UNIVERSITY OF NAIROBI
FINAL YEAR PROJECT
DEPARTMENT OF ELECTRICAL AND INFORMATION ENGINEERING
UNINTERRUPTED DC POWER SUPPLY
PROJECT NO: 96
By
GICHIA SALOME WANJIRU
REG. NO: F17/29036/2009
SUPERVISOR: DR. H. A. OUMA
EXAMINER: MR C. OMBURA
A PROJECT REPORT SUBMITTED TO THE DEPARTMENT OF ELECTRICAL AND
INFORMATION ENGINEERING IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS OF BSc. ELECTRICAL AND ELECTRONIC ENG. OF THE
UNIVERSITY OF NAIROBI
May, 2014
F17/29036/2009
DECLARATION OF ORIGINALITY
FACULTY/ SCHOOL/ INSTITUTE: Engineering
DEPARTMENT: Electrical and Information Engineering
COURSE NAME: Bachelor of Science in Electrical & Electronic Engineering
NAME OF STUDENT: GICHIA SALOME WANJIRU
REGISTRATION NUMBER: F17/29036/2009
COLLEGE: Architecture and Engineering
WORK: UNINTERRUPTIBLE DC POWER SUPPLY
1) I understand what plagiarism is and I am aware of the university policy in this regard.
2) I declare that this final year project report is my original work and has not been submitted
elsewhere for examination, award of a degree or publication. Where other people’s work
or my own work has been used, this has properly been acknowledged and referenced in
accordance with the University of Nairobi’s requirements.
3) I have not sought or used the services of any professional agencies to produce this work.
4) I have not allowed, and shall not allow anyone to copy my work with the intention of
passing it off as his/her own work.
5) I understand that any false claim in respect of this work shall result in disciplinary action,
in accordance with University anti-plagiarism policy.
Signature: ………………………………………………………………………………………
Date: ……………………………………………………………………………………………
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DEDICATION
This project is dedicated to my mother and sister for the moral and financial support and also to
those who have guided and inspired me throughout my journey of education.
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ACKNOWLEDGEMENT
First and foremost, I wish to thank the God for guiding me and being by my side throughout my
studies
I acknowledge the input by my supervisor, Dr. H.Ouma, for the useful comments and
suggestions which have led to the improvement of this project and for the guidance and moral
support that he granted unto me during the development of this project.
An assemblage of this nature could never have been attempted without reference to and
inspiration from the works of others whose details are mentioned in reference section. I also
acknowledge all of them.
Last but not the least to all of my friends and classmates who were patiently extended all sorts of
help for accomplishing this undertaking.
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ABSTRACT
This project is carried out to investigate the viability of and analyze relative power conception of
a system to supply DC power directly to selected office/ household electronic devices commonly
connected to the main supply for operation, maintaining operation from the mains but on power
outage supply should be directed as DC to the device.
It is basically a means to maintain power supply to devices connected to the mains thereby
avoiding any interruptions due to power outage.
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TABLE OF CONTENTS
DECLARATION OF ORIGINALITY ......................................................................................................... ii
DEDICATION ............................................................................................................................................. iii
ACKNOWLEDGEMENT ........................................................................................................................... iv
ABSTRACT.................................................................................................................................................. v
TABLE OF CONTENTS ............................................................................................................................. vi
LIST OF FIGURES ................................................................................................................................... viii
LIST OF ABBREVIATIONS ...................................................................................................................... ix
CHAPTER 1 ................................................................................................................................................. 1
INTRODUCTION .................................................................................................................................... 1
1.1 BACKGROUND ............................................................................................................................ 1
1.2PROBLEM STATEMENT .............................................................................................................. 2
1.4 PROJECT SCOPE .......................................................................................................................... 2
1.5 JUSTIFICATION ........................................................................................................................... 3
1.6 CHAPTER BREAKDOWN ........................................................................................................... 3
CHAPTER 2 ................................................................................................................................................. 4
LITERATURE REVIEW ......................................................................................................................... 4
2.1 INTRODUCTION .......................................................................................................................... 4
2.2 STORAGE BATTERY ................................................................................................................... 6
2.3 BATTERY CHARGER ................................................................................................................ 12
2.4 AC RECTIFICATION .................................................................................................................. 17
2.5 RELAY ......................................................................................................................................... 23
2.6 DC AND AC UPS COMPARISON ............................................................................................. 25
CHAPTER 3 ............................................................................................................................................... 27
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DESIGN .................................................................................................................................................. 27
3.1 BATTERY CHARGER ................................................................................................................ 28
3.2 RECTIFICATION ........................................................................................................................ 29
3.3 TRANSFORMER ......................................................................................................................... 30
3.4 BRIDGE RECTIFIER................................................................................................................... 31
3.5 FILTER ......................................................................................................................................... 32
3.6 CHARGING CIRCUIT................................................................................................................. 33
3.7 HEAT SINK OF THE TRANSISTOR ......................................................................................... 37
3.8 BATTERY MONITOR USING LM3914 .................................................................................... 37
3.9 RELAY SWITCH ......................................................................................................................... 40
CHAPTER 4 ............................................................................................................................................... 41
RESULTS, ANALYSIS AND SIMULATION ...................................................................................... 41
4.1 BATTERY CHARGER ................................................................................................................ 41
4.2 MONITORING CIRCUIT ............................................................................................................ 42
CHAPTER 5 ............................................................................................................................................... 47
CONCLUSION AND RECOMMENDATION ...................................................................................... 47
CONCLUSION ................................................................................................................................... 47
RECOMMENDATION ...................................................................................................................... 48
REFERENCES ........................................................................................................................................... 49
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LIST OF FIGURES
FIGURE 2.0 AC UPS ................................................................................................................................... 4
FIGURE 2.1 DC UPS ................................................................................................................................... 5
TABLE 1: CURRENT SUPPLY WITH HOURS ...................................................................................... 11
FIGURE 2.2 CHARGING GRAPH............................................................................................................ 16
FIGURE 2.3 BLOCK DIAGRAM OF AC RECTIFICATION .................................................................. 17
FIGURE 2.4 STEP DOWN TRANSFORMER .......................................................................................... 17
FIGURE 2.5 BRIDGE RECTIFIER ........................................................................................................... 18
FIGURE2.6 CENTRE TAPPED RECTIFIER ........................................................................................... 19
FIGURE 2.7 FILTER CAPACITOR .......................................................................................................... 22
FIGURE 2.8 VOLTAGE REGULATOR ................................................................................................... 22
FIGURE 3.0 GENERAL SCHEMATIC OF DC UPS................................................................................ 27
FIGURE 3.0 GENERAL SCHEMATIC OF DC UPS................................................................................ 27
FIGURE 3.1 BATTERY CHARGING CIRCUIT FLOW CHART ........................................................... 28
FIGURE 3.2 AC RECTIFICATION .......................................................................................................... 29
TABLE 2 BATTERY Ah RATING ........................................................................................................... 31
TABLE 3 OUTPUT VOLTAGES VARIATION ....................................................................................... 35
FIGURE 3.3 CHARGING CIRCUIT ......................................................................................................... 36
FIGURE3.4 MONITORING CIRCUIT WITH LM3914 ........................................................................... 38
FIGURE4.0 SIMULATED BATTERY CHARGER WAVEFORMS ....................................................... 41
FIGURE 4.1 SIMULATED BATTERY MONITOR ................................................................................. 43
FIGURE 4.2 ACTUAL RESULTS FOR MONITORING CIRCUIT AT 14.4V ....................................... 44
FIGURE 4.3 ACTUAL CHARGE MONITOR WITH BATTERY AT 12.4V .......................................... 45
FIGURE 4.4 PCB FABRICATION FOR THE CHARGING CIRCUIT ................................................... 45
FIGURE 4.5 PCB FABRICATION FOR THE CHARGE INDICATOR CIRCUIT ................................. 46
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LIST OF ABBREVIATIONS
DC – DIRECT CURRENT
AC – ALTERNATING CURRENT
UPS – UNINTERRUPTIBLE POWER SUPPLY
KVA – KILOVOLT AMPERE
IC – INTEGRATED CIRCUIT
I – CURRENT
V – VOLTAGE
R – RESISTANCE
L – INDUCTANCE
I/P – INPUT
O/P – OUTPUT
PCB – PRINTED CIRCUIT BOARD
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CHAPTER 1
INTRODUCTION
1.1 BACKGROUND
An uninterruptible dc power supply is an electrical apparatus that provides emergency power to
load when input source, typically the main source power fails. It provides near-instantaneous
protection from input power interruptions by supplying energy stored in batteries.
The disturbances which normally occur in commercial supply are as follows:
1. Transients – occur due to lightning, switching of power network which may result in
instantaneous rise of voltage.
2. Momentary over- and under- voltage which may be due to large changes of loads in
power systems.
3. Generation of harmonics or distortion of waveforms.
4. Electromagnetic interference (EMI) or Radio frequency interference (RFI) or noise are
introduced in the supply line due to lighting, power network switching, continuous
switching by some equipment like static inverters.
The uninterruptible power supply (UPS) is the best solution to power conditioning for critical
loads, such as real-time data processing computers, air route traffic, control centers, industrial
process control system because they are very sensitive to the nature of power supply for their
operation, protection of the equipment and continuity of a process or transfer of information.
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1.2PROBLEM STATEMENT
To eliminate the problem of power outages and data loss by maintaining power to the critical
loads. To try and reduce or eliminate the whole process of double conversion method of
accepting AC, rectifying to DC for passing through the rechargeable battery, then inverting back
to AC for powering equipment.
Thus the objectives from the problem statement are:
1. To investigate the viability of and analyze relative power conception of a system to
supply DC power directly to selected electronic devices commonly connected to the main
supply for operation.
2. To protect the critical loads from power interruptions.
3. To analyze a complete circuit design.
4. To make comparison between simulation & experimental result.
1.4 PROJECT SCOPE
This project entails the following:
1. Developing a circuitry that shows how to implement an uninterrupted DC power
supply.
2. Designing a circuit that incorporates all the key components required in the design of
a UPS.
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3. Comparison between results of experimental & hardware is used for analysis &
verification.
1.5 JUSTIFICATION
To analyze and test showing that many ac units can operate from dc without modification
making it possible to apply DC ups for existing computers specially designed for dc operation.
1.6 CHAPTER BREAKDOWN
This project is organized into five chapters:

Chapter one gives the introduction to the project, the project objectives and the
project scope.

Chapter Two is the literature review which describes the dc uninterruptible power
supply, its functions, types and its components.

Chapter three gives a complete design, description, roles and rating of dc power
supply components.

Chapter Four describes Testing of the components and Commissioning of the dc
power supply.

Chapter Five gives the conclusion of the whole project, if the objective and scope of
the project were achieved. It outlines the future works. The project ends by
outlining the appendices and the references used.
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CHAPTER 2
LITERATURE REVIEW
2.1 INTRODUCTION
There are several types of UPS systems; these include the on-line, line-interactive. An online
UPS uses a double conversion method of accepting AC, rectifying to DC for passing through the
rechargeable battery, then inverting back to AC for powering equipment.
FIGURE 2.0 AC UPS
A line-interactive UPS maintains the inverter in line and redirects the battery’s DC current path
from the normal charging mode to supplying current when power is lost. In a standby (off-line)
system the load is powered directly by the input power and the backup power circuitry is only
invoked when the utility power fails. The offline UPS provides surge protection and battery
backup.
Basically, the online UPS is the same as in the line-interactive UPS. It typically costs more due
to it having a much greater current AC-DC battery-charger/rectifier .It’s a double conversion
UPS due to the rectifier driving the inverter even when AC powered.
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In this case we need to rectify the AC power into DC power so as to directly supply DC to the
equipment. The charge stored in the battery on power outage can be supplied directly to the
equipment.
FIGURE 2.1 DC UPS
For the Kenyan case, the system designed should supply power to the load at 240 Vrms and
50Hzfrequency.
This system cannot provide complete power protection and the mode of operation has to be
changed on power failure, i.e. from line to battery and as such its application is restricted to small
systems where operation is not highly critical.
Though the system is simple and economical, it has a major disadvantage that if some fault
develops in the charger or rectifier the whole system gets out of order and no power is available
even when the power supply is present.
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An uninterruptible dc power system consists of: literature review

An automatic battery charger- To charge and protect the battery

Battery- to store the power to be used when the mains are not available

Monitoring system- To display on the battery levels.

An automatic transfer switch- to switch the load between the mains and backup power
depending on which is available.
2.2 STORAGE BATTERY
A battery is an electrical device which is a combination of several electrochemical cells, used to
convert stored chemical energy into electrical energy or vice versa for rechargeable batteries. A
rechargeable battery is made up of several cells. Since a single cell is basically 2 Volt, 12Volt
means it contains 6 cells. The reliable operation of UPS systems depends on the battery to a large
extent. If the battery is not well selected the overall life of the system is affected. When a battery
is connected to the load, energy stored in it gets utilized, this is known as ‘discharging of
battery’. The energy stored gets depleted after some time leaving a discharged battery. The
battery can be given energy from an external source to restore its energy again in a process called
‘charging of the battery’
The voltage and ampere-hour capacity of batteries required for UPS systems depend on; KVA of
load connected to UPS and load power factor, required back-up time, recommended battery
voltage range, minimum allowable v/cell after discharge. The choice of number of cells and its
capacity are chosen mostly from economical consideration.
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2.2.1 TYPES OF BATTERIES.
Batteries can be categorized in terms of the materials used to build them. They differ in terms of
capacity, cost and area of usage. In this categorization, there are 4 major types;

Lead-Acid Battery

Nickel-Cadmium(Ni-Cd) Battery

Nickel-Metal Hydride(Ni-Mh) Battery

Lithium-Ion (Li-Ion) Battery.
Lead-acid batteries are the most common batteries in the world today. The firstLead acid
batteries were developed by a French physician Gaston Plante in 1859 and are the oldest and
most widely used electrical storage units. They can take a fair amount of abuse, high discharge
rates and fast charging, but need to be maintained at full charge when not used to ensure that
degeneration of their plates and depth of their discharge is not compromised. For long term
storage lead acid battery has the most favorable characteristics, losing 40% of their charge over
one year as compared to 6 months with Ni-MHbatteries.
Lead-acid batteries are used extensively in power systems today as reliable sources of power.
This is because of their cells characteristics. Advantages offered by them compared to other
battery types are:

Low cost

High reliability
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
Tolerant to overcharging to a certain extend

Low internal impedance thus can be quickly charged and discharged

They have a higher discharge rate thus can deliver higher currents

Indefinite shelf life if store without electrolyte

Can be left on trickle/ float charge for prolonged periods

Available in a wide range of sizes and capacities

Recyclable

There is steady supply worldwide.
They however have the following disadvantages,

They are very heavy and bulky.

Columbic charge efficiency is only 70%but can be 85% to 90% for special
designs

If not charge properly, they are in danger of overheating thus not suitable for
fast charging

They must be stored in charged state once the electrolyte has been introduced to
avoid deterioration of active chemicals.
A basic lead acid battery is composed of a lead dioxide cathode, sponge metallic lead anode and
sulfuric acid electrolyte. However, there are two main types of lead- acid batteries

Wet lead acid battery- uses liquid electrolyte

Dry lead acid battery- uses dry/ paste electrolyte
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However there has been further development in this technology over the years to yield other
varieties with better characteristics. Some of them are;
1. Lead calcium batteries- lead acid battery modified by adding calcium to the
electrolyte
2. Lead antimony batteries- lead acid battery with electrodes modified by adding
antimony
3. Valve regulated lead acid (VRLA) batteries or sealed lead acid (SLA) batteriesconstruction is designed to prevent electrolyte loss through spillage, evaporation and
gassing thus prolonging battery life and easing maintenance. It has a specially
designed electrolyte that reduces gassing by stopping the release of oxygen and
hydrogen into the atmosphere. This is done by a recombination system that involves a
catalyst that causes the released oxygen and hydrogen to recombine into water. The
valves are for letting gas escape only under extreme conditions. Its commonly used
lead acid battery because of the following advantages over other types of lead acid
batteries

They are low cost

Its more safer

reliable, robust and tolerant to abuse

tolerant to overcharging to a certain extend

low internal impedance thus enabling fats charging

indefinite shelf life

wide range of sizes and capacities available,
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
They can deliver very high currents and can be left on trickle or float charge for
prolonged periods.
Sealed-lead acid batteries are generally used in emergency power back-ups and in
telecommunication equipment. The following are the most commonly used sealed lead acid
batteries

6V/4.3Ah

12V/6.5Ah

12V/7Ah

12V/7.6Ah
All these batteries require 20 hour charging. To maximize their performance and life, constant
charging method is recommended.
2.2.2 CURRENT RATING / CAPACITY OF LEAD-ACID BATTERY
Lead-acid batteries are rated in terms of how much current the battery can supply for a fixed
period of time. Battery capacity is rated in ampere-hour (Ah). Generally Ah value is based on 8
hours discharge time. The capacity of a battery depends on the number of plates used in each cell
of the battery. For example a 15 plate battery will have 90Ah and a 21 plate battery will have
150Ah rating
A 200Ah battery can provide the load current of 25A for 8 hours (200/8=25). The battery can
supply less current for long period or more current for a shorter period. For example, from table
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1 below, an 180Ah battery can supply the given current for given the number of hours
continuously.
CURRENT(AMPERES)
TIME(HOURS)
1
180
2
90
3
60
4
45
5
36
6
30
8
22.5
10
18
12
15
15
12
20
9
25
7.2
30
6
40
4.5
60
3
90
2
120
1.5
150
1.2
180
1
TABLE 1: CURRENT SUPPLY WITH HOURS
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2.3 BATTERY CHARGER
A battery charger is a device used to put energy into a cell or (rechargeable battery) by forcing an
electric current through it. Battery chargers typically have two tasks to accomplish.

To restore battery capacity, often as quickly as possible.

To maintain capacity by compensating for self-discharge.
A key factor in prolonging battery life and obtaining optimum performance from it is proper
charging environment. This is only possible if the charging current and voltage are properly
controlled and matched to the battery temperature.
To maintain capacity on a fully charged battery, a constant voltage is applied. The voltage must
be high enough to compensate for self-discharge, yet not too high as to cause excessive overcharging and heating of the battery cells.
2.3.1 CHARGING PROCESS OF BATTERIES.
Charging a battery is a matter of replenishing the depleted supply of energy that the battery has
lost during discharge or use. This replenishing process can be accomplished by different charger
implementations depending on the type of battery to be charged and there advantages and
disadvantages.
Controlled charging is very important because the life expectancy of a battery is decreased by
reducing the amount of overcharging and deep discharging. Over charging causes grid corrosion
on the positive plates and gassing that exhausts the electrolyte,while discharging to deep causes
the negative plate to form sulfate. In addition to decreasing the battery’s life, these adverse
actions can reduce the amount of Amp-hours it can deliver.
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Types of chargers are classified as:

constant voltage charger

constant current charger

"Multistage" constant voltage/current charger.
Constant Voltage charger
Constant voltage charging is one of the most common methods of charging lead acid batteries.
The idea behind this approach is to keep a constant voltage across the terminals of the battery at
all times.
Initially, a large current is drawn from the voltage source, but as the battery charges and
increases its internal voltage, the current slowly fold and decays exponentially. When the battery
is brought up to a potential full charge, which is usually considered around 13.8V, the charging
voltage is dropped down to a lower value that will provide a trickle.The charger maintains the
battery power as long as it is plugged into the charger.
The main advantage of this method is that it provides a way to return a large bulk of the charge
into the battery very fast.
The main drawback of this charger is that to complete a full charge would take a much longer
time since the current is exponentially decreased as the battery charges. A prolonged charging
time must be considered as one of the issues to this design
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CONSTANT CURRENT SOURCE CHARGER
Constant current charging is another simple yet effective method for charging lead acidbatteries.
A current source is used to drive a uniform current through thebattery in a direction opposite of
discharge.A constant current source is very easy to implement; therefore, the final solution
would require a very simple design.
The major disadvantage to this approach of this method is that the battery is always being
charged at a constant rate and when it is close to being fully charged, the charger would force
extra current into the battery, causing overcharge.
MULTI-STAGE CHARGERS
Multistage chargers combines the advantages of both constant current and constant voltage
chargers in the charging of a battery to achieve maximum charge time, with minimum damage to
the charging cell thus prolonging battery life and performance. The stages are:
Stage 1: Deep Discharge Charging Pulse Mode
The Charger starts charging at a small current of about 2mA to bring the battery voltage to a safe
threshold voltage of about 10 volts. This has effect of removing loose sulfate formed during deep
discharge state of the battery.
Stage 2: Constant Current Mode
The charger changes to constant current charging of 2 to 2.5A whenthe battery voltage has
reached the threshold voltage and charges until the battery voltage reaches about 14.4V.
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Stage 3: Constant Voltage Mode
The charger holds the battery at 14.4V and the current slowly reduces.When the current reaches
at 0.5 C (C= Battery Capacity), this pointcalled the Switching Point. The Switching Point is one
of the greatfeatures of this battery charger that it can adjust the currentautomatically according to
the battery capacity.
Stage 4: Standby Voltage Mode
The charger maintains the battery voltage at 13.8V and current slowly reduces to zero. Charger
can be left connected indefinitely without harming the battery.If the battery voltage drops to
13.8V, the charger changes from anymode to Constant Current mode and restart charging. The
charging cyclewill go through Stage 2 to Stage 4.
This four-stage charging technique takes advantage of the speed of a fast charge, but only when
within safe charge levels. Once the battery is beyond the margin of error created by fast
charging, the trickle charge takes over. This was the scheme we chose for our system.
As much as multi-stage charger in terms of its features is the best way to charge a battery to
maximize its life and capacity, for the complexity and the control logic needed to implement this
kindof solution is complex and is only possible through specially designed charging controller
ICs like UC2906 and UC3906.
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FIGURE 2.2 CHARGING GRAPH
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2.4 AC RECTIFICATION
The first step in charging a battery is to rectify the AC mains supply to Dc supply and regulate it
to charge the battery. From the figure below, a regulated DC supply consists of:
FIGURE 2.3 BLOCK DIAGRAM OF AC RECTIFICATION
2.4.1 Transformer
It steps down high voltage AC mains to low voltage AC. In this step the voltage is still AC.
FIGURE 2.4 STEP DOWN TRANSFORMER
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2.4.2 Rectifier
The conversion of ac to DC is known as rectification. The rectifier circuits comprise of either
diodes or thyristors. A rectifier is also known as ac-dc converter.
2.4.2.1 Uncontrolled rectifiers
Diode rectifiers are known as uncontrolled rectifiers because for a fixed value of ac input for low
power devices where control of output voltage is not required.
2.4.2.2 The diode bridge rectifier
This is an example of a full wave rectifier that has 4 diodes connected in two cycles; positive half
cycle and the negative half cycle. In the positive half cycles two diodes say D3 and D4 are
forward biased and conduct, while the remaining two diodes D1 and D2 are reverse biased and
behave as open circuits. During negative half cycle, diodes D1 and D2 are forward biased and
conduct while D3 and D4 are reverse biased and behave as open circuits.As shown in the
diagram below showing the bridge rectifier.
FIGURE 2.5 BRIDGE RECTIFIER
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2.4.2.3 The full wave rectifier with centre tapped transformer
Ithas two diodes connected in parallel with the load. The load current flows from A to C
through D1 and R. During the positive half cycle diode D1 is forward biased and D2 is
reverse biased. During negative half cycle D2 is forward biased and D1 is reverse biased.
The load current flows from B to C through D2 and R. The directionof load current during
both half cycles is as shown.
FIGURE2.6 CENTRE TAPPED RECTIFIER
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ADVANTAGES OF A BRIDGE RECTIFIER CIRCUIT

It does not require a centre tapped transformer which is costly

The peak inverse voltage in bridge circuit is half of that in full wave circuit
using centre tapped transformer.
DISADVANTAGES OF BRIDGE CIRCUIT

It requires 4 diodes as compared to 2 in other circuit.

More voltage drop, more losses, poor efficiency & poor voltage regulation as
compared to circuit using centre tapped transformer. This is because in a
bridge circuit 2 diodes are always in circuit while in centre tapped only one
diode is in circuit at a time.
Thus bridge rectifier is suitable for high voltages.
CONTROLLED RECTIFIERS
Rectifier circuits that are using thyristors are known as controlled rectifiers. This is due to the
reason that by changing the firing angle of thyristor the output dc voltage can be controlled. The
thyristor can be triggered at any angle α in the positive half cycle. The thyristor blocks during the
negative cycle. Just before triggering the voltage across the thyristor is the same as the input.
During conduction the voltage drop across the thyristor is only about 1V. Since thyristors are
available in high voltage, high current rating, controlled rectifiers are mostly used in many high
powered device.
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The uncontrolled rectifier has a higher efficiency, better power factor and lower input current
distortion compared with a controlled rectifier.
Rectifier should be properly sized to satisfactorily perform these two tasks: Supply the line at full
load and charge the batteries at maximum charge current.
2.4.3 FILTER CAPACITOR
The function of this capacitor, known as a reservoir capacitor (or smoothing capacitor) is to
lessen the variation in (or 'smooth') the rectified AC output voltage waveform from the bridge.
There is still some variation, known as ripple. One explanation of 'smoothing' is that the
capacitor provides a low impedance path to the AC component of the output, reducing the AC
voltage across, and AC current through, the resistive load. In less technical terms, any drop in the
output voltage and current of the bridge tends to be cancelled by loss of charge in the capacitor.
This charge flows out as additional current through the load. Thus the change of load current and
voltage is reduced relative to what would occur without the capacitor. Increases of voltage
correspondingly store excess charge in the capacitor, thus moderating the change in output
voltage / current.
The filter removes ripple from the dc supply to provide a stabilized smooth dc supply to the load.
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FIGURE 2.7 FILTER CAPACITOR
2.4.4 VOLTAGE REGULATOR- this maintains the DC output to the required constant level.
FIGURE 2.8 VOLTAGE REGULATOR
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2.5 RELAY
A relay is an electrical switch that uses an electromagnet to move the switch from the OFF to ON
position instead of a person moving the switch. It takes a relatively small amount of power to
turn on a relay but the relay can control something that draws much more power.
Relays (and switches) come in different configurations. The most common is Single Pole Single
Throw (SPST) is the simplest with only two contacts. Single Pole Double Throw (SPDT) has
three contacts. The contacts are usually labeled Common (COM), Normally Open (NO), and
Normally Closed (NC).
The Normally Closed contact will be connected to the Common contact when no power is
applied to the coil. The Normally Open contact will be open (i.e. not connected) when no power
is applied to the coil. When the coil is energized the Common is connected to the Normally Open
contact and the Normally Closed contact is left floating. The Double Pole versions are the same
as the Single Pole version except there are two switches that open and close together.
The relay selected should have contacts that can handle the voltage and current requirements of
the load. Keeping in mind that some loads (such as motors) draw much more current when first
turned on than they do at steady state. For example if you want to turn on the AC units with a
12VDC power supply get a 12VDC coil or if with 240VAC get a 240VAC coil. Note: Coils will
be rated for either AC or DC operation.
A Relay has limited lifetime (number of times it can open and close before failure) this expected
lifetime is usually given in the relay datasheet. If the relay won't be used much (say to control the
headlights on a car) a 20,000 cycle lifetime would last about 18 years if used three times a day. If
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the same relay was used to control a home AC unit which switches on and off much more often
it would wear out in a few years. Some relays have lifetimes of over a million cycles.
A relay coil is not only an electromagnet but it's also an inductor. When power is applied to the
coil the current in the coil builds up and levels off at its rated current (depends on the DC
resistance of the coil, I = V/R). Some energy is now stored in the coil's magnetic field (E =
05LI2). When the current in the coil is turned off this stored energy has to go somewhere. The
voltage across the coil quickly increase trying to keep the current in the coil flowing in the same
direction (V = Ldi/dt). This voltage spike can reach hundreds or thousands of volts and can
damage electronic parts.
By adding a fly back diode the current has a path to continue flowing through coil until the
stored energy is used up. The diode also clamps the voltage across the coil to about 0.7V
protecting the electronics. The stored energy dissipates quickly in the diode (E = V*I*t). The
current stops flowing and the relay turns off. The diode should be able to handle the coil current
for a short time and switch relatively fast. Note: A resistor or zener diode can be placed in series
with the diode to use up the stored energy quicker. This increases the amplitude of the voltage
spike above 0.7V but the energy is used up quicker (i.e. the voltage spike won't last as long).
Usually it doesn't matter if the relay takes 1ms or 100ms to turn off. Modern relay however are
already protected so the is no need to add a fly back diode.
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2.6 DC AND AC UPS COMPARISON
2.6.1 INTRODUCTION
[15] In comparison the direct current uninterruptible power supply, the DC UPS, offers the
unsurpassed opportunity of simple parallel redundancy and direct contact between the load and
the backup battery. Besides the obvious advantages of vastly increased reliability the DC UPS
also excels in energy conservation and economy, simply by being simple, straightforward and
avoiding unnecessary conversion steps.
2.6.2 UPS SYSTEM
[15] The DC UPS is very simple in implementation and operation. The only parameter which
requires management and supervision is the voltage. This concept provides direct connection of
the battery to the load, which is a great advantage for reliable service. In AC UPS, an ac bypass
switch adds to the complexity.
2.6.3 ENERGY EFFICIENCY COMPARISON
[15] The total efficiency of a direct current system can be made greater than in present ac
systems owing to elimination of the extra conversion step of the inverters. The centralization of
rectifiers and PFC circuits can be made more efficient than when each single device includes
rectifying the AC to DC with (more or less effective) power factor correction.
2.6.4 RELIABILITY COMPARISON
[14, 15] Now, due to IT/telecom convergence, AC UPS is introduced on a large scale in
telecommunication systems and other mission critical use, reliability has become more important
for UPS technology than before. In the INTELEC® white paper from 1998, “Powering the
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Internet, Datacom Equipment in Telecom Facilities” a comparative calculus of unavailability
was made comparing the AC UPS with the DC UPS as used in their respective standard
configurations. The difference is found to be as large as 7600 times in advantage of the DC
UPS. In large-scale operation and use, this difference will give a valuable contribution to overall
performance and economy on system level for the operators and the public, when using internet.
This comparison was done in 1998 and for 48 VDC. The same topology is valid for 350 VDC
systems and gives the same results.
2.6.5 CONCLUSION
It is shown that in most fields studied the DC UPS compares advantageously over AC UPS.
Because of the advances in electronics, a mains frequency inverter is seldom a necessary part of
a UPS system. Such an inverter in the UPS system contributes negatively to efficiency, cost and
reliability of the system. Various types of by-pass switches often included also make the system
complicated and vulnerable.
The simple implementation of DC UPS systems makes better use of energy and other resources
which in the end will contribute to meeting the challenge of global warming as well as it gives
clear advantages in security and operation of advanced technology equipment.
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CHAPTER 3
DESIGN
In this chapter, the process of designing the dc ups was met and explained. The project aimed to
design an uninterruptible dc supply at all times. The system was also to charge and protect the
battery.
When mains supply was available the systems was to work by connecting the load to the mains
supply and also charge the battery. When the mains supply was not available, the system had to
supply the DC power to the load. All these was to happen without human intervention (an
automatic system)
The project was categorized in four major parts:
1. Battery charger – to charge an protect the battery
2. Battery- to store electrical energy inform of chemical energy
3. Battery monitoring circuit
4. A relay switch – to switch the utility power and the inverter power across the load
The general schematic of the system is shown figure 3 below
HB1
IO1
HB3
IO2
IO1
IO2
BATTERY CHARGER
HB4
IO1
IO2
HB5
HB6
IO3
AUTOMATIC TRANSFER SWITCH
IO1
IO2
MAIN SUPPLY
FIGURE 3.0 GENERAL SCHEMATIC OF DC UPS
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IO1
LOAD
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The different sections will first be designed differently then combined to give the final circuit.
The operation of the circuit will be clearly explained. The designed circuit will then be simulated
using Proteus 7 Professional software’s where it would analyze and tested to verify if it meets the
desire specifications.
The circuit will then tested on a bread-board and then assemble on PCB where physical
measurement will be made.
3.1 BATTERY CHARGER
The rechargeable battery used in this dc ups system design required constant charging to keep
the battery fully charged. Since lead acid batteries are easily available in the local market, this
project was designed based on their characteristics. To charge this battery, the AC mains supply
was rectified and converted into a DC supply. These DC supply was then used to charge the
battery. The flow chart in figure 3.2 below details this process.
240V AC main
HB1
IO1
HB2
IO2
IO1
HB3
IO2
IO1
HB4
IO2
STEP DOWN TRANSFORMER FULL BRIDGE RECTIFIER SMOOTHING FILTER
IO1
IO2
REGULATOR
Regulater dc
HB5
IO1
V1
12 V
BATTERY CHARGER INDICATOR
FIGURE 3.1 BATTERY CHARGING CIRCUIT FLOW CHART
In this design a Transformer, bridge rectifier and smoothing capacitor were used in main charger
circuit to obtain DC voltage. LM317 voltage regulator with a comparator was then used to obtain
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controlled charge. To indicate battery charge level, LM3914 integrated circuit and Led’s were
used.
3.2 RECTIFICATION
FIGURE 3.2 AC RECTIFICATION
This module converts the AC input of 230Volts, to a usable DC output greater than 17V for the
charger circuit. The design includes an internal transformer, bridge rectifier and a filter capacitor.
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3.3 TRANSFORMER
The physical behavior of electricity is that it flows from high voltage to low voltage. We want
the current to flow from the transformer to battery. If the transformer voltage is lower than
battery voltage, the opposite situation will take place (the current will flow from the battery to
transformer). Although the pass transistor in the circuit will not let this happen, neither the
battery will be charged.
Since the battery voltage was 12V. The minimum supply from the transformer would be 12V
plus 3V regulator drop plus 1.4Vrectifier drop (2 diodes) plus 10% safety.
12+3+1.4+ (12x0.1) =17.6V ~ 18V
A step down of between 18V to 20V was therefore sufficient since current limitation was
handled by the regulator. It was not dangerous to use higher powers since the regulator can
handle up to 40V but using lower power will not be sufficient to charge to battery.
The Power of the 18V transformer is related to current of the battery to be charged. For example,
the current of a 10watt 18v transformer is 0.55A (10/18=0.555A) from the equation current
equals to power/voltage. As said in chapter 2the charging current is approximately 1/10 and 1/20
of battery capacity but closer to 1/10.
Therefore, 0.55 times 10 equals to 5.5 and 0.55 times 20 equals to 11.Since it will be closer to
5.5, 10watt 18V step down transformer is sufficient for 7A battery.
Table 2 below relates different Ah of a 12V battery and the minimum watts of a transformer.
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BATTERY
TRANSFORMER
12V, 1.3Ah
3W
12V, 2.2Ah
4W
12V, 4Ah
6W
12V, 7Ah
10W
TABLE 2BATTERY Ah RATING
In this design, the secondary voltage was taken to be 24V since such transformers are locally
available. To get a 20VAC voltage from 240VAC, the secondary to primary turn ratio of the
transformer was given by:
V1÷ 𝑽𝟐= 240÷ 24 = 10
The transformer utilization factor is the ratio of DC load power to the Ac rating of the secondary.
𝒖. 𝒇 = 𝑷𝒅𝒄 ÷ 𝑷𝒂𝒄
It is also the same as the rectifier efficiency and it is given in section
3.4 BRIDGE RECTIFIER
The next stage in the AC/DC conversion process involved inverting the negative cycles of the
AC input. This process required the use of a full wave rectifier diode bridge. The required
specification for the bridge rectifier was based on the input voltage and current. The rectifier
would have to be able to handle the peak voltage of 20V as well as 2 A that the charging circuit
would be pulling. The 2W04G rectifier was used for simulation purposes.
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-
Input voltage =24V Ac
Output dc voltage =0.9VS
= 0.9 × 24 = 21.6𝑉
- Output ac voltage = Vs = 24V
- Ripple factor =𝑹𝑭 = {(𝑽𝒓𝒎𝒔
)𝟐 − 𝟏} ^ 0.5 = 0.483
𝑽𝒅𝒄
-
Efficiency = Pdc/Prms =18/20*100% = 90%
3.5 FILTER
Next the value of the capacitance needed to minimize the voltage ripple was to be found. The Ac
output from the transformer consisted of 24Vrms at 50Hz. The regulator would be drawing a
maximum current of 2A to charge the battery. The required minimum capacitor value was
calculated using the formula:
𝑪=
𝑰𝑶𝑼𝑻
𝟐 × 𝑭 × 𝑹𝑭 × 𝑽𝑰𝑵
I out= Imax = 1.5A. This was based on the fact that the current that a lead acid battery starts
gassing when charged with is 2A. Therefore considering that this is a constant voltage charger,
1.5A can be considered to be the maximum current that can be allowed through to the battery.
= 1.5/ (2*50*0.484*24)
= 1291.32uF, 2200uF, 50v capacitor was used
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3.6 CHARGING CIRCUIT
General Description
The full charger feedback control circuit implements a three stage charger algorithm: constant
current state, constant voltage full charge state, and constant voltage float charges state. This
circuit will require an input voltage of at least 17 volts to output the 14.7V for charging because
of the 2V drop across the regulator.
The comparator is used to provide feedback of the current that the battery is drawing from the
circuit: as the battery charges, the current drawn decreases. The current sensing resistor is used to
convert that current into voltage, which can be used to compare to a reference within the circuit.
This will be the logic needed for the state switching mechanism. The full charge state will
provide 14.7V or 2.45V/Cell on the battery and float charge will provide 13.8V or 2.3V/Cell.
The battery will try to draw maximum current, in this case:
(14.7V-10.5V)/.1Ohm= 42A (assuming the battery is completely dead)
The current limiting of the voltage regulator will force the current to 3A. The charger will
continuously pump this 3A until the battery current falls below the limit of 500mA. This will
bring the voltage of the battery above the reference point, therefore causing the comparator to
turn on the transistor switch, pulling the output voltage to the float charging level.
The charging voltage of a lead acid battery is between 2.3-2.4V per 2V cell. So, for 12V battery
(6cells of 2V) the charging voltage was given by:
6 2.3=13.9V~ 𝑡ℎ𝑒 𝑚𝑖𝑛𝑖𝑚𝑢𝑚 𝑐ℎ𝑎𝑟𝑔𝑖𝑛𝑔 𝑣𝑜𝑙𝑡𝑎𝑔𝑒
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Our circuit will include an LM350, 3-Amp, adjustable voltage regulator, but since the model for
this regulator was not available in proteus, a comparable adjustable regulator LM117/317HV
was used for simulation and practical purposes.
Step 1: Testing voltage regulator
The input to the regulator was chosen to be at least two volts higher above the set output voltage.
In the figure above the resistor values were chosen so that the voltage regulator is configured to
the output 14.5V DC for testing purposes only. An interesting note, the simulator applies a 1.3V
potential between the adjustment and the output pins. This somewhat varies from the
specification of the LM117/317HV, which tries to maintain 1.25V across the same pins. Using
the simulator’s voltage across R1, the voltage was calculated to be 14.5V:
Vout = 1.3V + (1.3V/R1)*R2
This was confirmed by the simulation.
Step 2: Varying Vout through a single “switch”
Controlling the output voltage to the battery is an essential part of multi-stage charging circuitry.
Adding a transistor will act as a switch to ground, and therefore vary the equivalent resistance of
R4 which will provide control over the voltage regulator.
When the “switch” is turned on, R5 is connected to ground, putting it in parallel with
R4. This lowers the equivalent resistance of both resistors, and therefore lowers the voltage
across both of them. Since the voltage is lower, the total output voltage is brought down the same
amount. The voltage on the first stage needed to be 14.7V or
2.45 V/Cell and during the float state, 13.8V or 2.3V/Cell.
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The current through R1, R3 and R2 is
1.25V/250Ohms=.005 A
The output voltages were calculated to be:
TRANSISTOR
R4// R5
VR4
Vout
OFF
1000
2.8
14.4
ON
820
2.296
13.8
TABLE 3 OUTPUT VOLTAGES VARIATION
Simulating the circuit the output voltage was measured to be 13.8 and 15.1 V when it was
completely off. This .4V difference is due to the fact that the potential difference between the
adjusting terminal and the output pin is 1.3V rather than 1.25V; therefore producing a larger
current, while increasing the output voltage. Another possible reason could be the voltage drop
across the transistor which does not completely pull the resistor to ground when it is on, and also
has leakage currents.
Step 3: Reference Controlled Switch
Providing automatic switching based on voltage level was implemented using a comparator. The
logical state change happens when the battery reaches a certain potential below the full charging
voltage.
The current through R1 and R5 was calculated to be 5 mA. Breaking the 250Ohm resistor into
240Ohm resistor in series with a 10Ohm, provides a 50mV reference below the output voltage.
This reference is used as the negative input for the comparator, and the battery voltage as the
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positive input. Whenever the battery charge is greater than the reference point, the comparator
turns on the transistor, and therefore lowers the output voltage to the float charger state.
This phenomenon was observed as predicted in the simulations above.
Step 4: Providing feedback through the current sensing resistor
This final step was to build the complete circuit. It proved to be a challenging task.
Even though logically the circuit made sense, we were unable to get the expected output. After
trying to appease the simulator for long hours, we came to the conclusion that our analysis is
probably correct and that feedback loop through the current sensing resistor R6, added enough
complexity to the circuit that the simulator did not provide an accurate answer.
FIGURE 3.3 CHARGING CIRCUIT
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3.7 HEAT SINK OF THE TRANSISTOR
Maximum expected to be dissipated in the pass transistor is given by:
P=Vce × Ic where Vce= Vin-Vmin= 24-14=10V
P= 10× 1.5= 15w
Maximum junction temperature of the transistor, Tj = 150 degrees.
Maximum room temperature, Ta= 27 degrees if the heat sink is outside the casing or 40 degrees
if it’s going to be inside.
Maximum permissible thermal resistance for the heat sink, Rth= (Tj-Ta)/P
Rth= (150-27) 15= 8.2 /W
For the heat sink inside the casing Rth= 150-40÷ 15 = 7.33𝐶/𝑤
3.8 BATTERY MONITOR USING LM3914
To be able to observe the level of charge in the battery, a circuit of displaying that was required.
A simple and cheap way of doing that was using LEDS and an LED driver the circuit in figure
3.6 below was therefore designed to observe battery charge condition. The LM3914 is designed
for 12V battery since for higher input voltage values, the LM3914 LED driver integrated circuit
does not work.
There are ten led’s pins on the LM3914 IC integrated circuit therefore the design had 10 LEDS
of different colors to display the level of charge in the battery.
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FIGURE3.4 MONITORING CIRCUIT WITH LM3914
From the LM3914 datasheet, the LEDS are to be connected to 1,18,17,16,15,14,13,12,11,10. Pin
numbers 2 and 8 are to be connected to ground; pin 3 is to be connected positive voltage. Pins 4
and 6 were connected pot 1 which is a 5k potentiometer which would be adjusted to in order to
make the minimum acting voltage. This pot 1 is connected to ground (First pin of pot 1
connected to pin number 6, second pin of pot 1 connected to number 4 and third pin of pot 1
connected to ground).
Pin number 5 is input signal and was connected to pot 2 (20k potentiometer) in order to make a
maximum acting voltage adjustment. The pot was connected positive voltage (First pin and
second pin of potconnected to positive voltage and third pin of pot 2 connected topin number 5).
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Pin number 7which is reference was connected top in number 6.Pin number 9 is used to choose
either dot or bar mode. In order to makethis choice, jumper is put between pin number 9 and
+voltage. Monitor is on bar mode, whenjumper is connected. When jumper is removed, that is
when pin number 9 is disconnected is adot mode.
The diode D4 is to protect the circuit in the event of reverse polarity connection of the battery.
Voltage ratio between comparators is equal. Minimum acting point was adjusted by RV2 (5k).
By this process of adjustment led which is connected pin number 1 should lit up for minimum set
voltage value this value was taken to be 10V. Maximum acting point of monitor circuit is
adjusted by Rv1 (20k). In this case led which was connected to number 10 should lit up during
adjustment process. This maximum voltage was 13.9V, the value at which the battery is fully
charged. Thusminimum and maximum values which circuit will measure are set accordingly.
In this design, 3 red, 4 yellow and 3 green LEDS were used. The red LEDs were connected to
pin 1, 18 and 17 of LM3914. The yellow LEDS were connected to pin 13 to 17 of LM3914. The
green LEDS were connected to pins 10 to 12 of LM3914
The difference between two measured voltages (minimum and maximum) is divided intoten
equal pieces inside LM3914. The first LED (red) will lit up at 10% of that difference and the last
LED (green ) will lit up at 100% of this difference.
It was chosen 10Vfor minimum value when only the first RED LED lights and 13.9V
formaximum valuewhen LED lights’ meaning the battery is fully charged during charging
process. It was chosen 12.5v as the value at which the red to yellow LEDs light.
By this circuit one could be able to see and determine the state of charge of the battery.
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3.9 RELAY SWITCH
To be able to switch the load between the mains power and the battery power, a relay was used.
Based on the expected current to be sourced by the load (2.5A), a double pole double throw
(DPDT) 5pin, 5A relay was selected. The relay was IEC255, 12Vdc coil, capable of switching a
240V AC, 5A or 28V DC 5A power. An AC coil relay would also work well but since there was
a DC power in the battery charger, a DC coil relay was used.
To be able to switch between the two power sources without human intervention, the coil pins
were connected the mains power source. Since the relay coil is 12V DC, DC power source was
tapped from the outputs of the rectifier used in the charging of the battery. The mains power lines
were connected to normally open pins. The battery power lines were connected to normally
closed pins. The load lines were connected to common pins.
By this connection, when the mains were available, the relay coil would be energized thus the
common pins connected to normally open pins. The load would therefore be connected to the
mains. When the mains were not available, the relay coil would be de-energized thus the
common pins would be connected normally closed pins thus the load connected to the inverter
power.
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CHAPTER 4
RESULTS, ANALYSIS AND SIMULATION
4.1 BATTERY CHARGER
The circuit was simulated and the filtered dc wave forms and the regulated charging were as
shown in figure 4.0 below. From the filtered waveform, the level of ripple was minimized by the
filter capacitor.
FIGURE4.0 SIMULATED BATTERY CHARGER WAVEFORMS
From the filtered waveform, the level of ripple was minimized by the filter capacitor. The blue
waveform indicated the filtered waveform. The yellow waveform indicated that the battery was
being charged by a regulated DC voltage.
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The charging circuit was connected to a load which in this case was anAC/DC supply radio
requiring 6V DC supply upon lack of mains supply. This was done via a 7806 voltage regulator
(A voltage regulator is an electrical regulator designed to automatically maintain a constant
voltage level. The LM78XX series of three terminal regulators is available with several fixed
output voltages making them useful in a wide range of applications.)
4.2 MONITORING CIRCUIT
The circuit was run in simulation program(Proteus Isis), the following figures aretaken from this
program.
Calibration of Circuit: Minimum acting point is adjusted by P1 (20k). By this process of
adjustment led which is connected top in number 1 is lit up for minimum voltage value.
Maximum acting point of monitor circuit is adjusted by P2 (5k). In this case led which is
connected top in number 10 is lit up during adjustment process. These adjustments are done
more one time. The minimum and maximum values which circuit will measure are applied
accordingly. The adjustment is made with P1 and P2. Voltage ratio between comparators is
equal. Difference between two measured voltages is divided into ten equal pieces inside
LM3914. To understand working principle of the circuit it is necessary to look into pins and
inner structure of LM3914.
The simulated charge indicator was as seen in figure 4.1.below. When the battery voltage was
14.4V all the Led’s were light.
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FIGURE 4.1 SIMULATED BATTERY MONITOR
When the battery was 12.5v only the red and yellow LEDs lit up. When the battery was 10v only
the red LEDs were on.
As seen below from the attached actual results, the simulated and actual results the battery
charger and indicator were tested and worked as expected. In the first case below the charger
indicator was connected to a 14.4v DC supply which is the maximum. And in the second setup
shown below the charge indicator was connected to a battery with a charge of 12.4v therefore the
red and yellow LEDs lit up as in the simulations.
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FIGURE 4.2 ACTUAL RESULTS FOR MONITORING CIRCUIT AT 14.4V
When the circuit was connected to an actual lead acid battery charged at 12.4v the red and
yellow LEDs lit up as expected. This is shown in the figure 4.3.
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FIGURE 4.3 ACTUAL CHARGE MONITOR WITH BATTERY AT 12.4V
The PCB designs were made and implemented as seen below
FIGURE 4.4 PCB FABRICATIONFOR THE CHARGING CIRCUIT
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FIGURE 4.5 PCB FABRICATION FOR THE CHARGE INDICATOR CIRCUIT
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CHAPTER 5
CONCLUSION AND RECOMMENDATION
CONCLUSION
This project intended to design of an uninterruptible DC power supply. The project aimed at
investigating the viability of a system to supply DC power directly to selected office/ household
electronic devices commonly connected to the main supply for operation. The project was to
meet the desired objectives. Although the design didn’t meet all the objectives, some of them
were met.
The first stage was to design a battery charger that would charge and protect the battery. This
was designed. The battery charger was able to charge the battery and also indicate the level of
charge in the battery. Multistage charging was implemented, by adding a transistor which acted
as a switch to ground and therefore by varying the equivalent resistance providing control over
the voltage regulator. Automatic switching based on voltage level was implemented using a
comparator, whereby the logic state change happens when the battery reaches a certain potential
below full charging voltage.
The second stage was to design a battery monitor. The charge indicator helped to be able to
determine the level of charge in the battery. When the battery voltage was below 11v only the
red Led lit indicating low voltage. When the battery voltage was above 12v the red and yellow
Led’s lit and when the battery voltage was fully charged, all the Led’s lit.
The final stage was to design an automatic transfer switch to transfer the load between battery
power and mains power. Using a double pole double throw relay, the load could be successfully
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be switched between the battery power and the mains power without human intervention. In this
design, the mains power was given priority over the inverter power, therefore so long as the
mains were available, the load would be connected to them. The load would only be connected to
the battery when the mains were not available.
RECOMMENDATION
Although this design went a long way in designing an uninterruptible DC supply, some
objectives were not met. A simple lead acid battery charger system was designed successfully.
The proposed charger can work in constant voltage or constant current mode although constant
voltage mode is the most preferred. The battery charger has many advantages like successful 3stage charging, over charge protection, battery discharge protection and a simple design.
However the battery charger would be difficult to operate in hotter temperatures. Further we can
improve the heat sink to dissipate the heat better and also indicators can be designed to indicate
bulk charge and float charge states.
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REFERENCES
1. Electrical Engineering Uncovered Prentice Hall by Dick White., Roger Doering,
NewJersey,
2. Electric Circuits, International Edition by James W. Nilsson, Susan A. Riedel,
3. http://en.wikipedia.org/wiki/Relay
4. Introduction to Power Electronics. By Hart, D, Upper Saddle River and NJ: Prentice Hal
5. Modern Power Electronics
6. http://en.wikipedia.org/wiki/Lead%E2%80%93acid_battery
7. http://www.mpoweruk.com/leadacid.htm
8. Elements of electronic design by Clifford D. Ferris
9.
Introductory electronics devices and circuits 7th addition by Robert .T. Paynter
10. LM117/317HV DATASHEET
11. LM3914 DATASHEET
12. NPN 2N3055 DATASHEET
13. LM741 DATASHEET
14. IntelecAdhoc group, the INTELEC® white paper, “Powering the Internet, Datacom
Equipment in Telecom Facilities”, INTELEC® 1998.
15. “Comparison of the AC UPS and the DC UPS solutions for critical loads” rev F, Dec 20,
2007
16. Dr. B.R. Gupta and VandanaSinghal “Power Electronics” sixth edition 2010
17. Reference: http://www.electronics-tutorials.ws/diode/diode_6.html
18. LM78XX DATASHEET
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19. http://wiki.xtronics.com/index.php/Sealed_Lead_Acid_Battery_Applications
20. Class notes FEE 541
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