Regulated Power Supply

UNIT – II: REGULATED POWER SUPPLY.
Concept of voltage regulation, unregulated & regulated power supply, block
diagram of regulated power supply, Zener diode and its characteristics , Design of
Zener diode voltage regulator. Transistor series voltage regulator, Concept of IC,
Three pin IC regulator block diagram , 78xx series & 79xx series.
1. What is D.C Power supply?
D.C Power supply: In practice, d.c. power for electronic circuits is most
conveniently obtained from commercial a.c. lines by using rectifier-filter system,
called a d.c. power supply.
2. What is Voltage regulation?
The variation of output voltage w.r.t the amount of load current drawn from the
power supply is known as Voltageregulation and is given by




% Voltage regulation= or
Where VNL= DC output voltage at no-load VFL= DC output voltage at full-load
Problem 1: If the d.c. output voltage is 400V with no-load attached to power
supply but decreases to 300V at full-load, find the percentage voltage regulation










Problem 2: A power supply has a voltage regulation of 1%. If the no-load voltage
is 30V,what is the full-load voltage ? Solution.
Let VFL be the full-load voltage














2. What is Load regulation?
In the absence of properly designed voltage regulator the voltage across the load
cannot constant with the variation in the load current.
The change in load voltage with change in load current is called load regulation
and it is given by




% load regulation=
Where (Vdc)NL= DC output voltage at no-load
(Vdc)FL= DC output voltage at full-load
Dr P I Mandi, Dept. of Electronics , Basaveshwar Science College,Bagalkot # 2019 -20

2. What is Line Regulation?
Therefore, change in the load voltage due to the change in the line voltage is
called Line Regulation.
It is measure of how sensitive the output voltage is to the changes in the
a.c.voltage(line voltage). It is defined as,




%Line Regulation= x100
3. What is Stability Factor ?
3. Stability Factor or Voltage Stability Factor(Sv):
The change in load voltage due to the change in output voltage(Line Voltage) is
called Voltage stability Factor(Sv).
4. What is Regulated Power Supply? A d.c. power supply which maintains the
output voltage constant irrespective of a.c. mains fluctuations or load variations

is known as regulated d.c. power supply

The regulated power supply is a combination of three circuits viz, (i) bridge

rectifier (ii) a capacitor filter C and (iii) zener voltage regulator.

5. Draw the block diagram of Regulated Power Supply












5. What is the need of Regulated Power Supply?
Need of Regulated Power Supply
1. Variations in a.c. line voltage causes changes the d.c. output voltage.
2. The internal resistance of ordinary power supply is relatively large.


6. What is Integrated Circuit (IC)?
Integrated Circuit is a one in which circuit components such as transistors,

diodes, resistors, capacitors etc are automatically part of a small semiconductor
chip.


It consists of number of circuit components (transistors, diodes, resistors,
capacitors etc) and their inter connection in a single small package to perform a
complete electronic function.







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7. What is the significance of last two digit in 78xx series?

In 78XX series the last two digits XX indicates the output voltage. Suppose 7815
represents 15 Voltsregulator


8. What is Fixed Voltage regulator:


78XX series have +ve fixed voltage regulators and these are available in 7 options
of output voltage such as 5,6,8,12,15,18 and 21Volts.

Note that 7900 series provides fixed regulated voltages from – 5V to – 24 V.


Scale of integration Abbreviation No. of components
Small SSI 1-20
Medium MSI 20-100
Large LSI 100-1000
Very large VLSI 1000-10000
Super large SLSI 10,000-100000

















The size of the IC is extremely small. No components of an IC are seen to project
above the surface of the chip. This is because all the components are formed
within the chip.
Advantages:
(i) Increased reliability due to lesser number of connections.
(ii) Extremely small size due to fabrication of various circuits in a single
chip.
(iii) Lesser weight and space required.
(iv) Low power requirements.
Low cost because of simultaneous production of hundreds of alike circuits on a
small semiconductor wafer.
Classifications:
(i) Monolithic
(ii) Thin film
(iii) Thick film

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(iv) Hybrid
















5 Marks Questions:


1. Explain the Zener Diode Characteristics.

Zener Diode Characteristics:


A properly doped crystal diode which has a sharp breakdown voltage is known
as a zener diode.


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. 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
Semiconductor Diode.
Fig. 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.

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(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.







2. Explain Zener Diode as a Voltage Regulator


Zener Diode as Voltageregulator


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. (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.


















(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. (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) Suppose that input voltage is constant but the load resistance RL
decreases. This will cause an increase in load current. The extra

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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.

Voltagedrop across R = Ei − E0
Current through R, I = IZ + IL
3. Explain TransistorSeries VoltageRegulator.

TransistorSeries VoltageRegulator:
Figure shows a simple series voltage regulator using a transistor and zener diode.
The circuit is called a series voltage regulator because the load current passes
through the series transistor Q1 as shown in Fig. The unregulated d.c. supply is
fed to the input terminals and the regulated output is obtained across the load.
The zener diode provides the reference voltage.

























Operation: The base voltage of transistor Q1 is held to a relatively constant
voltage across the zener diode. For example, if 8V zener (i.e., VZ = 8V) is used,
the base voltage of Q1 will remain approximately 8V.
Referring to Fig. Vout = VZ − VBE
(i) If the output voltage decreases, the increased base-emitter voltage
causes transistor Q1 to conduct more, thereby raising the output
voltage. As a result, the output voltage is maintained at a constant
level.
(ii) If the output voltage increases, the decreased base-emitter voltage
causes transistor Q1 to conduct less, thereby reducing the output
voltage. Consequently, the output voltage is maintained at a constant
level. The advantage of this circuit is that the changes in zener
current are reduced by a factor β. Therefore, the effect of zener
impedance is greatly reduced and much more stabilised output is
obtained.
Limitations (i) Although the changes in zener current are much reduced, yet
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the output is not absolutely constant. It is because both VBE and VZ decrease
with the increase in room temperature.


(ii) The output voltage cannot be changed easily as no such means is provided.

4. Draw the block diagram of Regulated Power Supply and explain.


Regulated Power Supply:


A d.c. power supply which maintains the output voltage constant irrespective of
a.c. mains fluctuations or load variations is known as regulated d.c. power
supply.












A regulated power supply consists of an ordinary power supply and voltage

regulating device. Fig. shows the block diagram of a regulated power supply.
The output of ordinary power supply is fed to the voltage regulator which
produces the final output. The output voltage remains constant whether the load

current changes or there are fluctuations in the input a.c. voltage.

Fig. shows the complete circuit of a regulated power supply using zener diode as

a voltage regulating device.The regulated power supply is a combination of three
circuits viz., (i) bridge rectifier (ii) a capacitor filter C and (iii) zener voltage
regulator.


The bridge rectifier converts the transformer secondary a.c. voltage (point P)
into pulsating voltage (point Q). The pulsating d.c. voltage is applied to the
capacitor filter.


This filter reduces the pulsations in the rectifier d.c. output voltage (point R).
Finally, the zener voltage regulator performs two functions.


(a) it reduces the variations in the filtered output voltage.

(b) it keeps the output voltage (Vout) nearly constant whether the load

current changes or there is change in input a.c. voltage. Below fig. shows
the waveforms at various stages of regulated power supply. Note that
bridge rectifier and capacitor filter constitute an ordinary power supply.

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However, when voltage regulating device is added to this ordinary power
supply, it turns into a regulated power supply.






5. Explain Fixed Positive VoltageRegulators
Fixed Positive VoltageRegulators
This IC regulator provides a fixed positive output voltage. Although many types
of IC regulators are available, the 7800 series of IC regulators is the most popular.
The last two digits in the part number indicate the d.c. output voltage. For example
[See Table below], the 7812 is a + 12V regulator whereas the 7805 is a + 5V
regulator. Note that this series (7800 series) provides fixed regulated voltages from
+ 5 V to + 24V.

















Fig. shows how the 7812 IC is connected to provide a fixed d.c. output of + 12V.
The unregulated input voltage Vi is connected to the IC’s IN terminal and the IC’s
OUT terminal provides + 12V. Capacitors, although not always necessary, are
sometimes used on the input and output as shown in Fig. The output capacitor (C2)

acts basically as a line filter to improve transient response. The input capacitor (C1)
is used to prevent unwanted oscillation.

Fixed Negative VoltageRegulators


This IC regulator provides a fixed negative ouput voltage. The 7900 series of IC

regulators is commonly used for this purpose. This series (7900) is the
negative-voltage counterpart of the7800 series. Note that 7900 series provides fixed
regulated voltages from – 5V to – 24 V.


Fig. shows how 7912 IC is connected to
provide a fixed d.c. output of – 12 V.The

unregulated negative input voltage Vi is
connected to IC’s IN terminal and the IC’s
OUT terminal provides – 12 V.Capacitors
used in the circuit perform the same function

Dr P I Mandi, Dept. of Electronics , Basaveshwar Science College,Bagalkot # 2019 -20

as in a fixed positive regulator.




















































































Dr P I Mandi, Dept. of Electronics , Basaveshwar Science College,Bagalkot # 2019 -20