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1.0 INTRODUCTION TO MECHATRONICDEVICES
Introductiontomechatronicsdevices.
INTRODUCTION OFMECHATRONIC DEVICES
OBJECTIVES
General objective:To understandthe basicsof mechatronicdevices.
Specificobjectives:Atthe endof the unityou shouldbe able to:
• Listthe typesof mechatronicdevices.
• Describe the basicsof mechatronic.
• Explainthe mechatronicdevices.
1.1 INTRODUCTION OFMECHATRONIC.
Mechatronicis a termcoinedbythe Japanese todescribe the integrationof mechanical and
electronicengineering.More specifically,itreferstoamultidisciplinaryapproachtoproductand
manufacturingsystemdesign.Itrepresentsthe nextgenerationmachines,robotsandsmart
mechanismsforcarryingoutwork ina varietyof environments-predominantlyfactory
automation,office automationandhome automationasshowninfigure 1.1(a).
As a discipline,mechatronicencompasses electronicsenhancingmechanics( toprovide high
levelsof precisionandreliability) andelectronicsreplacingmechanics(toprovide new functions
and capabilities).Some exampleswheremechanicshasbeenenhancedbyelectronicsare
numericallycontrolledmachinestoolswhichcutmetal automatically,industrial robotsand
automaticbanktellers.The productswhere electronicsreplacesmechanicsinclude digital
watches,calculatoror others.

However,the productsthatreallyblurthe distinctionbetweenelectronicsandmechanicsare
machinesandrobotsdrivenbynumerical control.Japanisthe firstcountryinthe worldtohave
masteredthe NCmachinestechnologyandasa resultthe Japanese machine tool industryhas
flourished.Thisisbecause the Japanesehave masteredmechatronics,the fusionof precision
mechanicsandelectronicsindesign,engineeringandmanufacturing,whichare popularly
depictedbythe Japanese asshowninfigure 1.1(b).
1.2 SCOPEOF MECHATRONICS
Since the 1970s , there hasbeena dramaticchange in the technologyof these products,mainly
an increasingcontentof electricandelectronicsystemsintegratedwiththe mechanical partsof
the products,mechatronic.Example of productswhichhave alreadymovedtomechatronic
technology fromsimplemechanical productsare;
a. Machine toolsincorporatingcomputernumerical control (CNC),electricservodrives,
electronicmeasuringsystems,precisionmechanical parts,suchasball screws,
anti frictionguide waysand eachothers?
b.Electronicwatchesincorporatingfine mechanical partsandsophisticated
electroniccircuits.
c. Electronicconsumerproducts – washingmachines,electroniccookingappliances,
fax,plainpapercopiersandothers.
In the lasttwentyyears,the productiontechnologyhasseenthe introductionof highprecision
measuringinstrumentssuchaselectronicgaugesandmeasuringinstruments,inprocessgauge
and qualitycontrol instruments,lasermeasuringsystemsandotherstoensure highdimensional
accuracies,as well asincreasedproductivityonthe shopfloor.
In the domainof factory automation,mechatronicshashadfar reachingeffectsin
manufacturingandwill gainevenimportance infuture.Majorconstituents of factory
automationinclude CNCmachines,robots,automationsystemsandcomputerintegrationof all

functionsof manufacturing.Properapplication,utilizationandmaintenance of these high
technologyproductsandsystemsisanimportantaspectthat enhancesthe productivityand
qualityof productsmanufacturedbythe customers.Toensure correctselectionof equipment,
an accurate estimationof the techno-economicsof variousmanufacturingsystems,
developmentsinthe hightechnologymachinesandequipmentare studiedindetail.Also,
propermaintenance of variousmechatronicelements,diagnosticscanincrease the lifeof the
variousmechatronicelements,whichinturnwill enhancethe life of the productorsystem.Such
inputsinmechatronicscanbe bestgivenbythe manufacturersof hi-techmachinesand
manufacturingsystems.Infact,the machine tool manufacturersare now beingcalleduponto
offera total manufacturingforsolutioninproduction,bythe customers,ratherthansupplyof
justthe standalone machines.Thistrendisalreadyevidentinmanyof the advancedcountries.
Evidently,the designandmanufacturingof future productswill involve acombinationof
precisionmechanical andelectronicsystemsandmechatronicswill formthe core of all activities
inproducts andproductiontechnology.
1.3 TYPES OF MECHATRONICDEVICES.
In thismodule,we willdiscussaseveral typesof mechatronicdeviceswhichisinusedby
workingdesign.The typesof mechatronicdevicesisusedsuchasswitches,relay,solenoid,
powerdiode,powertransistor,thyristor,gate controllerswitch,rectifier,chopper,transducer
and others.These typesof deviceswillbe discussedinthismodule byunitinthismodule.
FIGURE I
FIGURE 2

2.0 SWITCH
Basic function, configurations, types, movement methods, usage
and specification according to NEMA (National Electrical
Manufacturers Association).
SWITCH
OBJECTIVES
General objective : To understand the concept and basic application of a switch.
Specific objectives : At the end of the unit you should be able to:
· Identify the basic function of switch .
· Identify the type of switches.
· Describe the application of switch in industrial.
· Draw the symbol of a switch.
2.1 INTRODUCTION OF SWITCH.
The switch is a mechanical, electrical or electronic device that opens or close a circuit.
Switching may also be called making or breaking the circuit.
2.2 BASIC FUNCTION OF SWITCH AND SWITCH DIAGRAM.
The switches function as a device that opens or close a circuit. The closing of a switch
is called making the circuit. The opening of switch is called breaking the circuit. The
output of system depends on the switching pattern of the converter switches and the
input voltage (or current). Similar to the linear system, the output quantities of a
converter can be expressed in terms of input quantities, by spectrum multiplication.
The arrangement of single –phase diagram of converter and simple hold circuit are
shown in figure 2.2(a) and 2.2(b) below.

2.3 TYPE OF STRUCTURE AND CONSTRUCTION OF SWITCH
The structure and construction of power electronics switch can divided into four
parts:
i. Design of power circuits.
ii. Protection of power devices.
iii. Determination of the control strategy.
iv. Design of logic and gating circuits
In the analysis, the power devices are assumed to be ideal switches unless stated
otherwise; and effects of circuit stray inductance, circuit resistances, and source
inductance are neglected. The practical power devices and circuit are also affected.
However, in the early stage of design, the simplified analysis of circuit is very useful
to understand the operation of the circuit and to establish the characteristics and
control strategy.
Before a prototype is built, the designer should investigate the effects of the circuit
parameters (and devices imperfections) and should modify the design if necessary.
Only after the prototype is built and tested, the designer can be confident about
validity of the design and can estimate more accurately some of the circuit
parameters (e.g., stray inductance).
2.4 TYPES OF SWITCH
There are many different type of switches. Some switch types are push buttons, limit
switch, slide switch, rocker switch, precision switch and toggle switch.
2.4.1 PUSH BUTTONS SWITCH.
A pushbutton is a switch activated by finger pressure. Two or more contacts open or
close when the button is depressed. Pushbuttons are usually spring loaded so as to
return to their normal position when pressure is removed. Figure 2.4(a), show that
the mechanical-interlocked pushbutton with NO (normally open) and NC (normally
close) contacts, rated to interrupt an ac current of 6A one million times.

2.4.2 LIMIT SWITCH
A limit switch is a low-power snap-action device that opens or closes a contact,
depending upon the position of mechanical part. Other limit switches are sensitive to
pressure, temperature, liquid level, direction of rotation and so on. Figure 2.4(b)
show that the limit switch with one NC contact, rated for ten million operations,
position accuracy is 0.5 mm.
2.4.3 SLIDE SWITCH.
A slide switch is a switch activated by sliding of two or more contacts open or close
when the button is depressed. Pushbutton are usually spring loaded so as to return to
their normal position when pressure is removed. Figure 2.4(c), show that the
mechanical-interlocked sliding plate/rod with NO (normally open) and NC
(Normally close) contacts.

2.4.4 ROCKER SWITCH.
A rocker switch is a switch activated by 3 finger which includes 2 finger push button
for opening and closing of 2 circuits. Two contacts open or close for 2 circuits when
the button are pressed and depressed. Pushbutton are usually spring loaded in the
bottom to return to their normal position when pressure is removed. Figure 2.4(d),
show that the mechanical-interlocked pushbutton with ON (normally open) and NC
(Normally close) contacts through 2 circuit involved.
2.4.5 PRECISION SWITCH.
A precision switch is a switch activated by roller loaded on the top of pushbutton to
open or close a circuit. Only one contact open or close when the button is depressed
by load moving on top of the roller. Pushbutton are usually spring loaded so as to
return to their normal position when load on top of roller is removed. Figure 2.4(e),
show that the mechanical-interlocked pushbutton with ON (normally open) and NC
(Normally close) contacts, rated to interrupt an ac current of mechanical effect to the
top of roller device.

2.4.6 TOGGLE SWITCH.
A toggle switch is a switch activated by ball bearing moving for open or close a circuit.
The movement of roller makes a contact to open or close a circuit when the toggle is
depressed. A toggle usually parallel with the force loaded so as to open or close.
Figure 2.4(f), show that the mechanical-interlocked toggle pushbutton with NO
(normally open) and NC (Normally close) contacts.
2.5 PRINCIPLE OPERATION OF SWITCH
Switches should follow the international system of units (SI) which defines the
ampere (the fundamental unit of electric current) as the constant current which, if
maintained in two straight parallel contactors of infinite length and negligible
circular cross section place in a mechanism part that will produce a force between
two parallel contactors. By international agreement, the value of the international
ampere for switches was based on the electrolytic deposition of silver form a silver
nitrate solution. The resistance standard of switches is absolute measurement of the
ohm and is carried out by the international standards laboratories, which preserve a
group of primary resistant standard. For the voltage standard the saturated cell has a
temperature dependence, and the out put voltage change about -40 m V/0C from the
nominal 1.0185 V. The output of a converter depends on the switching pattern of the
converter switches and the input voltage (on current). Similar to linear system, the
output quantities of a converter can be expressed in terms of the output quantities,
by spectrum multiplication.

3.0 RELAY
Function, application, specification referring to NEMA, logic circuit
and function of relay ladder diagrams.
RELAY
OBJECTIVES
General Objective : To apply the concept of relay.
Specific Objectives : At the end of the unit you should be able to:
· Identify the main uses for a relay
· Identify the application of relay
· Show how the control relay is constructed mechanically
· Identify the specification of relay according to NEMA standard
· Draw the relay and switch logic circuit
· Identify the function of ladder-type diagram of relay
· Draw the schematic diagram and the ladder-type diagram of relay
3.1 INTRODUCTION OF RELAYS
The relay is an electromechanical device. The relay offers a simple ON/OFF
switching action and response to a control signal.
3.2 RELAY PRINCIPLE
The electrical relay offers a simple ON/OFF switching action in response to a control
signal. Figure 3.2 illustrates the principle. When a current flows through the coil of
wire a magnetic field is produced. This pulls a movable arm that forces the contact to
open or close. This might then be used to supply a current to a motor or perhaps an
electric heater in a temperature control system. Time-delay relays are control relays
that have a delayed switching action. The time delay is usually adjustable and can be
initiated when a current flows through the relay coil or when it ceases to flow
through the coil.

3.3 APPLICATION OF RELAYS IN INDUSTRIES
Relays are used in the control of fluid power valves and in many machine sequence
controls such as boring, drilling, milling and grinding operations.
3.4 SYMBOL OF RELAY
Below is the common electrical symbols of relay based on the function of relay.
3.5 SPECIFICATION OF RELAY
The standard voltage for relay used in machine control is 120 volt. The coils on
electromechanical devices such as relays, contactors and motor starters are designed
so as not to drop out (de-energize) until the voltage drops to minimum of 85% of the
rated voltage. The relay coils also will not pick up (energize) until the voltage rises to
85% of the rated voltage. This voltage level is set by the National Electrical
Manufacturer Association (NEMA).

3.6 RELAY AND SWITCH LOGIC CIRCUIT
Relays are widely applied in electromagnetic devices. Figure 3.6(a) and 3.6(b) shows
a typical relay appearance. When the relay is not energized, the spring keeps the
armature away from the coil. This produces an air gap and the main contact presses
against the normally closed contact. When the relay is energized, the armature is
attracted and moves toward the coil. This eliminates the air gap and the main contact
touches the normally open contact and completes that circuit. The circuit with
normally closed contact is opened. The relay acts as a single-pole double throw
switch. Many different contact arrangements are possible.
Relays require a given current for pull-in. Once they pull in, less current is required
to hold them in the closed position. This is because the air gap is eliminated when the
armature pulls in. The air has quite a bit more reluctance than the iron circuit and
eliminating it means that less mmf is required to overcome the spring tension.
The switch logic circuit application in relay ( Figure 3.6( c ) shows a relay with two
NO contacts). One contact is used as an interlock around the START push button.
Thus, an interlock ircuit is a path provided for electrical energy to the load after the
initial path has been opened. The second relay contact is used to energize a light.
Remember that when a relay coil is energized, the NO contacts close. The circuit can
be de-energized by operating the STOP push-button switch.

Figure 3.6 (d) shows the addition of a selector switch, fuse, pilot light and a second
relay. When the selector switch is operated to the ON position, electrical energy is
available at the two vertical sides of the circuit. The green light is energized, showing
that the operation has been completed.
One additional relay contact is added in the circuit from relay 1 CR. This contact
closes when the relay1 CR is energized and it, in turn, energizes a second relay coil 2
CR. The operating circuit can be de-energized by operating the STOP push-button
switch.

4.0 SOLENOID
Functions, applications, symbol and control circuit.
SOLENOID
OBJECTIVES
General objective : To apply the concept of a solenoid.
· Identify the main function of a solenoid
· Identify the application of a solenoid
· Describe the application of solenoids with operating valves
· Draw the symbol of a solenoid with a standard symbol
· Draw a control circuit showing the energizing of a solenoid through the closing of
relay contact, using a
control relay, two pushbutton switches and a solenoid
4.1 INTRODUCTION OF SOLENOID
The general principle of the solenoid action is very important in machine control.
Solenoid is an electromechanical device. Electrical energy is used to magnetically
cause mechanical movement.
4.2 PRINCIPLE OPERATION OF SOLENOID
A solenoid is a coil with an iron core and moveable iron plunger. When the coil is
energized, the plunger is attracted by the coil. It “pulls in”, and this motion can be
used to activate another mechanism. The solenoid shown in figure 4.2 (a), is used in
many electrically activated devices such as valves, locks, punches and marking
machines.
Solenoid is made up of three basic parts
a. Frame
b. Plunger.
c. Coil.

The frame and plunger are made up of laminations of high-grade silicon steel. The
coil is wound of an insulated copper conductor. Solenoids for alternating current use
are now available as oilimmersed types. Heat dissipation and wear conditions are
improved with this design.
4.3 SYMBOL OF A SOLENOID.

4.4 TYPES OF SOLENOID.
There are many kinds of solenoids. Figure 4.4, shows one type of solenoid in
industrial use.
4.5 BASIC FUNCTION OF SOLENOID CONTROL CIRCUIT
Operating the START push-button switch close the circuit to the coil of relay ICR as
shown in figure 4.5(a). The coil is now energized. Relay contact 1CR-1 as shown
below then closes, interlocking around the START push-button switch. Contact 1CR-
2 closes, energizing solenoid A. The circuit can be de-energized by operating the
REVERSE push-button switch.
In figure 4.5(b), a time-delayed relay is added. Operating the START push button
switch closes the circuit to the coil of relay 1CR. The coil is now energized. Contact
1CR-1 closes, interlocking around the START push button switch. Contact 1CR-2
closes, energizing solenoid 1A. Contact 1CR-3 closes, energizing the coil of timing
relay 1TR. After a time delay, as set on the timing relay, the timing constant contact

closes, energizing solenoid 1B. The circuit can be de-energized by operating the
REVERSE push-button switch. Note that if for some reason the REVERSE push
button switch is operated before the time set on the timing relay expires, solenoid 1B
will not be energized , as the timing relay coil will be de energized . With the relay
coil de-energized , the timing contact remains in the normal open condition.
4.6 APPLICATION OF SOLENOIDS
Solenoids are used to control fluid flow in hydraulic or pneumatic system. Solenoids
are also applied in many electrically activated devices such as valves, locks, punches
and marking machines.
5.0 ELECTRONIC CONTROL DEVICES
Power diode, power transistor, gate-controlled switch, Gate
Controlled Switch (Gate-turn-off SCR) GCS (GTO) and
programmable unijunction transistor.
ELECTRONIC CONTROL DEVICES (PART 1)
General objective : To apply the concept of electronic control devices.
 Identify the power of diodes
 Define the symbol of diodes
 List the types of diodes
 Define the characteristics of diodes
 Define the application of diodes
 Draw the construction of diodes

 Identify the power transistor concept
5.1 INTRODUCTION OF POWER DIODE
A diode is a component that allows current to freely flow through it in one direction
but essentially stops any current from flowing in the reverse direction. Power diodes can be
assumed as ideal switches for most applications but practical diodes differ from the ideal
characteristics and have certain limitations. The power diodes are similar to pn-junction
signal diodes.
5.2 PRINCIPLE AND CONSTRUCTION OF DIODE
According to the introduction above, you can see that a PN junction has all the
characteristics of a diode. Thus, from previous discussion on the PN junction, its
characteristics apply to the diode. The legend for Fig. 5.2(a) could just as well be
“ capacitance of a reverse-biased diode”.
Nearly all diodes used in electronics today are silicon PN junctions. Silicon is preferred
because of its low current leakage. There are also a few diodes made from germanium PN
junctions. They are primarily used when the lower potential barrier voltage of the germanium
junction is advantageous.
Figure 5.2 (b) shows a cutaway view of a typical diode that is capable of carrying 2
A of current. One of the diode leads connects to the P-type material and the other to the N-
type material. The silicon PN junction is very small: about the diameter of the lead wire and a
few thousands of an inch thick. The body of the of this plastic-encased diode is about 1/8
inch in diameter and about 5/16 inch long. The band of the left end of the body identifies the
cathode end of the diode. The cathode end of a diode is N-type material and the anode end is
always P-type material. Thus, for a forward-bias diode, the anode must be positive to the
cathode.

5.3 SYMBOL OF DIODES.
The schematic symbol for diode is shown in figure 5. 3 (a). The cathode is
represented by the vertical straight line and the anode by the triangle. Figure 5.3 (b) and
5.3 (c) illustrate forward-biasing and reverse-biasing respectively.
5.4 TYPES OF POWER DIODES
A number of specific types of diodes are manufactured for specific applications in
electricity and electronics. Some of the more common types are rectifier diodes, zener diodes,

varactor diodes, switching diodes and signal diodes. All of these diodes except the zener
diode are represented by the symbol shown in Fig.5.3. The zener diode symbol, as shown in
Fig. 5.4, uses a different line configuration to represent the cathode.
Ideally, a diode should have no reverse recovery time. However, the manufacturing cost of
such a diode will increase. In many applications, the effects of reverse recovery time will not
be significant, and inexpensive diodes can be used. Depending on the recovery characteristics
and manufacturing techniques, the power diodes can be classified into three categories. The
characteristics and practical limitations of each type restrict their applications:
a. Standard or general-purpose diodes
b. Fast-recovery diodes
c. Schottky diodes
5.4.1 Standard or general-purpose diodes.
The general-purpose rectifier diodes have relatively high reverse recovery time,
typically 25 µs, and are used in low-speed applications, where recovery time is not
critical. These diodes cover current rating from less than 1 A to several thousands of
amperes, with voltage ratings from 50 V to around 5 kV. These diodes are generally
manufactured by diffusion. However, alloyed types of rectifiers that are used in welding
power supplies are most cost-effective and rugged, and their ratings can go up to 300 A
and 1000 V.
5.4.2 Fast-recovery diodes
The fast-recovery diodes have low recovery time, normally less than 5 µs. They are
used in dc-dc and dc-ac converter circuits, where the speed of recovery is often of critical
importance. These diodes cover current ratings from less than 1 A to hundreds of amperes,
with voltage ratings from 50 V to around 3 kV.
For voltage ratings above 400V, fast-recovery diodes are generally made by diffusion
and the recovery time is controlled by platinum or gold diffusion. For voltage ratings below
400V, epitaxial diodes provide faster switching speeds than that of diffused diodes. The

epitaxial diodes have a narrow base width, resulting in a fast recovery time of as low as 50 ns.
Fast-recovery diodes of various sizes are shown in Figure 5.4.2.
5.4.3 Schottky diodes
The charge storage problem of a pn-junction can be eliminated (or minimized) in
a Schottky diode. It is accomplished by setting up a “ barrier potential” with acontact
between a metal and a semiconductor. A layer of metal is deposited on a thin epitaxial layer
of n-type silicon. The potential barrier simulates the behavior of a pn-junction. The rectifying
action depends on the majority carriers only, and as a result there are no excess minority
carriers to recombine. The recovery effect is due solely to the self-capacitance of the
semiconductor junction.
The recovered charge of a Schottky diode is much less than that of an equivalentpn-
junction diode. Since it is due only to the junction capacitance, it is largely independent of the
reverse di/dt. A Schottky diode has a relatively low forward voltage drop.

5.5 CHARACTERISTICS OF DIODE
5.5.1 Peak Inverse Voltage
A diode can withstand only so much inverse voltage before it breaks down.
The peak inverse voltage (PIV) ranges from 50 V to 4000 V, depending of the
construction. If the rated PIV is exceeded, the diode begins to conduct in reverse and in
many cases, is immediately destroyed.
5.5.2 Maximum Average Current
There is also a limit to the average current a diode can carry. The maximum
current may range from a few hundred milliamperes to over 4000 A, depending of
construction and size of the diode. The nominal current rating depends upon the
temperature of the diode, which, in turn, depend upon the way it is mounted and how it is
cooled.
5.5.3 Maximum Temperature
The voltage across a diode times the current it carries is equal to the power lost,
which is entirely converted into heat. The resulting temperature rise of the diode must
never exceed the permissible limits, otherwise the diode will be destroyed.
5.6 APPLICATION OF DIODES.
Diodes have many applications, some of which are found again and again, in one
form or another, in electronic power circuit. In the sections that follow, we will analyze a
few circuit that involve only diodes. They will illustrate the methodology of power circuit
analysis while revealing some basic principles common to many industrial applications.
Examples of the applications are :

a. battery changer with series resistor
b. battery changer with series inductor
c. single-phase bridge rectifier
d. filter
e. three-phase, 3-pulse diode rectifier
f. three-phase, 6-pulse diode rectifier
g. effective line current: fundamental line current
h. distortion power factor
i. displacement power factor harmonic content
5.7 POWER TRANSISTORS
Power transistors have controlled turn-on and turn-off characteristics. The transistors,
which are used as switching elements, are operated in the saturation region, resulting in a low
on-state voltage drop. The switching speed of modern transistors is much higher than that of
thyristors and they are extensively employed in dc-dc and dc-ac converters, with inverse
parallel-connected diodes to provide bidirectional current flow. However, their voltage and
current ratings are lower than those of thyristors and transistors are normally used in low to
medium power applications. The power transistors can be classified broadly into four
categories:
1. Bipolar junction transistors (BJTs)
2. Metal-oxide-semiconductor field-effect transistors (MOSFET)
3. Static induction transistors (SITs)
4. Insulated-gate bipolar transistors (IGBTs)
BJTs or MOSFETs, SITs or IGBTs, can be assumed as ideal switches to explain the
power conversion techniques. A transistor switch is much simpler than a forced-commutated
thyristor switch. However, the choice between a BJT and a MOSFET in the converter circuits
is not obvious, but either of them can replace a thyristor, provided that their voltage and
current ratings meet the output requirements of the converter. Practical transistors differ from
ideal devices. The transistors have certain limitations and are restricted to some applications.
The characteristics and ratings of each type should be examined to determine its suitability to
a particular application.
A bipolar transistor is formed by adding a second p- or n-region to a pn-junction
diode. With two n-regions and one p-region, two junctions are formed and it is known as
an NPN-transistor, as shown in Fig. 5.7 (a). With two p-regions and one n-region, it is
called as a PNP-transistor, as shown in Fig 5.7 (b). The three terminals are named as
collector, emitter, and base. A bipolar transistor has two junctions, collector-base junction
(CBJ) and base-emitter junction (BEJ). NPN-transistors of various sizes are shown in Fig
5.7 (c).

5.7 STEADY-STATE CHARACTERISTICS
Although there are three possible configurations-- common-collector, common base, and
common-emitter, the common-emitter configuration, which is shown in Fig. 5.8(a) for
anNPN-transistor, is generally used in switching applications. The typical input
characteristics of base current, IB, against base-emitter voltageVBE, are shown in Fig.
5.8(b). Figure 5.8 (c)shown the typical output characteristics of collector current, Ic,
against collector-emitter voltage, VcE. For a PNP-transistor, the polarities of all currents
and voltages are reversed.

There are three operating regions of a transistor: cutoff, active, and saturation. In the
cutoff region, the transistor is off or the base current is not enough to turn it on and both
junctions are reverse biased. In the active region, the transistor acts as an amplifier, where
the collector current is amplified by a gain and the collector-emitter voltage decreases
with the base current. The CBJ is reverse biased, and the BEJ is forward biased. In the
saturation region, the base current is sufficiently high so that the collector-emitter voltage
is low, and the transistor acts as a switch. Both junctions (CBJ and BEJ) are forward
biased. The transfer characteristic, which is a plot of VCE against IB, is shown in Fig
5.8(d).
ELECTRONIC CONTROL DEVICES (PART 2)
General objective : To understand the concept of electronic control devices.

 Identify the Phase-Control Thyristors
 Identify the Gate-Turn-Off Thyristors
 Define the symbol of GTO
 Identify the Programmable Unijunction Transistor (PUT)
 Define the symbol of PUT
 Identify the Uni-junction Transistor (UJT)
6.1 PHASE-CONTROL THYRISTORS (SCRs)
This type of thyristors generally operates at the line frequency and is turned off by
natural commutation. The turn-off time,tq, is of the order of 50 to 100 µs. This is most suited
for low-speed switching applications and is also known as converter thyristor. Since a
thyristor is basically a silicon-made controlled device, it is also known as silicon-controlled
rectifier (SCR).
The on-state voltage , VT,varies typically from about 1.15 V for 600 V to 2.5 V for
4000 V devices; and for a 5500-A 1200-V thyristor it is typically 1.25 V. The modern
thyristors use an amplifying gate, where an auxiliary thyristor TA is gated on by a gate signal
and then the amplified output of TA is applied as a gate signal to the main thyristor TM. This
shown in Fig. 6.1. The amplifying gate permits high dynamic characteristics with typical
dv/dt of 1000 V/µs and di/dt of 500 A/µs and simplifies the circuit design by reducing or
minimizing di/dt limiting inductor and dv/dt protection circuits.
6.2 THYRISTOR TURN-OFF
A thyristor which is in the on-state can be turned off by reducing the forward current
to a level below the holding current IH. There are various techniques for turning off a thyristor.
In all the commutation techniques, the anode current is maintained below the holding current

for a sufficiently long time, so that all the excess carriers in the four layers are swept out or
recombined.
6.3 GATE-TURN-OFF THYRISTOR
A gate-turn-off thyristor (GTO) like an SCR can be turned on by applying a positive
gate signal. However, it can be turned off by a negative gate signal. A GTO is a latching
device and can be built with current and voltage ratings similar to those of an SCR. A GTO is
turned on by applying a short positive pulse and turned off by a short negative pulse to its
gate. The GTOs have advantages over SCRs: (1) elimination of commutating components in
forced commutation, resulting in reduction in cost, weight, and volume; (2) reduction in
acoustic and electromagnetic noise due to the elimination of commutation chokes; (3) faster
turn-off, permitting high switching frequencies; and (4) improved efficiency of converters.
In low-power applications, GTOs have the following advantages over bipolar
transistors: (1) a higher blocking voltage capability; (2) a high ratio of peak controllable
current to average current; (3) a high ratio of peak surge current to average current, typically
10:1; (4) a high on-state gain (anode current/gate current), typically 600; and (5) a pulsed
gate signal of short duration. Under surge conditions, a GTO goes into deeper saturation due
to regenerative action. On the other hand, a bipolar transistor tends to come out of saturation.
A GTO has low gain during turn-off, typically 6, and requires a relatively high negative
current pulse to turn off. It has higher on-state voltage than that of SCRs. The on-state
voltage of a typical 550-A 1200-V GTO is typically 3.4 V. A 160-A 200-V GTO of
type 160PFT is shown in Fig. 6.3.

6.3 PROGRAMMABBLE UNIJUNCTION TRANSISTOR (PUT)
The programmable unijunction transistor (PUT) is small thyristor shown in Fig.
6.4(a). A PUT can be used as a relaxation oscillator as shown in Fig. 6.4(b). The gate voltage
VG is maintained from the supply by the resistor divider R1 and R2, and determines the peak
voltage Vp. In the case of the UJT, Vp is fixed for device by the dc supply voltage. But Vp of
a PUT can be varied by varying the resistor divider R1 and R2. If the anode voltage VA is less
than the gate voltage VG, the device will remain in its off-state. If VA exceeds the gate
voltage by one diode forward voltage VD, the peak point is reached and the device turns on.
The peak current Ip and the valley point current Iv both depend on the equivalent impedance
on the gate RG = R1 R2 /( R1 + R2) and the dc supply voltage Vs. in general , Rk is limited to
a value below 100 Ω.
6.5 UNIJUNCTION TRANSISTOR (UJT)

The unijunction transistor (UJT) is commonly used for generating triggering
signals for SCRs. A basic UJT-triggering circuit is shown in Fig. 6.5(a). A UJT has three
terminals, called the emitter E, base-one B1, and base-two B2. Between B1 and B2 the
unijunction has the characteristics of an ordinary resistance. This resistance is the
interbase resistance RBBand has values in the range 4.7 to 9.1 kΩ. The static
characteristics of a UJT are shown in Fig. 6.5(b). When the dc supply voltage Vs is
applied, the capacitor C is charged through resistor R since the emitter circuit of the UJT
is in the open state. The time constant of the charging circuit is T1 = RC. When the
emitter voltage VE, which is the same as the capacitor voltage vc , reaches the peak
voltage Vp, the UJT turns on and capacitor C will discharge through RB1 at a
determined by the time constant T2 = RB1C. T2 is much smaller than T1. When the
emitter voltage VE decays to the valley point Vv, the emitter ceases to conduct, the UJT
turns off, and the charging cycle is repeated. The waveforms of the emitter and triggering
voltages are shown in Fig. 6.5 (c).

6.0 THYRISTOR
di/dt, dv/dt and reverse recovery time. Gating requirements;
thyristor gate characteristics, Vg/Ig, gate characteristics upper limit
value and maximum permitted gate voltage.
THYRISTOR
General objective : To understand the concept of thyristor.
 Identify the element of di/dt, dv/dt
 Identify the reverse recovery time for the ON and OFF thyristor method
 Identifythe characteristicsof thyristorgate
7.1 INTRODUCTION OF THYRISTOR
Although transistors can be used as switches, their current carrying capacity
is generally small. There are many applications in which it would be advantageous to
have a high-speed switch which could handle up to 1000 A. Such a device is known
as the thyristor. It also has the advantage of not having any moving parts nor arcing.
A thyristor is an electronic device similar to a transistor switch. It has four layers and
can only be switched on; it cannot be switched off. Circuits can be used to switch off a
thyristor but the most simple arrangement is to let the current fall to zero which
arises when used with an a.c. supply.
7.2 PRINCIPLE OF THYRISTOR
The basic parts of the thyristor are its four layers of alternate p-type and n-
type silicon semiconductors forming three p-n junctions, A,B and C, as shown in
Figure. 7.2(a). The terminals connected to the n1 and p2 layers are the cathode and
anode respectively. A contact welded to the p1 layer is termed the gate. The
CENELEC Standard graphical symbol for the thyristor is given in, Figure. 7.2 (b).
The direction of the arrowhead on the gate lead indicates that the gate contact is
welded to a p-region and shows the direction of the gate current required to operate
the device. If the gate contact is welded to an n-region, the arrowhead should point
outwards from the rectifier.
Whenthe anode is positive withrespecttothe cathode,junctionsA andC are forward-
biasedandtherefore have averylow resistance, whereasjunctionsBisreverse-biasedand
consequentlypresentsaveryhighresistance,of the orderof megohms,tothe passage
of current.On the otherhand,if the anode terminal ismade negativewithrespecttothe
cathode terminal,junctionBisforward-biasedwhileA andC act as two reverse-

biased junctionsinseries.
7.2 di/dt PROTECTION
A thyristor requires a minimum time to spread the current conduction
uniformly throughout the junctions. If the rate of rise of anode current is very fast
compared to the spreading velocity of a turn-on process, a localized “hot-spot”
heating will occur due to high current density and the device may fail, as a result of
excessive temperature.
The practical devicesmustbe protectedagainsthigh di/dt.Asanexample,letusconsider
the circuitin Figure.7.3. Understeady-state operation,Dm conductswhenthyristor T1 isoff.
If T1 isfiredwhen Dm isstill conducting, di/dtcanbe veryhighandlimitedonlybythe stray
inductance of the circuit.

7.4 DV /Dt PROTECTION
If switch S1 in Figure. 7.4 (a) is closed at t = 0, a step voltage will be applied
across thyristor T1 and dv/dt may be high enough to turn on the device. The dv/dt
can be limited by connecting capacitor Cs, as shown in Figure. 7.4(a). When
thyristor T1 is turned on, the discharge current of capacitor is limited by resistor Rs
as shown in Figure. 7.4(b).
With an RC circuit known as a snubber circuit, the voltage across the thyristor
will rise exponentially as shown in Figure. 7.4(c) and the circuit dv/dt can be found
approximately from

7.5 CHARACTERISTICS OF THYRISTOR
Let usnow considerthe effectof increasingthe voltage appliedacrossthe thyristor,with
the anode positive relativetothe cathode.Atfirst,the forwardleakage currentreaches
saturationvalue due tothe actionof junctionB.Ultimately,abreakover isreachedandthe
resistance of the thyristorinstantlyfallstoaverylow value,asshownin Figure.7.5.The forward

voltage dropisof the orderof 1 -2 V and remainsnearlyconstantovera wide variationof
current.A resistorisnecessaryinseries withthe thyristortolimitthe currenttoa safe value.
7.6 THYRISTOR PRINCIPLE
We shall nowconsiderthe effectuponthe breakovervoltage of applyingapositive
potential tothe gate as in Figure.7.6(a).WhenswitchSis closed, abiascurrent,IB, flowsviathe
gate contact andlayersp1 and n1 and the value of the breakovervoltage of the thyristor
dependsuponthe magnitude of the biascurrentinthe wayshowninFigure.7.6(b).Thus,with
IB = 0, the breakovervoltage isrepresentedbyOA andremainspracticallyconstantatthisvalue
until the biascurrentis increasedtoOB.For valuesof biascurrentbetweenOBandOD, the
breakovervoltage fallsrapidlytonearlyzero.Analternative methodof representingthiseffect
isshownin Figure.7.6(c).

Figure. 7.6(b) : Variation of breakover voltage with bias current
If the thyristor is connected in series with a non-reactive load, of resistance R,
across a supply voltage having a sinusoidal waveform and if it is triggered at an
instant corresponding to an angle Ø after the voltage has passed through zero from a

negative to positive value, as in Figure. 7.6 d(a) , the value of the applied voltage at
that instant is given by
υ = Vm sin Ø
Up to that instant, the voltage across the thyristor has been growing from
zero to υ. When triggering occurs, the voltage across the thyristor instantly falls to
about 1 – 2V and remains approximately constant while current flows, as shown
Figure. 7.6d(a). Also, at the instant of triggering, the current increases immediately
from zero to i , where
i = υ – p.d. across thyristor
R
= υ when the p.d. across thyristor « υ
R
If Ø islessthan ∏/2,the current increasestoa maximum Im andthendecreasestothe
holdingvalue,whenitfalls instantlytozero,asshowninFigure.7.6 d(b).The average value of
the current overone cycle isthe shadedareaenclosedbythe currentwave dividedby 2∏.

7.7 LIMITATION TO THYRISTOR OPERATION
Because of the nature of the constructionof the thyristor,there issome capacitance
betweenthe anode andthe gate.If a sharplyrisingvoltage isappliedtothe thyristor,thenthere
isan inrushof charge correspondingtothe relaion i = C (dv/dt).Thisinrushcurrentcanswitch
on the thyristor,andit can arise inpractice due to surgesin the supplysystem, forexample due
to switchingorto lighting.Thusthyristorsmaybe inadvertentlyswitchedon,andsuch
occurrencescan be avoidedbyprovidingC – R circuitsin orderto divertthe surgesfromthe
thyristors.

7.0 AC–DC CONVERTER (RECTIFIER) AND DC–DC
CONVERTER (CHOPPER)
Uncontrolled rectifier circuits; half-wave and full wave. Halfcontrolled
rectifier circuits. Controlled rectifier circuits; half-wave
and full wave. Function of choppers; commutator principles,
operation of chopper circuits, mark space ratio or time ratio
control, step-up and the step-down chopper.
RECTIFIER AND CHOPPER
General objective : To understand the concept of a rectifier.
 Identify the power of an uncontrolled rectifier, semi-controlled rectifier,
controlled rectifier and chopper.
 Identify the uncontrolled rectifier and chopper circuit.
8.1 INTRODUCTION OF RECTIFIER
The process of converting alternating current (or alternating voltage) into
pulsating direct current (or pulsating direct voltage) is known as rectification.
Rectification is accomplished with the help of diodes. Circuits which provide
rectification are called rectifier circuits. Rectifier circuits can provide either half-
wave rectification or full-wave rectification.
8.2 PRINCIPLE OF RECTIFIER
Assume a half-wave rectifier output is to be used to supply current to a load.
The output of the rectifier gives the expected half-cycle of sinusoidal output once
every cycle except that conduction of the rectifier diode is not allowed to begin at the
start of the cycle but after an angular measure of θ radians has occurred. The
resulting current waveform is shown in Fig. 8.2(a).
If the angle θ can be varied form 0 to  /2 radiants (or even from 0 to  radians)
then the mean value of current taken by the load can be varied as can the rms current to
be derived.

8.3 SEMI-CONTROLLED RECTIFIER
For control of electric power or semi control power conditioning, the
conversion of electric power from one form to another is necessary and the switching
characteristic of the power device permit these conversions. The static power
converter may be considered as a switching matrix. The power electronics semi-
control rectifier circuits can classified into two types:
i. Diode rectifiers
ii. AC - DC converters (controlled rectifiers)
8.4 HALF-WAVE RECTIFICATION
The result of half-wave rectification is illustrated in Fig 8.4 (a), and the circuit
which performs the rectification is drawn in Fig 8.4 (b). The ground symbol in 8.4 (c) is
the reference point for voltages referred to in the discussion which follows.

8.5 FULL-WAVE RECTIFICATION
Full wave rectification can be provided with two diodes and a center-tapped
transformer as shown in Fig. 8.5 (a) , or it can be accomplished with four
diodes and a nontapped transformer (see Fig. 8.5 (b) ).
Figure 8.5.1(a) shows the direction and path of current flow for the ½ cycle
when the polarity of the transformer is as marked. Notice that only D1 is conducting
and that only the top half of the transformer is providing power. This is because D2 is
reverse-biased.
During the second ½ cycle (see 8.5.1 (b) ), the polarities of the transformer
windings are reversed. Therefore, D1 is now reverse-biased and D2 allows the current to
flow in the indicated direction and path. Notice that current through R1 is in the same
direction for each ½ cycle.

Figure 8.5(b) shows a full-wave, bridge rectifier circuit. Notice that
this circuit provides twice as much dc voltage as does the previous full-wave circuit
when both circuits use the same transformer. The bridge rectifier circuit does not use
the center tap of the transformer and it requires four diodes.
During ½ cycle, two of the diodes in Fig. 8.5(c ) conduct and allow the full
secondary voltage to force current through load resistor R1. the remaining two diodes are
reverse-biased and thus prevent the diode bridge from short-circuiting the transformer
secondary.
8.6 INTRODUCTION OF CHOPPER.

A dc chopper is the equipment that can be used as a dc transformer to step up
or step down a fixed dc voltage. The chopper can also be used for switching- mode
voltage regulators and for transferring energy between two dc resources. However,
harmonics are generated at the input and load side of the chopper, and these
harmonics can be reduced by input and output filters.
8.7 PRINCIPLE OF CHOPPER.
A chopper can operate on either fixed frequency chopper or variable frequency.
A variable-frequency chopper generates harmonics of variable frequencies and a
filter design. A fixed – frequency chopper is normally used. A chopper circuit uses a
fast turn off as a switch and requires commutation circuitry to turn it off. The
circuits are the outcome of meeting certain criteria: (1) reduction of minimum on-
time limit, (2) high frequency of operation, and (3) reliable operation.
8.8 TYPE AND BASIC OPERATION OF CHOPPER FUNCTION
CIRCUIT
The development of alternative switching (e.g., power transistors, GTO
s), the applications for type and circuit of choppers are limited to high power levels and
especially, to traction motor control. Some of chopper type and circuit used by traction
equipment manufactures are discussed in this section.
8.8.1 IMPLUSE-COMMUTATED CHOPPERS
The impulse-commutated chopper is a very common circuit with two thyristors as
shown in figure 8.8(a) and is also known as a classical chopper. At the beginning of
operation, thyristor T2 is fired and this causes the commutation capacitor C to charge
through the voltage Vc , which should be supply voltage Vs in the fist cycle. The
plate Abecomes positive with respect to plate B. The circuit operation can be divided
into five modes, and the equivalent circuits under steady-state conditions are shown in
Fig. 8.8(b). We shall assume that the load current remains constant at a peak
value Im during the commutation process. We shall also redefine the time origin, t = 0, at
the beginning of each mode. Mode 1 begins with T1 is fired. The load is connected to the
supply. The commutation capacitor C reverses also its charge through the resonant
reversing circuit formed by T1, D1, and Lm.

8.8.2. IMPULSE-COMMUTATED THREE-THYRISTOR CHOPPERS
The problem of undercharging can be remedied by replacing diode D1 with
thyristorT3, as shown in Fig. 8.8(c). In good chopper, the commutation time, tc, should
ideally be independent of the load current. tc could be made less dependent on the load
current by adding an antiparallel diode Df across the main thyristor as shown in Fig. 8.8(c)
by dashed lines. A modified version of the circuit is shown in Fig. 8.8(d)., where the
charge reversal of the capacitor is done independently of main thyristor T1 by
firing T3 . There are four possible modes and their equivalent circuits are shown in Fig.
8.8(e).

8.8.2. RESONANT PULSE CHOPPERS
A resonant pulse chopper is shown in Fig. 8.8(f). As soon as the supply is
switched on, the capacitor is charged to a voltage Vc through Lm, D1, and load. The
circuit operation can be divided into six modes and the equivalent circuits are shown in
Fig. 8.8(g).

8.9 SKETCHING CURRENT AND VOLTAGE WAVE
There are no fixed rules for designing or sketching of copper circuit and the
design varies with the types of circuit used. The designer has a wide range of choice
and values of LmC components are influenced by the designer’s choice of peak
resonant reversal current, and peak allowable voltage of the circuit. The voltage and
current ratings LmC components and devices is left to the designer based on the
considerations of price, availability, and safety margin. In general, the following
steps are involved in the design:
a. Identify the modes of operations for the copper circuit.
b. Determine the equivalent circuits for the various modes.
c. Determine the currents and voltages for modes and their waveforms.
d. Evaluate the values of commutation components LmC that would satisfy the devices.
A chopper with a highly inductive load is shown in Fig. 8.9(a). The load
current ripple is negligible (I=0). If the average load current is Ia, the peak load
current is Im=Ia+ I= Ia The input current, which is of pulsed shape as shown in Fig
8.9(b).
The wave forms for currents and voltages are shown in figure 8.9(b). In the
following analysis, we shall redefine the time origin t=0 at the beginning of each mode.

Fig 8.9(c): Equivalent circuit for modes.
Mode 1 begins when main thyristor T1 is fired and the supply is connected to
the load. This mode is valid for t = kT.
Mode 2 begins when commutation thyristor T2 is fired. The commutation
capacitor reverses its charge through C, Lm, and T2.
Mode 3 begins when T2 is self-commutated and the capacitor discharges due
to resonant oscillation through diode D1 and T1. Assuming that the capacitor current
rises linearly from 0 to Im and the current of thyristor T1 falls from Im to 0 in time tx.
Mode 4 begins when current through T1 falls to zero. The capacitor continues
to discharge through the load at a rate determined by the peak load current.
Mode 5 begins when the freewheeling diode Dm starts conducting and the
load current decays through Dm. the energy stored in commutation
inductance Lm and source inductance Ls is transferred to capacitor C.

Mode 6 begins when the overcharging is complete and diode D1 turns off. The
load current continues to decay until the main thyristor is refired in the next cycle. In
the stedy-state condition Vc = Vx.
8.10 DEFINITION OF MARK SPACE RATIO (TIME RATIO CONTROL)
The sequence of events within the frequency counter is controlled by the time
ratio base, which must provide the timing for the following events: resetting the
counter , opening the count gate, closing the count gate, and storing the counted
frequency in the latch. The resetting of the counter and storing the count are not
critical events as long as they occur before and after the gate period, respectively. The
opening and closing of the count gate, on the other hand, determine the accuracy of
the frequency counter and are very critical in its timing.
Since the accuracy of the frequency counter depends directly on the accuracy of
the time ratio base signal, the time base is driven from a accurate crystal controlled (e.g;
oscillator). This element of the time base is typically a temperature compensated crystal
oscillator operating at several megahertz. A crystal oven could be used to supply a similar
accuracy, except that the oven require the application of power to provide the correct
frequency and is available for use immediately after power-on. Fig. 8.10(a) shows a
simplified diagram of temperature-compensated crystal oscillator.
8.11 COMPARING STEP-UP AND STEP-DOWN CHOPPER DOWN.
A chopper can be considered as dc equivalent to an ac transformer with a
continuously variable turns ratio. Like a transformer, it can be used to step-down or
step-up a dc voltage source.

8.11.1 PRINCIPLE OF STEP-UP OPERATION
A chopper can be used to step-up a dc voltage and an arrangement for step-up
operation is shown in Fig. 8.11(a). When switch SW is closed for time t1, the inductor
current rises and energy is stored in the indicator, L. if switch is opened for time t2, the
energy stored in the inductor is transferred to load through diode D1 and the inductor
current falls. Assuming a continuous current flow, the waveform for the inductor current
is shown in Fig. 8.11(b). For values of k tending to unity, the output voltage becomes
very large and is very sensitive to changes in k, as shown in Fig. 8.11(c).

8.11.1 PRINCIPLE OF STEP-DOWN OPERATION
The principle of operation can be explained by Fig. 8.11(d). When switch SW is
closed for time t1 , the output voltage Vs appears across the load. If the switch remains off
for a time t2 , the voltage across the load is zero. The waveforms for the output voltage
and load current are also shown in Fig. 8.11(e). The chopper switch can be implemented
by using a (1) power BJT, (2) power MOSFET, (3) GTO, or (4) forced-commutated
thyristor. The practical devices have a finite voltage drop ranging from 0.5 to 5 V, and for
the sake of simplicity we shall neglect the voltage drops of these power semiconductor
devices.

7.0 AC–DC CONVERTER (RECTIFIER) AND DC–DC
CONVERTER (CHOPPER)
Uncontrolled rectifier circuits; half-wave and full wave. Halfcontrolled
rectifier circuits. Controlled rectifier circuits; half-wave
and full wave. Function of choppers; commutator principles,
operation of chopper circuits, mark space ratio or time ratio
control, step-up and the step-down chopper.
RECTIFIER AND CHOPPER
General objective : To understand the concept of a rectifier.
 Identify the power of an uncontrolled rectifier, semi-controlled rectifier,
controlled rectifier and chopper.
 Identify the uncontrolled rectifier and chopper circuit.

8.1 INTRODUCTION OF RECTIFIER
The process of converting alternating current (or alternating voltage) into
pulsating direct current (or pulsating direct voltage) is known as rectification.
Rectification is accomplished with the help of diodes. Circuits which provide
rectification are called rectifier circuits. Rectifier circuits can provide either half-
wave rectification or full-wave rectification.
8.2 PRINCIPLE OF RECTIFIER
Assume a half-wave rectifier output is to be used to supply current to a load.
The output of the rectifier gives the expected half-cycle of sinusoidal output once
every cycle except that conduction of the rectifier diode is not allowed to begin at the
start of the cycle but after an angular measure of θ radians has occurred. The
resulting current waveform is shown in Fig. 8.2(a).
If the angle θ can be varied form 0 to  /2 radiants (or even from 0 to  radians)
then the mean value of current taken by the load can be varied as can the rms current to
be derived.
8.3 SEMI-CONTROLLED RECTIFIER
For control of electric power or semi control power conditioning, the
conversion of electric power from one form to another is necessary and the switching
characteristic of the power device permit these conversions. The static power
converter may be considered as a switching matrix. The power electronics semi-
control rectifier circuits can classified into two types:
i. Diode rectifiers
ii. AC - DC converters (controlled rectifiers)
8.4 HALF-WAVE RECTIFICATION

The result of half-wave rectification is illustrated in Fig 8.4 (a), and the circuit
which performs the rectification is drawn in Fig 8.4 (b). The ground symbol in 8.4 (c) is
the reference point for voltages referred to in the discussion which follows.
8.5 FULL-WAVE RECTIFICATION
Full wave rectification can be provided with two diodes and a center-tapped
transformer as shown in Fig. 8.5 (a) , or it can be accomplished with four
diodes and a nontapped transformer (see Fig. 8.5 (b) ).
Figure 8.5.1(a) shows the direction and path of current flow for the ½ cycle
when the polarity of the transformer is as marked. Notice that only D1 is conducting
and that only the top half of the transformer is providing power. This is because D2 is
reverse-biased.
During the second ½ cycle (see 8.5.1 (b) ), the polarities of the transformer
windings are reversed. Therefore, D1 is now reverse-biased and D2 allows the current to
flow in the indicated direction and path. Notice that current through R1 is in the same
direction for each ½ cycle.

Figure 8.5(b) shows a full-wave, bridge rectifier circuit. Notice that
this circuit provides twice as much dc voltage as does the previous full-wave circuit
when both circuits use the same transformer. The bridge rectifier circuit does not use
the center tap of the transformer and it requires four diodes.
During ½ cycle, two of the diodes in Fig. 8.5(c ) conduct and allow the full
secondary voltage to force current through load resistor R1. the remaining two diodes are
reverse-biased and thus prevent the diode bridge from short-circuiting the transformer
secondary.

8.6 INTRODUCTION OF CHOPPER.
A dc chopper is the equipment that can be used as a dc transformer to step up
or step down a fixed dc voltage. The chopper can also be used for switching- mode
voltage regulators and for transferring energy between two dc resources. However,
harmonics are generated at the input and load side of the chopper, and these
harmonics can be reduced by input and output filters.
8.7 PRINCIPLE OF CHOPPER.
A chopper can operate on either fixed frequency chopper or variable frequency.
A variable-frequency chopper generates harmonics of variable frequencies and a
filter design. A fixed – frequency chopper is normally used. A chopper circuit uses a
fast turn off as a switch and requires commutation circuitry to turn it off. The
circuits are the outcome of meeting certain criteria: (1) reduction of minimum on-
time limit, (2) high frequency of operation, and (3) reliable operation.
8.8 TYPE AND BASIC OPERATION OF CHOPPER FUNCTION
CIRCUIT
The development of alternative switching (e.g., power transistors, GTO
s), the applications for type and circuit of choppers are limited to high power levels and
especially, to traction motor control. Some of chopper type and circuit used by traction
equipment manufactures are discussed in this section.

8.8.1 IMPLUSE-COMMUTATED CHOPPERS
The impulse-commutated chopper is a very common circuit with two thyristors as
shown in figure 8.8(a) and is also known as a classical chopper. At the beginning of
operation, thyristor T2 is fired and this causes the commutation capacitor C to charge
through the voltage Vc , which should be supply voltage Vs in the fist cycle. The
plate Abecomes positive with respect to plate B. The circuit operation can be divided
into five modes, and the equivalent circuits under steady-state conditions are shown in
Fig. 8.8(b). We shall assume that the load current remains constant at a peak
value Im during the commutation process. We shall also redefine the time origin, t = 0, at
the beginning of each mode. Mode 1 begins with T1 is fired. The load is connected to the
supply. The commutation capacitor C reverses also its charge through the resonant
reversing circuit formed by T1, D1, and Lm.

8.8.2. IMPULSE-COMMUTATED THREE-THYRISTOR CHOPPERS
The problem of undercharging can be remedied by replacing diode D1 with
thyristorT3, as shown in Fig. 8.8(c). In good chopper, the commutation time, tc, should
ideally be independent of the load current. tc could be made less dependent on the load
current by adding an antiparallel diode Df across the main thyristor as shown in Fig. 8.8(c)
by dashed lines. A modified version of the circuit is shown in Fig. 8.8(d)., where the
charge reversal of the capacitor is done independently of main thyristor T1 by

firing T3 . There are four possible modes and their equivalent circuits are shown in Fig.
8.8(e).

8.0 SENSOR
Sensors; position and shift, Velocity and acceleration, pressure
and level, temperature, visual sensor (CCD, CID), contact sensor
(limit switch), non-contact sensor (proximity sensor), photoelectric
sensor, microwave sensor and laser sensor.
SENSOR / TRANDUCER
General objective : To understand the function of transducer and sensor .
 List the advantages and disadvantages of transducer and sensor.
 Describe the application of transducer and sensor.
 Identify the types of transducer and sensor.
 Describe the specification of sensor and transducer.
9.1 INTRODUCTION OF TRANSDUCER.
Transducer is a device that changes a quantity to another quantity. It has a few
elements which are able to change a signal quantity to another signal quantity, for
example it changes the pressure to the displacement, the displacement to the
electrical movement force and others. In other words, transducer is a device that
relates the electrical to the non-electrical. Translates physical parameters to electrical
signals acceptable by the acquisition system. Some typical parameters include
temperature, pressure, acceleration, weight displacement and velocity. Electrical
quantities, such as voltage, resistance or frequency also may be measured directly.
Sensor is a part of transducer.
9.2 CLASSIFICATION OF TRANSDUCER.
An electronic instrumentation system consists of a number of components
which together are used to perform a measurement and record the result. An
instrumentation system generally consists of three major elements, an input device,
a signal-conditioning or processing devices, and an output device. The input device
receives the quantity under measurement and delivers a proportional electrical
signal to the signal-conditioning device. Here the signal is amplified, filtered or
otherwise modified to a format acceptable to the output device. The output device
may be simple indicating meter, an oscilloscope, or a chart recorder for visual display.
It may be a magnetic tape recorder for temporary or permanent storage of the input
data or it may be a digital computer for data manipulation or process control. The
kind of system depends on what is to be measured and how the measurement result
is to be presented.
The input quantity for most instrumentation systems is non-electrical. In order to
use electrical methods and techniques for measurement, manipulation or control, the non-
electrical quantity is converted into an electrical signal by a device called

a transducer. One definition states “a transducer is a device which, when actuated in one
transmission system, supplies energy in the same form or in another form to a second
transmission system”. This energy transmission may be electrical, mechanical,
chemical, optical or thermal..
This broad definition of transducer includes, for example, devices that convert
mechanical force or displacement into an electrical signal. These devices form a very
large and important group of transducers commonly found in the industrial
instrumentation area, and the instrumentation engineer is primarily concerned with
this type of energy conversion. Many other physical parameters (such as heat, light
intensity, humidity) may also be converted into electrical energy by means of
transducers. These transducers provide an output signal when stimulated by a non-
mechanical input, a thermistor reacts to temperature variations, a photocell to
changes in light intensity, an electron beam to magnetic effects, and so on. In all
cases, however, the electrical output is measured by standard methods, yielding the
magnitude of the input quantity in terms of an analog electrical measure.
Transducers may be classified according to their application, method of energy
conversion, nature of the output signal and so on. All these classifications usually result
in overlapping areas. A sharp distinction between and classification of types of
transducers is difficult.
9.3 SPECIFICATION OF TRANSDUCER.
In a measurement system the transducer is the input element with the critical
function of transforming some physical quantity to a proportional electrical signal.
Selection of the appropriate transducer is therefore the first and perhaps most
important step in obtaining accurate results. A number of elementary questions
should be asked before a transducer can be selected, for example: what is the
physical quantity to be measured?, which transducer principle can best be used to
measure this quantity?, and what accuracy is required for this measurement?
The first question can be answered by determining the type and range of the
measurand. An appropriate answer to the second question requires that the input
and output characteristic of the transducer be compatible with the recording or
measurement system. In most cases, these two questions can be answered readily,
implying that proper transducer is selected simply by the addition of an accuracy
tolerance. In practice, this is rarely possible due to the complexity of the various
transducer parameters that affect the accuracy. The accuracy requirements of the
total system determine the degree to which individual factors contributing to
accuracy must be considered. Some of these factors are,
a. Fundamental transducer parameters – type and range of measurand, sensitivity, excitation.
b. Physical conditions – mechanical and electrical connection, mounting provisions, corrosion
resistance
c. Ambient conditions – nonlinearity effects, hysteresis effects, frequency response, resolution.

d. Environment conditions – temperature effects, acceleration, shock and vibration.
e. Compatibility of the associated equipment – zero balance provisions, sensitivity
tolerance,
impedance matching, insulating resistance.
Categories (a) and (b) are basic electrical and mechanical characteristics of
the transducer. Transducer accuracy, as an independent component, is contained in
categories (c) and (d). Category (e) considers the transducer’s compatibility with its
associated system equipment.
The total measurement error in a transducer-activated system may be
reduced to fall within the required accuracy range by the following techniques,
a. Using in-place system calibration with corrections performed in the data reduction.
b. Simultaneously monitoring the environment and correction the data accordingly.
c. Artificially controlling the environment to minimize possible errors.
Some individual errors are predictable and can be calibrated out of the system.
When the entire system is calibrated, these calibration data may then be used to
correct the recorded data. Environmental errors can be corrected by data reduction if
the environmental effects are recorded simultaneously with the actual data. Then the
data are corrected by using the known environmental characteristics of the
transducers. These two techniques can provide a significant increase in system
accuracy.
Another method to improve overall system accuracy is to control artificially the
environment of the transducer. If the environment of the transducer can be kept
unchanged, these errors are reduced to zero. This type of control may require either
physically moving the transducer to a more favorable position or providing the required
isolation from the environment by a heater enclosure, vibration isolation or similar means.
9.4 PRINCIPLES OF SENSOR/TRANSDUCERS OPERATION.
9.4.1 DISPLACEMENT SENSOR.
The concept of converting an applied force into a displacement is basic to
many types of transducers. The mechanical elements that are used to convert the
applied force into a displacement are called force-summing devices. The force-
summing members generally used are the following;
a. Diaphragm,
b. Bellows
c. Bourdon tube, circular or twisted.
d. Straight tube
e. Mass cantilever, single or double suspension.
f. Pivot torque.

Example of these force-summing devices as shown in figure 9.4.1(a).
The displacement created by the action of the force-summing device is converted
into a change of some electrical parameter. The electrical principles most commonly used
in the measurement of displacement are,
a. Capacitive.
b. Inductive
c. Differential transformer
d. Ionization
e. Oscillation.
9.4.1.1 CAPACITIVE SENSOR.
Since the capacitance is inversely proportional to the spacing of the parallel plates,
any variation causes a corresponding variation in capacitance. This principle is applied in
the capacitive transducer of figure 9.4.1.1. A force applied to a diaphragm that functions

as one plate of a simple capacitor, changes the distance between the diaphragm and static
plate. The resulting change in capacitance could be measured with an ac bridge, but it is
usually measured with an oscillator circuit. The transducer, as part of the oscillatory
circuit, causes a change in the frequency of the oscillator. This change in frequency is a
measure of the magnitude of the applied force.
The capacitive transducer has excellent frequency response and can measure
both static and dynamic phenomena. Its disadvantages are sensitivity to temperature
variations and possibility of erratic or distorted signals due to long lead length. Also,
the receiving instrumentation may be large and complex and it often includes a
second fixed-frequency oscillator for heterodyning purposes. The difference
frequency, thus produced can be read by an appropriate output device such as an
electronic counter.
9.4.1.1 INDUCTIVE SENSOR.
In the inductive transducer the measurement of force is accomplished by the
change in the inductance ratio of a pair of coils or by the change of inductance in a
single coil. In each case, the ferromagnetic armature is displaced by the force being
measured, varying the reluctance of the magnetic circuit. Figure 9.4.1.2 shows how
the air gap is varied by a change in position of the armature. The resulting change in
inductance is a measure of the magnitude of the applied force.

The coil can be used as a component of an LC oscillator whose frequency then
varies with applied force. This type of transducer is used extensively in telemetry
system, with a single coil that modulates the frequency of a local oscillator.
Hysteresis errors of the transducer are almost entirely limited to the
mechanical components. When a diaphragm is used as the force-summing member,
as shown figure 9.4.1.2(a), it may form part of the magnetic circuit. In this
arrangement the overall performance of the transducer is somewhat degraded
because the desired mechanical characteristics of the diaphragm must be
compromised to improve the magnetic performance.
The inductive transducer responds to static and dynamic measurements, and it
has continuous resolution and a fairly high output. Its disadvantages are that the
frequency response (variation of the applied force) is limited by the construction of the
force-summing member.
9.4.1 VELOCITY SENSOR.
The velocity transducer essentially consists of a moving coil suspended in the
magnetic field of a permanent magnet, as shown in figure 9.4.2. A voltage is
generated by the motion of the coil in the field. The output is proportional to the
velocity of the coil, and this type of pickup is therefore generally used for the
measurement of velocities developed in a linear, sinusoidal, or random manner.
Damping is obtained electrically, thus assuring high stability under varying
temperature condition.

9.4.1 PRESSURE AND LEVEL SENSOR.
9.4.3.1 BOURDON-TUBE PRESSURE GAUGE.
A Bourdon tube is long thin-walled cylinder of non-circular cross section
sealed at one end, made from materials such as phosphor bronze, steel and
beryllium-copper. A pressure applied to the inside of the tube causes a deflection of
the free end, proportional to the applied pressure. The most common shape
employed is the C-type, as shows in figure 9.4.3.1. Increased sensitivity can be
achieved by using spiral and helical-shaped tubes. The displacement is converted
into a pointer rotation over a scale by means of a gear-and-lever system. Remote
indication of pressure can easily be achieved by using any of the displacement
transducers.

Static and low-frequency pressure up to 500MN/m2. The frequency range is
limited by the inertia of the Bourdon tube if electrical displacement transducers are
used.
9.4.3.1 DIAPRAGHM PRESSURE TRANSDUCER.
Flat diaphragms are very widely employed as primary sensing elements in
pressure transducers using either the center deflection of the diaphragm or the
strain induced in the diaphragm. They can be conveniently fabricated as flush-
mounted sensing elements providing a clean smooth face, ideal for use in dirty
environments and for surface pressure sensing.
For high-pressure transducers, very stiff diaphragm must be used to limit the
center deflection to less than one third of the diaphragm thickness, otherwise non-linear
result. For lower pressure ranges, up to a few bars, beryllium-copper corrugated
diaphragm and bellow are also used to give the higher sensitivity required.

9.4.3.1 PIEZO-ELECTRIC PRESSURE TRANSDUCER.
Pressure transducers using the piezo-electric effect use a similar design to the
quartz load cells. The quartz discs being compressed by a diaphragm which is in
direct contact with the pressure being measured. The high sensitivity of the quartz-
crystal modules permits the transducers to be manufactured in extremely small size.
One standing feature of quartz transducer is their high sensitivity.
Piezo-electric pressure transducers can be operated at temperatures up to
240OC, although care must be taken to compensate for zero-drift with temperature.
Special water-cooled transducers are available as shown in figure 9.4.3.3. and these
particularly useful for high-temperature application.

9.4.1 TEMPERATURE SENSOR.
9.4.1.1 RESISTANCE THERMOMETERS.
Resistance-temperature detectors or resistance thermocouple employ a
sensitive element of extremely pure platinum, copper or nickel wire that provides
definite resistance value at each temperature within its range. The relationship
between temperature and resistance of conductors in the temperature range near
0O C can be calculated from the equation,
Rf = Rref (1+  t)
Where ,
Rf = resistance of the conductor at temperature t (oC)
Rref = resistance at the reference temperature, usually 0 oC.
 = temperature coefficient of resistance.
t = difference between operating and reference temperature.

Almost all metallic conductors have a positive temperature coefficient of
resistance so that their resistance increases with an increase in temperature. Some
materials, such as carbon and germanium, have a negative temperature coefficient of
resistance that signifies that the resistance decrease with an increase in temperature.
A high value of  is desirable in a temperature-sensing element so that a substantial
change in resistance occurs for a relatively small change in temperature. This change
in resistance (R) can be measured with a Wheatstone bridge, which may be
calibrated to indicate the temperature that caused the resistance change rather than
the resistance change itself.
The sensing element of a resistance thermometer is selected according to the
intended application. Platinum wire is used for most laboratory work and for industrial
measurements of high accuracy. Nickel wire and copper wire are less expensive and
easier to manufacture than platinum wire elements, and they are often used in low-range
industrial applications.
Resistance thermometers are generally of the probe type for immersion in the
medium whose temperature is to be measured or controlled. A typical sensing element for
a probe-type thermometer is constructed by coating a small platinum or silver tube with
ceramic material, winding the resistance wire over the coated tube, and coating the
finished winding again with ceramic. This small assembly is then fired at high
temperature to assure annealing of the winding and then it is placed at the tip of the probe.
The probe is protected by a sheath to produce the complete sensing element.
The Wheatstone bridge has certain disadvantages when it is used to measure the
resistance variations of the resistance thermometer. These are the effect of contact
resistances of connections to the bridge terminals, heating of the elements by the
unbalance current, and heating of the wires connecting the thermometer to the bridge.
Slight modifications of the Wheatstone bridge, such as the double slide-wire bridge,
eliminate most of these problems. Despite these measurement difficulties, the resistance
thermometer method is so accurate that it is one of the standard method of temperature
measurement within the range of -183oC to 630oC. Table 9.4.4.1 summarizes the
characteristics of the three most commonly used resistance materials.

9.4.1.1 THERMOCOUPLES.
A thermocouple consists of pair of dissimilar metal wires joined together at one
end ( sensing, or hot, junction) and terminated at the other end (reference, or cold,
junction) which is maintained at a known constant temperature (reference temperature).
When a temperature difference exists between the sensing junction and the reference
junction, an emf is produced that causes a current in the circuit. When the reference
junction is terminated by a meter or recording instrument, as in figure 9.4.4.2 (a), the
meter indication will be proportional to the temperature difference between the hot
junction and the reference junction. This thermoelectric effect, caused by contact
potentials at the junctions, is known as the Seeback Effect.

To ensure long life in its operating environment, a thermocouple is protected
in an open or closed end metal protecting tube or well. To prevent contamination of
couple when precious metals are used (platinum and its alloys), the protecting tube
is both chemically inert and vacuum tight. Since the thermocouple is usually in a
location remote from the measuring instrument, connections are made by means of
special extension wires called compensation wires. Maximum accuracy of
measurement is assured when the compensating wires are of the same material as
the thermocouple wires.
The simplest temperature measurement using a thermocouple is by
connecting a sensitive millivoltmeter directly across the cold junction. The deflection
of the meter is then almost directly proportional to the difference in temperature
between the hot junction and the reference junction. The simple instrument has
several serious limitations, mainly because the thermocouple can only supply very
limited power to drive the meter movement.
The most common method in thermocouple temperature measurements involves using a
potentiometer.
9.4.1.1 THERMISTOR.
Thermistors or thermal resistors are semiconductor devices that behave as
resistors with a high , usually negative, temperature coefficient of resistance. In some
cases, the resistance of a thermistor at room temperature may decrease as much as 6
per cent for 1OC rise in temperature. This high sensitivity to temperature change
makes the thermistor extremely well suited to precision temperature measurement,
control and compensation. Thermistors are therefore widely used in such
applications, especially in the lower temperature range of -100OC to 300 OC.
Thermistor are composed of a sintered mixture of metallic oxides, such as
manganese, nickel, cobalt, copper, iron and uranium. Smallest in size are the beads with a
diameter of 0.15mm to 1.25mm. Beads may be sealed in the tips of solid glass rods to

form probes that are somewhat easier to mount than beads. Disks and washers are made
by pressing thermistor material under high pressure into flat cylindrical shapes with
diameter from 2.5 mm to 25mm.Washer can be stacked and placed in series or in parallel
for increased power dissipation.
Three important characteristic of thermistors make them extremely useful in
measurement and control applications, the resistance-temperature characteristic,
the voltage-current characteristic and the current-time characteristic.
9.4.1 PHOTOELECTRIC SENSOR.
The photoelectric transducer makes use of the properties of a photoemissive
cell or phototube. The phototube is a radiant energy device that controls its electron
emission when exposed to incident light. The construction of a phototube is shown in
figure 9.4.5 (a), its symbol is given in the schematic diagram of figure 9.4.5(b).

The large semicircular element is the photosensitive cathode and the thin wire
down the center of the tube is the anode. Both elements are placed in a high-vacuum
glass envelope. When a constant voltage is applied between the cathode and the anode,
the current in the circuit is directly proportional to the amount of light, or light intensity,
falling on the cathode. The curves of 9.4.5 (c) show the anode characteristics of a typical
high-vacuum phototube.
Notice that the voltage above approximately 20V the output current is nearly
independent of applied anode voltage but depends entirely on the amount of incident light.
The current through the tube is extremely small, usually in the range of a few
microamperes. In most cases therefore, the phototube is connected to an amplifier to
provide a useful output.
The photoelectric transducer of figure 9.4.5(d), uses a phototube and a light
source separately by a small window whose aperture is controlled by the force-summing
member of the pressure transducer. The displacement of the force-summing member
modulates the quantity of incident light on the photosensitive element. According to the
curve of figure 9.4.5 (c ), a change in light intensity varies the photoemissive properties at
a rate approximately linear with displacement. This transducer can use either a stable
source of light or an ac modulated light.

The advantages of this type of transducer are its high efficiency and its
adaptability to measuring both static and dynamic conditions. The devices may have poor
long-term stability, does not respond to high frequency light variations, and requires a
large displacement of the force-summing member.
9.4.1 LASER VELOCIMETERS.
One of the application of laser sources is for measuring velocity with great accuracy and
large dynamic range. The technique has been improved and is now commercially
available. The laser beam is split (see figure 9.4.6), to provide two coherent sources that
interfere optically at the point where velocity is to be measured. It is necessary to make
an optical comparison since interference is not possible electronically and we do not as
yet have detectors for such high frequencies.

A sensor viewing this point sees a small circular fringe pattern that varies in
amplitude as scattering changes. If the medium is moving across the field of view, the
sensor detects passing fringes and produces short bursts of signal. The period of the
cycles in a burst is a measure of the velocity. Extensive electronic processing is needed to
produce accurate flow measurements on such vague signals. The main advantages of
laser flow meters is that the velocity of a volume of fluid only 10-3 mm3 is viewed. The
method is most useful in turbulence and profile studies. It is essential that some, but not
many scattering particles exist to provide a signal for the detector. Often air bubbles or a
colloidal solid are injected to enhance the signal strength.

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