BEEE-UNIT 3.pdf

HOUSE WIRING
• House wiring is to deal with the distribution system
within the domestic premises.
• The wiring requirements may vary among the different
consumers.
• House wiring is generally done for consumption of
electrical energy at 230 V, 1-phase or at 400 V,3-phase.
• In the three phase system, the total load in the house is
expected to be divided among the three phases.
• An earth wire is also run connecting all the power
plugs from where large quantity of electrical energy is
tapped by using electrical appliances like heater,
electric iron, hot plate, etc. This chapter deals with the
wiring materialsand accessories, different systems of
wiring and earthing methods.
2

WIRING MATERIALS AND
ACCESSORIES
1.Switches: A switch is used to make or break the
electric circuit.
2.Lamp Holders: A lamp holder is used to hold the
lamp for lighting purposes.
3. Power point: Power points (plugs, wall sockets)
need to be installed throughout the house in
locations where power will be required.
4.Main Switch: This is used at the consumer’s
premises so that he may have self-control of the
entire distribution circuit.
3

5.Circuit Breakers: Domestic Electrical Circuit
Breakers provide essential protection to house
from electrical hazards. It is essential to
use domestic circuit breakers to get protected
from electrical overloads and short circuit
conditions. The most widely used electrical
circuit breakers are Miniature circuit breakers
(MCBs) , Residual current circuit breaker
(RCCB) and Mounded Case Circuit Breaker
(MCCB)
4

Electrical wiring system is classified into five
categories:
1. Cleat wiring
2. Casing wiring
3. Batten wiring
4. Conduit wiring
5. Concealed wiring
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LED lamp
An LED lamp or LED light bulb is an electric
light for use in light fixtures that produces light
using one or more light-emitting diodes (LEDs).
LED lamps have a lifespan many times longer
than equivalent incandescent lamps, and are
significantly more efficient than most fluorescent
lamps, with a luminous efficacy of up to 303
lumens per watt.However, LED lamps require an
electronic LED driver circuit when operated from
mains power lines. Many LEDs use only about
10% of the energy an incandescent lamp
requires.
15

Safety Precautions when Working with
Electricity
1. Never touch or try repairing any electrical equipment or
circuits with wet hands. It increases the conductivity of
electric current.
2. Never use equipment with damaged insulation or broken
plugs.
3. If you are working on any electrical socket at your home
then always turn off the mains.
4. Always use insulated tools while working.(never use
aluminium or steel ladder )
5. Electrical hazards include exposed energized parts and
unguarded electrical equipment which may become
energized unexpectedly -carries warning signs like “Shock
Risk”. Always be observant such electrical signs.
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6. when working electrical circuit always use appropriate
insulated rubber gloves and goggles.
7. Never try repairing energized equipment. Always check
that it is de-energized first by using a tester. When an
electric tester touches a live or hot wire, the bulb inside
the tester lights up showing that an electrical current is
flowing through the respective wire.
8. Know the wire code of your country.
9. Always use a circuit breaker or fuse with the
appropriate current rating. Circuit breakers and fuses are
protection devices that automatically disconnect the live
wire when a condition of short circuit or over current
occurs. The selection of the appropriate fuse or circuit
breaker is essential.
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NOTE:
1. MC instruments are used for the measurement of DC Quantities only.
2. MI instruments are used for the measurement of both DC & AC Quantities.

OVERVIEW OF SEMICONDUCTORS
• Depending on their conductivity, materials can be
classified into three types as conductors,
semiconductors and insulators. Conductor is a
good conductor of electricity. Insulator is a poor
conductor of electricity. Semiconductor has its
conductivity lying between these two extremes.
Energy Band of Semiconductor
In terms of energy band shown in Fig., the valence
band is almost filled (partially filled) and
conduction band is almost empty.
35

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A comparatively smaller electric field (smaller than required for
insulator) is required to push the electrons from the valence band
to conduction band. At low temperatures, the valence band is
completely filled and the conduction band is completely empty.
Therefore a semiconductor virtually behaves as an insulator at
low temperature. However even at room temperature some
electrons crossover to the conduction band giving conductivity to
the semiconductor. As temperature increases, the number of
electrons crossing over to the conduction band increases and
hence electrical conductivity increases. Hence a semiconductor
has negative temperature coefficient of resistance.

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Classifications of Semiconductors
Intrinsic Semiconductor: A pure
semiconductor is called intrinsic
semiconductor.
Extrinsic Semiconductor: Due to the poor
conduction at room temperature, the intrinsic
semiconductor, as such, is not useful in the
electronic devices. Hence the current
conduction capability of the intrinsic
semiconductor should be increased. This can
be achieved by adding a small amount of
impurity to the intrinsic semiconductor, so that
it becomes impurity semiconductor or extrinsic
semiconductor. This process of adding impurity
is known as doping.

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N-type Semiconductor: A small amount of pentavalent
impurities such as arsenic, antimony or phosphorus is
added to the pure semiconductor (germanium or silicon
crystal) to get N-type semiconductor. Thus, the addition
of pentavalent impurity (antimony) increases the number
of electrons in the conduction band thereby increasing
the conductivity of N-type semiconductor. As a result of
doping, the number of free electrons far exceeds the
number of holes in an N-type semiconductor. So electrons
are called majority carriers and holes are called minority
carriers
P-type Semiconductor: A small amount of trivalent
impurities such as aluminium or boron is added to the
pure semiconductor to get the P-type semiconductor. The
number of holes is very much greater than the number of
free electrons in a P-type material, holes are termed as
majority carriers and electrons as minority carriers.

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THEORY OF PN JUNCTION DIODE
In a piece of semiconductor material, if one half is doped by P-
type impurity and the other half is doped by N-type impurity,
a PN junction is formed. The plane dividing the two halves or
zones is called PN junction. As shown in Fig., the N-type
material has high concentration of free electrons while P-type
material has high concentration of holes. Therefore at the
junction there is a tendency for the free electrons to diffuse
over to the P-side and holes to the N-side. This process is
called diffusion.

40
As the free electrons move across the junction from N-type to P-
type, the donor ions become positively charged. Hence a positive
charge is built. on the N-side of the junction. The free electrons
that cross the junction uncover the negative acceptor ions by filling
in the holes. Therefore a net negative charge is established on the
P-side of the junction. This net negative charge on the P-side
prevents further diffusion of electrons into the P-side. Similarly,
the net positive charge on the N-side repels the holes crossing from
P-side to N-side. Thus a barrier is set up near the junction which
prevents further movement of charge carriers, i.e. electrons and
holes. This is called potential barrier or junction barrier V0. V0 is
0.3 V for germanium and 0.72 V for silicon. The electrostatic field
across the junction caused by the positively charged N-type region
tends to drive the holes away from the junction and negatively
charged P-type region tends to drive the electrons away from the
junction. Thus the junction region is depleted to mobile charge
carriers. Hence it is called depletion layer.

41
Under Forward Bias Condition
When positive terminal of the battery is connected to
the P-type and negative terminal to the N-type of the
PN junction diode, the bias applied is known as
forward bias. Under the forward bias condition, the
applied positive potential repels the holes in P-type
region so that the holes move towards the junction and
the applied negative potential repels the electrons in
the N-type region and the electrons move towards the
junction. Eventually when the applied potential is more
than the internal barrier potential, the depletion region
and internal potential barrier disappear.

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V–I Characteristics of a Diode under Forward Bias
For VF > V0, the potential barrier at the junction
completely disappears and hence, the holes cross the
junction from P-type to N-type and the electrons cross
the junction in the opposite direction, resulting in
relatively large current flow in the external circuit.

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Under Reverse Bias Condition
When the negative terminal of the battery is connected to
the P-type and positive terminal of the battery is connected
to the N-type of the PN junction, the bias applied is known
as reverse bias. Under applied reverse bias, holes which
form the majority carriers of the P-side move towards the
negative terminal of the battery and electrons which form
the majority carrier of the N-side are attracted towards the
positive terminal of the battery. Hence the width of the
depletion region which is depleted of mobile charge carriers
increases. Thus the electric field produced by applied
reverse bias, is in the same direction as the electric field of
the potential barrier. Hence, the resultant potential barrier
is increased, which prevents the flow of majority carriers in
both directions. Therefore, theoretically no current should
flow in the external circuit. But in practice, a very small
current of the order of a few microamperes flows under
reverse bias.

44
V–I Characteristics of a Diode under Reverse Bias
For large applied reverse bias, the free electrons from the N-type
moving towards the positive terminal of the battery acquire
sufficient energy to move with high velocity to dislodge valence
electrons from semiconductor atoms in the crystal. These newly
liberated electrons, in turn, acquire sufficient energy to dislodge
other parent electrons. Thus, a large number of free electrons are
formed which is commonly called as an avalanche of free
electrons. This leads to the breakdown of the junction leading to
very large reverse current. The reverse voltage at which the
junction breakdown occurs is known as breakdown voltage.

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ZENER DIODE
Zener diode is heavily doped than the ordinary diode. From
the V–I characteristics of the Zener diode, shown in Fig., it
is found that the operation of Zener diode is same as that
of ordinary PN diode under forward biased condition.
Whereas under reverse-baised condition, breakdown of the
junction occurs. The breakdown voltage depends upon the
amount of doping.

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If the diode is heavily doped, depletion layer will be thin and,
consequently, breakdown occurs at lower reverse voltage and
further, the breakdown voltage is sharp. Whereas a lightly doped
diode has a higher breakdown voltage. Thus breakdown voltage
can be selected with the amount of doping. The sharp increasing
current under breakdown conditions are due to the following two
mechanisms.
(1) Avalanche breakdown
(2) Zener breakdown.
Avalanche Breakdown
As the applied reverse bias increases, the field across the junction
increases correspondingly. Thermally generated carriers while
traversing the junction acquire a large amount of kinetic energy
from this field. As a result the velocity of these carriers increases.
These electrons disrupt covalent bonds by colliding with immobile
ions and create new electron-hole pairs. These new carriers again
acquire sufficient energy from the field and collide with other
immobile ions thereby generating further electron–hole pairs. This
process is cumulative in nature and results in generation of
avalanche of charge carriers within a short time.

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This mechanism of carrier generation is known as Avalanche
multiplication. This process results in flow of large amount of current
at the same value of reverse bias.
Zener Breakdown
When the P and N regions are heavily doped, direct rupture of
covalent bonds takes place because of the strong electric fields, at the
junction of PN diode. The new electron-hole pairs so created increase
the reverse current in a reverse biased PN diode. The increase in
current takes place at a constant value of reverse bias typically below
6 V for heavily doped diodes. As a result of heavy doping of P and N
regions, the depletion region width becomes very small and for an
applied voltage of 6 V or less, the field across the depletion region
becomes very high, of the order of 107 V/m, making conditions
suitable for Zener breakdown. For lightly doped diodes, Zener
breakdown voltage becomes high and breakdown is then
predominantly by Avalanche multiplication. Though Zener breakdown
occurs for lower breakdown voltage and Avalanche breakdown occurs
for higher breakdown voltage, such diodes are normally called Zener
diodes.
Applications of Zener diode: Used as Voltage Regulator or Stabilizer.

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APPLICATIONS OF PN JUNCTION DIODE
RECTIFIERS, CLIPPERS, CLAMPERS ect..
RECTIFIERS-Rectifier is defined as an electronic device used for
converting ac voltage into dc voltage
Half-wave Rectifier
It converts an ac voltage into a pulsating dc voltage using only one half
of the applied ac voltage. The rectifying diode conducts during one half
of the ac cycle only. Figure shows the basic circuit and waveforms of a
half wave rectifier.

58
CLIPPERS
The circuit with which the waveform is shaped by removing (or
clipping) a portion of the input signal without distorting the
remaining part of the alternating waveform is called a clipper. These
circuits find extensive use in radars, digital computers, radio and
television receivers etc.
1.Positive clipper
When the input voltage is positive, diode conducts and acts as short-
circuit and hence there is zero signal at the output, i.e. the positive
half cycle is clipped off. When the input signal is negative, the diode
does not conduct and acts as an open switch, the negative half cycle
appears at the output as shown in Fig.

59
2.Negative clipper
When the input signal is positive, the diode does not
conduct and acts as an open switch, the positive half
cycle appears at the output as shown in Fig.. When the
input voltage is negative, diode conducts and acts as
short-circuit and hence there is zero signal at the
output, i.e. the negative half cycle is clipped off.

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3.Biased Positive clipper
In the biased positive clipper as shown in Fig., the diode conducts
as long as the input voltage is greater than +VR and the output
remains at +VR until the input voltage becomes less than +VR.
When the input voltage is less than +VR, the diode does not
conduct and acts as an open switch. Hence all the input signal
having less than + VR as well as negative half cycle of the input
wave will appear at the output, shown in Fig

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4.Biased Negative clipper
In the biased negative clipper shown in Fig. , when the
input voltage Vi ≤ VR the diode conducts and clipping
takes place. The clipping level can be shifted up and
down by varying the bias voltage (–VR).

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CLAMPERS
Clamping network shifts (clamps) a signal to a different
dc level, i.e. it introduces a dc level to an ac signal.
Hence, the clamping network is also known as dc
restorer. These circuits find application in television
receivers to restore the dc reference signal to the video
signal.

63
Consider the clamper circuit shown in Fig. A sine wave with
maximum amplitude of V is given as the input to the network.
During the positive half cycle, the diode conducts, i.e. it acts like a
short circuit. The capacitor charges to V volts. During this interval,
the output which is taken across the short circuit will be Vo = 0 V.
During the negative half cycle, the diode is open. The output voltage
can be found out by applying Kirchhoff’s law.
–V – V – V0 = 0
Therefore, V0 = –2 V
The analysis of the clamper circuit can be done as follows.
Determine the portion of the input signal that forward biases the
diode. When the diode is in short circuit condition, the capacitor
charges up to a level determined by the voltage across the capacitor
in its equivalent open circuit state. During the open circuit
condition of the diode, it is assumed that the capacitor will hold on
to all its charge and therefore voltage. In the clamper networks, the
total swing of the output is equal to the total swing of the input
signal. This is negative clamper.

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Positive clamper: Consider the clamper circuit shown in Fig. A sine
wave with maximum amplitude of 10 V is given as the input to the
network. During the negative half cycle, the diode conducts, i.e. it acts
like a short circuit. The capacitor charges to10 V volts. During this
interval, the output which is taken across the short circuit will be Vo = 0
V. During the positive half cycle, the diode is open. The output voltage
can be found out by applying Kirchhoff’s law.
10V +10 V – V0 = 0
Therefore, V0 = 20 V

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BIPOLAR JUNCTION TRANSISTOR [BJT]
A Bipolar Junction Transistor (BJT) is a three terminal
semiconductor device in which the operation depends on the
interaction of both majority and minority carriers and hence the
name Bipolar. It is used in amplifier and oscillator circuits, and as
a switch in digital circuits. It has wide applications in computers,
satellites and other modern communication systems.

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TRANSISTOR BIASING
Usually the emitter-base junction is forward biased and collector-base
junction is reverse biased. Due to the forward bias on the emitter-
base junction an emitter current flows through the base into the
collector. Though, the collector-base junction is reverse biased, almost
the entire emitter current flows through the collector circuit.

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