TOPIC : VOLTAGE STABILIZER USING ZENER DIODE
Normally we use voltage stabilizers to provide a constant voltage from a d.c source across a load. Zener diode can be used as a voltage regulator.
A properly doped crystal diode, which has a sharp break down voltage, is known as zener diode. It is operated always under reverse bias condition in the breakdown region.
Zener breakdown takes place in very thin junctions. When both P and n regions are heavily doped, the depletion layer becomes very narrow. In a very thin depletion layer, the electric field across the depletion layer becomes as high as 107 Vm-1 with only small applied voltage. An electric field of such a high magnitude exerts a large force on the valence electrons of the atoms in the depletion layer. Consequently, covalent bonds are broken and as large number of electron-hole pairs are produced. These carriers are then accelerated by the applied voltage. Hence the reverse current increases rapidly. This process by which covalent bonds in the depletion region are directly broken by the strong electric field is called zener breakdown and the reverse voltage at which breakdown takes place is called zener voltage (Vz).
By the careful adjustment of the doping, the characteristic beyond the breakdown voltage is obtained almost vertical. In other words, in this region the reverse voltage across the diode remains almost constant for a large variation of reverse current. So the zener diode can be used as a voltage stabilizer. Therefore it is interesting to find the variation of output voltage on different input voltage in a voltage stabilizer.
To construct a voltage stabilizer and to study its characteristics.
METHOD OF STUDY
Connections are made as shown in figure.
R is the current limiting resistor, which keeps the current through zener diode within the permissible limit and RL is the load across which we get the regulated output. To make this connection, an external d.c source, a Rheostat, two voltmeters, a Zener diode, a Resistor and a key are needed.
The input (dc) voltage is applied to the zener diode.So that the diode is reverse biased.The voltage across RL will be always Vz, the zener voltage of the diode provided, the input voltage does not go below Vz, the zener voltage. Measurement of output voltage for different input voltages across the zener diode is taken.
The output voltages across the zener diode for different input voltages are given below:
Zener voltage = 6 V
Resistance of the resistor= 330 ?
From the above observation we could understand that the voltage across RL(Load) will be always Vz (here ˜6V) the zener voltage of the diode provided the input voltage does not go below Vz,the zener voltage. If input voltage is less than that of zener voltage, the output shows same voltage as in the input. But if input voltage is greater than that of Zener voltage, the output gives only zener voltage. So zener diode can be used as a voltage regulator to provide a constant voltage from a d.c source across a load RL.
? Suppose the input voltage Vi increases beyond Vz. Since the zener is in the breakdown region. The voltage across the zener diode and hence the output voltage across the load remains constant as Vz. The excess voltage, Vi-Vz. is dropped across the current limiting resistor R. if I is the current from the sources.
Vi- Vz = IR
To put it in another way, as Vi increases beyond Vz, I = IL + IZ increases. The zener will conduct the increase of current in I while the load current IL remains constant. Hence the output voltage across RL= I L X RL remains constant irrespective of the change in the input voltage Vi.
? Suppose the load resistance RL changes.then also the output voltage remains constant at Vz.
In the case if the load Rl increases, there is a decrease in the source current I at constant VI
It can be shown that the load current. IL decreases and the decreases is due to the decrease in the total current I and the increase in the zener current Iz. Thus the voltage across the load remains constant as VZ.
Voltage stabilizers (zener diodes) can be used as a voltage regulator to provide a constant voltage from a d..c source across a load RL.
By using voltage stabilizers, we can save our electrical equipments to a large extent. Because whenever the input voltage in the circuit increases, by thundering, short circuiting or by any other way, zener diode give only its zener voltage to the appliance as output voltage. Otherwise the appliances will burn out due to high voltage.
• Zener diode must be reverse biased.
• The depletion layer must be kept very thin.
• Zener diode must connect parallel to the load.
• By using voltage stabilizers in electrical appliances, we can save them to a large extent from thundering, short circuiting etc;-
Fundamentals of PHYSICS CLASS XII BY Was Publication.
Practical Physics class XI & XII by sunil A Pillai – An Anamika Publication.
NCERT Text of class XII.
Plus two physics by Xavier and Roy.
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