1

TABLE OF CONTENTS

DECLARATION………………………………………………………………………………………………………………………I

PREFACE……………………………………………………………………………………………………………………………… II

DEDICA-TION……………………………………………………………………………………………………………………………………………..

ACKNOWLEDGEMENT

CHAPTER 1

1.0 INTRODUCTION 1.1 PROMBLEM STATEMENT 1.2 PROJECT OBJECTIVES 1.3 PROJECT AIMS 1.4 BLOCK DIAGRAMS 1.5 SPECIFICATIONS 1.6 PROJECT ANALYSIS 1.7 COMPONENT LIST 1.8 COMPONENT DESCRIPTION 1.9 COMPARATOR 1.10 ZENER DIODE 1.11 DIODE 1.12 RELAY 1.13 TRANSFORMER 1.14 CAPACITOR 1.15 FUSE 1.16 TRANSISTOR

CHAPTER 2

2.0 PROJECT DESIGN 2.1 COMPARATOR 2.2

3

CHAPTER ONE

1.1 INTRODUCTION:

Power instability in domestic, commercial & domestic applications, causes massive losses in time & equipment failure due damages such as single phasing, brownouts & under voltages. This necessitates the need for Automation of Electrical power generation or having other alternatives power sources to back up utility power.

Most commercial & industrial processes are dependent on electrical power supply & if change-over systems are manual time is wasted & also can cause device/machine damage due to human error during the change over operations.

This project aims to eliminate the need for manual human mitigation measures against sin-gle phasing, manual switching from mains to generator, brownouts etc.

1.2 PROBLEM STATEMENT

The motivation behind this project was the frequent burn outs & other equipment damages caused by poor power quality while working as maintenance supervisor at Kenchic ltd.

The main challenges usually were single phasing faults due to lose connections from the main utility transformer, accidental loss of one or two phases as a result of external activi-ties e.g. trees falling on the mains, single phasing due to fuse burnouts in one or two phases etc.

I intend to eliminate the above mentioned problems by installing this circuit at the main in-coming point which feeds power to equipments rather than having phase failure relays on each individual machine/component which is not cost effective.

1.3 PROBLEM OBJECTIVE

1. To design & test a circuit that will offer stabilized power supply in case of low voltag-es in one or two supply phases

2. To design & test a circuit that will protect equipments from failure & other faults due to single phasing, brownouts & low voltages in one or two phases.

1.4 PROJECT AIMS

1. The circuit should be able to provide stabilized power supply on all phases even when voltage on one or two phases is low or unavailable.

2. The circuit should protect equipment from undesired effects & failures due to unsta-ble power supply

3. The circuit should act as a stable ac power supply unit.

4

1.4 BLOCK DIAGRAM

1. UTILITY MAINS POWER SUPPLY

This stage shall consist of the power supply being monitored. The 415 volts available from the mains shall be stepped down by means of a transformer to safe working voltages of 12 volts ac.

2. THE INTERLOCKING MECHANISM

The purpose of this stage is to offer protection against simultaneous operation of the two contactors supplying power to the load. This stage shall consist of a simple mechanical link between the two contactors supplying power to the load. Further electrical interlock shall be added in the control circuit for added protection.

3. THE VOLTAGE SENSING CIRCUIT

This circuit shall compare two voltage values and then give an error based on margin of fluc-tuation. It is built around a comparator & a zener diode. The comparator is connected in the inverting mode since output is not equal to input. The error occurring from the above opera-tion shall then be amplified then used to switch on the switching transistor as appropriate.

4. LOGIC CONTROL STAGE

When the error signal is produced it shall be analyzed at this stage & an output relayed to the switching transistors. This signal will be analyzed internally by the IC74 & will therefore

Utility mains power supply

Load

Interlocking mechanism

Voltage sens-ing circuit

Logic control stage

Switching transis-tors

Change over & Electrical isola-tion

Generator or other auxil-iary power supply

5

provide an output if the error falls out of the preconditioned voltage levels set by the zener diode.

5. SWITCHING TRANSISTORS

This stage comprises of transistors that will conduct only when the output from the compar-ator is high. This will cause the transistor to energize a relay & thus disconnect faulty power to the load & automatically connect it to another phase where power is okay.

6. CHANGEOVER & ELECTRICAL ISOLATION

This stage comprises of a relays. The first set of relays shall connect one phase to another incase of loss of one or two phases, or if voltage in one or two phases fall below some prede-termined voltage levels. The second relay shall be used to switch on the generator incase of total power failure. This changeover system shall reconnect power back to the mains supply once normalcy returns.

7. LOAD

This consists of a variety of equipments utilizing power e.g. three phase motors, lighting systems etc

1.6.0 COMPONENT LISTS

COMPONENT NAME SPECIFICATIONS QUANTITY Step-down transformer 220v-240v, 300m.A 3 Fuse F1, F2, F3. 5 A 3 Comparator IC1, IC2, IC3. IC741 3 Transistor T1, T2, T3. BC557 3 Relay RL1, RL2, RL3. 1C/O 3 Zener Diode ZD1, ZD3, ZD3. 5.1V 3 Variable Resistors VR1, VR2, VR3 10K 3 Resistor R1, R2, R4, R5, R7, R8. 3.3K 6 Resistor R3, R6, R9. 10K 3 Diode 9 Capacitors 1000μf 25v 5 Capacitors 470μf 35 v 7 Wires

1.6.1 COMPONENT DESCRIPTION

A.)VOLTAGE COMPARATOR

DEFINATION:

A voltage comparator is a device that compares two voltage inputs & determines which of the two is greater. Inputs to the comparator can be analog but the output is digital. The out-put is high when the voltage on the positive input is greater than the voltage on the negative input & low when the positive input voltage is less than the negative input voltage. This is the case regardless whether the comparator is set for non-inverting or inverting operation.

6

The comparator used for this project is LM741. It’s a general purpose Operational amplifier which can be used as a comparator, differentiator, integrator etc. the device data sheets are attached in appendix A as shown.

The schematic representations of the internal components of LM741 are as shown below.

SCHEMATIC REPRESENTATION

PIN CONNECTION DIAGRAM

+

C130pF

Q20PNP

Q19NPN

Q18NPN

Q17NPN

Q16NPN

Q15NPN

Q14NPN

Q13PNP

Q12PNP

Q11NPN

Q10NPN

Q9PNP

Q8PNP

Q7NPN

Q6NPN

Q5NPN

Q2NPN

Q1NPN

Q3PNP

Q4PNP

R1250

R1150

R1050

R925

R87.5k

R74.5k

R539k

R45k

R350k

R21k

R11k

7

THE NON- INVERTING OPERATION

The negative input terminal is assigned Vref. When Vin exceeds Vref the output Vout goes from LOW to HIGH.

GRAPHICAL REPRESENTATION

1 8

2 7

3 6

4 5

Off sett Null

8

THE INVERTING OPERATION

The reference voltage is assigned to the positive input. When input voltage, Vin exceeds reference Voltage Vref the output Vout goes from HIGH to LOW.

GRAPHICAL REPRESENTATION

9

B.) ZENER DIODE

The zener diode is a special type of diode that can operate in the reverse direction if voltage is larger than ibreakdown voltage.

Basically a zener diode is a reverse biased heavily doped silicon or germanium P-N junction diode which operates in the breakdown region where current is limited by both external re-sistance &

When a zener diode breaks down both zener & avalanche effects are present but one or the other predominates depending on voltage levels. At reverse voltage about 6 volts zener ef-

fects predominate while at reverse voltage above 6 volts avalanche effects predominate.

Zener breakdown occurs due to breaking of covalent bonds by the strong electric field set up in the depletion region by the reverse voltage. It produces an extremely large no of elec-trons & holes which make up the saturation current now the zener current Iz whose value is limited by current resistance. It is independent of applied voltage.1

V/I CHARACTERISTICS

V/I characteristics area as shown.

1

10

The forward characteristics are those for normal forward biased junction diode

Vz = zener breakdown voltage

Iz min = minimum current to sustain breakdown

Iz max= maximum zener current limited by maximum power dissipation

The various diode representations are as shown below

VF

IZ

MIN

IZ

IZ

MA

X

IF

VZ

V/I CHARACTERISTICS OF A ZENER DIODE

11

Zener Voltages

Zener voltages are available from about 2.4volts –to- 200volts. These voltages are tempera-ture dependent.

The power dissipation of the zener diode is given by VzIz and can vary from 150mW to 50W.

ZENER BIASING

For proper working of a zener diode in any circuit, it must be

1. Reverse biased 2. Have voltages across it greater than Vz. 3. Be in a circuit where current is less than Iz max..

ZENER DIODE APPLICATIONS

1. Voltage regulation 2. Fixed reference in biasing networks 3. Comparison & meter calibration 4. Peak clippers &voltage limiters 5. Meter protection against accidental application of excessive voltages 6. Wave form reshaping

For this particular project we shall use the zener diode for voltage regulation & comparison applications.

VOLTAGE REGULATION

Voltage regulation is the circuit’s ability to maintain constant output voltage when either in-put voltage or load current varies. A zener diode working in the breakdown region can be used as a voltage regulator.

DIODE CIRCUIT SYMBOL

DIODE EQUIVALENT CIRCUIT

DIODE APPROXIMATE CIRCUIT

12

In fig I above Vin is the input voltage whose variations are to be regulated. The zener diode is reverse connected across Vin when P.D across diode is greater than Vz it conducts & draws relatively larger current through series resistance R.

The load resistance RL across which a constant voltage Vout is required is connected in paral-lel with the diode. Total current I passing through R equals sum of diode current & load cur-rent.

=Iz + IL

Under all conditions

Vout= Vz

Hence Vin= IR + Vout=IR + Vz

CASE 1:

RL is kept fixed but supply voltage Vin is increased slightly. I will increase. This increase will be absorbed by zener diode without affecting IL.the in-crease in Vin will be dropped across R thereby keeping Vout constant

If Vin falls the diode takes a smaller current & voltage drop across R is reduced thus keeping Vout constant hence when Vin changes I & IR drop change in such a way to keep Vout= Vz constant

CASE 2:

RESISTOR

IZ

VZ VOUT

IZ IZ

V IN

13

Vin is fixed but changes occur in Iz.when IL increases diode current Iz de-creases thereby keeping I & hence IR drop constant.Vout remains unaf-fected.

If IL decreases diode current Iz increases to keep I hence IR drop con-stant. Vout remains unaffected

Vout= Vin –IR= Vin-(Iz+ IL)

NOTE R = Vin -Vout

(IZ+IL)

When diode current reaches maximum value IL becomes zero. In such a case

R = Vin -Vout

IZ max

ZENER DIODE AS A REFERENCE ELEMENT

In some electronic circuits it is desirerable to maintain a constant volt-age between two points and use it as a reference for comparing other voltages against it. The difference between the two is amplified & used to perform some form of control function.

This application is mainly used in power supply voltage regulators, measurement circuits, servo mechanisms etc. the constant voltage char-acteristics in its breakdown region makes zener diode desirerable for this application

Fig below shows a circuit in which zener diode has been used as a refer-ence element.

14

The reference voltage equals zener breakdown voltage. Value of R is also chosen so that diode operates well within its breakdown region. The dif-ference between Ein - Eref gives the control output.

C.)THE P-N JUNCTION DIODE

The pn junction diode is a two terminal deviceconsisting of a pn junction formedin either Ge or Si crystals. This junction is formed by increasing the concentration of electrons or holes population in certain regions of a monocrystallline Ge or Si chip. The various diode representations are shown below

Note; the arrow head indicates conventional current flowwhen the diode is forward biased. It is the same direction as the hole flow.

P N Anode Cathode

Cathode Anode

Comparator & OP-AMP circuit

Control o/p

EIN

E

R

EREF

IZ

15

WORKING OF A PN JUNCTION

This is a one way device which offers resistance when forward biased & behaves almost like an insulator when reverse biased. Mostly diodes are used for rectification of ac voltage into dc voltage.

V/I CHARACTERISTICS.

FORWARD CHARACTERISTICS

When the diode is forward biased and applied voltage is applied from zero hardly any cur-rent flows through the device initially. This is due to opposition of external voltage by inter-nal barrier voltage VB whose value is 0.75V for Si & 0.3V for Ge.

When VB is overcome, current through the diode increases rapidly with increase in battery voltage. Diode have limitations of forward voltage since a burn out will occur if a certain safe limit is exceeded.

REVERSE CHARACTERISTICS

Forward char-acteristics

Reverse Char-acteristics

Breakdown

i

+V

16

When reverse biased majority carriers are blocked only by a small minority current which flows through the diode. As reverse voltage is increased from zero, current reaches its max-imum or saturation value Io also known as leakage current. For Si it’s in order of nano amps(nA) & micro amps (μA) for Ge.

Value of Io is independent of applied reverse voltage but dependent on temperature , degree of doping & physical size of the junction.

When reverse voltage exceeds a certain value known as breakdown voltage VBR or zener voltage Vz, leakage current suddenly & sharply increases, The curve indicating zero re-sistance . any further increase in voltage would cause a burn out.

EQUATIONS OF STATIC CHARACTERISTICS

The V/I characteristics described above are called static characteristics because they de-scribe the dc behavior of a diode. The forward & the reverse characteristics have been com-bined into a single diagram shown above. These characteristics can be analyzed using the Boltzmann diode equation given below.

I=IO (eev/KT -1) amperes

Where Io = reverse diode saturation current

V = voltage across junction- positive for forward bias & negative for reverse bias.

K= Boltzmann constant = 1.38*1023 J/k

Crystal temperature in

ϻ= 1 for Ge & 2 for Si

Hence: I= Io (eev/kT-1)…………………Ge

I=Io (eev/2kT-1)………………….Si

DIODE PARAMETERS

BULK RESISTANCE (rb)

This is the sum of the resistance values of P & N type semi conductor ma-terials which the diode is made up of. Ie

Rb= rp+rn……………………………………1

17

Usually it is very small. It is given by

Rb=(Vf -Vb)/If……………………………………2

JUNCTION RESISTANCE (rj)

It’s the value the forward junction depends on magnitude of forward dc current.

rj = 25 mV/If mA………..Ge

= 50mV/If mA………….Si

DYNAMIC/ AC RESISTANCE

rac/rd = rb +rj

for large forward currents rj is negligible , hence rac=rB

for small If, rb is negligible as compared to rj , hence rac = rj.

FORWARD VOLTAGE DROP

This is a relationship given by,

Other parameters have already been described earlier i.e. reverse satu-ration current & reverse breakdown voltage.

DIODE APPLICATIONS

1. Rectification; conversion of ac current into dc current. 2. As a signal diode in communication circuitsfor modulating and

demodulating of small signals. 3. As zener diode in stabilizing circuits. 4. As varactor diode in voltage controlled tuning circuits eg radio & tv

receivers. Etc.

18

THE TRANSFORMER

This is a static electrical device that can change magnitude of an al-ternating voltage or current from one value to another. A transformer can used be used as a means of circuit separation & can also be used to change the apparent value of a capacitor, resistor, or an inductor.

A transformer usually consists of two windings, primary & secondary wound on a common laminated magnetic core as shown below.

The primary winding is connected to power supply while secondary

winding is connected to the load.

In a practical transformer there may be more than two windings as well as magnetic cores. In simple terms with no load flowing a trans-former can be represented by two windings on a common core as shown above.

Basically a transformer is a device that can;

1. Transfer electric power from one circuit to another. 2. Does so without change in frequency. 3. Accomplishes this by electromagnetic induction.

Mains transformers are the most common type of transformers. They are designed to reduce ac supply voltage 230-240 to a safe low voltage eg 48v, 24, 12v. etc.

E1 E2 V2 load

I1

V1

N1

N2

Secondary

Primary

19

A mains transformer can have two separate secondary coils which can be used separately to give independent supplies or connected in series to create a center tp coil or one coil with double voltages. Some mains transformers are center tapped half way through secondary coil so they can produce full wave rectified dc with just two diodes unlike a standard transformer which requires four diodes to produce full wave rectified voltage. Specifications of main transformer are usually,

Secondary output voltage Vs.

Maximum power Pmax which the transformer can pass in VA (volt-ampere). This also determines maximum output secondary cur-rent Imax. ie.

Where Vs = secondary voltage.(if there are two secondary coils maxi-mum power should be halved for each coil.)

Construction either p.c.b mounting, chassis mounting or torroidal mounting.

VOLTAGE TRANSFORMATION RATIO

A Transformer with equal number of turns will have secondary voltage slightly less than primary voltage & voltage ratio is 1:1. But if secondary voltage is half as many turns as primary , secondary voltage will be only one half primary voltage. Voltage ratio will be 2:1 Primary & secondary turns respectively.

CURRENT TRANSFORMATION RATIO

In a transformer of 1:1 ratio 1 load ampere flows for every load ampere in secondary because of cancellation ampere turns added in primary must be equal to demagnetizing ampere turns N2I2. If number of second-

20

ary turns is one tenth as great as primary turns, the current carrying ca-pacity of secondary coil should be 10 times as great as that of primary turns. The current transformation of a transformer is inversely propor-tional to turns ratio. i.e.

⌊

⌋

Note: in a step down transformer current is amplified in magnitude.

For an ideal transformer.

E1=V1 , E2=V2 there is no voltage drop in the wind-ings, therefore

There are no losses therefore volt-ampere input to primary equals volts ampere output to secondary.

However a practical transformer suffers from several losses such as

Iron losses.

Winding losses.

Magnetic leakage or magnetic reactance. TRANSFORMER LOSSES

1.) IRON LOSSES

The iron core is subjected to attenuating flux & therefore eddy currents & hysteresis losses occur in it. These two losses depend upon the supply

21

frequency , maximum flux density in the core, etc . in a transformer magnitude of iron losses is very small.

2.) WINDING RESISTANCE.

This is the resistance offered by the copper conductor windings in both primary & secondary windings. The primary resistance R1 & secondary resistance R2 act in series with the respective windings as shown below.

A power loss & IR voltage drop will occur when current flows thru wind-ings. This will affect the power factor & e1 will be less than V1while v2 will be less than E2.

3.) LEAKAGE REACTANCE

Primary & secondary currents produce flux. The flux Ф is the useful flux linking both windings & is also known as mutual flux. However flux pro-duced by primary & secondary winding doesn’t link the two windings (as shown in fig 2 below).this flux which links only one winding is known as leakage flux. Leakage flux paths are mainly through air.

VI E1

N1

N2

E2 V2

R1 X1 R2 X2

Ф

22

The effects of this leakage flux would be the same as though individual reactance were connected in series to each winding of the transformer that had no leakage flux as shown above. There is no power loss due to leakage reactance however presence of leakage reactance affects power factor as well as voltage loss due to IX drop.

THE CAPACITOR

A capacitor is a device consisting of two conducting surfaces separated by a layer of insulating medium called the dielectric. Its purpose is to store electrical energy by electrostatic stress in the dielectric. The die-lectric material must be a poor conductor of electricity.

Capacitance → this is the property to store electricity. This is defined as the amount of charge required to create a unit Pd between the plates. Suppose we give Q coulombs of charge to one unit of the two plate capac-

Conductor electrode

Dielectric

Conductor electrode

Connecting wire

Connecting Wire

I2 I1 Ф

Ф1 Ф2

V1

Leakage flux

23

itor & if a Pd of V volts is established between the two plates its capacitor is,

Hence capacitance is the charge required per unit potential difference by definition unit of capacitance is coulomb/volt also the farad i.e 1 farad = 1 coulomb volt.

1 farad is defined as the capacitanceof capacitor which requires a charge of one coulomb to establish a P.D of one volt between its plates.

BASIC OPERATION OF A CAPACITOR

When a capacitor is connected to a battery one electrode of the capacitor will be positively charged & the other will be negatively charged. Pres-ence of electrical charges on the electrode indicates charges on the die-lectric. These charges determine the permittivity of the dielectric.

K is a measurement tool also known as dielectric constant & is the ratio of permittivity in use to permittivity of free space i.e. a vacuum. The val-ue of permittivity is given as 8.85*10-12

-

-

+

+

24

THE FUSE

his is a current interrupting device. Basically it’s a low resistance resis-tor that acts as a sacrificial device to provide over current protection of either load or source circuit. Its essential component is a wire or strip that melts when too much current flows & causes the device to overheat & thus melts to interrupt the circuit in which it is connected. Short cir-cuiting, overloading, mismatched loads, or device failure are prime rea-sons for excessive current.

Wiring regulations define maximum current rating for particular cir-cuits. Over current protection devices are essential in electrical systems to limit threats to human life & damage to property.

CONSTRUCTION

A fuse consists of a metal strip or wire fuse element of small cross sec-tional area as compared to circuit conductors. It is usually mounted be-tween a pair of electrical terminals & usually enclosed by a non-combustible housing. It is usually arranged in series to carry all current passing through the protected circuit. The resistance of the element gen-erates heat due to current flow.

The size & construction of the element is empirically determined so that the heat produced for normal current does not cause the element to at-tain high temp & melt.

Fuse elements are usually made of zinc, copper, silver, aluminum or al-loys to provide stable & predictable characteristics. An ideal fuse would carry its rated current indefinitely & melt quickly on small excess cur-rent but should not be damaged by minor surge or in-rush currents. It should also not oxidize or change its behavior after years of service.

CHARACTERISTIC PARAMETERS

RATED CURRENT IN

25

This is the maximum current a fuse can carry without interrupting the circuit.

I2t VALUE

This is the amount of energy spent by a fuse element to clear an electri-cal fault. It is normally used for short circuit calculations. This value is normally provided by fuse manufacturers. For co-ordination of fuse op-erations with upstream & downstream devices, both melting I2t & clear-ing I2t are specified.

Melting I2t – Is the proportional energy required to begin melting the fuse element.

Clearing I2t – is the proportional total energy let through by the fuse when clearing a fault.

In other words I2t can be defined as a measure of thermal damage & magnetic forces that will be produced by a fault since it is mainly de-pendent on current & time for fuses as well as available fault level & sys-tem voltage. Therefore I2t is proportional to energy it lets through.

BRAKING CAPACITY

This is the maximum current that can be safely interrupted by a fuse. It should be higher than prospective short circuit current Pscc. Pscc is also defined as available fault current & is the highest current which can exist in a particular electrical system under a short circuit. It is usually de-termined by voltage & inductance of the supply system.

RATED VOLTAGE

Voltage rating for a fuse must be greater or equal to what would be the open circuit voltage.

TEMPERATURE DERATING

Ambient temperature will change a fuse operational characteristics e.g. a fuse rated at 1 ampere at 25 may conduct up to 10 – 20 % more cur-rent at -40 may open at a out 0 of its rated value at 100 c

RELAY.

26

This is basically an electrically operated switch. Many relays use the principle of electromagnetism although other principles may be used.

Mainly relays are used to control circuits by a low power signal (with complete isolation between control & controlled circuits) or when sev-eral circuits are controlled by one signal. A relay that can handle high power directly required by loads such as electric motors is called a con-tactor. Solid state relays have no moving parts but use semi-conductor devices to perform switching.

BASIC DESIGN & OPERATION

A simple electromagnetic relay consists of a coil of wire wound around a soft iron core. An iron choke provides a low reluctance path for magnetic flux, a movable iron armature & core or more sets of contacts. Armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts. It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit. In this condition one of the two sets of contacts in the relay is closed.

When an electric circuit is passed through a coil, it generates a magnetic field that activates the armature & the consequent movement of the movable contact either makes or breaks a connection with a fixed con-tact. When current is switched off armature is returned by a force ap-proximately half as strong as the magnetic force to its relaxed position by a spring.

When the coil is energized with a direct dc current a diode is placed across the coil to dissipate energy from the collapsing magnetic field at deactivation which would otherwise generate a voltage spike dangerous to semiconductors circuit components.

Solid state relays use thyristors & other solid state switching devices ac-tivated by control signal to switch controlled load instead of a solenoid. An opto-coupler (an LED coupled with a photo transistor) can also be used to isolate control & controlled circuits.

POLE & THROW

Relays are basically switches & therefore the terminology that applies to switches can apply also to relays. A relay switches one or more poles

27

each of which contacts can be thrown by energizing the coil in one of three ways.

NORMALY OPEN (NO) CONTACT.

This contact connects the circuit when the relay is activated & discon-nects the circuit when the relay is de-energized. It is also known as form A contact or make contact. It should be differentiated from the early make contactor the NOEM, which means that the contact close before the switch is fully engaged .

NORMALY CLOSED (NC) CONTACT

This contact disconnects the circuit when the relay is energized & con-nects the circuit when the relay is inactive. It is also known as form B or break Contact. This differs from the late break NCLB contact, which means that contact stays closed until the button / switch is fully dis- en-gaged.

CHANGE OVER CONTACT/ DOUBLE THROW (DOUBLE THROW)

Controls two normally open contacts &one normally closed contact with a common terminal. This form of a relay is also known as a form c con-tact or a transfer contact (break before make). If it utilizes make before break principle, then it is a form d contact.

OTHER DESIGNATIONS USED

SPST- single pole single throw. They have two terminals which are to be connected or be disconnected.

Coil

A

B

SPST switch

28

SPDT – single pole double throw. A common terminal connects either of two others including two for the coils. It has a total of five terminals.

DPST – Double pole single throw. Have two pairs of terminals equivalent to two spst switches activated by one coil. It has six terminals in total. Poles may have either form A or form B or one of each.

DPDT – Double pole double throw. Have two rows of change over termi-nals equivalent to two spdt switches activated by a single coil. Have a to-tal of eight terminals.

B1 B2

A2 A1

Coil

A B

C

Coil

29

APPLICATIONS

1. Amplifying a digital signal i.e. switching a large amount of power with small amount of power. E.g. controlling a high voltage circuit with a low voltage signal.

2. Detecting & isolating faults on transmission & distribution lines by operating & closing protection relays.

3. Isolating controlling circuits from controlled circuit when the two are at different potentials. APPLICATION CONSIDERATIONS

1. Number & type of contacts 2. Ratings of contacts e.g. small relays carry a few milliamps to

large relays rated up to 3000 amps either ac or dc. 3. Voltage ratings of contacts i.e. for commonly used relays

415vac, or 600 vac, automotive relays 50 vdc or special type high voltage type of about 150000v etc

4. Operating life time – number of times a relay is expected to operate reliably. This is both mechanical & electrical life. It is usually dependent on type of load being switched on.

5. Coil voltage. Machine tools relays 240 vac, 120vac or 240 vac. relays for switchgear may have 125v or 250vdc coils.

6. Packaging enclosure- consider whether to use open type, safe to touch, double voltage for isolation between circuits.

7. Operating environment. Minimum & maximum operating temperatures & other environmental considerations are tak-en into account. Such factors include humidity, corrosion etc.

C2 C1

B2 A2 B1 A1

Coil

30

8. Other accessories to be used. E.g. timers, auxiliary contacts pilot lamps test buttons etc.

RESISTOR

This is a passive two terminal electrical component that implements electrical resistance as its element.

Ohms law is a relationship between current through a resistor & voltage. It is stated as V =IR.

Where I = Amperes V= Volts R= Ohms

Resistors are common elements in electrical & electronic circuits. Resis-tors can be made of various compounds & films as well as resistance wires e.g. nickel chromium or can be implemented within integrated cir-cuits especially analog devices or be implemented in hybrid & printed circuits.

Electrical functionality of a resistor is specified by its resistance. The re-quired precision of a resistor may require attention to the manufactur-ing tolerance of the chosen resistor according to its specific application. The temperature coefficient of the resistor may also be of concern in some precision applications. Maximum power rating must not exceedthe anticipated power dissipation of the resistor in a particular circuit. in high voltage circuits attention must be paid to maximum working volt-age of a resistor.

SYMBOL

THEORY OF OPERATION

Resistor Rheostat Variable resistor (potentiometer)

31

Behavior of an ideal resistor is directed by ohms law V=IR. Ohms law states; Voltage (V) across a resistor is proportional to current (I) where constant of proportionality is the re-sistance(R). An equivalent ohms law can be stated as I=V/R. this formulation states that current (I) is proportional to voltage (V) to resistance R. SERIES & PARALLEL RESISTANCES SERIES CONFIGURATION.

In this configuration current through all resistors is the same but voltage across each resistor will be in proportion to its resistance. The Pd (voltage) seen across the network is the sum of all these voltages& this total resistance is the sum of these resistances. PARALLEL CONFIGURATION

R1 R2 Rn

R1 R2 Rn

R eq = R1 + R2……………..+Rn

32

POWER DISSIPATION At any instant, the power P consumed by a resistor R (ohms) is calculated as,

Total amount of heat released over a period of time can be determined from the integral of the power over that period of time.

∫

Therefore Average power dissipated over that particular time period is.

∫

For a periodic waveform power is given by.

Resistors are rated according to their maximum power dissi-pation. Resistors required to dissipate substantial amounts of power e.g. in power supplies, power conversion, power am-plifiers etc are known as power resistors. MEASUREMENTS Value of a resistor can be measured by an ohmmeter which may be one function of a digital multimeter. COLOUR CODE Axial lead resistors can have resistsnces determined by the colour code marked on them. The colour code gives infor-mation like resistances tolerance temperature coefficient wattage e.t.c. Colour bands can be 4, 5, or 6.the first two bands represent the first tw digits to measure their value.the third band of a 4-banded resistorrepresents the multiplier & the fourth the tolerance.whereas in five & six banded resistors the third band represents the third digit but the fouth & the fifth bands represents the multiplier & tolerance respectively.only the sixth band represent temperature coefficient in a six band

33

resistor. Measuring digits against their colour code is given below.

First two digits

Multiplier Tolerance Temperature coefficient

Black 0 Black 1 Not used Not used Brown 1 Brown 10 Brown +1% Brown 100 Red 2 Red 100 Red +2% Red 50 Orange 3 Orange 1K Not used Orange 15 Yellow 4 Yellow 10K Not used Yellow 25 Green 5 Green 100K Not used Green 0.5 Blue 6 Blue 1 M Not used Blue 0.25 Violet 7 Violet 10M Not used Violet 0.10 Grey 8 Not used Not used Not used White 9 - Not used Not used Silver 0.01 Silver +10 % Not used Gold 0.1 Gold +5% Not used Eg The value of a four band carbon resistor having colour bands red red red & silver will be. 22*100+10%=2200+10% TYPES OF RESISTORS Resistors are categorized into three categories,

1. Fixed resistors 2. Variable resistors 3. Special resistors

FIXED RESISTORS.

1. CARBON PILE

These resistors are made of stacks of carbon disks compressed between two metal contact plates. They are usually used where adjustable load is required e.g. testing of vehicle batteries, radio transmitters, speed con-trol in small motors. They can also be incorporated in automatic voltage regulators for generators to control field current & maintain relatively constant voltage.

34

2. CARBON FILM.

A carbon film is deposited on an insulating substrate & a helix cut in it to create a narrow resistive path. Varying shapes coupled with resistivity of amorphous carbon ranging from 500 - 00μῼm can be produced.

3.PRINTED CARBON RESISTOR

Carbon composition resistors can be printed directly onto pcb substrate as part of p.c.b. manufacturing.

4. WIRE WOUND RESISTORS

These resistors are commonly made by winding a metal wire usually ni-chrome around a ceramic, plastic, or fiber glass core. The wire ends are soldered or welded to two caps or rings attached to the ends of the core. This assembly is protected by a layer of paint, molded plastic, or an enamel coating baked at high temperature.

2. VARIABLE RESISTORS 1 . ADJUSTABLE RESISTORS

A resistor can have one or more fixed tapping points so that resistance can be changed by moving connecting wires to different terminals. Where continuous adjustment of the resistance values measurement is required a rheostat is used.

2 POTENTIOMETER

This is a three terminal resistor . it is essentially a voltage divider used for measuring electric potential(voltage). Common applications include volume control, position control, & in transducers.

3. DECADE RESISTANCE BOX

This is a box containing resistors of many values with one or many me-chanical switches which allow any one of various discrete resistance to be switched on.

35

3.SPECIAL RESISTORS

1.RESISTANCE NTC THERMISTORS.

They exhibit strong negative temperature coefficient making them use-ful for measuring temperature.

2. HUMISTOR

Their resistance varies with humidity.

3.METAL OXIDE VARISTOR

Its resistance falls to very low values when high voltages are aap-plied.this is used for protection of electronic devices.

4.PHOTORESISTORU

Its resistance varies with levels of light illumination

5.STRAIN GAUGE

Its resistance varies with changes in applied strain.

POTENTIOMETER

As mentioned earlier it’s a three terminal device with a sliding contact that forms an adjustable voltage divider.

CONSTRUCION

It comprises of a resistive element, a sliding contact that moves along the element making good electrical contact with one part of its electrical terminal at each end of the element, a mechanism that moves from one end to the other & a housing containing the element & wiper. Some are formed with the resistive element formed into an arc of a circle a little less than full turn & a wiper rotating around the arc & contacting it.the resistive element with terminals at each end is flat or angled. The wiper is conneted to the third terminal between the other two. Another one is the linear sliding potentiometer which has a wiper sliding along linear element instead of rotating.

36

The resistive element of commonly used potentiometers is made of graphite. Other materials used include resistance wire, carbon particles in plastic & ceramic/metal mixture known as cement. The condictive part of the potentiometer use conductive polymer resistor paste that contains hard wearing resins & polymers, solvents & lubricants in addi-tion to carbon that provides conductive properties.

THEORY OF OPERATION

The potentiometer can be used as a voltage divider to obtain manually adjustable voltage at the slider from a fixed input voltage applied across the two ends of the potentiometer.

Voltage across RL can be calculated by

If RL is large compared to other resistances then output voltage can be approximated by a simpler equation shown below.

Dividing throughout by RL & cancelling terms with RL as the denomina-tor.

R2

R1

RL

+

-

Vs

RL R2

R1

-

+ Vs

37

CHAPTER TWO

CIRCUIT DESIGN AND ANNALYSIS

In this chapter the most important components of this circuit shall be analyzed. Five stages which are of utmost importance to the circuit shall be considered. They include voltage rectification, voltage sensing circuit, logic control circuit, switching circuit & the change over circuit. The or-der of analysis is shown below.

VOLTAGE RECTIFICATION.

THE STEP-DOWN TRANSFORMER & DIODE CIRCUIT.

The circuit in consideration is as shown below.

This section converts the 240 Vac into 12 Vdc necessary to provide oper-ating voltage for the OP-Amp. Lets us consider one phase (red phase) of the total circuit. The red phase supplies the step-down transformer which shall reduce the 240Vac to 12Vac at 300 mA. This voltage is fur-

D2

240 V 50 Hz

D1 C1

Tx 1

12 V dc

Voltage rectification

Voltage sensing

Logic Control Circuit

Switching Circuit

Change Over Circuit

Ste

pp

ed

do

wn

ac

vo

lta

ge

12

Va

c

38

ther passed through the diode D1 for rectification to dc Voltage. Capaci-tor C1 is the filtering element to reduce the effect of ripples in the circuit. Diode D2 shall provide operating voltage to the relay when transistor one conducts to allow the voltage to pass as a result of the phase voltage going below the pre-determined level.

VLOTAGE SENSING & CIRCUIT

The main component in this stage is the IC741 which acts the brain of the circuit. The zener diode is used to provide a fixed reference voltage to the non-inverting terminal of the IC741.

OPERATION OF IC741 AS A VOLTAGE COMPARATOR

The inverting mode shall be used.

Vout IC 741

R1

Vi

n

R2

7

4 3

2

V+

V-

6 IC 741

R1

VR1

R2

7

4 3

2

39

A voltage operator is a circuit which compares two voltages & switches the output to either high or low state depending upon which voltage is higher.

The reference voltage is applied to the non-inverting input & voltage to be compared is applied to the inverting input. Whenever input voltage goes above Vref the output of the OP-AMP swings to negative saturation. Here the difference between the two voltages (Vin-Vref) is inverted & am-plified to infinity mode is Av = -Rf/R1. Since there is no feedback resistor, the gain will be close to infinity & the output voltage as negative as pos-sible . I this case it will be zero.

The voltage divider configuration is used to create a reference voltage. The inverting terminal of the OP-AMP is set at reference voltage which is

( )

In our case R1 =3.3k, R2 = VR1 = ?????, Vsupply = 12 Volts & Vref = 5.1 v .

We shall solve the above equation to get value of VR1= R2 .

Let VR1=R2 be equal to x. therefore the above equation becomes.

40

Therefore value of VR1 shall be set at 2.4 k. however due to unavailability of the 5.1 zener diode the next available one was taken which was a 5.6v. the setting of the potentiometer therefore changes to;

VR1 shall be set at 2.9K.

When the input signal Vs at the non-inverting terminal is greater than Vreference at the inverting input, then Vout will equal the positive supply.

Similarly when Vsupply is less than Vreference Vout will be equal to a negative supply which could be grounded & in this case it will be zero volts when-ever Vs is less than the reference voltage.

Output transition occurs when

Our Vref is set at 5.6v by the zener diode. Therefore for the output at pin 6 to remain high voltage at non-inverting terminal 2 should be anything more than 5.6V.we shall therefore set our Vi to be minimum of 5.8. Calcu-lating the threshold voltage (this is a voltage at which the comparator will cut the existing power circuit due to a condition of low voltage below an already predetermined level of 5.6 volts.)

Here R2 is the VR1. Therefore

41

If the output at pin 6 is to remain high threshold voltage shall not fall be-low 12 volts. If it does the output at pin 6 will be low causing transistor conducts therefore operating the relay & so transferring the load to an-other line.

The gain on an inverting amplifier is given by

In our case we shall have,

Also gain can also be given by the equation below

Considering our input voltage 5.8v output voltage can be calculated us-ing the above formulae for gain.

Therefore output voltage at pin 6 is 6.612 volts.

The output on pin number 6 is to provide a voltage which will provide current when passed through the resistor which shall be used to switch the common base configuration of the transistor to operate the relay. If output at pin 6 remains above 5.6 volts the transistor will not conduct so the relay will remain de-energised.As soon as supply voltage at inverting terminal 2 falls below 5.6 volts the output at terminal 6 will go low re-

42

sulting in the transistor conducting to operate the relay to perform the changeover operation.

THE SWITCHING CIRCUIT

The switching circuit comprises of the comparator output & the switch-ing transistors connected in common base mode. The operation is as de-scribed above.

THE COMMON BASE OPERATION OF A TRANSISTOR

Operation of a switching transistor depends on the basic operation of PNP transistor biasing. This is briefly explained below;

For proper working of a transistor it is essential to apply voltages of cor-rect polarity across its two junctions. For normal operation;

Emitter base junction is always forward biased

Collector base junction is always reverse biased

VE

E IB

IC IE

VCC

VCC

IE

IB

IC

P N P

VEE