CHAPTER ONE

1.0 INTRODUCTION

The innovation of solar chargers for mobile phones as a product of research and

development has been prompted by the challenge to uncover other possible means of charging

mobile phones especially where and when power supply becomes erratic or totally inaccessible.

This challenge has made solar charging which is one of the expedient alternative methods for

charging mobile devices a necessity. Although this charging idea at present has not been widely

known and accepted in this part of the world: specifically in Nigeria, it is the solution to the

erratic and incessant interruption of power supply to technological equipments – mobile phones

being our focus. This fact is further substantiated by the simple fact that Nigeria is located in the

tropics, which are areas that are typically known to have an abundant supply of sunlight all year

round. The solar phone charger is inevitable in Nigeria as a case study, considering the facts that

Nigeria is located in the tropics and at present, many parts of the country are suffering from an

unstable, unreliable, erratic and severely unavailability electric power supply which poses a great

deal of danger to electronic and electrical appliances and consequently shortens their life span, or

incapacitates them at the most critical moments when they are needed to perform the functions

why they were invented or manufactured in the first place.

An electric phone charger (referred to as from now onwards as a ‘regular charger’) is a

device used to “force” current into the battery of a mobile phone by converting pulsating ac

(alternating current) from an ac supply outlet, to dc (direct current) which is the type of current

required by a mobile phone. In a solar mobile phone charger, the ac supply outlet is eliminated,

since the required current and voltage is supplied by a dc cell known as a solar cell, which

converts solar energy into electricity. A solar cell or photovoltaic cell is a large area electronic device

that converts solar energy into electricity by the photovoltaic effect. A solar charger provides an

alternative source for charging mobile phones and furthermore harnesses the use of the abundant solar

energy available for human use. There are many variations in the circuit design of regular electric

chargers and the circuit design of solar chargers. For example, because a solar cell produces dc,

which is what mobile phones generally require, if the solar cell ratings, as much as possible,

closely matches the power requirements of the mobile phone, a transformer is not required,

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whereas a transformer is needed for a regular charger, since neither of the regular 220V or 110V

can be supplied to a mobile phone even if it is dc. Furthermore, regular chargers have an ac input

and a dc output which means, they definitely must have rectifier circuits and some sort of filter

components to remove ripples, these requirements are somewhat eliminated in the design of solar

chargers. These are some major differences in the design of regular electric phone chargers and

solar chargers, but generally, their mode of operation is the same. It is worth noting that while

regular chargers generally differ from solar chargers, regular chargers also differ one from

another and solar chargers themselves have differences in construction and circuit requirements.

These variations in their individual designs majorly depend on the level of efficiency required. A

solar charger could be designed by simply using a 6V solar cell, connected in series with a

suitable resistor at the positive side of the cell and practically charge a mobile phone, but for

efficiency, it is better to use the solar cell to charge a battery pack which serves as a charge

storage medium, which in turn is used to charge the mobile phone anytime. Another major

advantage of a solar charger is that, it is mobile and could be used anywhere, anytime as long as

there’s enough sunlight to make the solar cell produce the power requirements of the phone

being charged and this means that ‘on the move’ charging is made possible by a solar charger,

since it does not require a regular ac outlet electricity source. The major disadvantage of a

mobile charger, which has been innovatively eliminated in this project is that, a solar charger

cannot be used anywhere or anytime there’s no available or sufficient sunlight, because, the solar

cell requires sunlight to produce a considerable amount of current flow. This disadvantage can be

innovatively minimized by placing the solar cell under strong lights when solar energy is

insufficient or unavailable. A better solution to this problem is to add a rechargeable battery to

the circuit, which further makes our design complex. The solar cell is however used to charge

this battery, while the battery in turn charges our mobile phone. In practice, solar cells only

require a small amount of incident light to produce an output power, making it possible to charge

round the clock with or without sunlight using a rechargeable battery.

1.1 SOLAR CELLS AT A GLANCE

A solar cell or photovoltaic cell is a large area electronic device that converts solar energy into

electricity by the photovoltaic effect. Assemblies of cells are used to make solar modules, or

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photovoltaic arrays which are used to supply either higher current or voltage which cannot be

practically realizable from single cells to loads requiring higher power. Solar cells have many

applications. Cells are used for powering small devices such as electronic calculators, laptops,

mp3 players etc. Photovoltaic arrays generate a form of renewable electricity, particularly useful

in situations where electrical power from the grid is unavailable such as in remote area power

systems. Silicon has some special chemical properties, especially in its crystalline form.

An atom of silicon has 14 electrons, arranged in three different shells. The first two shells, those

closest to the center, are completely full. The outer shell, however, is only half full, having only

four electrons. A silicon atom will always look for ways to fill up its last shell (which would like

to have eight electrons). To do this, it will share electrons with four of its neighbor silicon atoms.

It's like every atom holds hands with its neighbors, except that in this case, each atom has four

hands joined to four neighbors. That's what forms the crystalline structure, and that structure

turns out to be important to this type of PV cell.

We've now described pure, crystalline silicon. Pure silicon is a poor conductor of electricity

because none of its electrons are free to move about, as electrons are in good conductors such as

copper. Instead, the electrons are all locked in the crystalline structure. The silicon in a solar cell

is modified slightly so that it will work as a solar cell.

A solar cell has silicon with impurities -- other atoms mixed in with the silicon atoms, changing

the way things work a bit. We usually think of impurities as something undesirable, but in our

case, our cell wouldn't work without them. These impurities are actually put there on purpose.

Consider silicon with an atom of phosphorous here and there, maybe one for every million

silicon atoms. Phosphorous has five electrons in its outer shell, not four. It still bonds with its

silicon neighbor atoms, but in a sense, the phosphorous has one electron that doesn't have anyone

to hold hands with. It doesn't form part of a bond, but there is a positive proton in the

phosphorous nucleus holding it in place.

When energy is added to pure silicon, for example in the form of heat, it can cause a few

electrons to break free of their bonds and leave their atoms. A hole is left behind in each case.

These electrons then wander randomly around the crystalline lattice looking for another hole to

fall into. These electrons are called free carriers, and can carry electrical current. There are so

few of them in pure silicon, however, that they aren't very useful. Our impure silicon with

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phosphorous atoms mixed in is a different story. It turns out that it takes a lot less energy to

knock loose one of our "extra" phosphorous electrons because they aren't tied up in a bond --

their neighbors aren't holding them back. As a result, most of these electrons do break free, and

we have a lot more free carriers than we would have in pure silicon. The process of adding

impurities on purpose is called doping, and when doped with phosphorous, the resulting silicon

is called N-type ("n" for negative) because of the prevalence of free electrons. N-type doped

silicon is a much better conductor than pure silicon is.

Actually, only part of our solar cell is N-type. The other part is doped with boron, which has only

three electrons in its outer shell instead of four, to become P-type silicon. Instead of having free

electrons, P-type silicon ("p" for positive) has free holes. Holes really are just the absence of

electrons, so they carry the opposite (positive) charge. They move around just like electrons do.

The interesting part starts when you put N-type silicon together with P-type silicon. Remember

that every PV cell has at least one electric field. Without an electric field, the cell wouldn't work,

and this field forms when the N-type and P-type silicon are in contact. Suddenly, the free

electrons in the N side, which have been looking all over for holes to fall into, see all the free

holes on the P side, and there's a mad rush to fill them in.

There are high efficiency cells which are a class of solar cells that can generate electricity at

higher efficiencies than conventional solar cells.

Solar cells are often electrically connected and encapsulated as a module. PV modules often have

a sheet of glass on the front (sun up) side, allowing light to pass while protecting the

semiconductor wafers from the elements (rain, hail, etc.). Solar cells are also usually connected

in series in modules, creating an additive voltage. Connecting cells in parallel will yield a higher

current. Modules are then interconnected, in series or parallel, or both, to create an array with the

desired peak DC voltage and current.

The power output of a solar array is measured in watts or kilowatts. In order to calculate the

typical energy needs of the application, a measurement in watt-hours, kilowatt-hours or kilowatt-

hours per day is often used. A common rule of thumb is that average power is equal to 20% of

peak power, so that each peak kilowatt of solar array output power corresponds to energy

production of 4.8 kWh per day.

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To make practical use of the solar-generated energy, the electricity is most often fed into the

electricity grid using inverters (grid-connected PV systems); in stand-alone systems, batteries are

used to store the energy that is not needed immediately.

1.2 APPLICATION AND IMPLEMENTATION

Simple Explanation

1. Photons in sunlight hit the solar panel and are absorbed by semiconducting materials,

such as silicon.

2. Electrons (negatively charged) are knocked loose from their atoms, allowing them to

flow through the material to produce electricity. Due to the special composition of solar

cells, the electrons are only allowed to move in a single direction. The complementary

positive charges that are also created (like bubbles) are called holes and flow in the

direction opposite of the electrons in a silicon solar panel.

3. An array of solar cells converts solar energy into a usable amount of direct current (DC)

electricity.

Because solar cells are semiconductor devices, they share many of the same processing and

manufacturing techniques as other semiconductor devices such as computer and memory chips.

However, the stringent requirements for cleanliness and quality control of semiconductor

fabrication are a little more relaxed for solar cells. Most large-scale commercial solar cell

factories today make screen printed poly-crystalline silicon solar cells. Single crystalline wafers

which are used in the semiconductor industry can be made into excellent high efficiency solar

cells, but they are generally considered to be too expensive for large-scale mass production.

Poly-crystalline silicon wafers are made by wire-sawing block-cast silicon ingots into very thin

(180 to 350 micrometer) slices or wafers. The wafers are usually lightly p-type doped. To make a

solar cell from the wafer, a surface diffusion of n-type dopants is performed on the front side of

the wafer. This forms a p-n junction a few hundred nanometers below the surface.

Antireflection coatings, which increase the amount of light coupled into the solar cell, are

typically next applied. Over the past decade, silicon nitride has gradually replaced titanium

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dioxide as the antireflection coating of choice because of its excellent surface passivation

qualities (i.e., it prevents carrier recombination at the surface of the solar cell). It is typically

applied in a layer several hundred nanometers thick using plasma-enhanced chemical vapor

deposition (PECVD). Some solar cells have textured front surfaces that, like antireflection

coatings, serve to increase the amount of light coupled into the cell. Such surfaces can usually

only be formed on single-crystal silicon, though in recent years methods of forming them on

multicrystalline silicon have been developed.

The wafer then has a full area metal contact made on the back surface, and a grid-like metal

contact made up of fine "fingers" and larger "busbars" are screen-printed onto the front surface

using a silver paste. The rear contact is also formed by screen-printing a metal paste, typically

aluminium. Usually this contact covers the entire rear side of the cell, though in some cell

designs it is printed in a grid pattern. The paste is then fired at several hundred degrees Celsius to

form metal electrodes in ohmic contact with the silicon. After the metal contacts are made, the

solar cells are interconnected in series (and/or parallel) by flat wires or metal ribbons, and

assembled into modules or "solar panels". Solar panels have a sheet of tempered glass on the

front, and a polymer encapsulation on the back. Tempered glass cannot be used with amorphous

silicon cells because of the high temperatures during the deposition process.

1.3 CHARGERS AT A GLANCE

A battery charger is a device used to put energy into a secondary cell or (rechargeable) battery

by forcing an electric current through it. The charge current depends upon the technology and

capacity of the battery being charged. Battery chargers come in different physical shapes, sizes

and various capacities. According to technological designs, chargers can be broadly classified

into any of the following categories, although, some chargers may fall into more than one

category. They are: Simple Chargers, Trickle Chargers, Timer based Chargers, Intelligent

Chargers, Fast Chargers, Inductive Chargers, Pulse Chargers, Solar Chargers, USB Chargers etc.

Most mobile phone chargers are not really chargers, only adapters that provide a power source

for the charging circuitry which is almost always contained within the mobile phone. Mobile

phones can usually accept a relatively wide range of voltages, as long as it is sufficiently above

the phone battery's voltage. However, if the voltage is too high, it can damage the phone while a

too low voltage generally will not charge the battery. Battery chargers for mobile phones and

other devices are notable in that they come in a wide variety of DC connector-styles and

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voltages, most of which are not compatible with other manufacturers' phones or even different

models of phones from a single manufacturer.

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CHAPTER TWO

2.0 LITERATURE REVIEW

2.1 SOLAR CELLS: A solar cell or photovoltaic cell is a large area electronic device that

converts solar energy into electricity by the photovoltaic effect. Photovoltaics is the field of

technology and research related to the application of solar cells for solar energy. Sometimes the

term solar cell is reserved for devices intended specifically to capture energy from sunlight,

while the term photovoltaic cell is used when the source is unspecified. Assemblies of cells are

used to make solar modules, or photovoltaic arrays. Solar cells have many applications. Cells are

used for powering small devices such as electronic calculators. Photovoltaic arrays generate a

form of renewable electricity, particularly useful in situations where electrical power from the

grid is unavailable such as in remote area power systems, Earth-orbiting satellites and space

probes, remote radiotelephones and water pumping applications. Photovoltaic electricity is also

increasingly deployed in grid-tied electrical systems. Similar devices intended to capture energy

from other sources include thermo-photovoltaic cells, betavoltaics cells, and optoelectric nuclear

batteries.

Current research is targeting conversion efficiencies of 30-60% while retaining low cost

materials and manufacturing techniques. They can exceed the theoretical solar conversion

efficiency limit for a single energy threshold material, that was calculated in 1961 by Shockley

and Queisser as 31% under 1 sun illumination and 40.8% under maximal concentration of

sunlight (46,200 suns, which makes the latter limit more difficult to approach than the former).

Solar cells are often electrically connected and encapsulated as a module. PV modules often

have a sheet of glass on the front (sun up) side, allowing light to pass while protecting the

semiconductor wafers from the elements (rain, hail, etc.). Solar cells are also usually connected

in series in modules, creating an additive voltage. Connecting cells in parallel will yield a higher

current. Modules are then interconnected, in series or parallel, or both, to create an array with

the desired peak DC voltage and current.

The power output of a solar array is measured in watts or kilowatts. In order to calculate the

typical energy needs of the application, a measurement in watt-hours, kilowatt-hours or kilowatt-

8

hours per day is often used. A common rule of thumb is that average power is equal to 20% of

peak power, so that each peak kilowatt of solar array output power corresponds to energy

production of 4.8 kWh per day.

To make practical use of the solar-generated energy, the electricity is most often fed into the

electricity grid using inverters (grid-connected PV systems); in stand alone systems, batteries are

used to store the energy that is not needed immediately.

There are a few approaches to achieving these high efficiencies:

Multijunction photovoltaic cell (multiple energy threshold devices).

Modifying incident spectrum (concentration).

Use of excess thermal generation (caused by UV light) to enhance voltages or carrier

collection.

Use of infrared spectrum to produce electricity at night.

Technologies include:

Silicon nanostructures

Up/Down converters

Hot-carrier cells

Thermoelectric cells

2.1.1 High efficiency cells

High efficiency solar cells are a class of solar cells that can generate electricity at higher

efficiencies than conventional solar cells. While high efficiency solar cells are more efficient in

terms of electrical output per incident energy (watt/watt), much of the industry is focused on the

most cost efficient technologies (cost-per-watt or $/watt). Still, many businesses and academics

are focused on increasing the electrical efficiency of cells, and much development is focused on

high efficiency solar cells. An example of this is the Three-dimensional solar cells that capture

nearly all of the light that strikes them and could boost the efficiency of photovoltaic (PV)

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systems while reducing their size, weight and mechanical complexity. The new 3D solar cells

capture photons from sunlight using an array of miniature “tower” structures that resemble high-

rise buildings in a city street grid. To increase efficiency of solar cells a system has been

developed, known as concentrating photovoltaic systems which in practice are not cells but

rather are methods that use a large area of lenses or mirrors to focus sunlight on a small area of

photovoltaic cells. If these systems use single or dual-axis tracking to improve performance, they

may be referred to as Heliostat Concentrator Photovoltaics (HCPV). The primary attraction of

CPV systems is their reduced usage of semiconducting material which is expensive and currently

in short supply. Additionally, increasing the concentration ratio improves the performance of

general photovoltaic materials. Despite the advantages of CPV technologies their application has

been limited by the costs of focusing, tracking and cooling equipment

2.1.2 The p-n junction

The most commonly known solar cell is configured as a large-area p-n junction made from

silicon. As a simplification, one can imagine bringing a layer of n-type silicon into direct contact

with a layer of p-type silicon. In practice, p-n junctions of silicon solar cells are not made in this

way, but rather, by diffusing an n-type dopant into one side of a p-type wafer (or vice versa).

If a piece of p-type silicon is placed in intimate contact with a piece of n-type silicon, then a

diffusion of electrons occurs from the region of high electron concentration (the n-type side of

the junction) into the region of low electron concentration (p-type side of the junction). When the

electrons diffuse across the p-n junction, they recombine with holes on the p-type side. The

diffusion of carriers does not happen indefinitely however, because of an electric field which is

created by the imbalance of charge immediately on either side of the junction which this

diffusion creates. The electric field established across the p-n junction creates a diode that

promotes current in only one direction across the junction. Electrons may pass from the n-type

side into the p-type side, and holes may pass from the p-type side to the n-type side, but not the

other way around. This region where electrons have diffused across the junction is called the

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depletion region because it no longer contains any mobile charge carriers. It is also known as the

"space charge region".

Fig 2.1: The equivalent circuit of a solar cell

To understand the electronic behaviour of a solar cell, it is useful to create a model which is

electrically equivalent, and is based on discrete electrical components whose behaviour is well

known. An ideal solar cell may be modelled by a current source in parallel with a diode; in

practice no solar cell is ideal, so a shunt resistance and a series resistance component are added

to the model. The resulting equivalent circuit of a solar cell is shown

Fig 2.2: Schematic symbol of a solar cell

Characteristic equation

From the equivalent circuit it is evident that the current produced by the solar cell is equal to that

produced by the current source, minus that which flows through the diode, minus that which

flows through the shunt resistor:

I = IL − ID − ISH ........................ Equation 2.1

where

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I = output current (amperes)

IL = photo generated current (amperes)

ID = diode current (amperes)

ISH = shunt current (amperes)

The current flowing through these elements governed by the voltage across them:

Vj = V + IRS ........................... Equation 2.2

where

V = voltage across the output terminals (volts)

I = output current (amperes)

RS = series resistance (Ω)

By the Shockley diode equation, the current diverted through the diode is:

............................. Equation 2.3

where

I0 = reverse saturation current (amperes)

n = diode ideality factor (1 for an ideal diode)

q = elementary charge

k = Boltzmann's constant

T = absolute temperature

For silicon at 25°C, volts.

By Ohm's law, the current diverted through the shunt resistor is:

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............................... Equation 2.4

where

RSH = shunt resistance (Ω)

Substituting these into the first equation produces the characteristic equation of a solar cell,

which relates solar cell parameters to the output current and voltage:

................ Equation 2.5

An alternative derivation produces an equation similar in appearance, but with V on the left-hand

side. The two alternatives are identities; that is, they yield precisely the same results.

In principle, given a particular operating voltage V the equation may be solved to determine the

operating current I at that voltage. However, because the equation involves I on both sides in a

transcendental function the equation has no general analytical solution. However, even without a

solution it is physically instructive. Furthermore, it is easily solved using numerical methods. (A

general analytical solution to the equation is possible using Lambert's W function, but since

Lambert's W generally itself must be solved numerically this is a technicality.)

Since the parameters I0, n, RS, and RSH cannot be measured directly, the most common application

of the characteristic equation is nonlinear regression to extract the values of these parameters on

the basis of their combined effect on solar cell behaviour.

2.1.2a Effect of physical size

The values of I0, RS, and RSH are dependent upon the physical size of the solar cell. In comparing

otherwise identical cells, a cell with twice the surface area of another will, in principle, have

double the I0 because it has twice the junction area across which current can leak. It will also

have half the RS and RSH because it has twice the cross-sectional area through which current can

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flow. For this reason, the characteristic equation is frequently written in terms of current density,

or current produced per unit cell area:

................. Equation 2.6

Where

J = current density (amperes/cm2)

JL = reverse saturation current density (amperes/cm2)

rS = specific series resistance (Ω-cm2)

rSH = specific shunt resistance (Ω-cm2)

This formulation has several advantages. One is that since cell characteristics are referenced to a

common cross-sectional area they may be compared for cells of different physical dimensions.

While this is of limited benefit in a manufacturing setting, where all cells tend to be the same

size, it is useful in research and in comparing cells between manufacturers. Another advantage is

that the density equation naturally scales the parameter values to similar orders of magnitude,

which can make numerical extraction of them simpler and more accurate even with naive

solution methods.

A practical limitation of this formulation is that as cell sizes shrink, certain parasitic effects grow

in importance and can affect the extracted parameter values. For example, recombination and

contamination of the junction tend to be greatest at the perimeter of the cell, so very small cells

may exhibit higher values of J0 or lower values of rSH than larger cells that are otherwise

identical. In such cases, comparisons between cells must be made cautiously and with these

effects in mind.

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2.1.2b Cell temperature

Fig 2.3: Effect of temperature on the current-voltage characteristics of a solar cell

Temperature affects the characteristic equation in two ways: directly, via T in the exponential

term, and indirectly via its effect on I0. (Strictly speaking, temperature affects all of the terms, but

these two far more significantly than the others.) While increasing T reduces the magnitude of

the exponent in the characteristic equation, the value of I0 increases in proportion to expT. The

net effect is to reduce VOC linearly with increasing temperature. The magnitude of this reduction

is inversely proportional to VOC; that is, cells with higher values of VOC suffer smaller reductions

in voltage with increasing temperature. For most crystalline silicon solar cells the reduction is

about 0.50%/°C, though the rate for the highest-efficiency crystalline silicon cells is around

0.35%/°C. By way of comparison, the rate for amorphous silicon solar cells is 0.20-0.30%/°C,

depending on how the cell is made.

The amount of photogenerated current IL increases slightly with increasing temperature because

of an increase in the number of thermally generated carriers in the cell. This effect is slight,

however: about 0.065%/°C for crystalline silicon cells and 0.09% for amorphous silicon cells.

The overall effect of temperature on cell efficiency can be computed using these factors in

combination with the characteristic equation. However, since the change in voltage is much

stronger than the change in current, the overall effect on efficiency tends to be similar to that on

voltage. Most crystalline silicon solar cells decline in efficiency by 0.50%/°C and most

amorphous cells decline by 0.15-0.25%/°C. The figure below shows I-V curves that might

typically be seen for a crystalline silicon solar cell at various temperatures.

2.1.3 Series resistance

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Fig 2.4: Effect of series resistance on the current-voltage characteristics of a solar cell

As series resistance increases, the voltage drop between the junction voltage and the terminal

voltage becomes greater for the same flow of current. The result is that the current-controlled

portion of the I-V curve begins to sag toward the origin, producing a significant decrease in the

terminal voltage V and a slight reduction in ISC. Very high values of RS will also produce a

significant reduction in ISC; in these regimes, series resistance dominates and the behaviour of the

solar cell resembles that of a resistor. These effects are shown for crystalline silicon solar cells in

the I-V curves displayed in the figure below.

2.1.4 Shunt resistance

Fig 2.5: Effect of shunt resistance on the current-voltage characteristics of a solar cell

As shunt resistance decreases, the flow of current diverted through the shunt resistor increases

for a given level of junction voltage. The result is that the voltage-controlled portion of the I-V

curve begins to sag toward the origin, producing a significant decrease in the terminal current I

and a slight reduction in VOC. Very low values of RSH will produce a significant reduction in VOC.

Much as in the case of a high series resistance, a badly shunted solar cell will take on operating

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characteristics similar to those of a resistor. These effects are shown for crystalline silicon solar

cells in the I-V curves displayed in the figure to the right.

2.1.5 Reverse saturation current

Fig 2.6: Effect of reverse saturation current on the current-voltage characteristics of a solar cell

If one assumes infinite shunt resistance, the characteristic equation can be solved for VOC:

......................... Equation 2.7

Thus, an increase in I0 produces a reduction in VOC proportional to the inverse of the logarithm of

the increase. This explains mathematically the reason for the reduction in VOC that accompanies

increases in temperature described above. The effect of reverse saturation current on the I-V

curve of a crystalline silicon solar cell are shown in the figure to the right. Physically, reverse

saturation current is a measure of the "leakage" of carriers across the p-n junction in reverse bias.

This leakage is a result of carrier recombination in the neutral regions on either side of the

junction.

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2.1.6 Ideality factor

Fig 2.7: Effect of ideality factor on the current-voltage characteristics of a solar cell

The ideality factor (also called the emissivity factor) is a fitting parameter that describes how

closely the diode's behaviour matches that predicted by theory, which assumes the p-n junction

of the diode is an infinite plane and no recombination occurs within the space-charge region. A

perfect match to theory is indicated when n = 1. When recombination in the space-charge region

dominate other recombination, however, n = 2. The effect of changing ideality factor

independently of all other parameters is shown for a crystalline silicon solar cell in the I-V

curves displayed in the figure to the right.

Most solar cells, which are quite large compared to conventional diodes, well approximate an

infinite plane and will usually exhibit near-ideal behaviour under Standard Test Condition (

). Under certain operating conditions, however, device operation may be dominated by

recombination in the space-charge region. This is characterized by a significant increase in I0 as

well as an increase in ideality factor to . The latter tends to erode solar cell output voltage

while the former acts to increase it. The net effect, therefore, is a combination of the increase in

voltage shown for increasing n in the figure to the right and the decrease in voltage shown for

increasing I0 in the figure above. Typically, I0 is the more significant factor and the result is a

reduction in voltage.

2.2 BATTERY CHARGERS

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A battery charger is a device used to put energy into a secondary cell or (rechargeable) battery

by forcing an electric current through it. The charge current depends upon the technology and

capacity of the battery being charged. For example, the current that should be applied to recharge

a 12 V car battery will be very different from the current for a mobile phone battery.

2.2.1 TYPES OF BATTERY CHARGERS

Battery chargers are of various types and shapes, depending on the aim or

target of manufacturers. They vary in physical shape and size, circuit components,

charging techniques, component ratings, input requirements and their output.

Generally, chargers are classified into the following categories:

2.2.1a Simple Chargers

A simple charger works by connecting a constant DC power source to the battery being charged.

The simple charger does not alter its output based on time or the charge on the battery. This

simplicity means that a simple charger is inexpensive, but there is a trade-off in quality.

Typically, a simple charger takes longer to charge a battery to prevent severe over-charging.

Even so, a battery left in a simple charger for too long will be weakened or destroyed due to

over-charging. These chargers can supply either a constant voltage or a constant current to the

battery.

2.2.1b Trickle Chargers

A trickle charger is a kind of simple charger that charges the battery slowly, at the self-discharge

rate. A trickle charger is the slowest kind of battery charger. A battery can be left in a trickle

charger indefinitely. Leaving a battery in a trickle charger keeps the battery "topped up" but

never over-charges.

2.2.1c Timer-based Chargers

The output of a timer charger is terminated after a pre-determined time. Timer chargers were the

most common type for high-capacity Ni-Cd cells in the late 1990s for example (low-capacity

consumer Ni-Cd cells were typically charged with a simple charger).

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Often a timer charger and set of batteries could be bought as a bundle and the charger time was

set to suit those batteries. If batteries of lower capacity were charged then they would be

overcharged, and if batteries of higher capacity were charged they would be only partly charged.

With the trend for battery technology to increase capacity year on year, an old timer charger

would only partly charge the newer batteries.

Timer based chargers also had the drawback that charging batteries that were not fully

discharged, even if those batteries were of the correct capacity for the particular timed charger,

would result in over-charging.

2.2.1d Intelligent Chargers

Output current depends upon the battery's state. An intelligent charger may monitor the battery's

voltage, temperature and/or time under charge to determine the optimum charge current at that

instant. Charging is terminated when a combination of the voltage, temperature and/or time

indicates that the battery is fully charged.

For Ni-Cd and NiMH batteries, the voltage across the battery increases slowly during the

charging process, until the battery is fully charged. After that, the voltage decreases, which

indicates to an intelligent charger that the battery is fully charged. Such chargers are often

labelled as a ΔV, or "delta-V," charger, indicating that they monitor the voltage change.

The problem is, the magnitude of "delta-V" can become very small or even non-existent if (very)

high capacity rechargeable batteries are recharged. This can cause even an intelligent battery

charger to not sense that the batteries are actually already fully charged, and continue charging.

Overcharging of the batteries will result in some cases. However, many so called intelligent

chargers employ a combination of cut off systems, which should prevent overcharging in the vast

majority of cases.

A typical intelligent charger fast-charges a battery up to about 85% of its maximum capacity in

less than an hour, then switches to trickle charging, which takes several hours to top off the

battery to its full capacity.

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2.2.1e Fast Chargers

Fast chargers make use of control circuitry in the batteries being charged to rapidly charge the

batteries without damaging the cells' elements. Most such chargers have a cooling fan to help

keep the temperature of the cells under control. Most are also capable of acting as a standard

overnight charger if used with standard NiMH cells that do not have the special control circuitry.

Some fast chargers, such as those made by Energizer, can fast-charge any NiMH battery even if

it does not have the control circuit.

2.2.1f Pulse Chargers

Some chargers use pulse technology in which a pulse is fed to the battery. This DC pulse has a

strictly controlled rise time, pulse width, pulse repetition rate (frequency) and amplitude. This

technology is said to work with any size, voltage, capacity or chemistry of batteries, including

automotive and valve-regulated batteries. With pulse charging, high instantaneous voltages can

be applied without overheating the battery. In a Lead-acid battery, this breaks-down stubborn

lead-sulphate crystals, thus greatly extending the battery service life.

Some chargers use pulses to check the current battery state when the charger is first connected,

then use constant current charging during fast charging, then use pulse charging as a kind of

trickle charging to maintain the charge.

Some chargers use "negative pulse charging", also called "reflex charging" or "burp charging".

Such chargers use both positive and brief negative current pulses. Such chargers don't work any

better than pulse chargers that only use positive pulses.

2.2.1g Inductive Chargers

Inductive battery chargers use electromagnetic induction to charge batteries. A charging station

sends electromagnetic energy through inductive coupling to an electrical device, which stores the

energy in the batteries. This is achieved without the need for metal contacts between the charger

and the battery. It is commonly used in electric toothbrushes and other devices used in

bathrooms. Because there are no open electrical contacts, there is no risk of electrocution.

21

2.2.1h USB-based

Since the Universal Serial Bus specification provides for a five-volt power supply, it's possible to

use a USB cable as a power source for recharging batteries. Products based on this approach

include chargers for cellular phones and portable digital audio players.

2.2.1i Solar chargers

Solar chargers employs solar energy in charging devices and are generally portable.

2.3 CIRCUIT COMPONENTS

The following components were employed in the design of the solar mobile phone charger and

since they come in various sizes and ratings, it is worthwhile to briefly discuss them.

2.3.0 RESISTORS: In electrical and electronic circuits, there is a need for either

varying amounts of current of voltage to be applied at specific portions of the circuit and to

various components. Resistors, as their name implies, are used in such instances to ensure

resistance to the flow of current to various portions of the circuit. Generally, these are

materials with specific values of resistance in the range between that of a conductor and an

insulator. Their values of resistance are expressed in ohms and their heat withstanding rating

is expressed in watts. Resistors are classified as either being fixed or variable. Fixed resistors

have a constant value while variable resistors also known as potentiometers or pots have

values that can be varied depending on requirements. Generally, resistors used in circuits are

linear while non-linear resistors are used for special applications. The resistance of any

material is given by:

R = ρL/A

Where R = Resistance in ohms (Ω), ρ = Resistivity of the material (Ω/cm)

L = Length of material (cm), A = Cross sectional area of material (cm2)

Resistors are connected either in series or parallel. For resistors connected in series, the

current that flows in each resistor is the same, and total resistance of the series connection is

22

equal to the sum of the individual resistors. If for example, n resistors of equal resistance are

connected in series, the total equivalent resistance will be given as:

RT = nR.

If n = 3 i.e. 3 resistors of same resistance connected in series,

RT = R1 + R2 + R3

For resistors connected in parallel, voltage across each resistor is the same, but current across

each differs. Therefore the total resistance RT of say 3 resistors in parallel is given by:

1/RT = 1/R1 + 1/R2 + 1/R3

Fixed Resistors are generally manufactured in four basic types which are carbon

composition, metal film, carbon film and wire wound. The carbon composition type is most

commonly used in electronic circuits. Generally, resistors are colour coded and this means

that to ascertain the value of a resistor, the colour codes must be understood. There are

typically four colours on a resistor. The first two colours denote the first and second digits of

the resistance value; the third colour indicates the multiplier while the fourth colour indicates

the tolerance value for the resistor. The table below gives the standard for colour coding

resistors.

23

Table 2.1 – Colour Coding of Resistors

24

COLOUR DIGIT MULTIPLIER TOLERANCE

Black 0 1 -

Brown 1 10 ±1%

Red 2 102 -

Orange 3 103 -

Yellow 4 104 -

Green 5 105 -

Blue 6 106 -

Violet 7 107 -

Gray 8 - -

White 9 - -

Gold - 0.1 ±5%

Silver - - ±10%

No Colour - - ±20%

2.3.1 CAPACITORS: These are passive circuit components which are made of two

metal plates called electrodes separated by an insulator material called a dielectric.

Capacitors can be used for various purposes which include: Filtering, Tuning, Bypassing

resistors, Generation of sinusoidal waveforms, Energy Storage etc. The capacitance of a

parallel plate capacitor is directly proportional to the relative dielectric constant of the

insulator and to the area of the plates. Furthermore, the capacitance is greater if the

separation between plates is small and vice versa. These are expressed by the following

equation:

C = KԐoA/d

Where C = Capacitance in farads, F K = Relative dielectric constant

Ԑo = Permittivity of free space (constant) which is 8.85 × 10-12 F/m

A = Area of plates, m2 d = distance between parallel plates, m

It is seen from the equation that capacitance may be increased by increasing the area of the

plates or the dielectric constant and by decreasing the separation between plates.

Although, capacitors are available in various types, shapes and sizes, they can be

generally grouped into four broad categories which are fixed, variable, chip and voltage

variable. Types of capacitors available are silver mica, electrolytic, ceramic and trimmer

capacitors. Electrolytic capacitors are generally employed in circuits where a large value of

capacitance in a small volume is required and these capacitors can be either polarised or non

polarised while silver mica capacitors generally have small capacitance and greater

mechanical stability – capacitance remaining constant at different temperatures with different

voltages and does not easily wear with age of capacitor. They generally have uniform

characteristics and will not break down at high voltages or high resistances. Trimmer

capacitors are a type of variable capacitors operated by a screw driver instead of a knob.

Their capacitance can be altered by pressing the plates tightly together which in turn alters

the distance between the plates.

2.3.2 INDUCTORS: These are circuit components in which a magnetic field is created

when current passes through an integral wire core, which may be an air core armature and

25

made of soft iron or some other ferromagnetic materials. If an applied e.m.f (electromotive

force) applied to the coil changes, a back e.m.f is induced which opposes the change (Lenz’s

law). The coil acts strongly against rapid changes in e.m.f and the strength of this effect is

called the “inductance” of the coil, measured in Henry.

The voltage, V applied across an ideal inductor equals the value of the inductance L (in

Henry) multiplied by the rate of change of current with respect to time (ampere/time). For an

indicator, the greater the voltage applied across the coil, the faster the current increases,

Hence:

V = L di/dt where V = voltage across the coil

L = inductance of coil di/dt = rate of change of current in ampere/sec

It should be noted that inductance increases with permeability of core, increase in number of

turns and with increase in area of core while increase in length for the same number of turns

decreases inductance since magnetic field will be less concentrated. Also it is worthy to note

that inductors are made of wires and the coil has dc resistance which is equal to the resistance

of the wire used in the winding of the core. The amount of resistance however, is less with

heavier wires and fewer turns.

2.3.3 DIODES: These are probably the simplest semiconductor devices. Most diodes

are made from a host crystal of silicon (Si) with appropriate impurity elements introduced to

modify, in a controlled manner, the electrical characteristics of the device. They are formed

when a p-type semiconductor is joined to an n-type semiconductor and doped. The resulting

component is a diode which allows flow of current in only one direction. To achieve this, the

diode must be forward biased with a voltage higher than the threshold voltage which is 0.6V

for silicon and 0.25V for germanium. The threshold voltage decreases at the rate of 2mV per

degree rise in temperature. Another type of diode is a schottky (unipolar) diode,

manufactured by placing a metal layer directly across the semiconductor. Diodes are mainly

used in rectification, which is the process of converting ac into dc. Power diodes are a type of

diodes which are able to carry current of several amperes. Types of diode include: power

diodes, zener diodes, Schottky diodes, Light emitting diodes, photo diodes etc.

26

2.3.4 TRANSISTORS: They are three terminal devices (emitter, base and collector)

used as amplifiers and as switches. Generally, there are two types which are NPN and PNP

transistors. A transistor is an active circuit component in that it is capable of modifying or

amplifying the input signal. They are delicate and heat sensitive. Typically, a transistor is like

two diodes with either their p-type or n-type materials joined together at the ends. Transistors

come in different types and specifications, of which some are BJTs (Bipolar Junction

Transistors), Unipolar transistors, FETs (Field Effect Transistors), JFETs (Junction FETs),

MOSFET (Metal Oxide Semiconductor Field Effect Transistor) etc.

The criteria for a properly biased transistor is that the base-emitter junction is forward biased

while the base-collector junction is reverse biased. There are manufacturer’s data books

readily available that contain all the necessary information about any particular type of

transistor, including possible substitutes if a particular transistor is not readily available.

Since transistors are heat sensitive, excessive heat should not be applied when soldering and

all terminals should be correctly identified before installing them. Although all transistors

have terminals that are easily recognised in circuit diagrams, distinguishing these terminals

on the actual devices is not always quite easy

2.3.5 FUSES: The fuse is a simple and reliable safety device. It is second to none in its

ease of application and its ability to protect people and equipment. The fuse is a current-

sensitive device. It has a conductor with a reduced cross section (element) normally

surrounded by an arc-quenching and heat-conducting material (filler). The entire unit is

enclosed in a body fitted with end contacts.

Most fuses have three electrical ratings: ampere rating, voltage rating, and interrupting

rating. The ampere rating indicates the current the fuse can carry without melting or

exceeding specific temperature rise limits. The voltage rating, ac or dc, usually indicates the

maximum system voltage that can be applied to the fuse. The interrupting rating (I.R.)

defines the Maximum short-circuit current that a fuse can safely interrupt. If a fault current

higher than the interrupting rating causes the fuse to operate, the high internal pressure may

cause the fuse to rupture.

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CHAPTER THREE

3.0 DESIGN AND CIRCUIT ANALYSIS

The design and construction requirements for this solar mobile phone charger are given

below. The circuit diagram is divided into two major parts. Using the battery as our point of

division, every component towards the left hand side (the solar cell, capacitors C1, C2, C3,

resistors R1-R6, Diode D1, Inductor L1 and transistors Q1 and Q2) are for trickle charging the

battery pack while the remaining components to the right of the battery including the battery

itself actually supply the voltage and current requirements that charge the mobile phone. The

addition of a battery to the circuit ensures that with or without sunlight, the circuit can charge the

mobile phone round the clock. The reason for the addition of a battery pack is to ensure as might

practically occur, that days of very little or no sunshine do not incapacitate the charger. On days

when there is sufficient sunlight as required by the solar cell, the battery pack is kept charging

and whether or not there is sufficient sunlight on other days, as long as the battery is present and

has been charged to a level, any connected mobile phone can be charged, without totally

depending on the intensity of sunlight available at that instance. In the actual construction of the

circuit, the following optional components were added for the sake of flexibility: 2 switches

SW1 and SW2, added to each upper diagonal extreme of the circuit and a 2A fuse between

switch SW2 and the output jack J1. This means that there is a switch SW1 immediately after the

photocell PC1 (the positive terminal of the cell) to stop the cell from supplying current

indefinitely to the battery at all times, thereby avoiding overcharging of battery. Another switch

SW2 is placed between resistor R6 and the output jack J1 to avoid indefinite supply of current as

long as a phone is connected to the output terminal. A 2A fuse is further added between switch

SW2 and the output jack J1 to disconnect the load from the circuit in case of excess current

exceeding 2A. Additionally, a light emitting diode may be installed close to the positive terminal

of the output to light up when SW2 is in the ON position.

28

Fig 3.1 – Schematic of a solar mobile phone charger (with battery)

The circuit shown above uses a small 3 volt solar cell to charge a 6 volt NiCad/NiMH battery

pack which, in turn, may be used to charge many models of cell phones and other portable

devices. The circuit "scavenges" energy from the solar cell by keeping it loaded near 1.5 volts

(maximum energy transfer value) and trickle charges the internal battery pack with current

pulses. The simple circuit isn't the most efficient possible but it manages a respectable 70% at

100 mA from the cell and 30% when the cell is providing only 25 mA which is actually pretty

good without going to a lot more trouble or using more exotic components.

3.1 MODE OF OPERATION:

When the voltage on the emitter of Q1 rises a little over 1.5 volts, both transistors turn on

quickly, snapping on due to the positive feedback through R5 and C2. The current increases in

L1 through Q2 until the voltage across the cell drops somewhat below 1.5 volts. The circuit then

switches off quickly and the voltage on the collector of Q2 jumps up, turning on D1, allowing

the inductor current to flow into the battery. Once the inductor has discharged into the battery,

the process starts over. The circuit can charge higher voltage batteries without any circuit

changes since the voltage will jump up quite high on the collector when the transistors turn off.

The circuit should not be operated without a battery attached. For a little more efficiency,

29

increase R5 in proportion to the voltage increase on the battery. (For example, double R5 for

charging a 12 volt battery.) A NiCad instead of a NiMH battery is preferable because they are

particularly forgiving of overcharging, simply converting the excess current into heat.

LIST OF REQUIRED CIRCUIT COMPONENTS

Ref. Description

PC1 3 volt solar cell from a sidewalk solar light

C1 22 µF, 10 volt (values not critical)

C2 100 pF, any voltage or type, typically ceramic

C310 µF, 16 volt or more for higher voltage battery

R1 1.5 k, any type

R2 3.9k, any type

R3 10k, any type

R4 180 ohm, any type

R5 4.7k, any type

R6 10 ohm PTC (see text).

L1 50 to 300 µH (see text)

D11N5818 schottky rectifier, just about any will do.

Q1 2N4403, or similar

Q2 2N4401, or similar

J1 output jack

B1 6 volt NiCad/NiMH battery w/fuse

Fig 3.2 – List of required circuit components

The photocell can be salvaged from an inexpensive solar sidewalk illuminator, gotten from other

solar products or bought in the market and it should have an open-circuit voltage of about 3 volts

and supplies about 100 mA in bright sunlight. The circuit can handle more current but cells that

supply more than 250 mA should be avoided. The inductor has a low resistance winding but a

30

surprising number of cores will work fairly well. The value of inductance isn't critical, perhaps

between 40 and 300 µH and during proper operation there will be a pulse waveform on the

collector of Q2 with several 10s of microseconds period. This prototype operates at about 40 µS

as shown and the inductance measures about 50 µH.

For experimenting with cores or other circuit values, the NiCad/NiMH battery can be replaced

with a zener diode of the same voltage and replace the solar cell with a 3 volt power supply with

a series resistor, about 22 ohms to simulate moderate sun. Measure the current in the zener diode

and compare that power (zener diode current times zener diode voltage) to the power coming

from the power supply (3 volts times power supply current) to see how the circuit is doing. When

the power in the zener diode is over half the power from the supply, the inductor is good enough.

It is mandatory that a fuse be added near one of the terminals of the battery to shield the load

from excessive current supply should something go wrong. Battery packs can supply dangerous

current levels, so the lead from the fuse to the battery terminal must be kept as short as practical.

It is also better to prevent damage to the load by adding a fuse which can be easily replaced

anytime should anything go wrong, than eliminate the use of a fuse and subject the load to

possible damage.

In addition to the fuse a 10 ohm PTC was added in series with the output to limit the available

power but also to allow the unit to charge some special models of Nokia phone that in practice

do not like a very low impedance battery as a charging source. (The phone simply displays

"battery not charging".) The PTC is actually soldered directly to the copper board and one end

of the fuse connects directly to the top side (if a switch is used, then the 10 ohm PTC connects

directly to the switch and the switch in turn connects to the fuse).

The circuit works great! It can simply be left on the dashboard of a car to charge until needed.

Several Nokia phones can be charged with this circuit without a problem. It is actually more

convenient than a cigarette lighter adapter because it can travel with the phone and it doesn't

need sunlight to charge the phone.

It was noticed that the circuit charges phones suspiciously fast and it was thought that it might

not be a bad idea if the output resistance is increased. Fast charging cell phone batteries shorten

31

their life, if I understand correctly. The tradeoff is between fast charging the cell phone battery

and possibly shortening its overall life span or charging it more slowly (which increases the time

it takes to fully charge the cell phone especially when the phone battery is highly discharged) and

increasing the lifespan. Most phones have sophisticated internal charging circuits but it is

suspected that the manufacturers sacrifice battery life for fast charging. With the construction of

this charger, phones will not need to be significantly discharged due to lack of electricity supply

anymore.

32

CHAPTER FOUR

4.0 CONSTRUCTION, TESTING AND DISCUSSION

Before the actual soldering of circuit board components was done, some other tasks were

carried out to ensure success and proper operation of the components. All components (resistors,

capacitors, transistors, fuse and the diode) were tested, and measured values compared with

ratings to ensure they were as closely matched as possible. During this exercise, it was

discovered that some of the components were either not working at all or were not even the

required specifications. This timely discovery helped in conserving the time that might have been

wasted if they had been directly soldered onto the board without carrying out these initial test

and confirmation. Generally, it was a very tedious task getting the solar cell and the 2N4403

transistor in the market as they proved to be very scarce products that are not yet readily

available.

The construction started by first deciding how big the final design would be and this

made it necessary to choose what part of the Vero board would be used, how close to one

another the components would be and where the output jack would be located. It was thought

that having a general idea on the final outlook of the circuit and the component overlay would

make it easier to decide where and how a component should be installed and soldered to avoid

removing already soldered circuit components, which in turn would lead to an untidy

construction. After having a mental idea of the final circuit, capacitor C1 was installed, followed

by resistors R1 and R2, then transistor Q1. The component installation proceeded gradually,

while constantly making reference to the circuit diagram, watching out for short circuit, open and

wrong connections, till the output jack was installed. The output jack used was the output jack of

Nokia phones. It was thought that since this brand of phones was a very popular brand, it would

be wise to use a jack that would offer the possibility of reaching a wider audience. Although, the

output pins where the jack was connected were designed in such a way that they could

accommodate other types of phone models, by simply pulling out the jack in use, a new jack for

a different phone model can be connected, plugged in, and charged (polarities should be

carefully noted). The output jack design was made to be easily detachable by making it possible

33

to pull out the jack connector, replacing it with a desired phone model jack and plugging it back

in place. It was later thought wise to install two switches SW1 and SW2. SW1 was installed

between the positive terminal of the solar cell and the positive terminal of capacitor C1 to avoid

the solar cell from charging the battery pack indefinitely and SW2 was installed between resistor

R6 and the positive terminal of the output jack, to make it possible to disconnect the phone from

charging even when physically plugged in. A light emitting diode was further added to indicate

when SW2 is in the open or close state. Installing capacitor C2 and R5 which formed out positive

feedback path was not so easy, but after so much thoughtful efforts, success was achieved. The

type of battery used was a 6V NiMH (Nickel Metal Hydride) battery. In place of this, a NiCad

(Nickel Cadmium) battery could be used and this might even be a better option, following the

information gathered online, that NiCad batteries are particularly forgiving of overcharging, by

simply converting the excess charge into heat. If the negative electrode is

overcharged, hydrogen gas is produced, If the positive electrode is overcharged, oxygen gas is

produced. The last thing to be installed was the solar cell.

4.1 CONSTRUCTION OF THE CASING

The casing of the solar mobile phone charger was salvaged from the transparent plastic-

like casing of slightly flexible material. By cutting it into the required sizes to serve as an

enclosure for the circuit board, the different sizes were glued together and taped at the sides to

form a firm transparent casing. The inductor L1 used for the circuit was sealed and cylindrical in

shape. This initially served as a setback in the design of the casing but the top part of the casing

was later cut open to allow only the upper part of the inductor shoot out a little above the case.

To allow easy access to switches SW1 and SW2, the sides of the casing, restricting access to

these switches were cut open. The whole assembly was made compact and rigid, bearing in mind

that making the assembly too heavy would be a turn off when considering its mobility.

34

4.2 CIRCUIT ASSEMBLY TESTING

After installing every component, it was time to test the assembly. A Nokia phone was

connected to the output jack and nothing happened. The switches were confirmed to be closed

and the output voltage of the solar cell was measured and found to be within required range. It

was later discovered that the NiMH battery had naturally discharged over time and the measured

voltage was just 1.2V which was far too low to charge a mobile phone. The assembly was later

left in the sun for a very long period of time to ensure that the battery stored enough charge to

charge the mobile phone. A 6V speaker was tested and it was found to produce signs of current

flow, but when the Nokia phone was plugged, it simply displayed “battery not charging”. It was

later discovered that some models of Nokia phones generally in practice do not like a very low

impedance battery as a charging source and the 10 ohm PTC was added in series with the output

not only to limit the available power but also to allow the unit to charge those special models of

Nokia phone that don’t like a very low impedance charging source. (The phone simply displays

"battery not charging

After the tests, it was realized that what determined if the unit would charge a phone or not was

not the amount of sunlight or power output of the solar cell, but rather, the output power of the

attached NiMH battery. So if the battery becomes too low, the phone will not charge.

35

CHAPTER FIVE

5.0 SUMMARY AND CONCLUSION

With a project requiring design and construction of a circuit, although it’s been quite an

experience, bearing in mind that as at the time of carrying out this project, solar charging

generally has not been widely accepted, how much less, solar charging a mobile phone, it has

been proven that it is very possible, flexible and cost effective to charge mobile phones using

solar chargers, without totally relying on utility power. It is an obvious fact that the tropics enjoy

more sunlight round the year and fortunately, Nigeria is one of such countries located in the

tropics, which means there is abundant sunlight available for use all round the year. Another fact

that cannot be ignored, that serves as a major factor that encourages the use of solar mobile

chargers is the fact that at present Nigeria suffers greatly from gross unavailability of electricity.

With these facts in mind, it is seen that there is an urgent need to tap into the abundantly

available energy of the sun, not only in charging mobile phones, but in every other aspect of

technology that requires constant supply of electricity.

Some measures of precaution were taken and it would not be wise to conclude without

mentioning them. Circuit design and construction tasks require that components be installed with

the correct polarity observed and the positive terminal connected to the highest potential.

Whenever an equivalent component is incorporated into a circuit, even though there are readily

available alternatives, care should be taken that they have characteristics closely matched with

those required. Transistor legs are fragile and can easily break off, if twisted unnecessarily. More

so, since they are heat sensitive devices, heat produced by soldering iron should not be excessive

and the correct biasing rules should be borne in mind. Dry joints (circuit connections which

appear physically connected but core not electronically connected) should be eliminated as much

as possible since these can waste a lot of precious construction time and leads to unnecessary

troubleshooting of circuit. Extra caution should be taken to ensure that hot soldering leads do not

drop in between connecting foils on the circuit board as this can bridge circuit connections and

produce either a total deviation from expected result or damage to the design.

The task of producing a working solar mobile phone charger has not been as easy as

initially envisaged. There were hurdles of getting the required information, circuit diagrams,

36

buying the required circuit components, fear of destroying components that were not readily

available, soldering ethics etc.

In conclusion, it has been a worthwhile experience and the effort and time invested into

this design and construction has really paid off. It has revealed that solar energy is in abundance

and can be harnessed for use in a lot of ways, even to the point of charging mobile devices which

removes total dependence on frequently unavailable and highly erratic electric power supply

from utility grids. It has also showed that mobile charging is possible and by mobile charging,

what is meant is ‘charging while on the move’.

RECOMMENDATIONS

I want to recommend that solar mobile phone chargers should be designed and

constructed indigenously by both students (for educational purposes) and corporate bodies (for

commercial purposes)

Also, I want to recommend that more research effort should focus on harnessing the

abundant energy from the sun into various useful means.

I want to recommend that practical hands on electronic circuit design and construction be

introduced early enough in schools to stir up early interest.

37

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