Zener Diode As A Voltage regulator

introduction

Zener diode as a voltage regulator is used to regulate the voltage. It is a special purpose pn junction diode which works in reverse bias mode. It has a sharp breakdown Voltage Vz. This diode is heavily doped and it is an electronic device. It makes the variable input voltage to a fixed voltage.

Doping Of Zener Diode And Its Effect

Zener diode is a heavy doping diode. The amount of doping in this diode effects the Zener breakdown voltage. If diode is lightly dope then depletion layer is large and it has a higher breakdown voltage. If it has higher doping then, depletion layer is thin and due to this Zener breakdown voltage is low.

Zener Breakdown occurs at low reverse bias voltage. And it is based on electron tunneling effects in which the covalent bonds break to form electron-hole pairs. After that this electrons are move from valence to conduction band. The breakdown which occur at high reverse voltage is due to the avalanche multiplication.

Important points About Zener Diode

  • Zener diode always works and connects in revere bias mode.
  • It is heavily dope pn junction diode.
  • In forward bias, it works as ordinary pn junction diode.
  • It has sharp breakdown voltage which is called as Zener breakdown voltage and use as a voltage regulator.
  • It does not burn until current through it not exceed a maximum rated current of the diode.
Zener Diode symbol and characteristics
Zener Diode As A voltage Regulator

I V Characteristics

The forward bias characteristics is similar to pn junction diode. When the voltage is less than the cut in or knee voltage, the current is almost negligible( in uA range). As voltage cross the knee voltage, the current sharply rises and switches to milli ampere range.

In reverse bias characteristics,

  • When the voltage across diode is less than the Zener breakdown voltage, the current through diode is in micro ampere range and device is in off state.
  • As we increase the voltage the current is increases. And when voltage become equal to or greater than the breakdown voltage then the current sharply rises and voltage remains constant.

Working

  • In forward bias, when voltage is greater than 0.7 volts it works as a short circuit device .
  • In reverse bias, it remains in off state when voltage across it less than the Zener breakdown voltage. It works as a open circuit device.
  • If voltage across it equal to or greater than the Zener breakdown voltage in reverse mode then the current increases sharply and voltage across it become equal to Zener breakdown voltage. The device switch to ON state. 

Zener diode as A voltage regulator circuit diagram

Zener Diode As A Voltage Regulator
Zener Diode As Voltage Regulator
Figure: Zener Diode As A Voltage Regulator Circuit

 In the given circuit, we have

  • Vi or Vin : as a source or input Voltage.
  • Vr : Voltage across series resistance R
  • V(L) : Voltage across load resistance R(L).
  • Vz : Zener diode voltage

working of Zener diode As A Voltage regulator circuit

As we apply Vin Voltage to the circuit, depending upon the polarity of input voltage the zener diode is reverse bias or forward bias. For properly working of the circuit, the polarity of applied voltage is such that it makes the zener diode reverse bias and the magnitude of voltage is greater than the Vz voltage. In such case, the output voltage V(L) is equal to zener diode voltage Vz as long as input voltage is greater than Vz. And the fluctuating excess voltage is drop across series resistance as Vr.

If the Vin is increased, the output voltage V(L) = Vz and the it causes increase in load current I(L). Due to this, the total current I increased and all the excess voltage drop across series resistance R.

We can analyse the Zener Diode As a voltage Regulator circuit in 3 different ways:

01 : When Input voltage Vin and resistance R is fixed

To know whether zener diode is in active state or not, we have to find voltage across diode,

which is given as

V = Vo = ( R(L) x Vi ) / ( R + R(L) )

If V < Vz then the zener diode is in off state and circuit will become

Zener Diode As A Voltage Regulator
Figure: When voltage across zener diode is less than Vz

Output voltage will be V(L) = ( R(L) x Vin ) / ( R + R(L) )

If V >= Vz then the zener diode is in active state and output voltage will be given by

V(L) = Vz

and Vr = Vin – Vz , I(r) = Vr / R ,

I(L) = V(L) / R(L) , I(z) = I(r) – I(L)

P(z) = I(z) x Vz

02 : Fixed input voltage Vin and variable load resistance R(L)

The load resistance should has minimum value so that the voltage across zener diode is greater or equal than the voltage Vz. So Vz limits the value of load resistance R(L).

To find the minimum value of R(L), the output voltage V(L) should be equal to Vz.

since V(L) = Vz , and we know Vz = ( R(L) x Vin ) / ( R + R(L)

this implies, R(L)min = ( R x Vz ) / ( Vin – Vz )

Any value greater than or equal to R(L)min will make sure that the diode is in active state. If we have minimum value of load resistance then load current I(L) will be maximum,

I(L)max = V(L) / R(L)min

When we have maximum load current then the zener current will be minimum or vice-versa.

03: When we have fixed value of load resistance and variable input voltage Vin

In this case we have a minimum value of input voltage Vin(min) so that zener diode works as a voltage regulator

since V = Vz = ( R(L) x Vin ) / ( R + R(L) )

this implies Vin(min) = ( ( R + R(L) ) x Vz ) / R(L)

the value of load resistance should be sufficient and it limits the maximum value of input voltage,

Vin(max) = Vr(max) + Vz

Vin(max) = ( Ir(max) x R ) + Vz

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