Zener Diode

 

·         Zener diode:

Construction:

  It is a heavily doped pn junction diode which is always used in reverse bias mode.

            The depletion layer is very narrow.

            Zener diode can operate in any of the three regions:

            i)forward, ii) leakage, iii) breakdown

Symbol of Zener Diode:



In Zener diode, the barrier separating the two bands is very narrow, hence tunnelling of electrons from p-side valence band to the n-side conduction band occurs.

This constitutes a reverse current from n-region to p-region which is called as Zener effect.

At a particular voltage, a very sharp reverse current increases sharply and this voltage is known as Zener breakdown voltage.

Because of this property, Zener diode is used as voltage regulators.


Operating Principle:

            A heavily doped p-n junction diode which has a sharp breakdown voltage is known as Zener diode.

            The sharp breakdown voltage of Zener diode called Zener voltage (Vz), decided by doping level at the time of manufacturing the Zener diode.

            These diodes can operate at voltages ranging from a few volts to several hundred volts.

            Circuit to study Zener diode operation:

            


            I-V graph of Zener Diode

            


            I-V Characteristics:

            In the forward region the Zener diode acts like an ordinary PN junction diode.

            It starts conducting at about 0.7V.

            When it is reverse biased, The region between zero and breakdown is called leakage region.

            In this region there is only a small leakage or reverse current.

            In Zener diode, the breakdown has a very sharp knee, followed by an almost vertical increase in current.

            The voltage is almost constant and approximately equal to Vz over most of the breakdown region.

            Vz is specified at a particular test current IZT

            For normal operation, the current through the diode should be limited by an external circuit to a suitable value.

            Otherwise large current can damage the Zener diode.

            Zener diodes are usually operated in the breakdown region.

           Zener diode as a voltage regulator.

            


           Specification Parameters:

            1) Zener voltage and power rating.

            2) below power rating, Zener diode can operate in breakdown region safely and above the given 

                 rating it will get destroyed.

            3) Izmax, RZT, IZT, Vz and power rating are the specification parameters for Zener diode.

            4) The Zener diodes are available in the market with power rating from ¼ Watt to more than 60

                 Watts.

            Applications:

            1) Mostly used in voltage regulator circuits.

            2) They are also used to keep reference voltages required in certain circuits like comparator, in wave shaping circuits to keep amplitude of signal waves at particular voltage limits.

Comments

Post a Comment

Popular posts from this blog

Optocoupler

Image
  Optocoupler :              An optocoupler is a device that uses a short optical transmission path to transfer a signal   between elements of a circuit, typically transmitter and receiver which are electrically isolated.              It is basically used as isolator circuit. Fig. Circuit Symbol of optocoupler Construction              Basically optocoupler is a pair of LED and phototransistor enclosed in a single package.                           Here LED and phototransistor are isolated so that light may travel across a barrier but electrical current may not. Working Principle Specifications and Parameters :              Device operatin...
Read more

Zener Diode, IC 78XX and 79XX as a Voltage Regulator

Image
 Zener Diode as a Voltage Regulator Fig. Voltage regulator circuit using Zener diode The Zener diode is always connected in reverse biased in series in a voltage regulator circuit as shown in above fig. The Zener diode is selected with a Vz equal to the voltage desired across the load. The Zener diode acts as a ‘bypass valve’ through which more current flows when an increase in input voltage or a decrease in load current occurs, maintaining the voltage at the output nearly constant at Vz. The resistance Rs drops the excess voltage from the input to give the desired output; hence this circuit cannot compensate for input voltages smaller than desired output. The selection of R is made by using the following equation Rs= (Vin-Vout) / Iin Where, I = Iz + I L When R L is not connected,  I L =0 i.e. I= Iz = Iz(max) Therefore, Zener must be capable of passing all the current I through it. For Zener to be remaining in breakdown, Iz must not be permitted to fall below Iz(min) to work...
Read more