diode.

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About Zener Diode

  • Basically a PN junction with a little increase in doping level.
  • A break down diode.
  • Known as constant voltage device. Because it operates at or above knee voltage and below maximum voltage.
  • Major applications as voltage regulator circuit, can also be used as reference voltage devices.
  • Zener diode are available with breakdown voltage in the range 2.5V to 300V.
  • Operated under reverse breakdown because when forward biased  it will be work as normal diode.
  • Cut in voltage (forward biased ) is 0.7V.
  • Latest Zener diode are fascinated with abrupt junction and when operated under reverse biased they will exhibit the property of tunnelling effect.


VI Characters of Zener Diode



Zener Diode works under reverse biased. The current at which zener diode works remains between the breakdown current IZ(min) and the maximum current rating IZ(max).

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 How to solve questions on Zener Diode


                     For ideal:    the current is zero if reverse biased. If V1 < VBreakdown .

                            When reverse voltage = Breakdown voltage ,The current suddenly increases.

                            When reverse voltage > Breakdown voltage, more and more current will be passing through zener diode but voltage across zener diode will almost be constant and it is around breakdown voltage.


Dynamic resistance 

                                       RZ =∆ VZ / ∆ IZ

For ideal Zener diode , dynamic resistance is zero.



 When Zener diode is Forward biased,Voltage across zener diode is  cut in voltage =0.7V

  When Zener diode is reverse biased, voltage V0= VBR (break Down)

when two Zener diode are in series,one reverse biased one forward biased ,the voltage across zener diode is Vcut-in+ VB = 0.7 +10 (suppose break down voltage is 10 V)  =10.7


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             At any junction current divides. The current through zener diode if reverse biased is Knee current. I – IKnee is distrusted to.

Generally,  I = Power voltage – Breakdown voltage


The currenet can be distributed in two forms :

                                 I = Iload min + Izener max

                                 I= Iload max +  Izener min


  • If we need minimum load resistance, the current should be maximum.
  • If we need minimum power rating, it means maximum current. We can take load resistance as zero.


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Questions From Gate

Ques 1 :

A zener diode regulator in figure, is to be designed to meet the specifications : IL = 10 mA, Vo = 10 volts and Vin varies from 30 to 50 volts. The zener diode has VZ = 10 volts and IZK (knee current) = 1 mA. For satisfactory operation


Solution :

We know, I through R,

Is I = Vin + V0



V0  =10V (It is reverse biased)

I total ≥ IL + Iknee current

         ≥ 10 + 1

          ≥ 11

From above,

Vin + V0   ≥ 11mA



When Vin =30V :


30-10 ≥ 11 mA i.e. R ≤ 1818Ω


When Vin =50V :


50-10 ≥ 11 mA i.e. R ≤ 3636Ω


We prefer first one as R is less than 1818Ω ,Then it will be less than 3636Ω

So option 1.

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Ques 2 :

In the circuit shown below, the knee current of ideal zener diode is 10 mA. To maintain 5 volts across the load RL, the minimum value of RL in Ω’s and the minimum power rating of the zener diode in mW respectively are

  1. 125 and 125
  2. 125 and 250
  3. 250 and 125
  4. 250 and 250


Solution :

We know , I = IL + I2

I = (10-5)/ 100 = 5/100 = 50 mA

 This current can be distributed in to 2 ways

I = IL max. + Iz min.

I = IL min. + Iz max.

50mA= 10mA + IL max.

IL max. = 40 mA

RL= 5V/Imax. = 5/40mA = 125

Power= ­Imax.. VL

             = 50mA * 5V

               = 125 mA


Answer a

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