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VACUUM
TUBE
RECTIFIERS

INTRODUCTION
For reasons associated with economics of
generation and transmission, the electric power
available is usually an a.c. supply. The supply voltage
varies sinusoidally and has a frequency of 50 Hz. It is
used for lighting, heating and electric motors. But
there are many applications where d.c. supply is
needed. When such a d.c. supply is required, the
mains a.c supply is rectified by using vacuum diodes.
A vacuum diode can do rectification due to
unidirectional conduction i.e. it will conduct current
only when plate is positive w.r.t. cathode. Although
vacuum diodes have been upstaged by crystal diodes
as rectifiers, they are still used in many applications.
TYPES OF VACUUM TUBE
RECTIFIERS
- Broadly, single phase vacuum tube
rectifiers may be classified into half-wave
rectifier and full-wave rectifier. It is usual
practice to give a.c supply for rectification
through a transformer. There are two
reasons for it. First, a transformer allows
us to step up or down the a.c voltage.
Second, it isolates the rectifier circuit
from power lines and thus reduces the
risk of electric shock.
HALF-WAVE RECTIFIER
A
half-wave rectifier employs a single
diode and conducts current during the
positive half-cycles of input a.c supply are
suppressed i.e. During negative halfcycles, no current is conducted and hence
no voltage appears across the load.
Therefore, current always flows in one
direction (i.e. d.c.) through the load,
though after every half-cycle.
 Suppose
an a.c. Supply v = Vm sin 0 is applied
to a half-wave rectifier. Let rp and RL be the
diode resistance and load resistance
respectively. Then the various circuit
computations will be :
Efficiency of rectification
FULL-WAVE RECTIFIER
A
full-wave rectifier employs at least two
diodes and conducts current through the
load in the same direction for both half
cycles of input a.c. Voltage. For the
positive half-cycle of a.c. Voltage, one
diode supplies current to the load and for
the negative half-cycle, the other does so;
the current being always in the same
direction through the load. Therefore, a
full-wave rectifier utilises both half-cycles
of
 Input
a.c. Voltage to produce the d.c. Output.
The circuits commonly used for full-wave
rectification are:
a. Centre-tap circuit
b. Bridge circuit
 Suppose
an a.c. Supply v = Vm sin 0 is
applied to a full-wave rectifier. Then the
various circuit computations are:
Efficiency of rectification
It
is clear that for a given a.c.
Supply, the output of a fullwave rectifier is double than
that of a half-wave rectifier.
For this reason, full-wave
rectifiers are invariably used
for conversion of a.c. Into d.c.
PEAK INVERSE VOLTAGE
(PIV)
 It
is the maximum reverse voltage
that can be applied to a vacuum
diode without damage to it. The PIV
consideration is of particular
importance in rectifier service. While
using a diode for rectification, care
should be taken that reverse voltage
across the diode during negative
half-cycle of a.c. Supply does not
exceed the PIV reting of the diode.
BASIC ELECTRONICS
Semiconductor Physics
INTRODUCTION
Certain substances like germanium, silicon etc. are neither good conductors like copper nor
insulators like glass. In other words, the resistivity of these materials lies in between conductors and
insulators. Such substances are called semi-conductors. In fact, it is not the resistivity alone that
decides whether a substances is semi-conductor or not. They have several unique properties which
distinguish them from conductors and insulators. One very important property of semi-conductors is
that with the addition of suitable metallic impurity ( e. g. arsenic, gallium etc.) to a semi-conductor, it’s
conducting properties change appreciably. This property is the key factor responsible for the
widespread use of semi-conductors in the electronic devices.
A large number of semi-conductor are known but the most commonly used are silicon (Si) and
germanium (Ge). Both these elements are tetravalent i.e they have four valence electrons. The different
atoms of a semi-conductors are held together in an orderly pattern through co-valent bonds. Therefore,
a piece of germanium or silicon is generally called a crystal
INTRINSIC SEMI-CONDUCTOR
A semi-conductor in an extremely pure form is called an intrinsic semi-conductor. Even at room
temperature some of the co-valent bonds in a pure semi-conductor break, setting up free electrons.
When aco-valent bond is broken due to thermal energy, the removal of one electrons leaves a vacancy
i.e. a missing electron in the co-valent bond. This missing electron is called a hole. For one electron set
free, one hole is created. Therefore, there energy creates hole-electron pairs; there being as many
holes as the free electrons. When electric f field is applied across a pure semi-conductor, the current
conduction takes place by free electrons and holes . The total current inside the semi conductor is the
sum of currents due to free electrons and holes.
EXTRINSIC SEMI-CONDUCTOR
The intrinsic semi-conductor has little current conduction capability at room temperature. To be useful in
electronic devices, the pure semi-conductor must be altered so as to significantly increase its
conducting properties. This is achieved by adding a small amount of suitable metallic impurity to pure
semi-conductor. It is then called an impure or extrinsic semi-conductor. The purpose of adding impurity
is to increase either the number of free electrons or holes in the semi-conductors crystal.
(i) If a pentavalent impurity (i.e having 5 valence electrons) is added to a pure semi-conductor, a
large number of free electrons are produced in the semi-conductor. The semi-conductor thus produced
is known as n-type semi-conductor. Although there are some holes in an n-type due to thermal energy,
yet the number of free electrons far outnumber the holes . Therefore. The current conduction in an ntype semi –conductor is predominantly by free electrons. It is customary to call the free electrons in ntype as the majority carriers and holes as the minority carriers and holes as the minority carriers.
(ii) If a trivalent impurity( (having 3 valence electrons) is added to a pure semi-conductors, a large
number of holes are produced in the semi-conductor. Although there will be some free electrons in a ptype semi-conductor due to thermal energy, yet the number of holes far exceeds the free electrons.
Therefore, current conduction in a p-type semi-conductor is predominantly by hole. It is obvious that in a
p-type semi-conductors holes are the majority carriers while free electrons are the minority carriers.
PN JUNCTION
When a p-type semi-conductor is suitably joined to n-type semi-conductor, the contact surface is called
a pn junction, Most of semi-conductor devices contain one or more pn junctions. As soon as a pn
junction is formed, there is tendency for the free electrons to diffuse over to junction I is formed , there is
tendency for the free electrons to diffuse over to p-side and holes to n-side. This diffusion process sets
up a potential barrier at the junction which prevents the further movement of charge carriers ( i.e. holes
and free electrons) across the junction, for germanium junction, the value of this potential barrier is 0.3 V
whereas it is 0.7 V for silicon pn junction.
(i)
When external voltage applied to the pn junction is such that it cancels the potential barrier, thus
permitting current flow, it is called forward biasing. In order to forward bias a pn junction. Connect
positive terminal of battery to p-type and negative terminal to n-type. As potential barrier voltage is
very small, therefore, a small forward voltage is sufficient to completely eliminate the barrier.
(ii)
When external voltage applied to a pn junction is such that potential barrier is increased, it is called
reverse biasing. In order to reverse bias a pn junction, connect positive terminal of battery to n-type
and negative terminal to p-type. No can flow across a reverse biased pn junction showing that it
offers very high resistance
ROSELLE TAN SANCHEZ
CHERRY LYN V. TIMBOL
JANE TRUBANOS
BSE-TLE III
Engr. Aquilino Noceda