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 to 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 stabilising device, called voltage stabiliser is used. Several stabilising circuits have been designed but only zener diode as a voltage stabiliser 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. 6.52. The
satisfactory explanation of this breakdown of the junction was first given by the American scientist C. Zener. 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 layer 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.
When an ordinary crystal diode is properly doped so that it has a sharp 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. 6.53 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 are just those of ordinary diode.
(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 out.
The analysis of circuits using zener diodes can be made quite easily by replacing the zener diode by its equivalent circuit.
(i) “On” state. When reverse voltage across a zener diode is equal to or more than break down voltage VZ, the current increases very sharply. In this region, the curve is almost vertical. It means that voltage across zener diode is constant at VZ even though the current through it changes. Therefore, in the breakdown region, an **ideal zener diode can be represented by a battery of voltage VZ as shown in Fig. 6.54 (ii). Under such conditions, the zener diode is said to be in the “ON” state.
(ii) “OFF” state. When the reverse voltage across the zener diode is less than VZ but greater
than 0 V, the zener diode is in the “OFF” state. Under such conditions, the zener diode can be
represented by an open-circuit as shown in Fig. 6.55 (ii).
A zener diode can be used as a voltage regulator to provide a constant voltage from a source whose voltage may vary over sufficient range. The circuit arrangement is shown in Fig. 6.56 (i). The Zener diode of zener voltage VZ is reverse connected across the load RL across which constant output is desired. The series resistance R absorbs the output voltage fluctuations so as to maintain constant voltage across the load. It may be noted that the zener will maintain a constant voltage VZ(= E0) across the load so long as the input voltage does not fall below VZ
When the circuit is properly designed, the load voltage E0 remains essentially constant (equal to VZ) even though the input voltage Ei and load resistance RL may vary over a wide range.
(i) Suppose the input voltage increases. Since the zener is in the breakdown region, the Zener diode is equivalent to a battery VZ as shown in Fig. 6.56 (ii). It is clear that output voltage remains constant at VZ (= E0). The excess voltage is dropped across the series resistance R. This will cause an increase in the value of total current I. The zener will conduct the increase of current in I while the load current remains constant. Hence, output voltage E0 remains constant irrespective of the changes in the input voltage Ei
(ii) Now suppose that input voltage is constant but the load resistance RL decreases. This will cause an increase in load current. The extra current cannot come from the source because drop in R
(and hence source current I) will not change as the zener is within its regulating range. The additional load current will come from a decrease in zener current IZ. Consequently, the output voltage stays at constant value.
Voltage drop across R = Ei− E0
Current through R, I = Iz+IL
The analysis of zener diode circuits is quite similar to that applied to the analysis of semiconductor
diodes. The first step is to determine the state of zener diode i.e., whether the zener is in the “on”
state or “off” state. Next, the zener is replaced by its appropriate model. Finally, the unknown
quantities are determined from the resulting circuit.
If V ≥ VZ , the zener diode is in the “on” state and its equivalent model can be substituted as shown in Fig. 6.58 (i). If V < VZ , the diode is in the “off” state as shown in Fig. 6.58 (ii).
(i) On state. Referring to circuit shown in Fig. 6.58 (i),
(ii) Off state. Referring to the circuit shown in Fig. 6.58 (ii),
This is the minimum value of load resistance that will ensure that zener is in the “on” state. Any value of load resistance less than this value will result in a voltage E0 across the load less than VZ and the zener will be in the “off” state.
(ii) ILmin and RLmax. It is easy to see that when load resistance is maximum, load current is minimum.
Now, Zener current, IZ= I − IL
When the zener is in the “on” state, I remains **fixed. This means that when IL is maximum, IZ will be minimum. On the other hand, when IL is minimum, IZ is maximum. If the maximum current that a zener can carry safely is IZM, then,
If the load resistance exceeds this limiting value, the current through zener will exceed IZM and the device may burn out.
Fixed RL and Variable Ei. This case is shown in Fig. 6.60. Here the load resistance RL is fixed while the applied voltage (Ei) changes. Note that there is a definite range of Ei values that will ensure that zener diode is in the “on” state. Let us calculate that range of values.
(i) Ei (min). To determine the minimum applied voltage that will turn the Zener on, simply calculate the value of Ei that will result in load voltage E0 = VZ i.e.,
(ii) Ei (max)
Now, current through R, I = IZ+IL
Since IL(= E0/RL= VZ/RL) is fixed, the value of I will be maximum when zener current is maximum i.e.,
Imax = IZM + IL
Now Ei= I R + E0
Since E0(= VZ) is constant, the input voltage will be maximum when I is maximum.
∴ Ei (max)= Imax R + VZ
Example 6.25. For the circuit shown in Fig. 6.61 (i), find :
(i) the output voltage (ii) the voltage drop across series resistance
(iii) the current through zener diode.
Solution. If you remove the zener diode in Fig. 6.61 (i), the voltage V across the open-circuit is
given by :
Example 6.26. For the circuit shown in Fig. 6.62 (i), find the maximum and minimum values of
zener diode current.
Solution. The first step is to determine the state of the zener diode. It is easy to see that for the
given range of voltages (80 − 120 V), the voltage across the zener is greater than VZ(= 50 V). Hence the zener diode will be in the “on” state for this range of applied voltages. Consequently, it can be replaced by a battery of 50 V as shown in Fig. 6.62 (ii).
Maximum zener current. The zener will conduct *maximum current when the input voltage is
maximum i.e. 120 V. Under such conditions :
Minimum Zener current. The zener will conduct minimum current when the input voltage is
minimum i.e. 80 V. Under such conditions, we have,
Example 6.27. A 7.2 V zener is used in the circuit shown in Fig. 6.63 and the load current is to
vary from 12 to 100 mA. Find the value of series resistance R to maintain a voltage of 7.2 V across
the load. The input voltage is constant at 12V and the minimum zener current is 10 mA.
The voltage across R is to remain constant at 12 − 7.2 = 4.8 V as the load current changes from
12 to 100 mA. The minimum zener current will occur when the load current is maximum.
If R = 43.5 Ω is inserted in the circuit, the output voltage will remain constant over the regulating range. As the load current IL decreases, the zener current IZ will increase to such a value that IZ+IL=110 mA. Note that if load resistance is open-circuited, then IL= 0 and zener current becomes 110 mA.
Example 6.28. The zener diode shown in Fig. 6.64 has VZ= 18 V. The voltage across the load stays at 18 V as long as IZ is maintained between 200 mA and 2 A. Find the value of series resistance R so that E0 remains 18 V while input voltage Ei is free to vary between 22 V to 28V.
Solution. The zener current will be minimum (i.e. 200 mA) when the input voltage is minimum
(i.e. 22 V). The load current stays at constant value IL = VZ/ RL = 18 V/18 Ω = 1 A = 1000 mA.
Example 6.29. A 10-V zener diode is used to regulate the voltage across a variable load resistor
[See fig. 6.65]. The input voltage varies between 13 V and 16 V and the load current varies between 10mA and 85 mA. The minimum zener current is 15 mA. Calculate the value of series resistance R.
Solution. The zener will conduct minimum current (i.e. 15 mA) when input voltage is minimum
(i.e. 13 V).
Example 6.30. The circuit of Fig. 6.66 uses two zener diodes, each rated at 15 V, 200 mA. If the
circuit is connected to a 45-volt unregulated supply, determine :
(i) The regulated output voltage (ii) The value of series resistance R
Solution. When the desired regulated output voltage is higher than the rated voltage of the
zener, two or more zeners are connected in series as shown in Fig. 6.66. However, in such circuits,
care must be taken to select those zeners that have the same current rating.
Example 6.31. What value of series resistance is required when three 10-watt, 10-volt, 1000 mA zener diodes are connected in series to obtain a 30-volt regulated output from a 45 volt d.c.
power source ?
Solution. Fig. 6.67 shows the desired circuit. The worst case is at no load because then zeners
carry the maximum current.
Example 6.32. Over what range of input voltage will the zener circuit shown in Fig. 6.68
maintain 30 V across 2000 Ω load, assuming that series resistance R = 200 Ω and zener current
rating is 25 mA ?
Solution. The minimum input voltage required will be when IZ= 0. Under this condition,
Therefore, the input voltage range over which the circuit will maintain 30 V across the load is
33 V to 38 V.
Example 6.33. In the circuit shown in Fig. 6.69, the voltage across the load is to be maintained at 12 V as load current varies from 0 to 200 mA. Design the regulator. Also find the maximum wattage rating of zener diode.
Solution. By designing the regulator here means to find the values of VZ and R. Since the load voltage is to be maintained at 12 V, we will use a zener diode of zener voltage 12 V i.e.,
VZ = 12 V
The voltage across R is to remain constant at 16 − 12 = 4 V as the load current changes from 0 to
200 mA. The minimum zener current will occur when the load current is maximum.
Maximum power rating of zener is PZM = VZ IZM = (12 V) (200 mA) = 2.4 W
Example. 6.34. Fig. 6.70 shows the basic zener diode circuits. What will be the circuit behaviour
if the zener is (i) working properly (ii) shorted (iii) open-circuited?
Solution. Zener diodes cannot be tested individually with a multimeter. It is because multimeters
usually do not have enough input voltage to put the zener into breakdown region.
(i) If the zener diode is working properly, the voltage V0 across the load (= 5 kΩ) will be nearly
6V [See Fig. 6.70 (i)].
(ii) If the zener diode is short [See Fig. 6.70 (ii)], you will measure V0 as 0V. The same problem
could also be caused by a shorted load resistor (= 5kΩ) or an opened source resistor (= 1 kΩ). The
only way to tell which device has failed is to remove the resistors and check them with an ohmmeter. If the resistors are good, then zener diode is bad.
(iii) If the zener diode is open-circuited, the voltage V0 across the load (= 5 kΩ) will be 10V.
Example 6.35. Fig. 6.71 shows regulated power supply using a zener diode. What will be the
circuit behaviour if (i) filter capacitor shorts (ii) filter capacitor opens?
Solution. The common faults in a zener voltage regulator are shorted filter capacitor or opened
(i) When filter capacitor shorts. When the filter capacitor shorts, the primary fuse will blow. The reason for this is illustrated in Fig. 6.71. When the filter capacitor shorts, it shorts out the load
resistance RL. This has the same effect as wiring the two sides of the bridge together (See Fig. 6.71).
If you trace from the high side of the bridge to the low side, you will see that the only resistance across the secondary of the transformer is the forward resistance of the two ON diodes. This effectively shorts out the transformer secondary. The result is that excessive current flows in the secondary and hence in the primary. Consequently, the primary fuse will blow.
(ii) When filter capacitor opens. When the filter capacitor opens, it will cause the ripple in the
power supply output to increase drastically. At the same time, the d.c. output voltage will show a
significant drop. Since an open filter capacitor is the only fault that will cause both of these symptoms, no further testing is necessary. If both symptoms appear, replace the filter capacitor.
Semiconductor diodes (or crystal diodes) have a number of advantages and disadvantages as compared to their electron-tube counterparts (i.e., vacuum diodes).
(i) They are smaller, more rugged and have a longer life.
(ii) They are simpler and inherently cheaper.
(iii) They require no filament power. As a result, they produce less heat than the equivalent
(i) They are extremely heat sensitive. Even a slight rise in temperature increases the current
appreciably. Should the temperature *exceed the rated value of the diode, the increased flow of
current may produce enough heat to ruin the pn junction. On the other hand, vacuum diodes function normally over a wide range of temperature changes. It may be noted that silicon is better than germanium as a semiconductor material. Whereas germanium diode should not be operated at temperatures higher than 80ºC, silicon diodes may operate safely at temperatures upto about 200ºC.
(ii) They can handle small currents and low inverse voltages as compared to vacuum diodes.
(iii) They cannot stand an overload even for a short period. Any slight overload, even a transient pulse, may permanently damage the crystal diode. On the other hand, vacuum diodes can stand an overload for a short period and when the overload is removed, the tube will generally recover
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