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In practice, it is often required to make or break
an electric al circuit in many operations. In some
applications, it is desirable that this make and break
should be very quick and without sparking. The
mechanical switches or electric-mechanical switches (i.e.
relays) cannot be used for the purpose due to two main
reasons. First, these switches have a high inertia which
limits their speed of operation which limits their speed of
operation. Second, there is sparking at the contacts
during breaking operation which result in the burning of
contacts. Modern researches have revealed that tubes
and transistors can serve as the switching devices. They
can turn ON or OFF power in an electrical circuit as a
very high speed without sparking. Such switches are
called electronic switches. The electronic square,
rectangular and saw tooth waves.
A transistor with proper associated circuitry can
be used as switch to turn ON or OFF current in
electrical circuit. Transistor switches have become
popular because of their high speed of operation and
absence of sparking. When a transistor is used as a
switch, it is so arranged in the circuit that either
maximum current (called collector saturation current)
flows through the load or minimum current (called
collector leakage current) flows through the load. In
other words, a switching transistor has two states viz.
 ON state or when collector saturation current flows
through the load
 OFF state or when collector leakage current flows
through the load.
Is an electronic circuit that generates square waves
(or other-non-sinusoidal waves such as rectangular, sawtooth waves). It is switching circuit which depends for
operation on the positive feedback. It is basically a twostage amplifier with output of one fed back to the input of
the other.
A multivibrator operates in two states (viz. ON and
OFF) controlled by circuit conditions. Each amplifier stage
supplies feedback to the other in such a manner that will
drive the transistor of one stage to saturation (ON state)
and the other to cut off (OFF state).
 Depending
upon the manner in
which the two states interchange
their states, multivibrators are
classified as:
 Astable
or
free
running
multivibrator
 Monostable
or
one-shot
multivibrator
 Bistable or flip-flop multivibrator
The astable multivibrator has no state stable. It
switches back and forth automatically from one state to the
other, remaining in each for a time determined by circuit
constant. Thus it is just as an oscillator since it requires no
external pulse for its operation.

A monostable multivibrator has only one state stable
and one quasi-stable (i.e. half-stable). The application of
input pulse triggers the circuit into quasi-stable state in
which it remains for a period determined by circuit
conditions. After this period of time, the circuit returns to its
initial stable state, the process is repeated upon the
application of each trigger pulse.

The bistable multivibrator has both the state stable. It
requires the applications of an external triggering pulse to
change the operation from either one state to the other. Thus
one impulse is used to generate half-cycle of square wave. It
is called flip-flop because of the two possible state it can
assume.

A circuit in which output voltage is directly
proportional to the derivative of the input is known as
a differentiating circuit. A differentiating circuit is a
simple RC series circuit output taken across R. It is
important to note that merely using voltage across R
does not make the circuit a differentiator; it is
necessary to proportion the time constant of the circuit
properly.
The RC product should be many
times smaller than the time period of
the input wave. A ratio of 1 to 10 is
usually sufficient to provide good
differentiation. The output waveform
from differentiating circuit depends
upon the time constant and shape of
the input wave. Thus if a d.c. or
constant input is applied to such a
circuit, the output will be zero.
Similarly, when the input is a square
wave, the output will consist of sharp
narrow pulses.
A circuit in which output voltage is
directly proportional to the integral of the
input is known as an integrating circuit. An
integrating circuit is a simple RC series
circuit with output taken across C. The
circuit is suitably designed so the RC
product is many times greater than the
time period of the input wave. A ratio of 1 to
10 usually provides good integration.
The
output waveform from an
integration circuit depends upon
the time-constant and shape of
the input wave. Thus, if the
input fed to an integrating
circuit is a square wave, its
output will be a triangular wave.
Similarly, if the input is a
rectangular wave, then the
output will be a saw-tooth wave.
The circuit with which the waveform is
shaped by removing (or clipping) a portion of
the applied wave is known as a clipping
circuit. The clippers can remove signal
voltages above or below a specified level. In
the operation of certain electronic circuits, it
is essential to have some particular
waveform. For, example we require square
waves for using in television,digital and
other electronic system.
The desired square wave can be
produced either by reshaping the existing
signals or by generating them directly by
multivibrators. A clipper does the job of
reshaping the existing signal. Thus a sine
wave can be converted into a square wave
by removing the positive and negative
peaks of sine waves through the use of
diodes. It may be noted that in order to get
a perfect square wave, the peak value of
the sine wave should be very much greater
than the required square wave.
A circuit that adds d.c.
component to a signal is known
as clamping circuit. It shifts the
signal as a whole, lining up either
positive or negative peaks to any
desired potential
 For
example, suppose the input
signal is a sine wave with peak-topeak value 20 V. if a clamper adds
10 V d.c component to it, the signal
will be shifted either vertically
upward or downward by 10 V,
depending upward, it is called a
positive clamper. On the other hand,
if the clamper shifts the signal
vertically downward, it is called
negative clamper.

A clamping circuit is frequency
used to store d.c. component of a
pulse which has been passed through
an a.c. amplifier or through any
other circuit which does not pass d.c.
Thus if a pulse having a d.c.
component is passed through a
transformer or an a.c. amplifier, the
d.c. component of the pulse can be
restored by using a clamping circuit.
D.c. restoration is often applied in
Television recievers.
POWER ELECTRONICS
INTRODUCTION
 Since
1950’s there has been a great upsurge in
the development and application of
semiconductor devices. Today, there are well
over 100 million semiconductor devices
manufactured in a year. These figures alone
indicate how important semiconductor devices
have become to the electrical industry. One
major field of application of semiconductor
devices in the recent years has been to control
large blocks of power in a system. This has led
to the development of a new branch of
engineering called power electronics.
 Power
electronics essentially deals with
the control of 50Hzpower flow in a
system. The most important
semiconductor devices which form the
core of power electronics are SCR, Triac,
Diac, and UJT. These devices can act as
controlled switches and can perform the
duties of rectification, inversion and
regulation of power flow in a load.
TRIAC

–a
triac is a 3 terminal semiconductor
switching device which can control
alternating current in a load. Triac is an
abbreviation for triode a.c. switch. ‘Tri’
indicates that the device has three terminals
and ‘a.c’ means that the device controls the
alternating current or can conduct current
in either direction.
.
One major difference between an SCR
and triac is that whereas SCR is a
unidirectional switch and can conduct in
one direction only, a triac is a
bidirectional switch and can conduct in
either direction. The triac is often
considered analogous to two SCRs
connected in parallel but oriented in
opposite directions ( inverse-parallel
connection).