Basic electrics 619 Automotive electrical components 620 Nature of electricity 621 Electron flow 622 Current flow 623 Types of electrical materials 623 Summary of basic electrics 624 Practical conductors, resistors and insulators 625 Factors affecting current flow 627 The language of electricity 627 Electrical circuits 630 Parallel and series connections 630 Voltage drop in a circuit 632 Technical terms 633 Review questions 633
Trang 1Basics of the electrical system
6
PART
35 Basic electrics
36 Effects and applications of electric currents
37 Basic electronics
38 The battery
Trang 3Basic electrics
Chapter 35
Automotive electrical components
Nature of electricity
Electron flow
Current flow
Types of electrical materials
Summary of basic electrics
Practical conductors, resistors and insulators
Factors affecting current flow
The language of electricity
Electrical circuits
Parallel and series connections
Voltage drop in a circuit
Technical terms
Review questions
Trang 4A motor vehicle contains its own complete electrical
system.
This stores electrical energy when the engine is
stopped It provides electricity to start the engine and
to keep it running It generates electricity once the
engine is running and distributes it to various parts of
the vehicle It operates a wide variety of electrical and
electronic devices.
This chapter deals with electrical fundamentals
and how they apply to motor vehicles An
under-standing of these is important when servicing electrical
components and systems.
Automotive electrical components
The electrical system of a motor vehicle can be
generally divided into engine electrics and body
electrics Some electrical components belong with the
engine electrics and others form part of the body
electrics.
Engine electrics include the starter to turn the
engine during starting, an ignition system (petrol
engine) to start the engine and keep it running by
providing an electric spark, a computer and other
components for an electronic fuel-injection system, an alternator to provide electric energy and to charge the battery, gauges and indicators to show engine con-ditions, as well as many other electrical devices The main engine components are shown in Figure 35.1.
Body electrics include lights to enable the vehicle
to be operated at night, wipers and heaters to keep the windscreen and rear window clean, horn and turn-signal indicators for safety, audio systems for entertainment, window winders and mirror controls for convenience, air-conditioner controls for comfort, and also many small but important devices such as switches, fuses, connectors and relays.
As well as all this, there is a complete system of wiring to connect all these parts.
Component locations
Figure 35.2 shows the general location of various electrical components of a passenger car The illus-tration provides an indication of the range of electrical components that are used, and highlights the importance of the electrical system in the operation of the motor vehicle.
figure 35.1 The main components of an engine electrical system HYUNDAI
Trang 5■ It is obvious that a motor vehicle could not operate
without electricity, and also obvious that an
under-standing of electrical fundamentals is essential.
Nature of electricity
Unfortunately, it is not possible to obtain a piece of
electricity and examine it to see how it operates, as can
be done with a mechanical part Therefore, electricity
must be approached in a different manner It is helpful
to first think of its origin and then to consider its
effects While electricity itself is not normally
notice-able to many of our senses (touch, sight or smell), its
effects, in most cases, can be readily observed.
Molecules, atoms and electrons
In order to understand the basic principles of
electricity, it is necessary to consider briefly the
composition of matter Matter is anything which we
know to exist and includes liquids, solids and gases.
All matter is made up of very minute particles
called molecules If a molecule of any material is
divided into its parts, then atoms will be obtained If an
atom was to be further subdivided, then it would be
found to be composed of three different, infinitely
small, particles: electrons, protons and neutrons
(Figure 35.3) The electrons have a small negative (–)
electrical charge, the protons have a small positive (+)
electrical charge and the neutrons have no charge.
All materials have atoms
All materials have atoms with electrons and protons, but these are arranged differently in different materials.
In fact, it is the arrangement of the electrons and protons within one atom which makes it different from another That is, materials are different only because of the basic structure of their atoms.
Figures 35.4 and 35.6 illustrate the atoms of two different materials and show the positive and negative charges These are minute charges, by themselves having no effect However, if a number of these can be caused to move, then their effect will be noticeable and this can be used for useful purposes.
Electrons are free to move
Electrons (negative charges) are already free to move within their own atoms, while the protons (positive charges) are fixed in the centre, or nucleus, of the atom The electron moves in an orbit around the nucleus, being attracted to it and held in orbit by
figure 35.2 Location of various parts of the electrical system
figure 35.3 Composition of matter – the diagram
repre-sents matter subdivided into its parts
Trang 6the attraction between its negative charge and the
positive charges in the nucleus.
Figure 35.4 represents an atom of hydrogen This is
a simple atom, with only one proton in the nucleus and
one electron in orbit moving around the nucleus.
The electron can be likened to a ball on a string
being swung around by hand The ball orbits the hand,
kept in the circular path by the string This is similar to
the electron (negative charge) being attracted and held
in orbit by the proton (positive charge).
It is seen from the above that a positive charge
attracts a negative charge, and it follows on from this
that a negative charge repels a negative charge In
other words, electrons repel each other, and electrons
and protons attract each other (Figure 35.5).
■ The rule is that like electrical charges attract and
unlike charges repel.
The atom of hydrogen, just considered, is the simplest form of atom It possesses only one free electron Other materials possess many more than this and, in some instances, the electrons orbit at a greater distance from the nucleus.
Electrons in copper
In copper, which is used extensively in electrical systems, there are many electrons in orbit, some being further from the nucleus than others (Figure 35.6) As a result of this, the outer electrons are very loosely held.
This allows free electron movement, and in a piece
of copper wire some electrons would be moving at random between the atoms in the copper at all times These are referred to as free electrons An atom may lose one of its free electrons, only to gain another from
an adjacent atom (Figure 35.7).
figure 35.4 An atom of hydrogen consists of a nucleus
with one proton (positive charge) and one electron (negative charge) – the electron circles or orbits
the proton like a ball on a string
figure 35.5 Unlike electrical charges attract each other
while like charges repel each other
figure 35.6 An atom of copper has many electrons in
orbit
figure 35.7 In copper wire, there are many free electrons
moving from atom to atom
Electron flow
If a piece of copper wire was to be connected across the terminals of a battery (Figure 35.8), then the free electrons in the copper wire would move in a regulated manner The negative battery terminal has a surplus of electrons (due to chemical action within the battery) while the positive battery terminal has protons and a shortage of electrons.
Trang 7Connecting the wire to the battery causes the
electrons at the negative battery terminal to force
against the free electrons in the wire, so that electrons
move from atom to atom within the wire This is
referred to as electron flow and is in a direction from
negative to positive.
■ The arrangement would need to include a bulb or
similar load to prevent excess flow and heat.
Understanding electron flow
To understand electron flow, imagine that a copper
wire consists of a number of atoms stretched along its
length.
An electron, on entering the wire from the battery,
repels an electron from the outer orbit of the first atom.
The electron from the battery is captured by the first
atom, while the displaced electron attaches itself to the
second atom, displacing another electron in order to do
so This displaced electron does the same thing to the
third atom and so on throughout the length of the wire.
The overall effect is a movement of electrons from
atom to atom through the wire This occurs many times
to many electrons to produce a flow of electrons, as
shown in Figure 35.9(a) and 35.9(b).
Current flow
Electron flow was shown to be from negative to
positive, and this can be considered to be a flow of
current.
However, long before electron flow was
understood, it was believed that current flowed from
positive to negative, that is, in the opposite direction to
the electrons Many rules were designed to suit this
direction of current flow and are still used For
automotive electrics, it is very convenient to use this
original direction of current flow that is positive (+) to
negative (–) This is often referred to as conventional
current flow (Figure 35.9(c)).
Most workshop manuals use conventional current flow, and for this reason, conventional current flow (positive to negative) will be used here.
Another reason is that the negative terminal of the battery is connected to the metal bodywork of the vehicle, which forms part of the electrical circuit It is therefore much easier to follow current flow in circuits from the positive (+) side of the battery to the negative (–) or earthed side, than to try to follow the flow of electrons.
Types of electrical materials
Various materials can be classified under a number of types, according to their ability to conduct electricity.
Conductors
Conductors are materials that contain a large number
of free electrons, that is, they readily allow current to flow.
figure 35.8 When the switch is closed, electrons move
through the circuit from one battery cell terminal to the other
figure 35.9 Representation of electron flow and
conven-tional current flow
Trang 8In some materials, the electrons are tightly held,
while in other materials (such as copper), they are
loosely held The materials with loosely held electrons
will be good conductors, while those with tightly held
electrons will be poor conductors.
Resistors
Poor conductors are referred to as resistors because
they resist electron movement They are usually used
for special purposes where it is required to reduce or
limit current flow Most metals are good conductors,
but special metal alloys and carbon are used for
resistors.
Table 35.1 shows a list of some common materials
and their resistivity This is a resistance value, shown
for the purpose of comparison Materials with a low
resistivity are classed as conductors, while those with a
high resistivity are classed as resistors.
Semiconductors
Certain unusual materials, such as germanium and silicon, are halfway between conductors and insulators These normally act as insulators, but will conduct under certain conditions They are referred to as semiconductors, being used in electronic components, such as transistors, diodes, and for special purposes These are discussed later in Chapter 37.
Capacitors
Capacitors are neither conductors nor insulators, although they often consist of both types of materials Large capacitors, such as those used with ignition systems and for noise suppression, are made of two strips of metal foil These are separated by specially treated paper, which acts as an insulator (Figure 35.10) Very small capacitors are used in electronic systems, but these are usually made of semiconductor materials.
Capacitors are used to hold electrical charges and prevent voltage surges The large plate area provides a form of reservoir into which electrons can flow, and this dampens any surge of voltage.
table 35.1 Comparison of the resistance of some
common materials
C
Co ondu uc ctto orrs s
R
Re es siis stto orrs s
Insulators
Another group of materials are those that will not
conduct electrons These are referred to as insulators.
They are very useful as they can be used to insulate (or
separate) one conductor from another, for example
plastic coatings on copper wire.
This group of materials includes almost all common
materials other than metals.
■ Insulators have their electrons tightly held, so that
no electron movement can take place.
figure 35.10 The construction of a large capacitor
Summary of basic electrics
A summary of the main points of basic electrics is as follows:
1 Electrons are negative (–) electrical charges and protons are positive (+) electrical charges They are present in all matter.
2 Electrons can move from atom to atom within the material This is referred to as electron flow.
3 Electrons are repelled by electrons, but electrons are attracted by protons That is, like charges repel
Trang 9each other, while unlike charges attract This causes
electron movement.
4 Conductors allow electrons to move freely That is,
they are materials through which a current will
flow.
5 Resistors allow electrons to move, but not as freely
as through other conductors They tend to resist
electron movement and so reduce current flow.
6 Insulators are materials through which electrons do
not move They are therefore non-conductors of
electrical current and are used to separate materials
which are conductors.
7 Electron flow is from negative to positive
How-ever, for practical purposes, current flow is
considered to be from positive to negative.
Practical conductors,
resistors and insulators
Conductors, resistors and insulators are used in various
automotive electrical components and also in electrical
measuring and testing equipment.
Practical conductors
All metals are conductors, but most of the conductors
in the motor vehicle are copper, which is a good
conductor.
Cables
Cables are used to connect the various components of
the electrical system (Figure 35.11) The conductors in
these consist of a number of strands of thin copper wire.
Where large currents are carried, large-diameter cables are used Battery and starter cables can be
10 mm in diameter, while lighting cables, which carry much lower currents, can be 3 mm in diameter Battery and starter cables are kept as short as practicable to avoid unnecessary resistance (Figure 35.12).
Vehicle body
The vehicle body and frame are used as an earth or ground for the electrical system, and so become a common conductor for the various electrical circuits The metal parts of the vehicle form such a large conductor that, for practical purposes, they have no resistance However, all the electrical connections to the body or frame (earths) must be clean and tight
to prevent resistance at the connection.
figure 35.11 Automotive wiring – cables are combined to
form a wiring harness with connectors to join them to components DAIHATSU
figure 35.12 Battery and starter cables – there are earth connections to the body and engine
Trang 10Special metals
Special metals are used for particular applications.
Tungsten is used for some contact points in distributors
and relays where arcing could occur Special alloy
steels are used for spark plug electrodes.
Some contacts used for particular purposes, such as
air-bag sensors, are gold plated to prevent corrosion.
Printed circuits
Printed circuits are used extensively in electronic units.
They are also used for vehicle instrument panels
(Figure 35.13).
Carbon resistors
These consist of a cylindrical piece of carbon, with a terminal or wire attached to each end The total resis-tance depends mainly on the length and diameter of the cross-section.
In the ignition system, carbon is used in spark-plug cables and high-tension leads Carbon resistors are used in some voltage regulators.
Rheostats
A rheostat is basically a variable resistor with a sliding
or rotary contact that moves across a wire-wound resistor The resistance between the end of the wire and the contact can be varied in this way (Figure 35.14).
figure 35.13 An instrument panel printed-circuit board – it
is connected to the electrical system by a plug and socket and normal copper-wire conductors MITSUBISHI
figure 35.14 Rheostats – the resistance R can be varied by
moving the contact across the winding
figure 35.15 Resistance – the fuel tank gauge unit has a
variable resistance (rheostat) which is operated by the float
Printed circuits do not have separate electrical
cables, instead, they have metal conductors printed on
an insulating board The conductors are not covered
with insulation, but they are insulated from each other
by the space between them Small components are
soldered to the conductors.
Their advantages, compared with using individual
wires, are compactness for electronic units, and lower
cost of production and installation.
Practical resistors
Wire-wound resistors
These consist of a number of turns of special alloy
wire, of high resistance, wound on a former of mica or
ceramic material These are used in some alternator
voltage regulators and also in electrical instruments.
The total resistance depends on the type of material
and the thickness and length of the wire.
The instrument panel light circuit is one of the places where a rotary rheostat is used The resistance
in the circuit is varied by turning a knob, so that the current through the bulb can be either reduced or increased Reducing the resistance increases the brightness of the lamps and vice versa.
A rheostat is also used in the fuel tank to operate most fuel gauges, but this is a sliding type rheostat (Figure 35.15).