C20 AEI-405

INDUSTRIAL ELECTRONICS

Prepared by:
S. SURYATEJA
Lecturer in EEE
GPM, Guntur

This document consists of page numbers 1 to 34, prepared for the purpose of examination only.

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 CHAPTER.1 POWER ELECTRONIC DEVICES
Thyristor Family Devices:
Thyristors are semiconductor devices that belong to a family of four-layer, three-terminal devices. The main types of
thyristors include:
1.Silicon Controlled Rectifier (SCR) 2.Silicon Controlled Switch (SCS)
3.Silicin Bidirectional Switch (SBS), 4.Silicon Unidirectional Switch (SUS)
5.DIode that can work on AC (DIAC) 6.TRIode that can work on AC (TRIAC)
7.Gate Turn OFF (GTO) SCR 8.MOS Controlled Thyristor (MCT)
9.Light Activated SCR (LASCR) 10.Reverse Conducting Thyristor (RCT)
ISI Circuit Symbols of:

SCR , SCS , SBS ,

SUS , DIAC , TRIAC

GTO SCR .
Construction AND Working of SCR:
A Silicon Controlled Rectifier (SCR), also known as a thyristor. It is a four-layer, three-terminal semiconductor
device. It is widely used in electronic and power control applications.
It is made up of four semiconductor layers of alternating p-type and n- type materials which form a PNPN structure.
Hence it has three p-n junctions J1, J2, and J3. The three terminals Anode (A), Cathode (K), and Gate (G) are
arranged in such a way that Gate (G) terminal is attached to the p-type layer nearer to the Cathode (K) terminal in
the PNPN structure. A typical structure of SCR with P-N-P-N layers is shown in figure.1.

Here, Anode (A) is a positively charged electrode and the conventional current enters into the device through this
terminal. Cathode (K) is a negatively charged electrode and conventional current leaves the device through this
terminal. Gate (G) is a control terminal that controls the flow of current between Anode (A) and Cathode (K).

In the structure of SCR with PNPN form, the Anode (A) terminal is connected to the first P-type layer, and Cathode
(K) terminal is connected to the last N-type layer. The gate (G) terminal is connected to the second p-type layer
nearer to Cathode (K) as shown in figure.1. The outer layers (first P-type and last N-type layer) of SCR are heavily
doped whereas middle P and N- type layers are lightly doped. In SCR, silicon is used as an intrinsic

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semiconductor to form P-type and N-type layers because silicon has a very small leakage current in comparison
to germanium.

Figure.1.Structure of SCR Figure.2.SCR pin diagram

Depending on the biasing given to SCR, there are three modes of operation. They are

1. Forward Blocking Mode (OFF State)

2. Forward Conducting Mode (ON State)
3. Reverse Blocking Mode (OFF State)

1. Forward Blocking Mode (OFF State):

In this mode of operation, a positive voltage (+) is given to the Anode (A) terminal of SCR, and a negative voltage
(-) is given to Cathode (K). The Gate (G) terminal is open-circuited as shown in the figure.3

Under this condition, junctions J1 and J3 are forward biased whereas junction J2 is in reverse biased condition.
The depletion region at junction J2 blocks the flow of current from junction J1 to junction J3 as it acts obstacle or
wall between them. However, a small amount of leakage current flows between these junctions J2 and J3.

Fig.3 Forward Blocking Mode Fig.4 Forward Conducting Mode Fig.5 Reverse Blocking Mode
When the applied voltage across the SCR reaches a breakdown voltage, the avalanche breakdown occurs due to
high energy minority carriers. The current starts flowing through the SCR at this breakdown voltage and there is no
current flow below the breakdown voltage because SCR offers very high resistance to the current below the
breakdown voltage and acts as an open switch by blocking the forward current. Hence it will be in an OFF state.

From above, it is observed that the SCR is in forward biasing condition but there is no current flow through it.
Hence this mode of operation is named forward blocking mode.

2. Forward Conducting Mode (ON State):

In this mode of operation, the SCR comes into the conduction mode from blocking mode. It can be done in two
ways, i.e. either by increasing the forward bias voltage (voltage across Anode and Cathode) beyond

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the breakdown voltage or by applying positive pulse or voltage at the Gate terminal. The biasing of SCR in this
mode is shown in the figure.4.

In the first case, the forward bias voltage applied between Anode and Cathode is increased beyond the breakdown
voltage, the depletion region breakdown occurs at J2, and the current starts flowing through SCR. In this condition,
the SCR will be in an ON state. After the occurrence of junction breakdown, the current flow in SCR increases
rapidly as shown in V-I Characteristics below.

In the second case, a small positive pulse or voltage VG is applied to the Gate terminal of SCR as shown in the
figure.4. When the gate voltage is applied to the gate terminal, the reverse biased junction J2 in forward blocking
mode will become forward biased, and the depletion region width becomes very narrow. In this condition, a small
forward bias voltage between Anode and Cathode can easily penetrate this narrow depletion region. Therefore on
applying a small forward bias voltage, an electric current starts flowing through the SCR and it will be in an ON
state.

Once the SCR starts conducting, no more gate voltage is needed to maintain it in the ON state. The minimum
current required to maintain the SCR in the ON state on the removal of gate voltage VG is called latching current.

Any one of these methods results in avalanche breakdown at junction J2 and hence the SCR turns into conduction
mode and acts as a closed switch thereby current starts flowing through it. Here, the SCR is forward biased and
current flows through it. Hence this mode of operation is named as forward conducting mode.

3. Reverse Blocking Mode (OFF State):

In this mode of operation, a positive voltage (+) is given to Cathode (K) terminal, and a negative voltage (-) is given
to Anode (A), Gate (G) terminal is an open circuit as shown in the figure.5.

Under this condition, junctions J1 and J3 are reverse biased whereas junction J2 is in forward biased condition. As
junctions J1 and J3 are reverse-biased, there is no current flow through the SCR. But due to the drift of the charge
carrier in a forward-biased junction J2, there is small leakage current flow in SCR which is not sufficient to
turn ON the device. Hence the SCR will be in an OFF state and acts as an open switch.

The SCR offers high impedance in this mode of operation until the applied voltage is less than the reverse
breakdown voltage VBR. If the reverse applied is greater than the reverse breakdown voltage, the avalanche
breakdowns occur at junction J2 and hence increase reverse current flow in the SCR device. This reverse current
causes more losses in SCR and produces heat on more increasing it. When the reverse voltage applied to SCR is
more than VBR, There will be considerable damage to the device.
V-I Characteristics of Silicon Controlled Rectifier (SCR):

The V-I characteristics of SCR are shown in the figure.6. In this V-I characteristic, the horizontal line represents
the amount of voltage applied VA across the SCR and the vertical line represents the amount of current flow IA in the
SCR. Here, the V-I characteristics of SCR are divided into three regions. They are:
1. Forward Blocking Region: The region OA in V-I characteristics is called the forward blocking region. This
region represents the forward-blocking mode of SCR operation. In this region, the forward bias voltage is given to
SCR where positive voltage is given to Anode, the negative is given to Cathode and Gate is open-circuited. In this
condition, the junctions J1 and J3 become forward biased whereas junction J2 becomes reverse biased. A small
leakage current flows from the Anode terminal to the Cathode terminal of SCR which is known as a forward leakage
current. The SCR does not conduct electric current and the device is in an OFF state in this region.

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Fig.6. V-I characteristics of SCR

2. Forward Conduction Region

The region BC in V-I characteristics is called the forward conduction region. This region represents the forward
conduction mode of SCR operation. In this region, the current flowing from Anode to Cathode increases rapidly.
When the forward bias voltage applied between Anode and Cathode is increased beyond the breakdown voltage, the
depletion region breakdown occurs at junction J2 and the current starts flowing through the SCR and it will be in the
ON state. The current flow in this region increases rapidly after junction J2 breakdown occurs. The voltage at which
the junction breakdown occurs when the Gate is open is known as forward breakdown voltage (VBF)

The region AB in V-I characteristics indicates that as soon as the SCR becomes ON, the voltage across the SCR
drops to some volts.

3. Reverse Blocking Region

The region OE in the V-I characteristics is called the reverse blocking region. This region represents the reverse
blocking mode of SCR operation. In this region, the reverse bias voltage is applied to SCR where a positive voltage
is given to Cathode, a negative voltage is given to Anode, and the Gate terminal is open-circuited. In this condition,
junctions J1 and J3 are reverse biased whereas the junction is forward biased. As junction J1 and J3 are in reverse
biased condition, there is no current flow through SCR. But due to the drift of the charge carrier in forward-biased
junction J2, there is small leakage current flow in SCR which is not enough to turn ON the device. Hence the SCR
will be in an OFF state in this region.

When the reverse bias voltage between Anode and Cathode is increased beyond the reverse breakdown voltage VBR,
an avalanche breakdown occurs, and the current increases rapidly. The region EF in V-I characteristics is known as
the reverse avalanche region.

Two-Transistor Model of SCR:

Basic operating principle of SCR, can easily be understood by the two transistor model of SCR, as it is a
combination of p and n layers. This is a pnpn thyristor. If we bisect it through the dotted line then we will get two
transistors i.e. one pnp transistor with J1 and J2 junctions and another is with J2 and J3 junctions as shown in figure.7.

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 Fig.7 Two-transistor model of SCR
The relation between the collector current and emitter current is shown below
Here, IC is collector current, IE is emitter current, ICBO is forward leakage current, α is common base forward current
gain and relationship between IC and IB is IC = β IB
Where, IB is base current and β is common emitter forward current gain.
Let’s for transistor T1 this relation holds
IC1 = α1 Ia + ICBO1 ……………(i)

And that for transistor T2

IC2 = α2 Ik + ICBO2 ………(ii) again IC2 = β 2 IB2
Now, by the analysis of two transistors model we can get anode current,
Ia = IC1 + IC2 [applying KCL ]
From equation (i) and (ii), we get,
Ia = α1 Ia + ICBO1 + α2 Ik + ICBO2 ………..(iii)
If applied gate current is Ig then cathode current will be the summation of anode current and gate current i.e.
Ik = Ia + Ig
By substituting this value of Ik in (iii) we get,
Ia = α1 Ia + ICBO1 + α2 (Ia + Ig ) + ICBO2
α2 Ig + ICBO1 + ICBO2
Ia= 1− (α1+ α2)
From this relation we can assure that with increasing the value of (α1 + α2 ) towards unity, corresponding anode
current will increase.
Now the question is how (α1 + α2 ) increasing? Here is the explanation using two transistor model of SCR.
At the first stage when we apply a gate current Ig, it acts as base current of T2 transistor i.e. IB2 = Ig and emitter
ICBO1
current of the T2 transistor IE2 = Ik. Hence establishment of the emitter current gives rise α2 as α2 = Ig
Presence of base current will generate collector current as IC2 = β2 X IB2 = β2 Ig
This IC2 is nothing but base current IB1 of transistor T1, which will cause the flow of collector current,
IC2 = β1 X IB1 = β1 β2 Ig
IC1 and IB1 lead to increase IC1 as Ia = IC1 + IB1 and hence, α1 increases. Now, new base current of T2 is
Ig + IC1 = (1+ β1 β2) Ig , which will lead to increase emitter current Ik = Ia + IC1 and as a result α2 also increases and
this further increases IC2 = β2 (1+ β1 β2) Ig
As IB1 = IC2 , α1 again increases. This continuous positive feedback effect increases (α1 + α2 ) towards unity and
anode current tends to flow at a very large value. The value current then can only be controlled by external
resistance of the circuit.

Ratings of SCR:
The ratings of a Silicon Controlled Rectifier (SCR) refer to the specific parameters and limits that define its
operating conditions. Here are some common ratings associated with SCRs:
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1. Forward Voltage (VF):
The maximum forward voltage is the maximum voltage drop across the SCR when it is in the conducting state.
2. Reverse Voltage (VR):
The maximum reverse voltage that the SCR can withstand without breaking down.
3. Forward Current (IF):
The maximum forward current is the maximum current that the SCR can carry when it is in the conducting state.
4. Reverse Current (IR):
The maximum reverse current that can flow through the SCR when it is in the off state.
5. Gate Trigger Current (IGT):
The minimum current required at the gate to trigger the SCR into the conducting state.
6. Gate Trigger Voltage (VGT):
The minimum voltage required at the gate to trigger the SCR.
8. Surge Current (ISM):
The maximum surge current, which is a short-duration current that the SCR can handle without being damaged.
10. Operating Temperature (Tj):
The range of temperatures within which the SCR is designed to operate.
11. Holding Current (IH):
The minimum current required to keep the SCR in the conducting state once it has been triggered.
These ratings are crucial for selecting and designing circuits that involve SCRs, ensuring that the device operates
within its specified limits to maintain reliability and prevent damage. It's important to refer to the datasheet provided
by the manufacturer for detailed information on a specific SCR's ratings.
Construction and Working of DIAC :
Basically, the DIAC is a two-terminal device; it is a combination of parallel semiconductor layers that allows
activating in one direction. This device is used to activating the device TRIAC. The basic construction of DIAC
consists of two terminals namely MT1 and MT2.
When the terminal MT1 is positive the direction of the flow of current will be in the order P1-N2-P2-N3 The junction
between P1 and N2 is forward biased, the junction between N2 and P2 is reverse biased and the junction between P2
and N3 is forward biased.
When the terminal MT2 is positive the direction of the flow of current will be in the order N1-P1-N2-P2. The junction
between N1 and P1 is forward biased, the junction between P1 and N2 is reverse biased and the junction between N2
and P2 is forward biased

Fig.8 DIAC Structure

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 Fig.9 DIAC V-I Characteristics
Characteristics of DIAC:

When the external voltage is applied at the terminals, Diac does not conduct immediately. Only a small leakage
current flows. When the applied voltage is further increased and when it crosses the breakover voltage the junction
which is reverse biased breaks and they start conducting.

Till the breakover voltage is reached they remain in forward and reverse blocking state. After the applied voltage is
increased above the breakover voltage avalanche breakdown takes place and the current increases. It happens for
both the polarity of voltages. To turn OFF the device the applied voltage is decreased below the breakover voltage.

Construction and Working of TRIAC:

Fig.10 TRIAC Structure Fig.11 V-I characteristics of TRIAC

TRIAC is a three-terminal device and the terminals of the TRIAC are MT1, MT2, and Gate. Here the gate terminal
is the control terminal. The flow of current in the TRIAC is bi-directional which means current can flow in both
directions. The structure of TRIAC is shown in figure.10. Here, in the structure of TRIAC, two SCRs are connected
in the antiparallel and it will act as a switch for both directions. In the above structure, the MT1 and gate terminals
are near to each other. When the gate terminal is open, the triac will obstruct both the polarities of the voltage across
the MT1 & MT2.

V-I characteristics of TRIAC :

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Since the TRIAC is a bidirectional device it’s the VI characteristics curve of TRIAC will be on the first and third
quadrant of the graph, which is similar to the VI characteristics of a Thyristor. When the terminal MT2 is set to be
positive with respect to the terminal MT1 the TRIAC will be operating in the forward blocking mode.
During the initial stage due to the resistance of the TRIAC, there will be a small leakage current flowing through the device
as the applied voltage is less than the breakdown voltage. When the voltage is increased and it reaches the breakdown
voltage the TRIAC is turned on and high current starts flowing through the device.
Apart from increasing the voltage of the device the TRIAC can be turned ON by applying the gate pulse, even if the applied
voltage is less than the breakdown voltage. The same operation can be carried out in the negative direction of the TRIAC
which can leave us with a mirror image of the same curve on the negative quadrant. The supply voltage at which the
TRIAC starts conduction will depend on the gate current applied to the TRIAC. If the gate current is higher, then the
voltage required to turn ON the TRIAC can be less. The characteristic curve that is given above shows the operation of
TRIAC in mode 1 on the first quadrant and mode 3 on the third quadrant.

TRIAC Triggering in different modes :

TRIAC can go to conduction state if the applied voltage is equal to the breakdown voltage, but the most preferred
way of turning on a TRIAC is by providing a gate pulse, either positive or negative. If the gate current is high, a very
small amount of voltage is enough to turn on the TRIAC. As the TRIAC is bidirectional and has an ability to get
turned on with both the polarities to the gate pulse it can operate in four different types of modes of operation as
listed below
1. MT2 is positive with respect to MT1 with a gate
polarity positive with respect to MT1.
2. MT2 is positive with respect to MT1 with a gate
polarity negative with respect to MT1.
3. MT2 is negative with respect to MT1 with a gate
polarity negative with respect to MT1.
4. MT2 is negative with respect to MT1 with a gate
polarity positive with respect to MT1.

1.MT2 is positive with respect to MT1 with a gate polarity positive with respect to MT1
When MT2 is positive with respect to MT1, the junction P1-N1 and P2-N2 are forward biased whereas the Junction
N1-P2 is reverse biased. When gate terminal the positive with respect to MT1, gate the current flows mainly through
P2-N2 junction like an ordinary SCR. When gate current has injected sufficient charge into P2 layer, reverse biased

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Construction AND Working of UJT:
Unijunction Transistor (UJT) is a three terminal semiconductor device. UJT is formed by a single junction of P type
and N type semiconductor material. It is used for switching applications and it cannot be used to amplify signals.
The channel is formed of N type which is lightly doped and P type material is infused on it and the doping
concentration of P type is very high. Thus it forms single PN junction and this is the reason for the name Unijunction.
The terminals Base2 (B2) ,Base1 (B1) are taken from the N channel through the Ohmic contacts and Emitter taken
from the heavily doped P type material. The Emitter terminal is closer to the Base2 terminal than the Base1 terminal.

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 Fig.16 UJT Working Fig.17 UJT V-I Characteristics
When no voltage is applied at the emitter, the resistance in the channel is high and the device is turned OFF, till the
applied voltage is higher than the triggering voltage.

When the PN Junction is forward biased, positive voltage is applied at the emitter terminal and Base 2 is made
positive with the Base1. The majority carrier which is the holes in the P type enters the N channel and since Base2 is
positive it gets repelled and attracted towards Base1 terminal. So the resistance decreases. The Emitter current
increases and reaches the peak and starts decreasing. After it reaches the valley point again it starts increasing.
When the PN Junction is reverse biased, the emitter current doesn’t flow and it is cut-off.

Characteristics of Unijunction Transistor:

Cut off Region:

In this region the applied input voltage is not sufficient to turn the device ON and the applied voltage didn’t reach
the triggering voltage.
Negative Resistance Region:
After the applied voltage reach the triggering voltage the device is turned ON and the emitter voltage reaches the
peak voltage and it drops to valley point, even though the emitter current increases.
Saturation Region:
In this region when the applied voltage increases, the emitter voltage and the emitter current increase gradually.

Intrinsic Stand-Off Ratio of UJT:

The intrinsic standoff ratio η is the ratio of R B1 to RBBO. It varies from 0.4 to 0.8 for different devices.

SCR Triggering using UJT:

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To fire the SCR, a short-time current pulse at its gate is to be applied. The delay of firing and controlling the instant
at which the pulse occurs within each half cycle is to be provided by the firing circuit. A UJT can be used to perform
this function as shown in figure.18.

Fig.18 SCR triggering using UJT circuit diagram Fig.19.Wave forms

The input voltage to UJT is applied through a capacitor C whose charging rate can be adjusted through resistance R3.
As the capacitor C charges, voltage VE increases until its value reaches the firing potential, forcing the UJT to conduct
discharging the capacitor through base resistance R5.

The spike of current occurring with the capacitor discharge is observed as a voltage spike developed across base
resistance R5, part of this current is applied to the gate of the SCR. The duration of the firing pulse can be varied by
adjusting the discharge rate of the capacitor through R5.

The switching speed of UJT determines the rise time of the triggering pulse. Discharge of capacitor C is accompanied
by a decrease in voltage VE till UJT stops conduction for values of VE below Vmin. The capacitor will again start
charging and the circuit is ready for the next operation.

Applications of SCR:
The main application of SCR is switching and power control. The followings are some applications that use
switching and power control properties of SCR.
1. It is used as a switch 2.It is used in AC voltage stabilizers
3.It is used in choppers (DC to Dc converters) 4.It is used for inverters (DC to AC converters)
5.It is used in battery charger 6.It is used for power control circuits
7.It is used in DC circuit breaker 8.It is used for AC power control with a solid relay
9.It is used to control motors speed 10.It is used to adjust the light dimmer
11.It is used in fan speed regulators

Applications of TRIAC:
TRIACs can be used in various applications such as
1.Control circuits like electric fan speed control and smaller motor controls
2.High Power lamp switching and light dimmers
3.AC power control domestic appliances

Application of DIAC:
The main application of DIAC is to trigger TRIAC.

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 CHAPTER.2 OPTO ELECTRONIC DEVICES
Optoelectronic devices are devices that use the interaction of light (optical signals) with electronic signals.
The Circuit Symbols of:

photo diode photo transistor LED LDR Photovoltaic cell opto-coupler

Photo Diode:
A photodiode is a semiconductor device that converts light into an electric current. Photodiode operates in reverse
bias condition i.e., the p – side of the photodiode is connected with negative terminal of battery (or the power supply)
and n – side to the positive terminal of battery. The basic working principle involves the generation of electron-hole
pairs in the semiconductor material when exposed to light. Typical photodiode materials are Silicon, Germanium,
Indium Gallium Arsenide Phosphide and Indium gallium arsenide.
Working of Photo Diode:
Generally, when a light is made to illuminate the PN junction, covalent bonds are ionized. This generates hole and
electron pairs. Photocurrents are produced due to generation of electron-hole pairs. Electron hole pairs are formed
when photons of energy more than 1.1eV hits the diode. When the photon enters the depletion region of diode, it hits
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the atom with high energy. This results in release of electron from atom structure. After the electron release, free
electrons and hole are produced.

Fig.20 Working of Photo diode Fig.21 V-I characteristics of Photo diode

In general, an electron will have a negative charge and holes will have a positive charge. The depletion energy will
have built-in electric field. Due to that electric field, electron-hole pairs move away from the junction. Hence, holes
move to anode and electrons move to the cathode to produce photocurrent.
V-I Characteristics of Photo Diode:
The V-I characteristics can be summarized in a graph where the x-axis represents the reverse bias voltage, and the y-
axis represents the photocurrent. The graph shows an exponential increase in photocurrent with increasing reverse
bias until it reaches saturation. When there is no light falling on the photodiode (dark conditions), a small current
called the dark current flows. This current is due to the thermal generation of electron-hole pairs within the
semiconductor material. Dark current increases with temperature When light falls on the photodiode, the photocurrent
is superimposed on the dark current. The photocurrent increases with the intensity of incident light. As the reverse
bias voltage increases, the photodiode becomes more efficient at converting light into current. However, there is a
limit beyond which further increase in reverse bias voltage leads to a saturation of the photocurrent. Excessive
reverse bias can lead to reverse breakdown, where the photodiode starts conducting heavily in the reverse direction.
This condition can damage the photodiode, so reverse bias should be limited to prevent breakdown.

Photo Transistor:
Photo Transistor is a three terminal semiconductor device which converts the incident light into photocurrent. Light
is incident on the base terminal and it is converted into current which flows through emitter and collector terminals.
Working of Photo Transistor:

From the figure.22 we can know that base is not connected to any external bias and only light is incident on the base
terminal. Collector terminal is connected to the positive side of external supply and output is taken from the emitter
terminal. When no light is incident on the base terminal only some leakage current flows and it is called as dark
current. When light is incident on the lens at the base collector junction, base current is generated which is
proportional to the intensity of the incident light.

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 Fig.22 circuit diagram Fig.23 V-I charactristics
V-I Characteristics of Photo Transistor:
From the figure.23 we can observe how the collector current varies with the intensity of the incident light. The
collector current increases with the intensity of the incident light. Collector current differs with the wavelength and
the intensity of the light. At high light intensities, the collector current may reach a saturation point where further
increases in light intensity do not result in a proportional increase in current. Saturation occurs when the
phototransistor is operating at its maximum capability. The V-I characteristics provide an operating range for the
phototransistor, indicating the voltage and current values within which the phototransistor can operate reliably.
Photo Multiplier:
A photomultiplier tube (PMT) is a highly sensitive device used for detecting low levels of light. It operates on the
principle of photoelectric emission and electron multiplication through a series of dynodes. A photomultiplier tube
is used in order to convert a low amount of electromagnetic radiation into a strong electric signal
Working of Photo Multiplier:
The PMT contains a photosensitive material called the photocathode, typically made of a material like cesium
antimonide. When photons of light strike the photocathode, they cause photoelectric emission, releasing electrons.
The emitted electron is accelerated towards the first dynode, which is at a higher positive potential. This process
occurs in a vacuum to prevent electron collisions with gas molecules. The dynode is a metal electrode that is
maintained at a positive potential relative to the photocathode. As the electron strikes the first dynode, it releases
secondary electrons through a process called secondary emission. The secondary electrons are accelerated towards
the next dynode, where each electron can release more secondary electrons. This process repeats through several
dynodes, resulting in a cascade or multiplication of electrons. The electron multiplication process continues through
multiple dynodes, leading to a significant amplification of the initial photoelectron. The total number of electrons at
the end of the dynode chain is much larger than the number of photons initially detected. The final dynode in the
chain is maintained at a high positive potential. The multiplied electrons are then attracted towards the anode, creating
a measurable current. The magnitude of this current is proportional to the intensity of the incident light. The amplified
current at the anode is the output signal of the photomultiplier. This signal can be further processed and used for
various applications, such as in scientific instruments, medical imaging devices, and low-light detection systems. The
photomultiplier requires a high-voltage power supply to maintain the potential difference between the dynodes. The
voltage applied determines the level of electron multiplication and, consequently, the sensitivity and gain of the
photomultiplier.

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 Fig.24 Working of Photo Multiplier
Applications of Photo Diodes:
1. Photovoltaic Power Generation 2. Light Sensors 3. Communication Systems 4. Barcode Readers
5. Photometry and Spectrometry 6. Medical Instruments 7. Smoke and Fire Detection 8. Security Systems
9. Environmental Monitoring
Applications of Photo Transistors:
1. Light Sensing 2. Optocouplers 3. Object Detection 4. Infrared Remote Controls 5. Light-Activated
Switches 6. Barcode Scanners 7. Position and Motion Sensing 8. Encoders
Applications of Photo Multipliers:
1. Scientific Research 2. Medical Imaging 3. Astronomy 4. Fluorescence Spectroscopy 5. Flow Cytometry
6. Nuclear Physics Experiments 7. Environmental Monitoring 8. Laser Rangefinders
9. Bioluminescence Measurement 10. High-Speed Imaging
Working of LDR:
A Light Dependent Resistor (LDR), also known as a photoresistor, is a type of resistor whose resistance changes
with the intensity of light incident upon it. LDRs are semiconductor devices that exhibit a decrease in resistance as
the light level increases. The working of an LDR is based on the principle of the photoconductivity of certain
semiconductor materials.

Fig.25 LDR Structure Fig.26 LDR characteristics

LDRs are typically made of semiconductor materials such as cadmium sulfide (CdS) or cadmium selenide (CdSe).
These materials have a property called photoconductivity, where their electrical conductivity changes in response to
light. When light falls on the semiconductor material of the LDR, the energy from the photons is absorbed by the
semiconductor atoms, causing some of the electrons in the atoms to move from the valence band to the conduction
band. The movement of electrons to the conduction band increases the electrical conductivity of the semiconductor
material. This increase in conductivity is directly proportional to the intensity of the incident light. As the conductivity
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of the semiconductor material increases, the resistance of the LDR decreases. This is because resistance and
conductivity are inversely proportional. In the absence of light or in low-light conditions, the LDR exhibits a higher
resistance, known as its dark resistance. This is the resistance level when the LDR is not exposed to light. When
exposed to light, the LDR's resistance decreases, allowing more current to flow through it. The degree of resistance
change depends on the intensity of the light.
Photovoltaic effect:
The photovoltaic effect is a process that generates voltage or electric current in a photovoltaic cell when it is
exposed to sunlight.
Working of Photovoltaic Cell:
A photovoltaic cell, commonly known as a solar cell, is a semiconductor device that converts sunlight directly into
electrical energy through the photovoltaic effect. The basic working principle involves the generation of an electric
current when photons of light strike the semiconductor material.
The core of a photovoltaic cell is made of a semiconductor material, usually silicon. Silicon is chosen for its ability
to release electrons when exposed to sunlight. When photons (particles of light) from sunlight strike the
semiconductor material, they transfer their energy to electrons in the material. This energy is sufficient to liberate
electrons from their bound states, creating electron-hole pairs. Due to the internal structure of the semiconductor
material, the liberated electrons and the positively charged holes created by their absence are separated. This
separation of charge carriers creates an internal electric field within the material. The electric field within the
semiconductor material forces the liberated electrons to move toward the n-type (negative) side, and the holes move
toward the p-type (positive) side. This directional flow of electrons constitutes an electric current. Metal contacts are
placed on the top and bottom surfaces of the semiconductor material to collect the flow of electrons and create an
external electric circuit. The generated electric current can be used to power an external load, such as a light bulb,
motor, or electronic device. The electricity generated is direct current (DC).

Fig.27 Working of Photovoltaic cell

Applications of Photovoltaic Cells:
1.Residential solar power systems 2.commercial and industrial solar power systems 3.solar street lighting
4.Off grid power systems 5.solar water pumping 6.solar powered vehicles 7. solar powered gadgets
Working of LED:
Light Emitting Diodes (LEDs) are semiconductor devices that emit light when an electric current is applied. The
process by which LEDs work is based on the phenomenon of electroluminescence.
The core of an LED is a semiconductor material. Most commonly used materials are compounds of gallium, arsenic,
phosphorus, and nitrogen, depending on the desired wavelength of light. The semiconductor material has a specific
energy band gap. Electrons within the material can exist in one of two energy bands: the valence band or the
17
conduction band. The energy gap between these bands is crucial for the operation of LEDs. When a voltage is applied
to the LED in the forward direction (anode to cathode), it creates an electric field across the semiconductor material.
The applied voltage provides enough energy to some electrons in the valence band, allowing them to move to the
conduction band. This process creates electron-hole pairs. As electrons move to the conduction band, they leave
behind holes in the valence band.

Fig.28. Working of LED

When an electron recombines with a hole, it releases energy in the form of a photon (light). The energy of the photon
is determined by the energy band gap of the semiconductor material. The released photons have a specific wavelength
corresponding to the color of light produced by the LED. The color is determined by the semiconductor material
used. For example, gallium arsenide emits red light, while gallium nitride emits blue light. Unlike traditional light
sources, LEDs emit light directly as a result of the electron-hole recombination process. This direct emission
contributes to the efficiency of LEDs. In color LED applications, such as RGB (Red-Green-Blue) LEDs, multiple
semiconductor materials are combined in a single LED to produce a range of colors. By adjusting the intensity of
each color, a broad spectrum of colors can be achieved.
Working of LCD:

Fig.29. Working of LCD

Liquid Crystal Displays (LCDs) are widely used in various electronic devices, including televisions, computer
monitors, smartphones, and digital watches. The basic working principle of an LCD involves the use of liquid crystal
molecules that can be controlled to modulate the passage of light, creating the images we see on the display.
18
1. Liquid Crystal Layer: The core component of an LCD is a layer of liquid crystals located between two layers of
glass or plastic. These liquid crystals are rod-shaped molecules that can align themselves in specific ways when
subjected to an electric field.
2. Substrate Layers: The glass or plastic layers containing the liquid crystal material are coated with transparent
electrodes. These electrodes apply an electric field to the liquid crystal layer.
3. Polarizing Filters: Two polarizing filters are placed on the outer surfaces of the glass or plastic layers. These filters
are aligned perpendicular to each other. The first filter allows only light vibrating in one direction to pass through,
while the second filter allows only light vibrating in a different direction to pass through.
4. Liquid Crystal Alignment: When no voltage is applied across the liquid crystal layer, the liquid crystal molecules
align themselves parallel to the surfaces of the glass or plastic layers. In this state, the liquid crystal layer twists the
polarization of incoming light.
5. Applying Voltage (Changing State): When a voltage is applied across the liquid crystal layer by activating the
electrodes, the electric field causes the liquid crystal molecules to untwist. This change in molecular alignment
disrupts the polarization of light passing through the layer.
6. Rotation of Polarization: As the liquid crystal molecules untwist, the polarization of light passing through the layer
is rotated. This rotation is crucial for controlling the intensity of light that reaches the second polarizing filter.
7. Color Filters: In color LCDs, there are color filters placed over each pixel. These filters are typically red, green,
and blue, and they determine the color of each pixel.
8. Controlled Light Transmission: By adjusting the voltage applied to each pixel, the amount of light passing through
the liquid crystal layer can be controlled. This modulation of light intensity, combined with color filters, allows the
display to create a wide range of colors and shades.
9. Pixel Matrix: The pixels on an LCD are arranged in a matrix, with each pixel corresponding to a specific location
on the display. By controlling the voltage applied to each pixel, the liquid crystal layer selectively allows or blocks
light, creating the desired image.
10. Backlighting: In many LCDs, a backlight source (such as LED or fluorescent lamps) is placed behind the liquid
crystal layer to provide illumination. The controlled modulation of light by the liquid crystals creates the visible
image.
In summary, the operation of an LCD involves manipulating the alignment of liquid crystal molecules using an
applied electric field to control the passage of light through the display. This ability to selectively modulate light
transmission allows LCDs to produce images and graphics in a wide range of electronic devices.
Working of Opto-Coupler:
An optocoupler, also known as an opto-isolator, is a device that combines a light-emitting diode (LED) and a
photodetector (like a photodiode, phototransistor, or photometric) to transfer electrical signals using light waves. The
primary purpose of an optocoupler is to provide electrical isolation between input and output circuits, preventing the
transfer of unwanted electrical signals, such as noise or high voltages, from one side to the other. This isolation is
particularly important in applications where safety, signal integrity, and protection against voltage spikes are crucial

19
 Fig.30 Working of Opto-Coupler
The input side of the optocoupler contains an LED. When a voltage is applied to the input terminals, the LED emits
light. The emitted light from the LED crosses the physical gap between the input and output sections. This gap
ensures electrical isolation between the two sides. On the output side, there is a photodetector (such as a photodiode,
phototransistor, or photometric). This component is sensitive to the emitted light from the LED. When light falls on
the photodetector, it induces a photoelectric effect, generating electron-hole pairs within the semiconductor material
of the photodetector. The generated electron-hole pairs result in a current or voltage signal on the output side,
depending on the specific design of the optocoupler.

Applications of Opto-Couplers:
1.Signal isolation in power supplies 2. Motor control circuits to isolate control signal
3.Data communication to isolate data communication signal 4.Used in test and measurement equipment
5. Instrumentation and Control systems 6. Switching power supplies 7. Microcontroller interface.
Applications of Light Emitting Diodes (LEDs):
1. Display Technology 2. Indicator Lights 3. Automotive Lighting 4. Street Lighting 5. Traffic Lights
6. Backlighting for Screens 7. Flashlights and Portable Lighting 8. Illumination in Buildings
9. Decorative Lighting 10. Gaming Peripherals 11. Photography and Video Lighting
12. Plant Growth Lighting.
Applications of Liquid Crystal Displays (LCDs):
1. Televisions and Monitors 2. Laptops and Tablets 3. Smartphones 4. Digital Cameras 5. Digital Signage
6. Medical Imaging 7. Automotive Displays 8. Industrial Control Panels 9. Gaming Consoles 10. E-Readers
11. ATM and Point-of-Sale Displays 12. Aircraft Cockpit Displays
Discrete Displays:
"Discrete displays" refers to individual visual elements that can be independently controlled to convey information.
These displays consist of distinct components, each representing a specific part of the overall visual output. Unlike
continuous displays, where information is presented in a continuous manner, discrete displays use individual
elements or segments to create characters, symbols, or graphical representations.
Common examples of discrete displays include: Dot matrix, Bar matrix , Bar graph and Seven segment display.
Discrete displays, such as dot matrix displays, bar matrix displays, bar graph displays, and seven-segment displays,
are used to visually convey information by illuminating specific elements. Each type has its unique configuration and
is suitable for different applications.
1. Dot Matrix Display:

20
A dot matrix display consists of an array of individual pixels arranged in rows and columns. Each pixel can be
independently controlled to emit light or remain off. The display can be thought of as a two-dimensional matrix of
LEDs or pixels. To display characters, symbols, or graphics, specific pixels are activated in patterns to create the
desired visual output.

Fig.31 5 x 7 dot matrix display Fig.32 Bar matrix display

2. Bar Matrix Display:
A bar matrix display is similar to a dot matrix display, but instead of individual pixels, it consists of bars or segments
that can be illuminated or turned off. Each bar represents a segment of the display, and by controlling the activation
of these segments, different characters or symbols can be displayed.
3. Bar Graph Display:
A bar graph display typically consists of a series of adjacent bars arranged horizontally or vertically. The length or
height of each bar corresponds to the quantity it represents. The bars can be selectively illuminated to visually
represent different levels or values.

Fig.33 Bar graph display Fig.34. Seven-Segment Display

4. Seven-Segment Display:
A seven-segment display consists of seven individual LED segments arranged in the shape of the digit "8." Each
segment can be independently illuminated, allowing the display of numerical digits (0-9) and some alphabetic
characters (A-F in hexadecimal).

21
 CHAPTER.3 ULTRASONICS
Ultrasonics Defination:
Ultrasonics refers to sound waves that have a frequencies greater than the upper limit of the audible range for humans,
that is, greater than about 20 kilohertz.
Properties of Ultrasonics:
Ultrasonic waves have several properties that make them useful in various applications. Here are some key properties
of ultrasonics:
1.Ultrasonic waves have frequencies higher than the audible range for humans.
2.These waves propagate through a medium, such as air, water, or solids.
3.They exhibit characteristics of both longitudinal and transverse waves.
4.These waves have shorter wavelengths compared to audible sound waves.
5.These waves can be directed and focused.
6.These waves can reflect off surfaces.
7.These waves are absorbed by different materials to varying degrees.
8.These waves can undergo refraction and interference.
Generation of Ultrasonics:

22
The generation of ultrasonics involves creating mechanical vibrations at frequencies above the upper limit of human
hearing, typically above 20,000 hertz (Hz). The following are the methods to generate ultrasonics.
1.Magnetostriction Oscillator/Generator:
When a ferromagnetic rod like iron or nickel is placed in a magnetic field parallel to its length, the rod experiences a
small change in its length. This is called magnetostricion effect.
The change in length (increase or decrease) produced in the rod depends upon the strength of the magnetic field, the
nature of the materials and is independent of the direction of the magnetic field applied.

Fig.35 Magnetostricion effect. Fig.36 The experimental arrangement

of Magnetostriction oscillator
XY is a rod of ferromagnetic materials like iron or nickel. The rod is clamped in the middle. The alternating magnetic
field is generated by electronic oscillator. The coil L1 wound on the right hand portion of the rod along with a variable
capacitor C. This forms the resonant circuit of the collector tuned oscillator. The frequency of oscillator is controlled
by the variable capacitor. The coil L2 wound on the left hand portion of the rod is connected to the base circuit. The
coil L2 acts as feed –back loop.
When High Tension (H.T) battery is switched on, the collector circuit oscillates with a frequency,

This alternating current flowing through the coil L1 produces an alternating magnetic field
along the length of the rod. The result is that the rod starts vibrating due to magnetostrictive effect.

The frequency of vibration of the rod is given by

where l = length of the rod Y = Young’s modulus of the rod material and  =density of rod material.
The capacitor C is adjusted so that the frequency of the oscillatory circuit is equal to natural frequency of the rod and
thus resonance takes plate. Now the rod vibrates longitudinally with maximum amplitude and generates ultrasonic
waves of high frequency from its ends.
2. Piezo-electric Oscillator/Generator:
If mechanical pressure is applied to one pair of opposite faces of certain crystals like quartz, equal and opposite
electrical charges appear across its other faces. This is called as piezo- electric effect.
If an electric field is applied to one pair of faces, the corresponding changes in the dimensions of the other pair of
faces of the crystal are produced. This is known as inverse piezo electric effect or electrostriction.

23
 Fig.37 Piezo- electric effect Fig.38 The circuit diagram of Piezo electric oscillator
The quartz crystal is placed between two metal plates A and B. The plates are connected to the primary(L3) of a
transformer which is inductively coupled to the electronics oscillator. The electronic oscillator circuit is a base tuned
oscillator circuit. The coils L1 and L2 of oscillator circuit are taken from the secondary of a transformer T. The
collector coil L2 is inductively coupled to base coil L1 . The coil L1 and variable capacitor C1 form the tank circuit
of the oscillator.
When H.T. battery is switched on, the oscillator produces high frequency alternating voltages with a frequency.

Due to the transformer action, an oscillatory e.m.f. is induced in the coil L3 .This high
frequency alternating voltages are fed on the plates A and B. Inverse piezo-electric effect takes place and the crystal
contracts and expands alternatively. The crystal is set into mechanical vibrations.
The frequency of the vibration is given by

Y = Young’s modulus of the crystal and ρ = density of the crystal.

The variable condenser C1 is adjusted such that the frequency of the applied AC voltage is equal to the natural
frequency of the quartz crystal, and thus resonance takes place. The vibrating crystal produces longitudinal ultrasonic
waves of large amplitude.
Applications of Ultrasonics:
1. Medical Imaging (Ultrasound) 2. Industrial Testing and Inspection 3. Ultrasonic Cleaning
4. Ultrasonic Welding 5. Distance Measurement and Level Detection 6. Pest Control
7. Sonar Systems 8. Ultrasonic Therapy 9. Food Processing 10. Material Disintegration

CHAPTER.4 INDUSTRIAL HEATING and WELDING

Different Industrial Heating Methods:
There are various industrial heating methods employed across different industries, each suited to specific
applications. The following are some common industrial heating methods:
1. Electric Heating: (i) Resistance Heating (ii) Induction Heating
2. Combustion Heating 3. Steam Heating 4. Microwave Heating
24
5. Radiant Heating 6. Convection Heating 7. Indirect Heating 8. Plasma Heating
9. Resistance Furnace Heating 10. Solar Heating.
Principle of Induction Heating:
The principle of operation of induction heating is based on Faraday’s law of electromagnetic induction and on the
concept of Joule or resistance or ohmic heating. The figure.39 represents the principle of induction heating.

Fig.39 Principle of Induction Heating

It consists of two coils primary and secondary which act as the primary and secondary winding of a transformer. The
primary is connected to an ac supply of high frequency, and the secondary is used to heat the workpiece. In figure.39
the workpiece to be heated will act as the single-piece short-circuited secondary winding.

When a high-frequency alternating current is given to the primary coil, it set up an alternating magnetic field. This
flux when links the workpiece, induces an emf in the workpiece resulting in the flow of eddy current through it,
similar to how eddy currents are induced in the secondary winding of the transformer.
We know that according to the Joule Heating effect, thermal energy is produced in a conductor when an electric
current flow through it i.e., power is dissipated in the form of heat. Thus eddy currents induced in the workpiece will
develop heat in it, thereby increasing the temperature of the workpiece.

Due to the low resistivity and high conductivity of metals, they are well suited for induction heating compared to
non-metals. In the case of non-magnetic material, the heat developed will be due to eddy current loss. Whereas in the
case of magnetic material, the heat developed will be due to both eddy current loss and hysteresis loss

Merits of Induction Heating:

The key advantages of induction heating are
1. Highly efficient 2. Precise temperature control 3.Localized heating without affecting the entire object
4.Clean and controllable 5. Induction heating is a non-contact process 6. Environmentally friendly
7. Reduced processing times 8. Very high heating rates.
Applications of Induction Heating:
1. Surface hardening of metal components 2. Annealing metals 3. Brazing and Soldering
4. Melting and Castings 5. Forging 6. Heat treatment 7. Shrink fitting
8. Cooking appliances 9. Heating of wires and tubes 10. Sealing of containers

Principle of Dielectric Heating:

25
An alternating current (AC) electric field is applied to the dielectric material. The electric field oscillates rapidly,
changing direction back and forth. When the dielectric material is subjected to the alternating electric field, the
electric dipoles within the material attempt to align themselves with the field. In simple terms, the positive and
negative charges within the molecules of the dielectric material try to follow the changes in the electric field. The
continuous reorientation of the electric dipoles in response to the changing electric field leads to molecular friction.
This molecular friction results in the generation of heat within the dielectric material. The heat produced due to
molecular friction is known as dielectric loss. This is the primary mechanism through which dielectric heating
achieves heating of materials.

Fig.41 Dielectric heating circuit diagram Fig.42 Dipole orientation

A dielectric material is basically a poor conductor or insulator such as wood, plastic, glass, ceramics, etc. We know
that each and every material is made up of molecules whose elemental particle is an atom. When the dielectric
material is not present in the electric field, the polar molecules in the material are randomly arranged as shown in
figure.42

When these molecules are subjected to the influence of the alternating electric field, the molecules in the material
undergo polarization. Due to the formation of dipoles on polar molecules, the dipole moments of the polar molecules
get properly oriented as shown in figure.42

The orientation of the polar molecules will be according to the direction of the impressed alternating electric field.
Since the electric field applied is alternating, the orientation of the polar molecules will be changed for every half
cycle of the supply. The speed at which the orientation of the polar molecules changes will depend upon the frequency
of the supply.

In doing so, atoms in the material get stressed due to inner atomic friction for which some energy will be wasted as
the dielectric loss. This dielectric loss in dielectric material when subjected to an alternating electric field will results
in the production of heat thereby increasing the temperature of the material.

Dielectrics used for Dielectric Heating:

1.Plastic 2. Paper and Wood 3. Rubber 4. Ceramics 5. Polymers and Composites 6. Glass 7. Textiles
Applications of Dielectric Heating:
Dielectric heating is widely used in various industrial processes, such as the curing of composites, drying of materials,
preheating in plastic molding, and in the food industry for processes like microwave cooking.

Types of Resistance Welding:

1.Spot welding 2.Projection welding 3.Seam welding 4.Butt welding 5.Flash welding.
Principle of Resistance Welding process:
Resistance welding is a type of electric welding in which heat produced to weld the two metal pieces will be due to
the resistance offered to the flow of current by the two metal pieces at the point of joint.
26
In resistance welding sufficiently strong electric current is sent through the two metal pieces to be welded using
electrodes, the resistance offered by the two metal pieces at the contact area develops heat and melts the metal to a
plastic state or liquid state. Then after a high mechanical pressure is applied by electrodes to press the two metal
pieces together which completes the weld

Fig.44 Resistance Welding

The heat developed is given by I2Rt, where 'I' is the current flow, 'R' is the resistance (in ohms) and 't' is the time for
which the current flows. The resistance in the equation is made of,
1.Resistance to the current path in the workpiece.
2.The resistance between the contact surfaces of parts being welded.
3.The resistance between the electrodes and workpieces.

Working of basic circuit of A.C. Resistance Welding Working:

The "line contactor" is basically a controlled switch which connects the ac mains voltage across the primary
winding of the "welding transformer" only during the welding interval. It otherwise disconnects the mains from the
primary.

The "welding transformer" is a step down transformer which supplies a reduced (steps down) voltage on the
secondary side but increases (steps up) the secondary current which is the "welding current". This current is usually
in the range of several hundred to several thousands of amperes, depending on the nature of weld

Fig.45 Basic circuit of A.C. resistance welding.

27
Thyrite resistor has a property that its resistance decreases with increase in the voltage across it. It is connected
across the primary winding of the welding transformer in order to protect its insulation against high voltage spikes
that are induced due to high rate of change of current in the primary winding when the contactor is suddenly turned
on or off.
The control circuit design depends upon the nature and precision of welding. The control circuit decides the instants
at which the line contactor is turned on and off j.e. the welding time. In order to get better quality of weld the control
Circuit may control : (a) time for pressure to build up at the electrodes. (b) time for metal to cool down before
electrode pressure is released. (c) time interval between successive welding operations.
The line contactor may use a "controlled switch" like ignitron tubes or thyristors in order to control the magnitude of
the welding current.

28
 CHAPTER.5 INVERTERS, SMPS and UPS
Need of Inverters:
Inverters play a crucial role in various applications by converting DC power into AC power at desired output voltage
and frequency.
Principle of Operation of Inverter:
The inverter is a kind of oscillator. It can produce a high-power AC output from a DC supply.

Fig.46 Principle of operation of inverter

Here we see in the circuit diagram here we use 12V battery, one transformer (Primary winding of transformer is
Center tapped), One two-way switch and 50 Hz oscillator.
Here 12V battery generate DC supply and inverter will change it into, AC supply of 220V, 50Hz to use to operate
any appliances.
The 12V DC supply from the positive terminal of the battery comes to the primary winding of transformer which is
center tapped. The two ends of the primary winding of transformer (A and B point) are connected to the two-ways
switch to the ground. If the switch connects to A point of the primary winding. The current flows from the battery
into upper half of primary winding (o) through A contact of th e switch to the ground. If switch turn from A point
into B point. This time the current number 1 stops flowing. Then, the current 2 flows to the ground through o and
contact B of the switch.
Here, 2 ways switch is controlled with the square wave oscillator it generates a frequency of 50 Hz. It causes the
switch to selects between A and B point with speed about 50 times per second. Also, the current 1 and 2 flows to the
transformer alternately at a rate of 50 times per second. So, the current flows into the transformer alternately look
like AC voltage.
We know that transformers work on the principle of Electromagnetic induction. When current flow in primary
winding EMF induced and a current will be induced into the secondary winding of transformer. Which it causes AC
voltage 220V 50Hz. Now, the voltage is use to be supplied to the various types of electrical equipment that operate
in 220 Volt AC supply.
Classification of Inverters:

29
 1. Based on Waveform Type: 2. Based on Output Voltage:
Sine Wave Inverters Single-Phase Inverter
Modified Sine Wave (Quasi-Sine Wave) Inverters Three-Phase Inverters
3. Based on Application: 4. Based on Power Rating:
Grid-Tied (Grid-Connected) Inverters Low-Power Inverters
Off-Grid (Stand-Alone) Inverter Medium-Power Inverters
High-Power Inverters
5. Based on Technology:
PWM (Pulse Width Modulation) Inverter
Multilevel Inverters

Working of Single-Phase Bridge Inverter using MOSFET:

In this type of inverter, four switches are used. The gate pulse for MOSFET 1 and 2 are same. Both switches are
operating at same time. Similarly, MOSFET 3 and 4 has same gate pulses and operating at same time. But, MOSFET
1 and 4 (vertical arm) never operate at same time. If this happens, then DC voltage source will be short circuited.

Fig.47 Circuit diagram Fig.48 waveforms

𝑇
For upper half cycle (0 < t < 0), MOSFET 1 and 2 get triggered and current will flow as shown in figure.49. In this
2
time period, the current flow from left to right direction.

Fig.49 During upper half cycle Fig.50 During lower half cycle
𝑇
For lower half cycle ( 0 < t < To), MOSFET 3 and 4 get triggered and current will flow as shown in figure.50. In this
2
time period, the current flow from right to left direction. The peak load voltage is same as DC supply voltage Vdc in
both cases.
Working of Voltage Source Inverter:
The circuit diagram of a single-phase HB-VSI with a load is shown in fig.51

30
Fig. 51 Circuit diagram Fig.52 Waveforms
In this type of topology, one thyristor T1 conducts for half of the time period and the other thyristor T2 conducts for
the other half of the time period of the output waveform. The diodes are connected anti-parallel with the thyristor
and will allow the current to flow when the main thyristor is turned off. Diode D1 will conduct when the voltage is
positive and the current is negative, and diode D2 will conduct when the voltage is negative, and the current is
positive. It is especially useful in the case of non-resistive loads. When the diode conducts, the energy is fed back to
the DC source and hence, these diodes (D1 & D2) are called flyback diodes.

Each thyristor is triggered via its gate. While analyzing the circuit, it is assumed that each thyristor conducts for the
duration its gate pulse is present and is commutated as soon as this pulse is removed. The gating signal for thyristor
T1 (ig1) and thyristor T2 (ig2) and the output voltage waveform of this inverter are shown in figure.52.

It can be observed that ig1 is applied for a period of 0 <t≤ (T/2), the thyristor T1 conducts during this period and the
load is directly connected to the source Vs/2 on the upper arm of the circuit and the output voltage equals Vs/2 during
this time. ig1 is removed at time T/2, thyristor T1 gets turned off, ig2 is applied and thyristor T2 starts conducting.
The load is then connected directly to the -Vs/2 on the lower arm (it has the opposite polarity as compared to the
upper arm). The output voltage is now -Vs/2 as shown in figure.52.

The resulting output waveform is an alternating square wave with a frequency of 1/T Hz and an amplitude of Vs/2.
The frequency is controlled by changing the value of the period, T.

Working of PWM Inverter:

PWM Inverter uses PWM(Pulse Width Modulation) techniques to control the output voltage of the inverter and are
characterized by constant amplitude pulses. The width of these pulses is, however, modulated to obtain inverter
output voltage control and to reduce its hormonic content. In PWM inverters, forced commutation is essential.

31
Fig.53 Circuit diagram Fig.54 Waveforms
The circuit diagram of PWM Inverter is shown in fig.53 The output voltage from single phase full bridge inverter is
shown in fig.54(a). When this waveform is modulated, the output voltage is of the form shown in fig.54(b). It consists
of a pulse of width 2d located symmetrically about 𝜋/2 and another pulse is located symmetrically about 3𝜋/2. The
range of pulse width 2d varies from 0 to 𝜋 ; i.e. 0< 2d < 𝜋. The output voltage is controlled by varying the pulse
width 2d. This shape of the output voltage wave shown in fig.54(b) is called quasi square wave.
Applications of Inverters:
1. Residential Power Backup 2. Solar Power Systems. 3. Wind Power System 4. Induction Heating
5. Electric Vehicles (EVs) 6. Uninterruptible Power Supply (UPS) 7. HVAC Systems 8. HVAC Systems
Working of SMPS with block diagram:
The major components that constitute SMPS are as follows:
1.Input rectifier and Filter (Diode rectifier and capacitor filter)
2.High-frequency switch (Power transistor or MOSFET)
3.Power transformer
4.Output rectifier and Filter (Diode rectifier and capacitor filter)
5.Control circuit (comparator and pulse width modulator

Initially, the unregulated ac input signal from the source is provided to the input rectifier and filter circuit. Here the
ac input signal is rectified to generate a dc signal and further smoothened to remove high-frequency noise component
from it. The dc output (still in unregulated form) is fed to the power transistor that acts as a high-frequency switch.

Fig.55 Block diagram of SMPS

Here the dc signal undergoes chopping (switching). This circuit acts as an ideal switch i.e., when the power transistor
(chopper circuit) is in on state, current passes through it with negligible voltage drop, and dc signal is obtained at the
output terminal of the transistor. However, under the off state of the power transistor, no current passes through it
and leading to cause maximal voltage drop within it. Thus, at the output side, no voltage will be present.

Hence, according to the switching action of the power transistor dc voltage will be obtained at its output side. The
chopping frequency plays a crucial role in maintaining the desired dc voltage level.

The obtained dc signal at the output of the chopper circuit is then fed to the primary winding of the high-frequency
power transformer. Here the step-down transformer converts the high voltage signal into a low voltage level which
is further provided as input to the output rectifier and filter unit. This simply filters out the unwanted residuals from
the signal in order to provide a regulated dc signal as the output.

32
The control circuitry present here acts as the feedback circuit for the complete unit. This involves a comparator along
with a pulse width modulator (PWM). The dc output from the rectifier and filter is fed to the control circuit where
the error amplifier which acts as a comparator, compares the obtained dc voltage with the reference value.

If the dc output is greater than the reference value then the chopping frequency is to be decreased. The decrease in
chopping frequency will reduce the output power and so the dc output voltage. However, if the dc output is less than
the reference value then the chopping frequency is increased. When chopping frequency is raised then the dc output
voltage will get increased.

Applications of SMPS:
1.Consumer Electronics 2. Computers and Servers 3.Telecommunications Equipments
4.Power Adapters and Charger 5. Industrial Power Supplies 6.LED Lighting 7. Medical Devices
8. Automotive Electronics 9. Renewable Energy Systems 10.Aerospace and Défense
11.Battery Charging Systems 12.Audio Amplifiers
Classification of UPS:
1. Online UPS 2. Off Line UPS 3. Line Interactive UPS
Storage Batteries used in UPS:
1. Lead-Acid Batteries 2. Lithium-Ion Batteries 3.Nickel-Cadmium (Ni-Cd) Batteries
4. Nickel-Metal Hydride (NiMH) Batteries 5.Flow Batteries 6. Thin Plate Pure Lead (TPPL) Batteries
Working of Online UPS:
EMI Filter: It is made using inductors and capacitors. The main function of this EMI filter circuit is to reduce or
filter electromagnetic interferences.
Rectifier Circuit: The rectifier circuit is used to convert AC to DC. As the UPS has a battery inside it, and the battery
can store only DC that is why we need to convert the input AC supply into DC.
DC Filter Circuit: The DC filter circuit is used to filter the impure DC that comes from the rectifier circuit. The DC
output from the rectifier has some AC components. So the filter circuit is used to remove those AC components from
the DC supply.
Battery: The battery is connected to the output of the DC filter circuit. When the UPS is connected to the power
supply the battery will charge.
Inverter Circuit: Now we have a DC supply but, we need an AC supply as output to drive the load. So the inverter
circuit is used to convert the DC to AC. The inverter Circuit is made of high-speed solid-state switches such
as MOSFET, SCR, etc. If your load required DC supply then the Inverter Circuit is not required.
AC Filter Circuit: The AC filter circuit is used to filter the impure AC coming from the inverter circuit.
Static Switch: A static switch is connected between the AC filter Circuit and the Critical Load. Which allows or
disallows the power flow from the UPS to load according to the given condition.

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Fig.56 Online ups block diagram
Another static switch is connected between the critical load and the main power supply after the EMI filter Supply.
This switch allows or disallows the power flow from the main supply to the load.

In the case of Online UPS, the lower static switch is normally ON and the upper static is normally OFF. So in normal
condition the power flow from the main supply to the load through the total UPS circuitry. When the main power
supply is not available, then the load takes power from the battery.

If the UPS is unable to deliver power to the load then the upper static switch will be ON and the lower Switch will
be OFF. So in this case, the power will directly flow from the main supply to the load.

Working of Off Line UPS:

In the case of Offline UPS, the upper static switch is normally ON and the lower static switch is normally OFF. So
in normal conditions, the power directly flows from the main supply to the load. At the same time, the battery will
charge. When the main power supply is not available, the upper static switch will be OFF and the lower static switch
will be ON. So the load takes power from the battery.

So the static transfer switch block is a key component in an Offline UPS. It detects the presence or absence of the
utility power. When the utility power is available or within acceptable limits, the switch routes the power directly to
the connected devices without involving the battery or the inverter. If the utility power fails or fluctuates beyond
acceptable levels, the transfer switch activates the battery backup. Other all the blocks or internal components for
both online and offline ups are the same

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Fig. 57 Off line ups block diagram
Applications of UPS:
1.Computers and Server 2.Telecommunications Equipments 3. Critical Data Centers
4. Medical Equipment 5. Industrial Control Systems 6. Emergency Lighting Systems
7. Security Systems 8. Financial Institutions 9. Retail Point-of-Sale (POS) Systems
10. Educational Institutions 11. Home Electronics 12. Internet Service Providers (ISPs)
13. Transportation Systems

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