T. Y. B.Sc. (PHYSICS) ELECTRONICS Dr. KALANGE A.E.

REGULATED POWER SUPPLY

The voltage regulators which are fabricated on a single silicon chip are called
monolithic or IC regulators. Advantages of these regulators are small size, easy to
use, high performance and low cost. Monolithic voltage regulators are three terminal
devices having three terminals denoted as input, output and common terminals.
These regulators are available in fixed or variable output voltage, positive or
negative output voltage.

Block Diagram of Three Pin Regulator

Fig. 1 shows simplified block diagram of three pin regulator. The

three terminals are Vin (input voltage), Vout (output voltage) and common terminal.
The dc unregulated voltage is applied to the terminal Vin, supply ground to
common and regulated output is taken at terminal Vout.

The block diagram in fig.1 is basically a series regulator. The two

resistors R1 and R2 form the sampling network to produce the feedback signal
proportional to the output voltage. Vref is the internally generated voltage reference
voltage. This reference voltage is compared with the feedback signal by the
comparator to produce the control voltage. This control signal is transferred through
the thermal shutdown and current limiting block to the series pass transistor. The
series pass transistor is operating as the control element. The voltage across the
control element is varied by the control signal to get a constant output voltage.

The thermal shutdown and current limiting block provides the

protection against increased internal temperature or over current. If the internal
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temperature exceeds a predetermined value, then the thermal shutdown block will
not allow the control signal to pass through to the series pass transistor. This will
turn off the chip automatically. The current limiting circuit protects the regulator
against over current.

Three Terminal Fixed Voltage Regulators

The three terminal voltage regulators are of two types: Fixed and adjustable
voltage regulators. In fixed voltage regulators we have positive voltage regulators
and negative voltage regulators. The 78XX series is a series of fixed positive voltage
regulators and the 79XX series is a series of fixed negative voltage regulators.

78XX Series:

78Xx series are three terminal, positive fixed voltage regulators. Here XX indicate
the output voltage. These regulators are available in seven different output voltages
such as 5, 6, 8, 12, 15, 18 and 24 volts. e.g. IC 7812 means +12V.

IC Number 7805 7806 7808 7812 7815 7818 7824

Output Voltage 5V 6V 8V 12V 15V 18V 24V

79XX Series:

79Xx series are three terminal, negative fixed voltage regulators. Here XX indicate
the output voltage. These regulators are available in nine different output voltages
such as 5, 6, 8, 12, 15, 18 and 24 volts in addition to this series provides -2 and -5.2V.
e.g. IC 7812 means +12V.

IC Number 7905 7906 7908 7912 7915 7918 7924

Output Voltage -5V -6V -8V -12V -15V -18V -24V

Standard connection for fixed positive voltage regulators (78XX):

The fixed voltage regulator IC’s have three terminals: input, ground and output. The
pin diagram of three pin regulator is shown in Fig.2 (a) and the standard connections
for a three pin fixed positive regulator is shown in fig.2 (b).

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Fig. 2(a): pin diagram of three pin regulator

Fig.2 (b): The standard connections for a three pin fixed IC regulator.

An unregulated dc voltage is applied at the input terminal of the IC. The

capacitor Ci connected between the input terminal and ground cancels out any
inductive effect due to long distribution leads. The output capacitor Co is used for
improving the transient response of the regulator IC i.e the response of the regulator
to the sudden changes in the load. This capacitor is also reduces the noise present at
the output.

The difference between the unregulated input voltage Vin and the output
voltage Vo is called drop out voltage. The drop out voltage need to be at least 2 volt
under all the operating conditions for proper operation of regulator.

Drop out Voltage= Vin -Vout

A Positive 12 V Power supply using IC 7812:

The circuit diagram of a 12 v regulated power supply using IC 7812is shown in Fig.3

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The bridge rectifier and capacitor input filter produce an unregulated dc

voltage which is applied at the Input terminal of IC 7812. As the minimum drop out
voltage is 2v for Ic 7812, the voltage applied at the Input terminal should be at least
14V. C1 is the filter capacitor and C2 is connected at the output terminal to improve
the transient response of the regulator.

Negative voltage regulators (79XX):

The fixed voltage regulator IC’s have three terminals: input, ground and output. The
pin diagram of three pin regulator is shown in Fig.2 (a) and the standard connections
for a three pin fixed negative regulator is shown in fig.2 (b).

Fig. 3(a): pin diagram of three pin regulator

Standard connection for fixed negative voltage regulators (79XX):

Fig.3 (b): The standard connections for a three pin fixed IC regulator.

An unregulated dc voltage is applied at the input terminal of the IC. The

capacitor Ci connected between the input terminal and ground cancels out any
inductive effect due to long distribution leads. The output capacitor C o is used for
improving the transient response of the regulator IC i.e the response of the regulator

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to the sudden changes in the load. This capacitor is also reduces the noise present at
the output.

The difference between the unregulated input voltage Vin and the output
voltage Vo is called drop out voltage. The drop out voltage need to be at least 2 volt
under all the operating conditions for proper operation of regulator.

Drop out Voltage= Vin -Vout

A Negative 12 V Power supply using IC 7912:

The circuit diagram of a -12 V regulated power supply using IC 7912 is shown in
Fig.4

Fig.4: A Negative 12 V Power supply using IC 7912

A full wave rectifier and capacitor filter Ci produces the unregulated negative
dc input to the regulator IC. At the output of 7912, we get a regulated output of -12V.

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Linear IC 723 Regulator:

The popular general purpose precision regulator is Linear IC 723 Regulator. It is a

monolithic linear integrated circuit in different physical packages.

The pin diagram along with the various packages is shown in the Fig. 1 (a), (b).

Important Features of IC 723:

1. It works as voltage regulator at output voltage ranging from 2 to 37 volts at

currents upto 150 mA.
2. It can be used at load currents greater than 150 mA with use of suitable NPN
or PNP external pass transistors.
3. Input and output short-circuit protection is provided.
4. It has good line and load regulation (0.03%)
5. Wide variety of applications of series, shunt, switching and floating regulator.
6. Low temperature drift and high ripple rejection.
7. Small size, lower cost
8. Relative ease with which power supply can be designed.
9. It provides a choice of supply voltage.

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Internal Structure of IC 723:

The functional block diagram of IC 723 can be divided into four major blocks

1. Temperature compensated voltage reference source, which is zener diode.

2. An op-amp circuit used as an error amplifier.
3. A series pass transistor capable of a 150 mA output current.
4. Transistor used to limit output current.

The functioning of the above blocks can be explained with the help of a simplified
functional block diagram of IC 723 as shown in the Fig. 2.

Temperature compensated zener diode, constant current source and reference

amplifier constitutes the reference element. In order to get a fixed voltage from zener
diode, the constant current source forces the zener to operate at a fixed point.

Output voltage is compared with this temperature compensated reference potential

of the order of 7 volts. For this Vref is connected to the non-inverting input of the
error amplifier.

This error amplifier is high gain differential amplifier. It’s inverting input is
connected to the either whole regulated output voltage or part of that from outside.
Error amplifier controls the series pass transistor Qi, which acts as variable resistor.
The series pass transistor is a small power transistor having about 800 mW
dissipation. The unregulated power supply source (< 36V d.c.) is connected to
collector of series pass transistor.

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Transistor Q2 acts as current limiter in case of short circuit condition. It senses drop
across Rsc placed in series with regulated output voltage externally.

The frequency compensation terminal controls the frequency response of the error
amplifier. The internal structure can be represented in more simplified form as
shown in the Fig. 2.111.

Both non inverting and inverting terminals of the error amplifier are available on
outside pins of IC 723. Due to this, device becomes versatile and flexible to use. Only
restriction is that internal reference voltage is 7 volts and therefore we have to use
two different circuits for getting regulated outputs of below 7 volts and above 7
volts.

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