CHAPTER ONE INTRODUCTION 1.0 INTRODUCTION The inefficiency on the part of power supply authority to adequately ensure the availability of power supply to consumers has led to a common practice where the three phases are combined to a single phase to power their single phase loads. (See figure 1.1). Most worrisome is method used in selecting these phases which is manually done. The risk of electrocutions in this practice is very high as every Tom, Dick and Harry always jumps at the cut-out fuse board as soon as a phase fails. Though the idea of consumers changing phases on their own is not quite safe for the power distribution equipment which could result in a situation where one phase could be overloaded while the other is under loaded. This creates an unbalanced situation in the power distribution network. Since our 1
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
CHAPTER ONEINTRODUCTION
1.0 INTRODUCTION
The inefficiency on the part of power supply authority to adequately ensure the
availability of power supply to consumers has led to a common practice where the
three phases are combined to a single phase to power their single phase loads. (See
figure 1.1). Most worrisome is method used in selecting these phases which is
manually done. The risk of electrocutions in this practice is very high as every
Tom, Dick and Harry always jumps at the cut-out fuse board as soon as a phase
fails. Though the idea of consumers changing phases on their own is not quite safe
for the power distribution equipment which could result in a situation where one
phase could be overloaded while the other is under loaded. This creates an unbal-
anced situation in the power distribution network. Since our people have wrong-
fully accepted this practice, to ensure safety, however, the idea of automating this
culture now forms the basis of this project. As soon as the power supply authori-
ties rises up to this challenge, the system to be developed in this project could still
be there but will not create any harm on the power distribution network.
Furthermore, an improvement on the manual method of phase change is the low
voltage detector that takes a low voltage below a preset value which could be
dangerous to our equipment as no power supply will be available and
consequently isolate the load.
1
Figure 1.1 manual looping of three phases to one phase
1.1 AIM
The aim of this project is to design and construct an automatic phase selector for
safety of human life and equipment.
1.2 OBJECTIVES
To detect the availability of voltage supply in existing phase
To measure the value of voltage in each phases
Compare the different values of the phases
Automatically lock and connects the best phase to the load
1.3 APPLICATIONS
The work finds applications in:
2
Power supply stability in cell sites, hospitals – during operation
Fire Prevention in homes
Security Efficiency in banks
Energy Efficiency
System Protection for file servers
Locations where Automatic selection of available phase for single phase
load required
As a safety equipment in industries and also where unmanned operations
are required
1.4 SCOPE OF THE PROJECT
The design of the project will involve the use of discrete components as well as
integrated circuits. It is will be a prototype which will be assembled on a board.
An average load of 25A will be considered in selecting main switching contactor,
however, with appropriate rating of the contactor the project can be used to control
heavier load switching without human involvement.
1.5 UNIQUENESS OF PROJECT
3
Being an improvement to the manual method of phase change, the device can be
used to secure maximum safety of equipments and human life.
CHAPTER TWO
4
LITERATURE REVIEW
2.0 HISTORICAL BACKGROUND
Older fuse boxes use rewirable fuses with no other protective device, and basic 4
ways boxes are very common. A lot of these boxes are made of brown-black
Bakelite, some times with a wooden base. Although their design is historic, these
were standard equipment for new installs as recently as the 1980s, so are very
common.
Users should be wary of these fuse boxes, as typically pulling a fuse carrier out
with the power on results in fingers grasping live connections, and these boxes are
wide spread even in modern installations.
The popular 4 way box usually usually takes heavy or sustained loads such as
immersion heaters and oven on a socket circuit. This arrangement is not a
recommended practice today, but it is common in existing installations.
Hence these fuse boxes do not have sufficient breaking capacity for safe reliable
operation in many premises.
5
Voltage sensorBlue phase
Voltage sensorYellow phase
Scanning circuit
Load interfaceSwitching circuit
Comparators
Single phase Load
Low voltage detector
Low voltage detect - or
Low voltage detector
Voltage sensorRed phase
2.1 BLOCK DIAGRAM OF UNIT
Figure 2.1:The Block Diagram of the Unit
6
Red phase
Blue Phase
Yellow
Neutral
2.2 METHODOLOGY
The approach to be adopted in the design of the automatic phase selector is to
develop a scanner and a full voltage presence sensor that is capable of detecting
the availability of supply in a phase automatically locking and connecting that
phase to the load. The block diagram of the unit is as shown in figure 1.2 above.
The scanner is made up of an astable multivibrator and a counter and the low
voltage detector is made up of a step down transformer, a rectifier, filter, a
regulator and a comparator. The switching will be done by electromechanical
relays and contactors. The power supply will be uninterrupted as the system must
run even when there is total power failure.
2.2.1 FUNCTIONAL PARTS OF THE BLOCK DIAGRAM
Voltage Sensor: The three phases usually available on a 220v supply will be
monitored by a voltage sensor on the device. The three phases are each having a
voltage sensor that is capable of determining the availability of supply in a phase.
The sole aim of this sensor is first to detect if there is voltage at all on the phases
and there by sending a signal to the scanner.
Low and Comparator Voltage Detector: The low voltage detector is made up of a
step down transformer, a rectifier, filter, a regulator and a comparator. The
regulated (Reference) and the unregulated voltages are compared. If the voltage is
low the unregulated voltage will be low since there is a linear relationship between
7
the input and the output voltages of a transformer. Now the output of the
comparator will be low if the input voltage is low hence the counter gate can be
opened to allow the counter to advance to the next available phase.
Scanning Circuit: The scanner is made up of an astable multivibrator and a
decade counter. The astable multivibrator is used as a clock for the counter though
gated such that the counter receives the clock pulse only when the load is not
powered.
Switching Circuit: The switching circuit turns ON or OFF the interface
electromechanical relays and contactors which closes only on the availability of
correct voltage in that phase and only one is connected at a time so the short
between phases is completely avoided.
2.3 REVIEW OF COMPONENTS USED
Electronic systems are made from units which are in turn made from components
Basic circuit components have been used in this project. They include resistors,
capacitors, diodes, transistors and integrated circuit chips like the 555 timer,
operational amplifier, in his chapter; I will review the theory behind these
components as well as their applications to this project
8
2.3.1 RESISTORS
Resistors are electronic devices designed to limit the amount of current flow in a
circuit. In other words they offer opposition to the flow of current in a circuit. This
ability to limit the flow of current in a circuit is called resistance of a resistor and
its unit is ohm. The symbols used to represent resistance is shown in figure 2.2
Fixed
[Variable]
Fig. 2.2 Resistor symbols
2.3.1.1 CLASSIFICATION OF RESISTORS`
Furthermore, the types of resistors that were used can be classified in terms of
their value. (i.e. fixed resistors and variable resistors), with each having different
maximum ohmic values, different tolerances, power or wattage ratings, as well as
stabilities.
2.3.1.2 FIXED RESISTORS
These are resistors whose ohmic values cannot be changed in a circuit except
when replaced. They are mainly of carbon wire wound types of resistors.
9
Their resistance values are usually marked on their bodies. However, available
nowadays are resistors with their resistance values given on their bodies in the
form of a colour code consisting of four or five coloured bands
2.3.1.3 VARIABLE RESISTORS
These are the type of resistors used when it is required to vary resistance while the
circuit is in use (e.g. as in volume control in Radio and TV. Sets). They consist of
a circular carbon track on an insulating base upon which a metal contact wiper
moves; these are commonly referred to as rotary variable resistors or simply
potentiometers.
The maximum resistance values of this type of resistors are often indicated on the
casing, either in colour code or by letters. Some variable resistor may have
additional indications such as LOG or ‘LIN’. The LOG stands for logarithmic with
the effect that the log of the resistance of the resistor is proportional to the position
of the movable metal contact wiper. Similarly, LIN stands for Linear and the effect
is that, the resistance of the resistor is directly proportional to the position of the
movable contact wiper on the track.
These rotary variable resistors have power ratings ranging from 0.25W to
5 W and are available in ganged (resistance boxes or decades) and single forms.
10
2.3.2 CAPACITORS
Capacitors are some of the widely used components in the electronic aspect of my
project. A capacitor is simply a device, which stores electrical energy or charges.
Essentially, a capacitor is made up of two sets of metal plates separated by an
insulator called dielectric. Capacitors derive their names from the kind of
dielectric employed in making them e.g.
(i) Electrolytic capacitor has electrolyte as dielectric
(ii) Air capacitor has air as dielectric
(iii) Mica capacitor has mica as dielectric
(iv) Paper capacitor has paper as dielectric
Capacitance is the measure of the capacitor ability to store charge.
The unit of capacitance is the Farad with a unit symbol ‘F’ and it is defined as the
capacitance of the capacitor when 1V p.d. gives it a charge of 1C.
There are however two major types of capacitors that were used which are:
2.3.2.1 THE FIXED CAPACITORS
The fixed capacitors have values that cannot be altered or varied. They are usually
classified according to the kind of insulator (dielectric) used, and they come in
various shapes and sizes ranging from the small button shaped or disc ceramic
capacitors (usually used in power stations and industries) and the electrolytic
capacitor. Electrolytic capacitors belong to a class of capacitor made through an
electrochemical process. It is made from rolling up two aluminum strips
11
sandwiched with strapped gauze cloth soaked in borax. The borax serves as an
electrolyte for the electrochemical process involved in the manufacture. This
type of capacitors have polarity indicated on their bodies the positive being
connected to the more positive part of a circuit and the negative to the more
negative part of the circuit. The symbol is as shown in figure 2.3
This instruction should be observed otherwise the capacitor will be damaged. They
usually have large capacitance of the order of about 5μF to hundreds of μF
Figure 2.3: The circuit symbol of a capacitor
2.3.2.2 THE VARIABLE CAPACITORS
There variable capacitors used is:
(i) Variable Trimmer Capacitors
This is a much smaller type of variable capacitors. It is made up of two half-moon
shaped metal plates separated with a piece of mica. One of the plates is fixed while
the other can be rotated with a screwdriver in and out of mesh with the fixed one.
This varies the area of the contact.
12
Typical values of these small variable capacitors are as from a minimum of about
2pF to maximum of about 30pF. Figure 2.4 shows the circuit symbol of a variable
capacitor
figure 2.4 circuit symbol of a variable capacitor
+ -
Fixed value capacitor Fixed value capacitor Variable capacitor
(Non – polarized) (Polarized)
Fig. 2.4 Capacitor Symbols
2.3.3 TRANSISTORS
Generally transistors are semiconductor devices. The transistors fall into the
category of bipolar transistor, either the more common NPN bipolar transistors or
the less common PNP transistor types. There is a further type known as a FET
transistor which is an inherently high input impedance transistor with behavior
somewhat comparable to valves. Modern field effect transistors or FET's including
13
JFETS and MOSFETS now have some very rugged transistor devices. However,
the FET is not used in this project.
The transistor has three legs, the Collector (C), Base (B), and Emitter (E).
Sometimes they are labeled on the flat side of the transistor. Transistors always
have one round side and one flat side. If the round side is facing you, the Collector
leg is on the left, the Base leg is in the middle, and the Emitter leg is on the right
for the small type while the power types has a flat face with metal at the back.
Transistor Symbol
The symbol of figure 2.5 is used in circuit drawings (schematics) to represent a
transistor.
Figure 2.5: The circuit symbol of an NPN transistor
2.3.3.1 BASIC CIRCUIT
The Base (B) is the On/Off switch for the transistor. If a current is flowing to the
Base, there will be a path from the Collector (C) to the Emitter (E) where current
14
output
Non-inverting input
Inverting input
+V
-V
can flow (The Switch is On.) If there is no current flowing to the Base, then no
current can flow from the Collector to the Emitter. (The Switch is Off.)
2.3.4 OPERATIONAL AMPLIFIER (OP-AMP)
An operational amplifier is 1C is a solid-state integrated circuit that uses external
feedback to control its functions. It is one of the most versatile device in
electronics. The term op-amp was originally used to describe a chain of high
performance dc amplifier that was used as basis for the analogue type computers
of long ago. The op-amp without any external devices is called ‘open loop’ mode,
referring actually to the so-called ideal operational amplifier with infinite open-
loop gain, input resistance, bandwidth and a zero output resistance. However, in
practice, no op-amp can meet these ideal characteristics. In fact, there is no such
thing as an ideal op-amp. Nowadays, the µA741 is a frequency compensated
device and although still widely used. The Bipolar are low-noise and replacing the
old style op-amps.
Shown in the figure 2.6 is an op-amp symbol as used today. It is common fashion
to omit the power supply connections as they are implied.
Figure 2.6 Symbol of an op-amp
15
As indicated in the symbolic diagram, it has two inputs and a single output.
Additional terminals are also made available to which resistors may be connected
to correct any offset dc levels. It is usually operated using symmetrical positive
and negative dc power supplies. The integrated type op-amp such as the µA741
may be represented by its schematic and the equivalent circuit of the figure 2.7 (a)
and (b) below.
2.3.4.1 A 741 OP-AMP
The A741 is a high performance operational amplifier with high open loop gain,
internal compensation, high common mode range and exceptional temperature
stability. The A741 is short-circuit protected and allows for nulling of the off set
voltage. Fairchild semiconductor manufactures it and the data provided are as
shown in table 2.1.
Absolute Maximum Parameters
Maximum means that the op-amp safety tolerate the maximum rating given in the
data section of such am op-amp without the possibility of destroying it.
Table 2.1 : μA741 manufacturer data
Maximum Ratings
Supply Voltage 18 volts
Internal power dissipation 500mw
Differential input voltage 30volts
Input voltage 15 volts
16
Voltage offset null/v 0.5 volts
Operating Temperature range 00 to + 700C
Storage temperature range -650C to + 1500C
Lead temperature, solder 60sec 3000C
Output short circuit Indefinite
2.3.4.2 DEFINITION OF TERMS
1. Supply voltage (+/-Vs) the maximum voltage (positive and negative)
that can be safely used to feed the op-amp.
2. Dissipation (pd): the maximum power the op-amp is able to dissipate by
specified ambient temperature (500mw at 800C)
3. Differential input voltage (Vid): This is the maximum voltage that can be
applied across the positive and negative inputs.
4. Input voltage: The maximum input voltage that can be simultaneous sly
applied between both input and ground also referred to as the common
mode voltage. In general, the maximum voltage is equal to the supply.
5. Operating temperature: This is the ambient temperature range for which
the op-amp will operate within the manufacturer’s specifications.
6. Output short-circuits Duration: This is the amount of time that an op-
amp’s output can be short circuited to either supply voltage.
Unlike the ideal op-amp, the op-amp that is used in more realistic circuits today
does not have infinite gain and bandwidth.
17
One additional parameter is worth mentioning, the transient response or rise time
is the time it takes for the output signal to go from 10% to 90% of its final value
when a step-function pulse is used as an input signal and is specified under closed-
loop conditions. From electronics circuit theory, the rise time is related to the
bandwidth of the op-amp by the relation:
BW = 0.35/rise time.
The uses of op-amp are numerous. In this project, it has been used as a
comparator.
2.3.5 THE OPERATIONAL COMPARATOR
A comparator is a circuit which compares a time varying input signal Vs (t) with a
reference signal VR with a view to determining whether or not the analogue input
signal is greater than or less than the reference signal. The reference signal
determines the level at which comparison is made when the input signal exceeds
VR, the comparator output takes on a value which is different in magnitude from
when Vs (t) is less than VR
Assume the inverting input is grounded at zero volt. If a voltage signal V s is
applied to the non-inverting input, then the difference voltage V i across the input is
(Vs – O) = Vs. In the absence of feedback, the voltage gain Ao of the Op amp is
infinite. Thus, since the output voltage Vo is AoVs, the slightest departure of Vs
from zero volt produces a very large output voltage. The output voltage is
18
however limited to the value of the power supply voltage (Vcc), unless it is
otherwise limited.
In the circuit of figure 2.9 (a), if Vs>O (ie Vs is positive), the output voltage Vo is
latched to the positive power supply +Vcc, and if Vs < O (ie negative), the output
will latch to the negative power supply – Vcc. This is illustrated in the associated
waveforms in figure 2.9 (b).
We should note that if the signal is applied to the inverting input, then the output
voltage will be inverted.
2.3.5.1 OFFSETS IN OP AMP
A re-occurring assumption in the analysis of op amp circuits is that the input
current is zero. Ideally, there should be no current flowing into the input of the
basic op-amp. In practice however, low-level input dc bias currents flow because
the transistors used in the internal amplifier draws bias currents needed for their
correct operation. If the currents at the input terminals are I1 and I2 when the
output voltage Vo is zero, then input bias current is given by
I bias = ½ (I1 + 12 ). It is of the order of 100nA for the µA741 op-amp.
Again I1 should equal 12. When I1 and 12 differ in value, the difference current
(11-12) = Ios is called the input offset current it is typically 10 to 20nA.
The flow of the bias current drops voltages across the internal resistors R1 and R2.
If the dc paths to ground are not identical for both input terminals due perhaps to
mismatch in the op-amp circuit elements, an offset voltage Vos is develop which is
19
amplified at the output. The offset voltage is defined as the input voltage needed
to reduce the output voltage to zero. Ideally, the op-amp output voltage should be
zero for zero input voltage, in practice however, the offset voltage is of the order
of 1mv for the µA741 op amp, but is also known to drift as a result of temperature
and power supply variations,. Such fluctuation in dc condition is not
distinguishable from the signal to be amplified. Thus in measurement
applications, serious measurement error can occur. Offset voltages can be
minimized by making the dc paths to ground as nearly equal as possible, using
offset adjust bias circuits.
2.3.6 THE 555 TIMER IC
The 555 timer IC was first introduced around 1971 by the Signetics Corporation as
the SE555/NE555 and was called “The IC Time Machine” and was also the very
first and only commercial timer ic available. It provided circuit designers and
hobbyists with a relatively cheap, stable, and user-friendly integrated circuit for
both monostable and astable applications. The schematic diagram of the internal
structure is as shown below in figure 2.8.
20
The 555 schematic is shown in figure 2.7
Definition of Pin Functions:
Refer to the internal 555 schematic of the Figure2.10 above.
Pin 1 (Ground): The ground (or common) pin is the most-negative supply
potential of the device, which is normally connected to circuit common (ground)
when operated from positive supply voltages.
Pin 2 (Trigger): This pin is the input to the lower comparator and is used to set
the latch, which in turn causes the output to go high. This is the beginning of the
timing sequence in monostable operation. Triggering is accomplished by taking
21
Positive Vcc 4.5 -16V
Ground
the pin from above to below a voltage level of 1/3 V+ (or, in general, one-half the
voltage appearing at pin 5).
Pin 3 (Output): The output of the 555 comes from a high-current totem-pole
stage made up of transistors Q20 - Q24. Transistors Q21 and Q22 provide drive
for source-type loads, and their Darlington connection provides a high-state output
voltage about 1.7 volts less than the V+ supply level used.
Pin 4 (Reset): This pin is also used to reset the latch and return the output to a
low state. The reset voltage threshold level is 0.7 volt, and a sink current of 0.1mA
from this pin is required to reset the device.
Pin 5 (Control Voltage): This pin allows direct access to the 2/3 V+ voltage-
divider point, the reference level for the upper comparator. It also allows indirect
access to the lower comparator, as there is a 2:1 divider (R8 - R9) from this point to
the lower-comparator reference input, Q13. Use of this terminal is the option of the
user, but it does allow extreme flexibility by permitting modification of the timing
period, resetting of the comparator, etc.
Pin 6 (Threshold): Pin 6 is one input to the upper comparator (the other being
pin 5) and is used to reset the latch, which causes the output to go low. Resetting
via this terminal is accomplished by taking the terminal from below to above a
voltage level of 2/3 V+ (the normal voltage on pin 5). The action of the threshold
pin is level sensitive, allowing slow rate-of-change waveforms. The voltage range
that can safely be applied to the threshold pin is between V+ and ground. A dc
22
current, termed the threshold current, must also flow into this terminal from the
external circuit.
Pin 7 (Discharge): This pin is connected to the open collector of an NPN
transistor (Q14), the emitter of which goes to ground, so that when the transistor is
turned "on", pin 7 is effectively shorted to ground. Usually the timing capacitor is
connected between pin 7 and ground and is discharged when the transistor turns
"on".
Pin 8 (V +): The V+ pin (also referred to as Vcc) is the positive supply voltage
terminal of the 555 timer IC. Supply-voltage operating range for the 555 is +4.5
volts (minimum) to +16 volts (maximum), and it is specified for operation
between +5 volts and + 15 volts.
2.3.6.1: MODES OF OPERATION OF 555 TIMER
The 555 timers have two basic operational modes: one shot and astable. In the
one-shot mode, the 555 acts like a monostable multivibrator (figure 2.9). A
monostable is said to have a single stable state--that is the off state. Whenever an
input pulse triggers it, the monostable switches to its temporary state. It remains in
that state for a period of time determined by an RC network. It then returns to its
stable state. In other words, the monostable circuit generates a single pulse of fixed
time duration each time it receives and input trigger pulse. Thus the name one-
shot. One-shot multivibrators are used for tuning some circuit or external
component on or off for a specific length of time. It is also used to generate delays.
23
When multiple one-shots are cascaded, a variety of sequential timing pulses can be
generated. Those pulses will allow you to time and sequence a number of related
operations.
The other basic operational mode of the 555 is as and astable multivibrator (figure
2.10). An astable multivibrator is simply an oscillator. The astable multivibrator
generates a continuous stream of rectangular off-on pulses that switch between
two voltage levels. The frequency of the pulses and their duty cycle are dependent
upon the RC network values.
In the application of the 555 timers in the monostable mode, the duration of the
output pulse in seconds is approximately equal to:
T = 1.1 x R1 x C1 (in seconds)………………………………………………2.13
On the other hand, the frequency of operation of the astable circuit is dependent
upon the values of R1, R2, and C1. The frequency can be calculated with the
formula:
f=1/ (.693xC1x (R1+2xR2))………………………………………………..2.14
Figure 2.8: The 555 as a monostable multivibrator.
24
Figure 2.9: The 555 as an astable multivibrator
2.3.7 DECADE COUNTER
The CD 4017 is called a counter or divider or decade counter. It is a very handy
chip for producing “Running LED effects” which is scanning.
It has 10 outputs. Output “0” goes HIGH on the rise of the first clock cycle.
On the rise of the second clock cycle, output “0” goes LOW and output “1” goes
HIGH. This process continues across the ten outputs and cycles to output “0” on
the eleventh cycle.
The “Carry Out” pin goes LOW when output “5” goes HIGH and goes HIGH
when output “0” goes HIGH.
When RESET (pin 15) is taken HIGH, the chip will make output “0” go HIGH
and remain HIGH.
When “Clock Inhibit” (pin 13) is taken HIGH, the counter will FREEZE on the
output that is currently HIGH
The CD 4017 is a divide-by-10 CMOS chip.
25
Minimum supply voltage 6v
Maximum supply voltage 15v
Max current per output 15mA
Maximum speed of operation 5MHz
figure 2.10: the decade counter
2.3.8 INVERTER (NOT GATE)
In digital devices there are only two values, usually referred to as 0 and 1. 1 means
there is a voltage (usually 5 volts) and 0 means the voltage is 0 volts.
An inverter (also called a NOT gate) is a basic digital device found in all modern
electronics. So for an inverter, as the name suggests, it's output is the opposite of
the input (Output is NOT the Input). If the input is 0 then the output is 1 and if the
input is 1 then the output is 0. We can summarize the operation of this device in a
26
truth table 2.2. Figure 2.12 shows the circuit symbol of the NOT gate
table2.2: the NOT truth table
Input Output
1 0
0 1
Figure 2.11: the circuit symbol of the NOT gate
2.3.9 THE OR GATE
The OR gate has an output of 1 when either A or B or both are 1. In other words,
there an output when any or all the inputs have a high logic. Figure 2.13 shows a
circuit analogy. the lamp will light up (logic 1) when either switch A or B or both
are closed. Obviously, the output would be 0 if and only if both its inputs are 0. In
terms of the switching conditions, it means that lamp would be OFF (logic 0) only
when both switches A and B are OFF. The above logic operation of the OR gate
can be summarized with the help of the truth table given in truth Table 2.3.The
electronic symbol for 2 two-input OR gate is shown in figure 2.13.
27
Table 2.3.9.1: OR GATE TRUTH TABLE
A B C
0 0 0
0 1 1
1 0 1
1 1 1
Figure 2.12:.The electronic symbol for 2 two-input OR gate
2.3.10 THE AND GATE
The AND gate gives an output of a high logic only when all its inputs are high.
The electronic (or logic) symbol for 2- input AND gate is shown in figure 2.14.
The AND gate has a ‘1’ output when both A and B are 1. Therefore, in the AND
gate its output would be ‘1’ only if all its inputs are all ‘1’s true. Its output would
be ‘0’ if any of its inputs is ‘0’. Table 2.4 shows the truth table for a 2-Input AND
gate.
Table 2.3.10.1 AND GATE TRUTH TABLE
28
A B C
0 0 0
0 1 0
1 0 0
1 1 1
Figure 2.13: The electronic (or logic) symbol for 2- input AND gate
2.3.11 CONTACTORS
Contactors are extremely useful when we have a need to control a large amount of
current and/or voltage with a small electrical signal. The contactor coil which
produces the magnetic field may only consume fractions of a watt of power, while
the contacts closed or opened by a magnetic field set up may be able to conduct
hundreds of times that amount of power to a load.
Contactors typically have multiple contacts, and those contacts are usually (but not
always) normally-open, so that power to the load is shut off when the coil is de-
energized. The contactor was used as a means of switching either of the phases to
the load.
CHAPTER THREE
29
DESIGN OF THE AUTOMATIC PHASE SELECTOR
3.0 INTRODUCTION
This chapter is dedicated to the application of basic theories of electrical and
electronic Engineering to the development of a circuit diagram that is capable of
physically realizing the objectives of this project.
From the block diagram of figure 2.1, the following units can be identified:
(i). The scanning unit
(ii). Voltage presence sensor for each phase.
(iii). Low voltage detector for each phase.
(iv). Switching and load interface unit
3. 1 THE SCANNING UNIT
The unit is made up of a decade counter (4017) restricted to seven counts. The
eight - output is used as reset to the beginning of the counting process.
Count 1, 3 and 5 are used to scan for the red, yellow and blue phase respectively.
The clocking of the counter is done via an oscillator configured using a 555 timer
IC in a free running (astable multivibrator)
The output of the oscillator is gated to the counter with the output of the phase
voltage presence sensor.
30
3.1.1 Design of 555 Astable multivibrator
The chosen astable NE555
The supply voltage 18V
The minimum current 20mA
Operating temperature 0 – 70 oC
The frequency of the oscillator in Hertz is given as f = 1
(R 1+2 R 2 ) 0∙ 693 ×C 1
Choosing a frequency of 1Hz, and setting capacitor C1 as 100µF, given
(R1 + 2R2) = 1100 x 0.6 93 x1 = 14430Ω
Taking R1 = 470Ω
2R2 = 14430 – 470
R2 = 13960 /2 =6980Ω
3.1.2 Design of the CD4017 Decade Counter
The count advances as the clock input becomes high (on the rising-edge). Each
output Q0-Q9 goes high in turn as counting advances. Counting to less than 9 is
achieved by connecting the relevant output (Q0-Q9) to reset, for example to count
0,1,2,3 connect Q4 to reset.
31
Power Dissipation (PD)
Dual-In-Line 700 mW
Small Outline 500 mW
Lead Temperature (TL) (Soldering, 10 seconds) 260°C
The circuit diagram of the scanning unit is as shown in figure 3.1.
Figure 3.1:.The circuit diagram of the scanning unit
The clock pulse (output of the, oscillator) cannot get to the center except when the
phase detection given an output of zero which is then inverted to 1.
When a phase is encountered and thus in voltage, the output of the phase detector
produces a 1 which is then inverted to a zero and consequently disallows the clock
phase generated by the oscillator to getting through.
32
3.2 VOLTAGE PRESENCE SENSOR.
This unit is expected to produce an output of ‘1’ when there is power in the
encountered phase.
This unit is made up of a step down transformer selected as 12V, 300mA a bridge
rectifier, a filter capacitor and regulator.
The power supply unit is as shown in Fig 3.2.
3.2.1 Design of the step down transformer
The chosen step down transformer has the following ratings
Voltage rating 240V/12V
Current rating 500mA
3.2.2 Design of the bridge rectifier diode
The chosen diode is D3SBA10
Peak reverse voltage 800V
Forward voltage drop 0.7V
This diode satisfies the requirement because the maximum expected voltage is far
less than the peak reverse voltage rating.
3.2.3 Design of the filter capacitor
The maximum expected voltage (Vmax) = √2Vr.m.s
Vmax = √2×12 = 16.97V
Peak to peak ripple voltage (∆ Vp-p) = 10% Vmax / 2