Tải bản đầy đủ

Automotive mechanics (volume i)(part 6, chapter35) basic electrics

617-634_May_chap 35


11:02 AM

Page 617

Basics of the electrical system

35 Basic electrics
36 Effects and applications of electric currents
37 Basic electronics
38 The battery


617-634_May_chap 35


11:02 AM

Page 618

617-634_May_chap 35


11:02 AM

Page 619


Chapter 35

Basic electrics

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

617-634_May_chap 35


11:02 AM

Page 620

620 part six basics of the electrical system
A motor vehicle contains its own complete electrical
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 understanding 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
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

figure 35.1

components for an electronic fuel-injection system, an
alternator to provide electric energy and to charge the
battery, gauges and indicators to show engine conditions, 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 turnsignal 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 illustration 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.

The main components of an engine electrical system


617-634_May_chap 35


11:02 AM

Page 621

chapter thirty-five basic electrics

figure 35.2


Location of various parts of the electrical system

■ It is obvious that a motor vehicle could not operate
without electricity, and also obvious that an understanding 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 noticeable 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.

figure 35.3

Composition of matter – the diagram represents matter subdivided into its parts

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

617-634_May_chap 35


11:02 AM

Page 622

622 part six basics of the electrical system
the 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 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.

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

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).

figure 35.6

An atom of copper has many electrons in

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).

■ The rule is that like electrical charges attract and
unlike charges repel.
figure 35.7

In copper wire, there are many free electrons
moving from atom to atom

Electron flow

figure 35.5

Unlike electrical charges attract each other
while like charges repel each other

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.

617-634_May_chap 35


11:02 AM

Page 623

chapter thirty-five basic electrics


figure 35.8

When the switch is closed, electrons move
through the circuit from one battery cell
terminal to the other

Connecting 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
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
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)).

figure 35.9

Representation of electron flow and conventional current flow

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

Types of electrical materials
Various materials can be classified under a number of
types, according to their ability to conduct electricity.
Conductors are materials that contain a large number
of free electrons, that is, they readily allow current to

617-634_May_chap 35


11:02 AM

Page 624

624 part six basics of the electrical system
In 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.
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
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.

table 35.1

Comparison of the resistance of some
common materials



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 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
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.

















Sulphuric acid

850 000

Distilled water

72 000 000

figure 35.10

The construction of a large capacitor


Summary of basic electrics

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.

A summary of the main points of basic electrics is as

■ Insulators have their electrons tightly held, so that
no electron movement can take place.

3. Electrons are repelled by electrons, but electrons
are attracted by protons. That is, like charges repel

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.

617-634_May_chap 35


11:02 AM

Page 625

chapter thirty-five basic electrics


each 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
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. However, 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.

figure 35.11

Automotive wiring – cables are combined to
form a wiring harness with connectors to join
them to components DAIHATSU

these consist of a number of strands of thin copper
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

Practical conductors
All metals are conductors, but most of the conductors
in the motor vehicle are copper, which is a good
Cables are used to connect the various components of
the electrical system (Figure 35.11). The conductors in

figure 35.12

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.

Battery and starter cables – there are earth connections to the body and engine

617-634_May_chap 35


11:02 AM

Page 626

626 part six basics of the electrical system
Special 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 resistance depends mainly on the length and diameter of the
In the ignition system, carbon is used in spark-plug
cables and high-tension leads. Carbon resistors are
used in some voltage regulators.
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 35.14

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

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.

Rheostats – the resistance R can be varied by
moving the contact across the winding

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).

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.

figure 35.15

Resistance – the fuel tank gauge unit has a
variable resistance (rheostat) which is
operated by the float

617-634_May_chap 35


11:02 AM

Page 627

chapter thirty-five basic electrics


Carbon pile rheostat
This type of rheostat is found in some workshop
instruments and is used mainly to control high currents.
It consists of a number of carbon discs, which are piled
one on top of the other. A knob and screw thread are
arranged to clamp the discs together (Figure 35.16).

figure 35.16

Carbon-pile rheostat

It operates on the principle that when the discs are
pressed firmly together, there will be less resistance
than when they are lightly in contact. In this way, the
resistance can be varied by adjusting the knob.

figure 35.17

A spark plug has conductors with a ceramic

Practical insulators
1. Plastics. Most cables are insulated with a covering
of soft plastic, with various colours being used to
identify the different cables in a circuit.
2. Varnish. A coating of varnish or lacquer is used on
the copper wire that is used for the windings of
coils, although cotton covering is sometimes used.
3. Ceramics. Ceramic is used as the insulator for spark
plugs. This material has the capacity to resist the
high temperatures of combustion to which spark
plugs are subjected (Figure 35.17).
4. Mica. This is another material which is sometimes
used where an insulator is needed that will
withstand heat, although it has been replaced by
resins and fibreglass.
5. Glass. Glass acts as an insulator in bulbs and also in
some fuses.
6. Fibre and nylon. These materials and also hard
plastics are used as washers and insulators on
various electrical components. Fibreboard and
fibreglass are used for printed circuit boards.

Factors affecting current flow
All conductors, even good ones, have some resistance,
and for each conductor, this is shown by its resistivity
(Table 35.1). Resistivity is the opposite to conductivity, which relates to the ability of a conductor to
carry current.

There are four factors which can affect the
resistance of a conductor and the current that it carries:
1. The material of the conductor. Some materials have
more resistance to electron flow than others.
2. The cross-sectional area of the conductor. A large
cross-section will have many more electrons that
are able to move through it at the same time, while
a small cross-sectional area will allow relatively
few electrons to move at the same time.
3. The length of the conductor. The longer the
conductor, the higher the resistance. As an
explanation of this, it can be considered that in a
long conductor, electron movement occurs from
atom to atom over a great distance, and as a result,
this causes resistance to the electron movement.
In short conductors, movement of the electrons
from one end to the other is relatively easy, as they
have to pass through very few atoms. Short
conductors, therefore, have little resistance.
4. The temperature of the conductor. With many
materials, the higher the temperature, the greater
the resistance, so heat has an effect on electron

The language of electricity
To discuss electricity and its behaviour, it is necessary
to know the language, or terms, that are used for
the various electrical units, and also to understand the

617-634_May_chap 35


11:02 AM

Page 628

628 part six basics of the electrical system
relationship of one unit to another. The three common
units are:
1. Ampere. Measures the rate of current flow (symbol
A or I).
2. Volt. Measures electrical pressure (symbol V or E).
3. Ohm. Measures electrical resistance (symbol W
or R).

Ohm’s law
Ohm’s law states the relationship between the three
basic electrical units. It simply says that: a pressure of
one volt will cause one amp to flow through a
conductor that has a resistance of one ohm.
This can be stated as a simple equation, using V for
volts, I for current and R for resistance:
V = IR (1 volt = 1 amp ¥ 1 ohm)

Current (amps)
Amps (or amperes) are a measure of the quantity of
current flowing per second, or more simply, the rate at
which current flows. When relatively few electrons
flow in a conductor, then the amperage is low. When
many electrons flow, then the amperage is high.

or by rearranging, the resistance (R) will be:
R = ᎏ (1 ohm = 1 volt Ϭ 1 amp)

and the current flow (I) will be:
I = ᎏ (1 amp = 1 volt Ϭ 1 ohm)

Voltage (volts)
A force is needed before the electrons will flow
through a conductor. The force is provided by a
concentration of electrons at the terminal of an
alternator or battery. When either of these is connected
to a circuit, then the concentration of electrons at the
negative terminal will cause movement of the electrons
in the conductor and produce a current flow.
Therefore, voltage represents a difference in electrical
potential, or an electrical pressure.
Voltage is measured with a voltmeter (Figure 35.18).

An easy way to recall these is by the use of the
diagram shown in Figure 35.19. Placing the finger to
cover the unit required will show the other two units of
the equation. For example, if it is desired to find the
equation for current (amps), the finger is placed on I to
give the equation:
I= ᎏ

figure 35.19

figure 35.18

A voltmeter in an instrument panel can keep
a check of battery voltage HYUNDAI

Resistance (ohms)
All materials, even good conductors, have some
resistance to electron flow. Carbon and certain special
alloys have a very high resistance to electron flow and
are known as resistors. Therefore, the rate of current
flow in a conductor will depend on its resistance, as
well as the voltage (pressure) applied to the conductor.

The relationship of voltage (V ), current (I )
and resistance (R )

By way of demonstration, if 2 volts from a battery
are applied to a resistance of 2 ohms, then from the
equation, the current flow will be found to be 1 amp.
If the voltage is now doubled, so that 4 volts are
applied to the resistance, then the current will also be
doubled, which gives 2 amps.
■ This shows that any increase in voltage will
produce a corresponding increase in current
Ohm’s law applied to vehicles
The following illustrate the application of Ohm’s law
to the headlamp of a motor vehicle.

617-634_May_chap 35


11:03 AM

Page 629

chapter thirty-five basic electrics


Example 1
If a vehicle headlamp bulb has a resistance of 2 ohms,
what current will flow when it is connected to a 12 volt
The appropriate equation is:
I= ᎏ
= ᎏ
= 6 amps

Example 2
If the headlamps of a vehicle using a 12 volt battery
have a current of 4 amps, what would be the resistance
of the headlamps?
The appropriate equation is:

figure 35.20

Headlamp assembly – the headlamp has a
65 watt bulb to provide bright light; the turnsignal lamp has a 25 watt bulb HOLDEN LTD

R= ᎏ
= ᎏ
= 3 ohms

Example 3
What voltage battery would be required to give a
current flow of 2 amps through a resistance of 6 ohms?
T The appropriate equation is:
V = IR = 2 ¥ 6 = 12 volts

Power (watts)
A watt is the unit of electrical power and this is found
by multiplying together the voltage and the current.
The power of any electrical component is expressed in
watts. The bulbs, such as those shown in Figure 35.20,
are a common component that have their power shown
in watts.
The wattage of bulbs is illustrated in Figure 35.21,
which has a circuit with three different-sized bulbs
connected in parallel. All the bulbs are connected to
the same 12 volt battery, but because they all have a
different wattage, they will draw different currents.
This is shown on the ammeter connected into the
circuit for each bulb. It can be seen that:
1. The smallest bulb has a current of 1 amp and is
therefore a 12 watt bulb (12 volts ¥ 1 amp =
12 watts).
2. The middle bulb is larger and draws 2 amps from

figure 35.21

Bulbs of different wattage connected to a
battery – each draws a different current

the 12 volt battery, so must have twice the wattage
of the previous bulb. It is therefore a 24 watt bulb.
3. The largest bulb draws 3 amps and is therefore a
36 watt bulb.
Resistance of the bulbs
The resistance of the bulbs can also be found by using
Ohm’s law (R = V/I). The small bulb which draws
1 amp has a resistance of 12 ohms, the middle-sized
bulb 6 ohms, and the largest bulb 4 ohms. Think about
the electrical units and how they are related, for
example, the relationship of resistance and current.

617-634_May_chap 35


11:03 AM

Page 630

630 part six basics of the electrical system
■ The bulb with the lowest resistance draws the
highest current and produces the brightest light.

Electrical circuits
An electrical circuit is a closed path through which
current (electrons) can flow.
A practical circuit has conductors (wires) that carry
electrical energy supplied by the battery or alternator,
to an electrical device such as lights, the starter motor,
the fuel pump, a fan or some other electrical device
that performs a useful service.
The complete circuit will therefore contain:
1. A source of supply – the battery or alternator.
2. A consuming device, or load – lamps, coil etc, all of
which have some resistance.
3. Cables and connectors – cables to conduct the
current and terminals to make the connections.
4. Control devices – switches, relays, rheostats etc,
which may be operated manually, electrically, or
5. Protective and safety devices – fuses and circuit
breakers (most circuits).
A motor vehicle has many circuits such as these,
with wires running to all parts of the vehicle.
Connecting instruments
Test instruments can be connected to a circuit to
measuring the rate of current flow (ammeter) or the
electrical pressure (voltmeter). It is essential that the
instruments are connected to the circuit in the correct
Figure 35.22 illustrates a simple circuit with an
ammeter and a voltmeter. Note that the ammeter
actually forms part of the circuit because it is joined
into the circuit to measure the current that flows

through it. The voltmeter is connected across the
battery terminals to show the battery voltage
(pressure). It is not connected directly into the circuit
like the ammeter.
■ Ammeters are connected into the circuit, but voltmeters are connected across the circuit or parts of
the circuit.

Parallel and series connections
Electrical components can be connected in series or in
parallel. When connected in series, the components
are end-on to each other. When connected in parallel,
they are, in effect, beside each other.
Figure 35.23 shows simple electrical circuits with
bulbs connected to a battery in a number of different
ways. Diagrams (a), (b) and (c) are series circuits,
while (d) is a parallel circuit.
The way in which the bulbs are connected will
affect the light produced by the bulbs as follows:
1. A single bulb is connected to the battery (Figure
35.23(a)). The bulb receives full battery voltage
and so will glow brightly. This is a simple series
2. Two bulbs are connected in series and connected to
the battery (Figure 35.23(b)). Put simply, the bulbs
now have to share the battery voltage, so neither of
them will be bright. They could produce a dull
glow, but this will depend on the size of the bulbs.
3. Three bulbs are in series and so there is even less
voltage for each (Figure 35.23(c)). None of the
bulbs will receive sufficient voltage to produce
4. Three bulbs are connected in parallel and are
connected to the battery (Figure 35.23(d)). With
this circuit, all three bulbs will receive full battery
voltage and all will glow brightly. If they are of the
same size and type, they will all be equally bright.
Any number of bulbs can be connected in
parallel. They will all receive full battery voltage
and so will produce their full brightness.
■ Automotive lights have parallel circuits.
Series circuits

figure 35.22

Simple electrical circuit with a bulb, an
ammeter (A) and a voltmeter (V)

A series circuit is one that has only one path through
which current can flow, even though a number of
components are connected together to form the
complete circuit.

617-634_May_chap 35


11:03 AM

Page 631

chapter thirty-five basic electrics

figure 35.25

figure 35.23

Bulbs connected in different ways
(a) single bulb, (b) two bulbs in series,
(c) three bulbs in series, (d) three bulbs in parallel


Simple parallel circuit – three resistors are
shown connected in parallel

will operate independently and will carry its share of
current from the battery.
To find the total current flowing from the battery,
it is only necessary to find the current for each branch
and add these together.
Using the formula (I = V/R) it is found that the
1 ohm branch will carry 12 amps, the 2 ohm branch
(with twice the resistance) will carry only half this
amount or 6 amps, and the 3 ohm branch (with three
times the resistance) will carry one-third of the current
of the 1 ohm resistor, or 4 amps. The total current for
the complete circuit is therefore 22 amps.
■ Refer also to the current flow in the parallel circuit
in Figure 35.21.

figure 35.24

Simple series circuit – three resistors are
shown connected in series to a 12 volt battery

Electron flow in the circuit

A simple series circuit is shown in Figure 35.24
with three resistors, but they could be any type of
electrical component.
The resistors are connected to a 12 volt battery,
end-on to each other, so that each resistor will help to
reduce the rate at which the current flows. They may
be considered as being a single resistor, and their
resistances are added together to obtain the total
resistance. In the figure shown, the total resistance is
6 ohms and the total current flow will be 2 amps.

If electron flow is considered in relation to Figure
33.25, then electrons will flow from the battery
through the circuit. When a junction is reached, some
electrons will continue on, and some will pass through
the branch.
If the branches have different resistances as shown,
each will carry its share of the current. More electrons
will flow through some branches than others. The one
with the least resistance will carry the most electrons,
and the one with the most resistance will carry the least

Parallel circuits

Motor vehicle circuits

The various parts of a circuit can also be arranged
parallel to each other, as shown in Figure 35.25.
When the connection to the battery is made, current
will flow through each part of the circuit. Each resistor

Many of the electrical circuits in a motor vehicle are
parallel circuits because, in most cases, their
components are connected in parallel within the source
of voltage.

617-634_May_chap 35


11:03 AM

Page 632

632 part six basics of the electrical system
The headlamps, for example, are not connected to
each other, although they have a common switch. Each
headlamp is connected in parallel with the battery and
each receives full battery voltage. If one headlamp
fails, the other will continue to function.
Some circuits have their components connected in
series within the circuit, even though they are parallel
circuits. For example, the windscreen wiper has a
fuse, switch and wiper motor connected in series. The
wiper motor is the only part of the circuit that draws
■ While the circuit is parallel, components in
the circuit, such as switches and controls, are
connected in series within the circuit.

Voltage drop in a circuit
In a series circuit, any component that has resistance
will cause a voltage drop. If a voltmeter is connected
across a resistor, then it will show a voltage reading.
This represents the drop in voltage (or simply the
difference in pressure or potential) between the ends of
the resistor.
As an explanation of this, consider that the voltage
is expended (used up) in forcing the electrons to flow
through the resistance. Therefore, if a voltage drop is
shown as 2 volts, then these 2 volts have been
expended in forcing electrons to flow through that part
of the circuit.
A law relating to voltage drop states that: the total
voltage in a circuit is equal to the sum of the voltage
drops around the circuit.
This law is illustrated in Figure 35.26, which shows
voltmeters connected to measure the voltage drop
across three resistors. If the voltmeter readings are
added together, then this will be equal to the voltage
applied to the circuit (12 volts).

Voltmeter readings
From the information shown on Figure 35.26, the
readings of the voltmeters (voltage drops) can be
found. The total resistance is 6 ohms, so the current
will be 2 amps (again using the formula I = V/R). This
current will flow through all three resistors.
For the 1 ohm resistor, with a current of 2 amps, the
voltage drop will be 2 volts (V = I/R). For the 2 ohm
resistor, it will be twice this (4 volts) and for the 3 ohm
resister it will be three times (6 volts). These add up to
12 volts, which is the battery voltage.
Practical application of voltage drop
Excessive voltage drop in a headlamp circuit, for
example, will result in low voltage being available at
the headlamps, and dim headlamps will result. The
defective part, or parts, of the circuit can often be
located by using a voltmeter and checking to find the
poor earth or similar bad connection that has unwanted
resistance that is causing excessive voltage drop.
Figure 35.27 shows how a voltmeter can be
connected across a switch to measure voltage drop.
The meter must be connected with the correct polarity,
that is, positive to positive and negative to negative.
The leads of the meter are usually coloured to show
Some voltage drop in circuits is unavoidable,
mainly because of the size and length of the cable and
the connections in the circuit.



figure 35.26

Voltmeters connected across resistors to
measure the voltage drop – the readings on
the three voltmeters, when added together, will be 12 volts

figure 35.27

Using a voltmeter to check the voltage drop
of a switch HYUNDAI

617-634_May_chap 35


11:03 AM

Page 633

chapter thirty-five basic electrics

Technical terms
Engine electrics, body electrics, electrical
components, atoms, electrons, negative charge,
positive charge, electron flow, current flow,
conventional current flow, conductor, semiconductor, resistor, resistance, insulator, capacitor,
cable, wire-wound resistor, carbon resistor, rheostat,
resistivity, ceramic, amp, ampere, volt, voltage,
ohm, watt, Ohm’s law, electrical circuit, parallel
circuit, series circuit, printed circuit, voltage drop.


What is a conductor?


What is an insulator?


Name some of the common insulators.


What is a resistor?


What is the purpose of a rheostat?


Explain Ohm’s law in simple terms.


If the voltage of a circuit is increased, what
effect will this have on the current?


What is meant by a series circuit?


Explain how three resistors can be connected in


If the resistors shown in Figure 35.25 were to be
2 ohms, 4 ohms and 6 ohms, what current would
flow in the circuit?


Sketch a series circuit with an ammeter, three
resistors (4, 6 and 8 ohms) and a battery.


If the ammeter in question 17 showed 2 amps,
what would be the battery voltage?

Review questions

What are the main parts of the engine electrical


Name the various components of a motor
vehicle that depend on electricity for their



What is an electron?


What is meant by a free electron?


What is voltage drop?


Explain briefly how electrons move in a copper


What is a practical application of voltage drop?



What is the conventional direction of current

What function does the metal of the body
perform in the vehicle’s electrical system?

617-634_May_chap 35


11:03 AM

Page 634

Tài liệu bạn tìm kiếm đã sẵn sàng tải về

Tải bản đầy đủ ngay