A rectifier with an appropriate filter serves as a good source of d.c. output.
However, the major disadvantage of such a power supply is that the output voltage
changes with the variations in the input voltage or load.
Thus, if the input voltage
increases, the d.c. output voltage of the rectifier also increases. Similarly, if the load
current increases, the output voltage falls due the voltage drop in the rectifying element,
filter chokes, transformer winding etc. In many electronic applications, it is desired that
the output voltage should remain constant regardless of the variations in the input
voltage or load.
In order to ensure this, a voltage stabilizing device, called voltage
stabilizer is used. Several stylizing circuits have been designed but only as
a voltage stabilizer will be discussed.
It has already been discussed that when the reverse bias on a crystal diode is
increased, a critical voltage, called breakdown voltage is reached where the reverse
current increases sharply to a high value.
The breakdown region is the knee of the
reverse characteristic as shown in Fig.1.
The satisfactory explanation of this breakdown of the junction was first given by the American scientist C. Zenger.
Therefore, the breakdown voltage is sometimes called zener voltage and the sudden
increase in current is known as zener current.
The breakdown or zener voltage depends upon the amount of doping. If the
diode is heavily doped, depletion laer will be thin and consequently the breakdown of
the junction will occur at a lower reverse voltage. On the other hand, a lightly doped
diode has a higher breakdown voltage; it is called a zener diode.
A properly doped crystal diode which has a sharp breakdown voltage is known
as a zener diode.
Fig.2 shows the symbol of a zener diode. It may be seen that it is just like an
ordinary diode except that the bar is turned into z-shape.
The following points may be noted about the zener diode:
(i) A zener diode is like an ordinary diode except that it is properly doped so
as to have a sharp breakdown voltage.
(ii) A zener diode is always reverse connected i.e. it is always reverse biased.
(iii) A zener diode has sharp breakdown voltage, called zener voltage Vz .
(iv) When forward biased, its characteristics adore just those of ordinary
(v) The zener diode is not immediately burnt just because it has entered the
*breakdown region. As long as the external circuit connected to the diode
limits the diode current to less than burn out value, the diode will not burn
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