Electrics Training 2w5c8
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TRAINING TIPS & TACTICS
Module 2 Electrical Principles
1
2
Contents
3
4
Introduction About this Pocket-guide
This guide is intended as a memory jogger for the knowledge you have gained during your training course. The guide includes a summary of the material covered in: Module 2 - Electrical Principles.
AC/DC
Note that all the material in this guide relates to DC circuits.
5
Danger, Warning, Caution & Note
In this guide, risk of injury or damage is indicated by the following headings: DANGER - indicates a risk of serious personal injury or death. WARNING - indicates risk of personal injury, or severe product damage. CAUTION - indicates risk of product damage. Note - draws attention to special methods or particular features.
Pay attention to these headings
Read and implement all DANGER, WARNING and CAUTION instructions.
6
Replacement parts
When replacement parts are required, it is essential that only Volvo genuine parts are fitted. If Volvo genuine parts are not used: - safety features embodied in the vehicle or components may be impaired. - performance and/or operation of the vehicle or components may be adversely affected - Volvo warranty may be invalidated.
7
Specification
Volvo are constantly seeking ways to improve their products, and alterations take place accordingly. Whilst every effort has been made to ensure the accuracy of this guide, it should not be regarded as an infallible guide to current specifications of any particular product. Neither Volvo, nor the supplier of this book shall, in any circumstances, be held liable for inaccuracy or the consequences thereof.
Copyright
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, transmitted or copied without written permission from Volvo Truck & Bus Ltd.
c
Volvo Truck & Bus Ltd 2004
8
Atoms 9
Electron
Proton Neutron
Nucleus
Shell or orbit
10
Matter
Element
Compound
Molecule
Matter is anything that occupies space and has weight. Matter can be solid, liquid and gaseous. An element is a substance which cannot be reduced to a simpler substance by chemical means. Examples are: gold, silver, iron, copper and oxygen. There are approx.100 known elements, and everything around us is made up of one or more of these elements. When two or more elements are chemically combined, the resulting mixture is called a compound. A molecule is a chemical combination of two or more atoms. In a compound,the molecule is the smalest particle thsat has all the characteristics of the compoond.
11
12
Atoms are everywhere
Molecules are made up of atoms. Atoms are extremely small - if one could be magnified a million times, it would only just be the size of the full stop at the end of this sentence. An atom is the smallest particle of an element that retains all the characteristics of the element. The atoms of one element differ from the atoms of all other elements Atoms are made up of only three types of particle protons, electrons and neutrons. All protons are exactly the same, all neutrons are exactly the same and all electrons are exactly the same. Since there are approx.100 known elements, so there are approx.100 kinds of atom in the entire universe. Whether these atoms make up water, ashes the air that we breathe, trees or tyres depends on how they are put together - i.e. the number and arrangement of the particles - their structure. 13
Electron
Proton Neutron
Nucleus
Shell or orbit
14
Atoms, and their particles exist by nature in every kind of material around us. Neither atoms nor their particles can be manufactured or destroyed. Nucleus
The neutrons and protons are in the centre of the atom in a nucleus, which has by far the greatest mass of the atom. Most of the atom is empty space. Protons and neutrons have almost exactly the same mass. Electrons have a mass about 1/1880 the mass of of a proton - e.g. in a person of 150 lb weight, 149lb 15oz are protons and neutrons, 1oz are electrons.
Positive r o t o n
Protons have a positive electrical charge.
Neutrons have no electrical charge.
15
Electron
Proton Neutron
Nucleus
Shell or orbit
16
e l e c t r o Negative
Electrons have a negative electrical charge. The charge on every electron is equal and constant and remains with the electron wherever it moves. Although a proton has 1800 times the mass of an electron, the charge on both is exactly the same, but opposite. It is one of the fundamentals of nature that no known physical process can produce a net change in electric charge. Electrons, with their charge, are the elementary particles of electricity. In a complete atom, the positively charged protons are always balanced by an equal number of negatively charged electrons - a complete atom is electrically neutral. Electrons move around the nucleus in areas of space called electron shells. Atoms can have up to seven electron shells.
17
-
+
Hydrogen atom Copper atom
18
The atoms of each different element have a different number of neutrons, protons, electrons and electron shells. - that’s what makes them different. A hydrogen atom, for example, has only one electron and one proton. An copper atom has 29 electrons and 29 protons. Each shell can hold only a certain number of electrons - the innermost shell can hold only two electrons, the next up to eight and - normally - the next up to eighteen and so on (but there are exceptions).
19
20
Free electrons
Electrons in the outermost shell can become free of their shell by application of an external force such as movement through amagnetic field, friction or chemical action. These are called free electrons. Because they are largely responsible for the way atoms bond together - and for the electrical properties of the material - they are the most important. A copper wire, for example, is full of atoms with free electrons, which are rapidly moving at random in all directions - even with no voltage applied. When a voltage is applied, the electrons gain additional velocity and start to flow in a uniform direction from negative to positive. This flow is commonly called electric current.
21
Copper wires good conductors
Nickel-chrome heating element - resistor
Insulators porcelain bakelite/nylon
22
Conductors
In some atoms free electrons are less firmly attached than in others. Materials made from elements with such atoms are good conductors of electrical current - e.g. most metals especially silver and copper.
Insulators
Materials which have atoms with electrons so firmly attached that they will not allow any current to flow through them, are insulators - e.g. rubber, plastic, wood, porcelain.
Resistors
In between are certain materials which will allow some current flow, but are not good conductors. Such materials are resistors - e.g. the nickle-chrome heating element of an electric fire.
23
Neutral charge
Negative charge
Positive charge
24
Electron flow
The movement - or flow - of electrons through a conductor is what is commonly known as electricity, and refered to as current. Electrons do not move directly from one end of a conductor to the other. They move from one atom to another - one electron is attached, one is lost to another atom and so on. To make electrons flow along a wire (produce an electrical current), there must be a pressure difference - or potential difference - between the ends of the wire. A battery can produce a potential difference. The chemical reactions in a battery cause electrons to leave the positive pole, and pile up at the negative pole. An excess of electrons creates a negative charge A shortage of electrons creates a positive charge.
25
>
>
> +
<
<
>
>
+
>
>
-
-
>
<
<
Current flow as originally assumed - conventional flow
Actual electron flow
26
Direction of current flow
Electrons negative to positive
When scientists first new about what we now call electric current, they didn’t know about the structure and behaviour of atoms and electrons. The poles or sides of a circuit were given the names positive and negative, with the reasonable assumption that positive had a lot of electric charge and negative had little or no charge. On this basis, current flow was thought to be from positive to negative. Eventually, it was discovered that what was really moving was electrons - from negative to positive. However, the original positive to negative flow had become a convention that would have been difficult to change. Circuit diagrams today are often shown on the basis of conventional flow. Despite this, electrons flow from negative to positive - as they always have. Just be sure which type of flow you are looking at.
27
Free electrons moving from atom to atom producing current
28
Current
The flow of electrons produced by connecting a battery is called current and is measured in Amperes or Amps. Smaller units in electronics are: - milli-Amps mA (1/1,000th of an Amp) and - micro-Amps uA (1/1,000,000th of an Amp). An Amp is the number of electrons flowing past a given point in one second.
29
2.0 mm
R = 2.5 ohms
1.0 mm
R = 5 ohms
Cross sectional area - thickness
< 0.5 m > R = 5 ohms < 1.0 m > R = 10 ohms
Length
30
Resistance
Cross sectional area and length
Resistance = opposition. In this case it is the opposition against electron flow in a circuit. Resistance is represented by the symbol R, and is measured in Ohms The factors which determine the resistance of a conductor: - cross sectional area (thickness). - length. - conductivity - or resistivity - of the material. - temperature As illustrated opposite - all other things remaining equal: - resistance is doubled if the length of wire is doubled, or the thickness is halved. - resistance is halved if the length of wire is halved or the thickness is doubled.
31
Chafed soild core
Severed multi-core strands
Good connection - no reduction of cross sectional area.
Poor connection reduction of cross sectional area.
32
Changing the length or thickness
Working with established electrical circuits, it is unlikely that we would deliberately change the thickness or length of a wire. However, mechanical faults - such as the examples below effectively reduce the thickness and, therefore, increase the resistance: - a wire solid core chafed or strands of a multi-strand core severed. - a dirty/corroded, loose or poor connection. Such ‘weak spots’ generate heat, and may cause a fire.
33
Resistivity or Conductivity
Resistivity or conductivity - or - is an inherent property of a material which depends on the behaviour and number of free electrons. It has nothing to do with the dimensions or shape of the material. As we have seen, metals such as silver or copper have many free electrons and therefore have low resistivity or good conductivity.
Effect of temperature
Temperature has an effect on resistivity. Generally - in metals - an increase in temperature results in an increase in resistivity - decrease in conductivity. This is because heat adds energy causing the atoms to vibrate erratically and obstruct the flow of electrons. Conversely, a decrease in temperature results in a decrease in resistivity. At very low temperatures some metals exhibit virtually no resistance at all - they are superconductors.
34
Using resistance
In some cases resistance is wanted - e.g: - when the required result is light or heat. The type of wire used for the filament of a light bulb or electric fire is deliberately selected because of it’s high resistivity. - when a voltage lower than the source is required in part or all of a circuit. A resister is a component deliberately used to provide a resistance in a circuit. In all other cases, resistance is definitely not wanted - e.g. - in a plain conductor such as copper wire, or in junctions/ connections/switches etc.
35
36
Ohms law
V=IxR
There is a relationship between voltage, current and resistance. Understanding this relationship will help with electrical diagnosis and repair. Ohms law states: the current in a circuit is directly proportional to the applied voltage, and inversely proportional to the resistance. This means: - if voltage goes up, current goes up - and vice versa. - if resistance goes up, current goes down - and vice versa.
Source voltage
Voltage is not affected by either current or resistance. It is either too low, too high or correct. If it is too low, current will be low. If it is too high, current will be high. If it is correct, current will be high if resistance is low, and current will be low if resistance is high.
37
Current
Resistance
Current is affected by either voltage or resistance. If voltage is high or resistance is low, current will be high. If voltage is low or resistance is high, current will be low. Resistance is not affected by either voltage or current. It is either too low, too high or correct. If it is low, current will be high at any voltage. If it is too high, current will be low if voltage is correct. When voltage stays the same - as in an automotive circuit current goes up as resistance goes down, and current goes down as resistance goes up. By ed devices reduce resistance, causing high current. Loose/dirty connections increase resistance, causing low current
38
39
V I R V = Voltage (Volts) R = Resistance (Ohm) I = Current (A)
V I R
V I
R
V I R
V=IxR
I=V R
R=V I
40
Ohms law equations
An easy way to the equations is to arrange the symbols in a triangle. If you know any two values an equation will give you the third. Cover the symbol for the value you want, and what remains uncovered is the equation.
An equation ‘wheel’ - which also includes power (Watts) and other combinations - can be found inside the back cover.
41
+ 12V
4 Ohms
+ 12V
What is the current flow in this circuit ?
42
Application of Ohms law Current
As an application of Ohms law, any voltage, current or resistance can be calculated if the other two values are known. This can help in fault diagnosis if, for some reason or other, actual measurement any one value is difficult. In the circuit above, current is unknown, voltage is supplied by 2 x 12 volt batteries, and resistance is 4 Ohms.
V I I=V R
R
Current = 24 4
Answer = 6 A
43
>
+
3A
12V
? + 12V
What is the Resistance in this circuit ?
44
Application of Ohms law -
In the circuit above resistance is unknown, voltage is supplied by 2 x 12 volt batteries, and current is 3 Amps.
resistance
V I R=V I
R
Resistance = 24 3
Answer = 8 R
45
>
+ 4A
? 3 Ohms
+ ?
What is the Voltage in this circuit ?
46
Application of Ohms law -
In the circuit above voltage is unknown, resistance is 3 Ohms,and current is 4 Amps.
voltage
V I R V=IxR
Voltage = 4 x 3
Answer = 12 V
47
W I V W = Power - Watts I = Current (A) V = Voltage - Volts
W I V
W I
V
W I
V
W=AxV
V=W A
A=W V
48
Energy and Power
Electricity is a form of energy, and energy can do work. Electric power refers to the rate at which a source of electricity produces energy, or the rate at which an electrical device converts electrical energy into some other form of energy - e.g. heat or light. Work done over a period of time is called Power, and is measured in Watts. Because a Watt is a small unit, power is often expressed per thousand Watts; 1 kilowatt (kW) = 1000 watts
Watt’s law
Power = Volts x Amps: so - increasing either the current or the voltage increases the power. The various equations can be shown using an Ohms law type triangle.
49
>
+ 3A
12V
? + 12V
What power will be consumed by the bulb in this circuit ?
50
In the circuit above voltage is supplied by 2 x 12 volt batteries, and current is 3 amps.
W I V W=IxV
Watts = 3 x 24
Answer = 72 W
51
+ 6V
100 W
+ 6V
How much current is required to get 100 Watts out of the bulb in this circuit ?
52
In the circuit above voltage is supplied by 2 x 6 volt batteries, and power is 100 W.
W I V I=
P V
Amps = 100 12
Answer = 8.33 A
53
+ 12V
100 W
+ 12V
54
Higher voltage = lower current
If we repeat the example above, but use 2 x 12 volt batteries ? Amps = 100 P Answer = 4.16 A 24 V So this circuit produces the same power, but with half the current - there is an advantage to this. Resistance in wires consumes power. The consumed power increases as current flow increases, as illustrated by the equation: I=
Increased current = increased power
P = I2 x R This confirms an increase in consumed power with an increase in resistance, but shows a much higher increase in consumed power with an increase in current. So reducing current by using a higher voltage can increase the efficiency of electrical systems.
55
56
Electrical Circuits Note: All circuits are drawn using the electrical symbols you will find on Volvo wiring diagrams
57
5
4
+ 2
1
3
+
Basic circuit
58
Circuit requirements
Electrons must flow from and return to the power source, so an unbroken circuit must exist. The basic components needed for a useful circuit are used in the diagram above: 1 Power source to produce the electron flow - battery, alternator. 2 Conductors to provide a path to and from the power source - wires, earth return. 3 A load which converts electricity into work - bulb, motor, heating element. 4 A control to allow the circuit to be opened or closed switch, relay. 5 A protection device to prevent circuit or component damage - fuse, circuit breaker.
59
+
+
Body/frame - earth return
60
Automotive circuits
In automotive circuits the body/frame of the vehicle is used as a conductor. One end of the wire from each load is connected to the body/frame, so current can flow back to the battery. To ensure integrity of the circuit, ‘earth’ straps are often used to make a connection from the body to the frame, and from major components - such as the engine - to the body or frame.
61
<
D
C
+ 12V
+ 12V
A
B
Series Circuit
62
Series circuit
A series circuit is the simplest circuit. All the components are connected with only one path to ground for current flow, so the electrons must flow through each component to get back to ground. The resistance of each component can be different. The current through each - and at any point in the circuit will be the same. The voltage across each each will be different. If the path is broken, no current flows - if one bulb blows, the other goes out. In the circuit above, electron flow is - A >B > C> D >A
63
< +
I=?
12V R1 = 2 Ohms
+
R2 = 4 Ohms
12V
64
Resistance in a series circuit
The total resistance in a series circuit is - the sum of all the resistances in the circuit. So, adding resistors increases the total resistance. In the above circuit the total is R1 + R2 = 6 Ohms
Current in a series circuit
In a series circuit, current is the same wherever in the circuit it is measured. In the circuit above, current is unknown, voltage is supplied by 2 x 12 volt batteries.
V I I=
V R1 + R2
R
Current = 24 6
Answer = 4 A
65
OV
OV
< +
10 A
12V 8V
+
16V
12V
66
Voltage drop
Voltage drop is the amount of voltage - or electrical potential - that is used or lost as current es through the loads. All voltage is used up in the circuit. When there is more than one load in a circuit, the voltage divides and is shared between the loads. The sum of all voltage drops (8V + 16V) must equal the supply voltage (24V). If the resistance of each load differs, so will the voltage drop - the higher the resistance, the greater the voltage drop. Voltage drop is measured at a point just before the load, and a point just after the load.
67
< + 12V R1 = 2 Ohms
+
R2 = 4 Ohms
12V
68
In the circuit above voltage is supplied by 2 x 12 volt batteries. The voltage drop across R1 and R2 can be found as follows. Total resistance 2 Ohms + 4 Ohms = 6 Ohms.
V I
Current
R
I=V R
Voltage drop across R1
V=IxR V = 4 x 2 = 8V
Voltage drop across R2
V=IxR V = 4 x 4 = 16V
V I
R
I = 24 = 4 A 6
69
<
F
D
E
+ 12V R1
R2
+ 12V
A
B
Parallel Circuit
C
70
Parallel circuit
A parallel circuit has more than one current path called a branch The voltage is the same along each branch. If the load in each branch is the same, the current will be the same. If the load in each branch is different, the current will be different. If one branch is broken, current will still flow in the other branch. In the circuit above there are two current branches: -A>B>E>F>A -A>B>C>D>E>F>A
71
< + R2
12V
R3
R1
+ 12V
72
Resistance in a parallel circuit
All resistances are the same value
The total resistance in a parallel circuit is always less than the smallest resistance in the circuit. This is because the current has more than one path. So, adding resistors decreases the total resistance. If all resistances are the same value - e.g R1, R2 and R3 are all 90 Ohms: Divide the resistance value by the number of resistances: 90 3
total resistance = 30 Ohms
73
< + R2
12V R1
+ 12V
74
Two resistances with different values
If there are only two resistances, each with a different value - e.g. R1 = 5 Ohms, R2 = 15 Ohms: Use product over sum: R1 x R2 R1 + R2
5 x 15 5 + 15
75 20
Answer 3.75 Ohms
75
< + R2
12V
R3
R1
+ 12V
76
More than two resistances with different values
If there are more than two resistances, each with a different value - e.g. R1 = 5 Ohms, R2 = 15 Ohms, R3 = 25 Ohms: Use:
1 1 1 R1 + R2 + R3 1 5 +
1 15 +
1 25
0.20 + 0.06 + 0.04
1 0.20 + 0.06 + 0.04 1 0.3
Answer = 3.3 Ohms
77
78
Components 79
Conductor: Diam - mm Cross sectional area - mm2
80
Wires
In a vehicle, wires are the main conductors of electric current, connecting one component to another. Wires act as a paths or pipes through which free electrons with their charge - can flow. Wires are full of movable charge - always. Wires distribute power from the battery to many devices located all over the vehicle. Wires also transmit signals to and from electronic control units and sensors. This means that there are many different types and thicknesses of wire in a vehicle. The diameter of a wire (mm), or cross sectional area (mm2) is an indication of how much current it carries - the larger the diameter, the more current it can carry e.g. the wires connected to the battery are very thick.
81
Twisted pair
Co-axial shielding
Multi-strand copper wires
82
The thickness of the insulation is an indication of the voltage in the wire - thick insulation means high voltage e.g. spark plug wires. Sensor signals
Multi-strand
Signals to and from sensors - often digital in form - are extremely susceptable to interference and corruption from external forces. Wires carrying such signals are normally shielded from interference by being arranged as a twisted pair, or by having a co-axial braided shield between the outer insulation and the insulation around the inner conductor. Almost all vehicle wires are made of copper, and are multistranded. Generally, the finer the strands, the lower the resistance and the more current the wire can carry. Multi-stranded wire is also more flexible and easier to handle.
83
_ - -- ---
+ ----- -
_ + 84
Batteries Circuit symbol
+
-
This symbol indicates a single battery cell of, nominally, 2V. A 12V battery symbol will have six such cells or, may be shown by two cells linked by dotted line.
A battery does not store electricity. It stores chemical energy and converts this to electrical energy when required. A battery does not make electrical charge, electrons or any other particle. The electrons - with their charge - are there by nature in the conductors. All conductors are full of movable charge - always. A battery acts as a pump which causes electric charge to flow. In the simple single cell above, there is a positive plate cathode - and a negative plate - anode. They are immersed in electrolyte. Electrolyte is a solution which is capable of conducting an electric charge.
85
>
>
_ --
+ ---
-
-
Electron flow in external circuit - battery supplying current
86
The chemical reactions between the different composition of the plates, and the electrolyte causes electrons to leave the cathode and move to the anode. The cathode now has an excess positive charge, and the anode has an excess negative charge. This results in an electrical difference - or potential difference - between the anode and cathode. This difference can be considered an unstable distribution of electrons, which can be rebalanced only by the excess electrons at the anode returning to the cathode. Note that electrons cannot move from one pole to another within the battery without a chemical reaction occuring. The only way they can return - from negative to positive - is via an outside circuit formed when a wire is attached to both terminals. In doing so, an electrical current - which can light a lamp - is created.
87
G
--
>
_
< +
---
-
-
Electron flow in external circuit - battery receiving current
88
However, as the battery discharges current, the plates become more chemically alike, and the electrolyte is diluted to mostly water. The transfer of electrons stops. There is now no potential difference to cause a current flow. Fortunately, in the type of battery used on vehicles, the chemical process can be reversed - it can be recharged.
Recharging
Whenever the engine is running, the alternator is recharging the battery by driving current into the battery in the opposite direction to the discharge flow. This charging continues until the plate coatings have been converted back to their original composition, and the electrolyte has been converted back to a solution of acid and water.
89
12V
-
+ 2V 2V
2V
2V
2V 2V
+ - + - + - + - + - + -
90
Lead acid battery
Automotive batteries are a type called lead acid. The plates are made of lead. In a charged battery, the positive plate is coated with lead peroxide, and the negative plate is coated with sponge lead. The electrolyte is a mixture of approx. 35% sulphuric acid and 65 % water The chemical reaction between these materials and the electrolyte can produce approx. 2 V per cell. To obtain a nominal 12V, six cells are connected in series.
Battery capacity
Battery capacity can be expressed in several ways, but either way it depends on the construction. Capacity is roughly proportional to the total surface area of the plates - that’s why batteries have several thin plates instead of a less number of thick plates. Note that temperature affects capacity. Because the chemical reactions are faster,capacity increases with temperature. 91
92
Cold cranking Amps
The function of an automotive battery is to provide a high current, to operate the starter motor, for a short time. The capacity measure of cold cranking Amps is, therefore one of the most important ratings. Cold cranking Amps is defined as the current that a battery will give at - 18° C for a given time and end voltage; Standard BS DIN SAE
Reserve capacity
Time 60 sec. 30 sec. 30 sec.
End voltage 8.4 9.0 7.2
This is defined as the time in minutes a fully charged battery at 25° C can supply 25A before the terminal voltage falls below 10.5 V .
93
94
Ampere hour rating
This is defined as the amount of current, in Ampere hours, a fully charged battery at 27° C can supply for 20 hours before the terminal voltage falls below 10.5 V. Example: a battery labelled as 50Ah should deliver 2.5 Amps for 20 hours. Note that when two batteries are connected in series, the voltage doubles, but the Ah capacity remains the same. Conversely, when two batteries are connected in parallel, the Ah capacity doubles, but the voltage remains the same.
95
6
4
3 1
5 2
96
Alternator
An alternator is a machine that converts mechanical energy into electrical energy. The alternator does not create electrons: it simply sets up the conditions for electrons, already in the conductors, to be made to flow. All conductors are full of movable charge - always. The main components are:
Rotor - 1
Two claw shaped iron pole pieces, each with six poles, enclose a copper wire coil (2) wound on a former. The coil is fed with a magnetising (exitation) current via the slip rings. The assembly is mounted on shaft, and acts as rotating electromagnet.
Slip-rings - 3
Collect the magnetising current from the carbon brushes (5). One slip ring is connected to one end of the rotor coil, and the other to the other end of the coil. 97
6
4
3 1
5 2
98
Stator - 4
Rectifier/ regulator - 5
Voltage output control
Three sets of copper wire coils are wound around a laminated iron core. The stator is attached to the alternator main housing.
The rectifier is a series of diodes. The main diodes are connected so that the AC current, produced by the alternator, is converted - rectified - to DC for the vehicle systems. Three auxilliary diodes convert AC to DC for the rotor coil magnetising current. The regulator senses the battery voltage, and adjusts the amount of magnetising current applied to the rotor coil. This varying current controls the output voltage, and keeps it at fairly constant level. Maximum charging voltage is controlled at approx. 28.5 V. Without regulation, the output would rise and fall with engine speed.
99
N S
N S
S N
N S
N S
N S
+ _ C
< 0
90
180
> 270
360
450
100
Alternating current
Frequency
Mechanical energy is converted to electrical energy using the principle of magnetic induction. This principle is based on the fact that whenever a conductor is moved within a magnetic field, so that the conductor cuts across the magnetic lines of force, voltage is generated in the conductor. The illustration above shows a simple coil which is cut by the magnetic north and south poles of a rotating magnet. The voltage generated in the coil changes direction as the poles by - N > S > N > S. If the coil is connected to a complete external circuit, the current will change direction at the same rate, or frequency, at which the poles rotate. This is alternating current - AC. The number of revolutions per second of the magnet corresponds to the frequency of cycles (C) of the current wave. Frequency is expressed in Hertz (Hz). E.g. 50 rps = 50 Hz. 101
R
B Y
+ Vo _ 0O
R Y B
360O
102
Electromagnet
In an alternator, the simple permanent magnet is replaced by an electromagnet - the rotor. The rotor is magnetised using a portion of the rectified DC output, which is collected by the slip rings from the brushes.
Three-phase output
To improve efficiency, and output, most alternators use three sets of six stator coils. Because the coils overlap, at least two coils are generating current at any instant. The three AC outputs are called phases, and generally referred to as red, blue and green. An 18-pole three-phase alternator and output wave is shown above. Each phase is generated separately from every third stator coil. The three phase voltages are plotted against time, or rotor angle.
103
D+
R B
+
R Y
B+
0 -
Y
B
D-
104
Rectification
Alternator drive
In a three-phase alternator, a pair of diodes for each phase rectifies both the positive and negative half cycles. This does not produce a perfectly flat DC line, as would be obtained from a battery. However, because the wave forms of the three phases overlap, the rectified DC current has very small pulsations, with a high average voltage level, and this is acceptable for most vehicle applications.
The mechanical rotational energy of the engine is transferred to the alternator by a belt driven from the crankshaft pulley - so the ultimate energy source - on the vehicle - is the diesel fuel.
105
5
2
1
2
2
2
2 4
3
106
Starter motor
The starter motor is a high power sliding gear motor. This type of motor operates in two stages to ensure that full power is not applied until the drive pinion is fully meshed with the flywheel ring gear. The main components are:
Field windings
Armature
There are two sets of field windings (1). The main winding is connected in series with the armature winding. The shunt winding connection with the armature can be switched from parallel connection to series connection. The armature (2) has a hollow shaft, through which es an engagement rod (3). The rod is actuated by a solenoid, and pushes the drive spindle (4) and pinion (5) forward during the first stage of motor actuation.
107
5
2
1
2
2
2
2 4
3
108
Control relay
The control relay (6) controls current supply to the field winding and solenoid windings.
Solenoid
When energised, the solenoid (7) armature moves forward and pushes the engagement rod forward.
Multi-disc clutch
The multi-disc clutch (8) provides the connection between the armature and the drive spindle which carries the drive pinion.
Actuation stage 1
When the starter switch is closed, the control relay is energised and directs current to the shunt field winding and to the solenoid windings. The shunt winding is now in series with the armature winding. The weak magnetic field established by the shunt winding causes the armature to rotate slowly. At the same time, the solenoid pushes the pinion forward via the engagement rod.
109
110
The slowly rotating pinion eventually meshes with the ring gear. Just before the pinion is fully in mesh: Actuation stage 2
- a mechanical trip lever allows closure of another in the control relay, and current is fed to the armature main field winding. - a finger on the solenoid actuates a microswitch, and switches the shunt winding from a series connection with the armature to a parallel connection. Full current now flows through the shunt winding, and through the main filed winding, and the the torque developed by both windings is applied to the pinion to crank the engine. The pressure of the pinion, pushing against the ring gear, presses the clutch plate together, allowing full torque to be applied. 111
Spade or Blade fuse
Glass fuse
112
Fuses Circuit symbol Rating
Used for
Fuse rating is designated in Amps, and marked on the fuse. Fuses are used to protect wiring and components from damage due to current overload. Fuses are a special type of wire conductor in a self contained connector. The conductor is a metal similar to solder, which has a melting point lower than the attached wires. The diameter of the conductor is selected very carefully so that when the rated current is reached, enough heat is rapidly generated to melt the conductor and break the circuit.
113
Colour
Rating - A
Purple Pink Orange
3 4 5
Brown Red Blue Yellow White Green
7.5 10 15 20 25 30
114
Because different circuits carry widely differing amounts of current, several different fuse ratings are needed.
Like for like
!WARNING! A replacement fuse must be exactly the same rating as the old one.
115
86 30
87 85 85 87a 30
86
87a numbering
87
116
Relays
A relay is a simple electromechanical, remote control switch. Relays have the following parts: - electromagnet - armature, that can be attracted by the electromagnet - spring - set of electrical s
numbering
Note that there is a strict convention for numbering the s: 86 Electromagnet winding supply 85 Electromagnet winding ground 30 Load power supply - input 87 Load power supply output, electromagnet energised 30 and 87 have continuity 87a Load power supply output, electromagnet deenergised - 30 and 87a have continuity
117
86 30
87 85 85 87a 30
86
87a numbering
87
118
When the electromagnet is de-energised, the return spring in the relay ensures that s 30 > 87a are normally closed (NC). Therefore, s 30 > 87 are normally open (NO). Normally, the load supply circuit is to be closed when the electromagnet is energised, so the load is connected to 87. Because current will be flowing whenever the relay is deenergised, it is not normal for the load to be connected to 87a. However, a load may be connected to 87a - and to 87 when the relay is being used as a changeover relay, to switch power from one circuit to another.
119
87a
20 - 50 Amps
87 30 86
0.25 amps
85
120
Application
Operation
In general, the use of a relay is to allow a small amount of current to switch a much larger amount of current. For example: a switch - and the wires - to handle the current needed for say, a heater element, would be quite large and heavy. It would be impractical to have such switches on the dashboard. So, a small lightweight switch, mounted on the dashboard, controls a remote relay. A relay has two separate and independent circuits. One circuit energises the electromagnet when the lightweight switch is closed. Once energised, the armature is attracted to the electromagnet, and acts as a heavy duty switch to close the load circuit. The lightweight switch, controlling the electromagnet, may be switching e.g. 0.25 Amps. The armature, controlling the load, may be switching 20 50 Amps. 121
122
Resistors Circuit symbol Rating
Used for
Resistors are rated by two main characteristics: - Resistance in Ohms - Power disipation in Watts Resistors are used to limit current or voltage. The resistor is the most basic electronic component. In a circuit, the resistor opposes the flow of current through itself. It does this by absorbing some of the electrical energy applied to it, and disipating that energy as heat. Therefore, the resistor provides a way of limiting, or controlling, the amount of current that can through the circuit. A resistor is a piece of material deliberately put into a circuit to produce a resistance of a specific value.
123
RESISTORS Fixed
Carbon/metal oxide
Variable
Wire wound
Rheostats
Potentiometers
Thermisters
>
>
>
T
Symbol
124
Fixed resistors
Carbon resistors are generally in two forms: Carbon film resistors are formed by coating a ceramic slug with a carbon film. The resistance value is adjusted by cutting a helical groove in the film - the width of cut determines the resistance. In carbon composition resistors, carbon is mixed with a binder - the more carbon, the lower the resistance. typical resistors have a power rating of 0.125 - 2 W For metal oxide resistors, a chemical like tin-chloride is applied to a ceramic slug. These resistors often have better temperature stability, and are typically used for the 1 - 2 W range.
125
7W
1W
10W
0.5W
2W
0.25W
50W for heat sink mounting
0.125W
126
Fixed resistors cont.
Power handling
In wire wound resistors, a continuous length of resistance wire is either wound around a former, or encased in ceramic. These resistors are accurate and heat stable, and are generally used for higher power circuits, typically 2 - 50 W. Resistors absorb power and generate heat. The power handling of a resistor is generally determined by its ability to disipate that heat. Maximum operating temperatures are determined by the materials used to make the resistor. To avoid damage, the temperature must be kept below the maximum value by heat disipation. If the resistor is not mounted on a heat sink, its physical size generally determines the power rating. Larger resistors have more surface area, and can disipate more heat than smaller resistors. Even large size resistors sometimes need heat disipation improving by mounting them on a large heat sink.
127
< + R1
12V
+
LOAD
12V
128
Varying current and voltage Resistors as current limiters
The vehicle batteries deliver a fixed voltage and current, but not all components or circuits require - or can withstand - full voltage or current. Resistors can be used to limit current flow, and match it to the load requirement. As we have seen - Ohms law - increasing resistance reduces current flow. Example: battery voltage is 24 V, load requires a current of 1.5 Amps: R = V/ I
V I
R
R = 24/1.5
R = 16
To limit the current through the load to 1.5 A, R1 needs to have a resistance of 16 Ohms.
129
< + R1
12V
LOAD
+
R2
12V
130
Resistors as voltage droppers
Some circuits or components may require a voltage different from the battery output. In most cases the voltage required is less than battery voltage. A single resistor in series with the supply voltage will cause a volt drop. However, because resistors are available in specific resistance values and tolerances, a single resistor of the right value to obtain the required volt drop may not be available. A solution to this problem is to use two resistors in series R1 and R2, and use the voltage appearing across one of them. Because this arrangement divides the battery voltage into two parts, it is called a voltage divider.
131
< + R1 1000Ohms
12V
A
+ 12V
LOAD
R2 1000 Ohms
B
132
Voltage drop
The voltage drop across all resistors in a series circuit will always be equal to the supply voltage. If R1 and R2 both have a value of 1000 Ohms, the voltage drop across each is the same. Each resistor drops half the supply voltage - 12V. If there were three resistors of equal value, each would drop one third of the supply voltage - 8V. So, a load connected between A - the junction of the two resistors - and B would receive a voltage of 12V. In this case the resistors are being used to divide the supply voltage. By selecting the correct value resistors, voltages, different from the supply voltage, can be obtained at the required point in the circuit.
133
Colour Black Brown Red Orange Yellow Green Blue Violet Grey White Gold Silver
1st digit 1 2 3 4 5 6 7 8 9
2nd digit 0 1 2 3 4 5 6 7 8 9
Zeros 1 10 100 1,000 10,000 100,000 1,000,000 10,000,000 100,000,000 1,000,000,000 x 0.1 x 0.01
Tolerance % +/-1% +/-2%
+/-5% +/-10%
134
Fixed resistor colour codes
The reistance value of fixed resistors is indicated by a series of coloured bands around the resistor. There are commonly four bands, but some high precision resistors may have five bands. Bands one and two give the digit or number value. Band three gives the number of zeros to be put behind the number. Band four gives the tolerance value as a percentage of the given resistance.
Example
Yellow Violet 4 7
Orange 000
Gold 5%
A resisitor with this colour banding has a resistance of: 47,000 Ohms or 47k, and a tolerance of 5%. The resisitance range is, therefore: 44.6k - 49.3k.
135
2 1
136
Variable resistors
Variable resistor is common term used to cover a variety of devices whose resistance can be varied, usually by mechanical rotary or linear movement. They have a resistance track (1), with a connection at both ends, and a wiper (2) which moves along the track, and also has a connection. The track may made from carbon, a ceramic /metal compound, a plastics material or a coil of wire. Such devices are called rheostats or potentiometers (pots). The are sometimes mixed, and the construction and appearance may be the same, but they are used in different ways.
Rheostats
Although all mechanical variable resistors may have three terminals, rheostats use only two - the wiper and one end of the track. Rheostats control resistance and, therefore, control current flow.
137
BRIGHT
DIM 1
2
138
Variable resistors
Before solid state electronic circuits became the norm, rheostats were typically used for lamp dimming. Terminal - (1) is connected to the fixed end of a resistor, and (2) to a sliding on the resistor. Moving the slider towards BRIGHT places a short length of the resistor in series with the lamp, and the lamp is bright. Moving the slider towards DIM places a longer length of the resistor in series with the lamp, and the lamp is dimmer.
139
5V REF SIGNAL
R
GROUND G Throttle position sender Engine control module
140
Variable resistors Potentiometers
Potentiometers use all three terminals. One end of the track is fed a reference voltage, and the other end is connected to ground. The wiper connection carries the signal voltage collected from the track. Unlike a rheostat, a pot is used to vary the voltage in a circuit. Pots are used to measure changes in position - e.g. the position of the throttle pedal in an electronic engine management system. Current flows from R, along the track to G. The wiper measures the changing signal voltage as it moves, and transmits this voltage to the ECM. The closer to R the wiper is, the higher is the measured voltage.
141
Thermister
142
Variable resistors Thermisters
Thermisters are resistors that change resistance as the temperature changes. Typical uses are - measuring coolant/oil/ air temperature.
Negative temperature coefficient
NTC thermisters: as temperature goes up, resistance goes down. NTC thermisters are commonly used in automotive applications.
Positive temperature coefficient
PTC thermisters: as temperature goes up, resistance goes up.
143
5V R2 VM
R1
Temperature sender
Engine control module
144
In the example circuit above, a regulated 5V is fed to one connection of thermister pellet R1, via R2. As the engine warms up, the resistance of NTC thermister R1 goes down. This results in a decreased volt drop across R1, and an increased volt drop across R2 - R2 now uses more voltage. The increased volt drop across R2 results in a lower voltage - VM - measured by the monitor circuit, which is seen by the ECM as an increase in engine temperature.
145
Polarity sensitive capacitors - large values, 1uF + + + +
10uF 63V
220uF 25V
Electrolytic
-
10u
+
+
Tantalum bead
Non polarity sensitive capacitors - small values , up to 1uF 0.1 M
0.1 uF
102
1st number = 1st digit 2nd number = 2nd digit 3rd number = number of 0’s
1000 pF
146
Capacitors Circuit symbol
+
Polarity sensitive
Non polarity sensitive
Rating
Capacitors are rated by capacitance and voltage. Capacitance is expressed in Farads, but a farad is a very large value of capacitance, so most common values are expressed as fractions of a farad: - e.g. one millionth of farad is a micro-farad shown as 1 uF
Used for
Capacitors are used: - to smooth varying DC supplies - voltage peaks are absorbed, and voltage dips are filled. - with resistors in timing circuits. - in filter circuits because capacitors easily AC signals, but block DC signals. - to store charge for instantaneous release - e.g. a camera flash.
147
+
+
148
In a way, a capacitor is similar to a small battery - they both store energy. A battery stores chemical energy: a capacitor stores electrical energy. Like a battery, a capacitor has two terminals. Inside the capacitor, the terminals are connected to two metal plates. The plates are separated by an insulating material called a dielectric. The dielectric can be air, plastic, paper or anything that does not conduct electricity. In the circuit above, a capacitor is connected to a battery via a switch. At the instant the switch is closed, electrons are attracted from the positive plate to the battery positive. The same number are pushed into the negative plate from the battery negative terminal.
149
DANGER
Although not usually found in automotive applications, large capacitors can store a potentially lethal charge, which can be dumped instantaneously if certain components or connections are touched. Such large capacitors are found in flash units and TV sets, and warning labels usually warn against inexpert handling.
150
The initial charging rate is very fast, but slows down the more the capacitor is charged. In small capacitors, the complete charging process happens almost instantaneously. Enough electrons move into and out of the capacitor to make the voltage equal to the battery voltage. The positive plate now has a shortage of electrons, and the negative plated has an excess of electrons - the capacitor is now charged. Because there is no current path between the plates, the charge remains on the plates even after the switch is opened. If the capacitor is now removed from the circuit, and the terminals connected together, the stored charge will be released in a fraction of a second.
151
+
+ C1
C2
C3
+
Four capacitors in parallel
C1
C2
C3
C4
C4
+
Four capacitors in series
152
The total capacitance of capacitors in parallel and series circuits is the opposite of that for resistors.
Capacitors in parallel
The total capacitance of capacitors in parallel is the sum of all capacitors: C total = C1 + C2 + C3 + C4
Capacitors in series
The total capacitance of capacitors in series is the sum of the reciprocals of all capacitors: C total =
1 + C1
1 + 1 + C2 C3
1 C4
153
154
Diodes
>
Circuit symbol <
Used for
permitted direction of < electron flow
Note - that the arrow head points against the direction of electron flow, but in the direction of conventional current flow.
Diodes are used for: - protection against incorrect polarity connections. - changing - rectifying - AC current to DC current, e.g. in an alternator. - protection against voltage spikes, e.g. connected to a relay to prevent arcing when s open. The most common type of diode is the semiconductor diode, made from silicon.
155
> +
>
< permitted direction < of electron flow
< permitted direction < of electron flow
+ +
+
Current flows - diode is forward biased
Current cannot flow - diode is reverse biased
156
Forward/ reverse bias
One way valve
A diode in a simple battery/lamp circuit will either allow or prevent current flow - depending on the polarity of supply voltage. When the supply voltage polarity is arranged so that electrons can flow from the negative terminal of the battery through the diode in the permitted direction, the diode is forward biased, and allows current flow. When the supply voltage polarity is reversed, the diode is reverse biased, and no current can flow. Diodes can be regarded as one-way valves or check valves. Like check valves, diodes are essentially pressure operated devices - voltage being the operating pressure.
157
0.7 V
>
>
>
>
>
>
24 V
+ 12V
12V
<
<
+ 0.0 V
23.3 V
+
<
<
12V
12V
+
Forward biased
Reverse biased
158
The essential difference between forward bias and reverse bias is the polarity of the voltage dropped across the diode. When the diode is forward biased and conducting current, a small amount of voltage is dropped across it - this being the voltage expended to push the current through the diode - approx 0.7 V. So, most of the voltage is dropped across the lamp.
Diode marking
When the diode is reverse biased, it drops all of the voltage and leaves none for the lamp. Cathode Anode
>
The cathode end of the diode is marked by a stripe, spot or raised marker. Some diodes may have code of coloured stripes, which can be used to identify the device type from an identification system for semiconductors.
159
160
Measurements & Tests 161
Multimeter Display
Push buttons
Rotary switch
Input terminals
162
Using a multimeter
Display
Push buttons
Rotary switch
Input terminals
Multimeters - like the model used and recommended by Volvo Technical Training - can be set to measure several types of electrical signal. The result of the measurement is shown on the display.
Push buttons select different functions.
The rotary switch is used to select the type of signal to be measured. The probe connecting leads are plugged into the input terminals, according to the signal to be measured.
163
164
Wire sizing chart
165
Multipliers Tera Giga Mega Kilo
T G M K
-----
(Unit)
milli micro nano pico
m u n p
1,000,000,000,000 1,000,000,000 1,000,000 1,000
1/1,000 1/1,000,000 1/1,000,000,000 1/1,000,000,000,000
166
Formula wheel
VxI 2
V R I2 x R IxR
PxR
V R
P V P R
Watts Amps P I R V Volts Ohms
P I
P I2
V2 P V I 167
168
Module 2 Electrical Principles
1
2
Contents
3
4
Introduction About this Pocket-guide
This guide is intended as a memory jogger for the knowledge you have gained during your training course. The guide includes a summary of the material covered in: Module 2 - Electrical Principles.
AC/DC
Note that all the material in this guide relates to DC circuits.
5
Danger, Warning, Caution & Note
In this guide, risk of injury or damage is indicated by the following headings: DANGER - indicates a risk of serious personal injury or death. WARNING - indicates risk of personal injury, or severe product damage. CAUTION - indicates risk of product damage. Note - draws attention to special methods or particular features.
Pay attention to these headings
Read and implement all DANGER, WARNING and CAUTION instructions.
6
Replacement parts
When replacement parts are required, it is essential that only Volvo genuine parts are fitted. If Volvo genuine parts are not used: - safety features embodied in the vehicle or components may be impaired. - performance and/or operation of the vehicle or components may be adversely affected - Volvo warranty may be invalidated.
7
Specification
Volvo are constantly seeking ways to improve their products, and alterations take place accordingly. Whilst every effort has been made to ensure the accuracy of this guide, it should not be regarded as an infallible guide to current specifications of any particular product. Neither Volvo, nor the supplier of this book shall, in any circumstances, be held liable for inaccuracy or the consequences thereof.
Copyright
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, transmitted or copied without written permission from Volvo Truck & Bus Ltd.
c
Volvo Truck & Bus Ltd 2004
8
Atoms 9
Electron
Proton Neutron
Nucleus
Shell or orbit
10
Matter
Element
Compound
Molecule
Matter is anything that occupies space and has weight. Matter can be solid, liquid and gaseous. An element is a substance which cannot be reduced to a simpler substance by chemical means. Examples are: gold, silver, iron, copper and oxygen. There are approx.100 known elements, and everything around us is made up of one or more of these elements. When two or more elements are chemically combined, the resulting mixture is called a compound. A molecule is a chemical combination of two or more atoms. In a compound,the molecule is the smalest particle thsat has all the characteristics of the compoond.
11
12
Atoms are everywhere
Molecules are made up of atoms. Atoms are extremely small - if one could be magnified a million times, it would only just be the size of the full stop at the end of this sentence. An atom is the smallest particle of an element that retains all the characteristics of the element. The atoms of one element differ from the atoms of all other elements Atoms are made up of only three types of particle protons, electrons and neutrons. All protons are exactly the same, all neutrons are exactly the same and all electrons are exactly the same. Since there are approx.100 known elements, so there are approx.100 kinds of atom in the entire universe. Whether these atoms make up water, ashes the air that we breathe, trees or tyres depends on how they are put together - i.e. the number and arrangement of the particles - their structure. 13
Electron
Proton Neutron
Nucleus
Shell or orbit
14
Atoms, and their particles exist by nature in every kind of material around us. Neither atoms nor their particles can be manufactured or destroyed. Nucleus
The neutrons and protons are in the centre of the atom in a nucleus, which has by far the greatest mass of the atom. Most of the atom is empty space. Protons and neutrons have almost exactly the same mass. Electrons have a mass about 1/1880 the mass of of a proton - e.g. in a person of 150 lb weight, 149lb 15oz are protons and neutrons, 1oz are electrons.
Positive r o t o n
Protons have a positive electrical charge.
Neutrons have no electrical charge.
15
Electron
Proton Neutron
Nucleus
Shell or orbit
16
e l e c t r o Negative
Electrons have a negative electrical charge. The charge on every electron is equal and constant and remains with the electron wherever it moves. Although a proton has 1800 times the mass of an electron, the charge on both is exactly the same, but opposite. It is one of the fundamentals of nature that no known physical process can produce a net change in electric charge. Electrons, with their charge, are the elementary particles of electricity. In a complete atom, the positively charged protons are always balanced by an equal number of negatively charged electrons - a complete atom is electrically neutral. Electrons move around the nucleus in areas of space called electron shells. Atoms can have up to seven electron shells.
17
-
+
Hydrogen atom Copper atom
18
The atoms of each different element have a different number of neutrons, protons, electrons and electron shells. - that’s what makes them different. A hydrogen atom, for example, has only one electron and one proton. An copper atom has 29 electrons and 29 protons. Each shell can hold only a certain number of electrons - the innermost shell can hold only two electrons, the next up to eight and - normally - the next up to eighteen and so on (but there are exceptions).
19
20
Free electrons
Electrons in the outermost shell can become free of their shell by application of an external force such as movement through amagnetic field, friction or chemical action. These are called free electrons. Because they are largely responsible for the way atoms bond together - and for the electrical properties of the material - they are the most important. A copper wire, for example, is full of atoms with free electrons, which are rapidly moving at random in all directions - even with no voltage applied. When a voltage is applied, the electrons gain additional velocity and start to flow in a uniform direction from negative to positive. This flow is commonly called electric current.
21
Copper wires good conductors
Nickel-chrome heating element - resistor
Insulators porcelain bakelite/nylon
22
Conductors
In some atoms free electrons are less firmly attached than in others. Materials made from elements with such atoms are good conductors of electrical current - e.g. most metals especially silver and copper.
Insulators
Materials which have atoms with electrons so firmly attached that they will not allow any current to flow through them, are insulators - e.g. rubber, plastic, wood, porcelain.
Resistors
In between are certain materials which will allow some current flow, but are not good conductors. Such materials are resistors - e.g. the nickle-chrome heating element of an electric fire.
23
Neutral charge
Negative charge
Positive charge
24
Electron flow
The movement - or flow - of electrons through a conductor is what is commonly known as electricity, and refered to as current. Electrons do not move directly from one end of a conductor to the other. They move from one atom to another - one electron is attached, one is lost to another atom and so on. To make electrons flow along a wire (produce an electrical current), there must be a pressure difference - or potential difference - between the ends of the wire. A battery can produce a potential difference. The chemical reactions in a battery cause electrons to leave the positive pole, and pile up at the negative pole. An excess of electrons creates a negative charge A shortage of electrons creates a positive charge.
25
>
>
> +
<
<
>
>
+
>
>
-
-
>
<
<
Current flow as originally assumed - conventional flow
Actual electron flow
26
Direction of current flow
Electrons negative to positive
When scientists first new about what we now call electric current, they didn’t know about the structure and behaviour of atoms and electrons. The poles or sides of a circuit were given the names positive and negative, with the reasonable assumption that positive had a lot of electric charge and negative had little or no charge. On this basis, current flow was thought to be from positive to negative. Eventually, it was discovered that what was really moving was electrons - from negative to positive. However, the original positive to negative flow had become a convention that would have been difficult to change. Circuit diagrams today are often shown on the basis of conventional flow. Despite this, electrons flow from negative to positive - as they always have. Just be sure which type of flow you are looking at.
27
Free electrons moving from atom to atom producing current
28
Current
The flow of electrons produced by connecting a battery is called current and is measured in Amperes or Amps. Smaller units in electronics are: - milli-Amps mA (1/1,000th of an Amp) and - micro-Amps uA (1/1,000,000th of an Amp). An Amp is the number of electrons flowing past a given point in one second.
29
2.0 mm
R = 2.5 ohms
1.0 mm
R = 5 ohms
Cross sectional area - thickness
< 0.5 m > R = 5 ohms < 1.0 m > R = 10 ohms
Length
30
Resistance
Cross sectional area and length
Resistance = opposition. In this case it is the opposition against electron flow in a circuit. Resistance is represented by the symbol R, and is measured in Ohms The factors which determine the resistance of a conductor: - cross sectional area (thickness). - length. - conductivity - or resistivity - of the material. - temperature As illustrated opposite - all other things remaining equal: - resistance is doubled if the length of wire is doubled, or the thickness is halved. - resistance is halved if the length of wire is halved or the thickness is doubled.
31
Chafed soild core
Severed multi-core strands
Good connection - no reduction of cross sectional area.
Poor connection reduction of cross sectional area.
32
Changing the length or thickness
Working with established electrical circuits, it is unlikely that we would deliberately change the thickness or length of a wire. However, mechanical faults - such as the examples below effectively reduce the thickness and, therefore, increase the resistance: - a wire solid core chafed or strands of a multi-strand core severed. - a dirty/corroded, loose or poor connection. Such ‘weak spots’ generate heat, and may cause a fire.
33
Resistivity or Conductivity
Resistivity or conductivity - or - is an inherent property of a material which depends on the behaviour and number of free electrons. It has nothing to do with the dimensions or shape of the material. As we have seen, metals such as silver or copper have many free electrons and therefore have low resistivity or good conductivity.
Effect of temperature
Temperature has an effect on resistivity. Generally - in metals - an increase in temperature results in an increase in resistivity - decrease in conductivity. This is because heat adds energy causing the atoms to vibrate erratically and obstruct the flow of electrons. Conversely, a decrease in temperature results in a decrease in resistivity. At very low temperatures some metals exhibit virtually no resistance at all - they are superconductors.
34
Using resistance
In some cases resistance is wanted - e.g: - when the required result is light or heat. The type of wire used for the filament of a light bulb or electric fire is deliberately selected because of it’s high resistivity. - when a voltage lower than the source is required in part or all of a circuit. A resister is a component deliberately used to provide a resistance in a circuit. In all other cases, resistance is definitely not wanted - e.g. - in a plain conductor such as copper wire, or in junctions/ connections/switches etc.
35
36
Ohms law
V=IxR
There is a relationship between voltage, current and resistance. Understanding this relationship will help with electrical diagnosis and repair. Ohms law states: the current in a circuit is directly proportional to the applied voltage, and inversely proportional to the resistance. This means: - if voltage goes up, current goes up - and vice versa. - if resistance goes up, current goes down - and vice versa.
Source voltage
Voltage is not affected by either current or resistance. It is either too low, too high or correct. If it is too low, current will be low. If it is too high, current will be high. If it is correct, current will be high if resistance is low, and current will be low if resistance is high.
37
Current
Resistance
Current is affected by either voltage or resistance. If voltage is high or resistance is low, current will be high. If voltage is low or resistance is high, current will be low. Resistance is not affected by either voltage or current. It is either too low, too high or correct. If it is low, current will be high at any voltage. If it is too high, current will be low if voltage is correct. When voltage stays the same - as in an automotive circuit current goes up as resistance goes down, and current goes down as resistance goes up. By ed devices reduce resistance, causing high current. Loose/dirty connections increase resistance, causing low current
38
39
V I R V = Voltage (Volts) R = Resistance (Ohm) I = Current (A)
V I R
V I
R
V I R
V=IxR
I=V R
R=V I
40
Ohms law equations
An easy way to the equations is to arrange the symbols in a triangle. If you know any two values an equation will give you the third. Cover the symbol for the value you want, and what remains uncovered is the equation.
An equation ‘wheel’ - which also includes power (Watts) and other combinations - can be found inside the back cover.
41
+ 12V
4 Ohms
+ 12V
What is the current flow in this circuit ?
42
Application of Ohms law Current
As an application of Ohms law, any voltage, current or resistance can be calculated if the other two values are known. This can help in fault diagnosis if, for some reason or other, actual measurement any one value is difficult. In the circuit above, current is unknown, voltage is supplied by 2 x 12 volt batteries, and resistance is 4 Ohms.
V I I=V R
R
Current = 24 4
Answer = 6 A
43
>
+
3A
12V
? + 12V
What is the Resistance in this circuit ?
44
Application of Ohms law -
In the circuit above resistance is unknown, voltage is supplied by 2 x 12 volt batteries, and current is 3 Amps.
resistance
V I R=V I
R
Resistance = 24 3
Answer = 8 R
45
>
+ 4A
? 3 Ohms
+ ?
What is the Voltage in this circuit ?
46
Application of Ohms law -
In the circuit above voltage is unknown, resistance is 3 Ohms,and current is 4 Amps.
voltage
V I R V=IxR
Voltage = 4 x 3
Answer = 12 V
47
W I V W = Power - Watts I = Current (A) V = Voltage - Volts
W I V
W I
V
W I
V
W=AxV
V=W A
A=W V
48
Energy and Power
Electricity is a form of energy, and energy can do work. Electric power refers to the rate at which a source of electricity produces energy, or the rate at which an electrical device converts electrical energy into some other form of energy - e.g. heat or light. Work done over a period of time is called Power, and is measured in Watts. Because a Watt is a small unit, power is often expressed per thousand Watts; 1 kilowatt (kW) = 1000 watts
Watt’s law
Power = Volts x Amps: so - increasing either the current or the voltage increases the power. The various equations can be shown using an Ohms law type triangle.
49
>
+ 3A
12V
? + 12V
What power will be consumed by the bulb in this circuit ?
50
In the circuit above voltage is supplied by 2 x 12 volt batteries, and current is 3 amps.
W I V W=IxV
Watts = 3 x 24
Answer = 72 W
51
+ 6V
100 W
+ 6V
How much current is required to get 100 Watts out of the bulb in this circuit ?
52
In the circuit above voltage is supplied by 2 x 6 volt batteries, and power is 100 W.
W I V I=
P V
Amps = 100 12
Answer = 8.33 A
53
+ 12V
100 W
+ 12V
54
Higher voltage = lower current
If we repeat the example above, but use 2 x 12 volt batteries ? Amps = 100 P Answer = 4.16 A 24 V So this circuit produces the same power, but with half the current - there is an advantage to this. Resistance in wires consumes power. The consumed power increases as current flow increases, as illustrated by the equation: I=
Increased current = increased power
P = I2 x R This confirms an increase in consumed power with an increase in resistance, but shows a much higher increase in consumed power with an increase in current. So reducing current by using a higher voltage can increase the efficiency of electrical systems.
55
56
Electrical Circuits Note: All circuits are drawn using the electrical symbols you will find on Volvo wiring diagrams
57
5
4
+ 2
1
3
+
Basic circuit
58
Circuit requirements
Electrons must flow from and return to the power source, so an unbroken circuit must exist. The basic components needed for a useful circuit are used in the diagram above: 1 Power source to produce the electron flow - battery, alternator. 2 Conductors to provide a path to and from the power source - wires, earth return. 3 A load which converts electricity into work - bulb, motor, heating element. 4 A control to allow the circuit to be opened or closed switch, relay. 5 A protection device to prevent circuit or component damage - fuse, circuit breaker.
59
+
+
Body/frame - earth return
60
Automotive circuits
In automotive circuits the body/frame of the vehicle is used as a conductor. One end of the wire from each load is connected to the body/frame, so current can flow back to the battery. To ensure integrity of the circuit, ‘earth’ straps are often used to make a connection from the body to the frame, and from major components - such as the engine - to the body or frame.
61
<
D
C
+ 12V
+ 12V
A
B
Series Circuit
62
Series circuit
A series circuit is the simplest circuit. All the components are connected with only one path to ground for current flow, so the electrons must flow through each component to get back to ground. The resistance of each component can be different. The current through each - and at any point in the circuit will be the same. The voltage across each each will be different. If the path is broken, no current flows - if one bulb blows, the other goes out. In the circuit above, electron flow is - A >B > C> D >A
63
< +
I=?
12V R1 = 2 Ohms
+
R2 = 4 Ohms
12V
64
Resistance in a series circuit
The total resistance in a series circuit is - the sum of all the resistances in the circuit. So, adding resistors increases the total resistance. In the above circuit the total is R1 + R2 = 6 Ohms
Current in a series circuit
In a series circuit, current is the same wherever in the circuit it is measured. In the circuit above, current is unknown, voltage is supplied by 2 x 12 volt batteries.
V I I=
V R1 + R2
R
Current = 24 6
Answer = 4 A
65
OV
OV
< +
10 A
12V 8V
+
16V
12V
66
Voltage drop
Voltage drop is the amount of voltage - or electrical potential - that is used or lost as current es through the loads. All voltage is used up in the circuit. When there is more than one load in a circuit, the voltage divides and is shared between the loads. The sum of all voltage drops (8V + 16V) must equal the supply voltage (24V). If the resistance of each load differs, so will the voltage drop - the higher the resistance, the greater the voltage drop. Voltage drop is measured at a point just before the load, and a point just after the load.
67
< + 12V R1 = 2 Ohms
+
R2 = 4 Ohms
12V
68
In the circuit above voltage is supplied by 2 x 12 volt batteries. The voltage drop across R1 and R2 can be found as follows. Total resistance 2 Ohms + 4 Ohms = 6 Ohms.
V I
Current
R
I=V R
Voltage drop across R1
V=IxR V = 4 x 2 = 8V
Voltage drop across R2
V=IxR V = 4 x 4 = 16V
V I
R
I = 24 = 4 A 6
69
<
F
D
E
+ 12V R1
R2
+ 12V
A
B
Parallel Circuit
C
70
Parallel circuit
A parallel circuit has more than one current path called a branch The voltage is the same along each branch. If the load in each branch is the same, the current will be the same. If the load in each branch is different, the current will be different. If one branch is broken, current will still flow in the other branch. In the circuit above there are two current branches: -A>B>E>F>A -A>B>C>D>E>F>A
71
< + R2
12V
R3
R1
+ 12V
72
Resistance in a parallel circuit
All resistances are the same value
The total resistance in a parallel circuit is always less than the smallest resistance in the circuit. This is because the current has more than one path. So, adding resistors decreases the total resistance. If all resistances are the same value - e.g R1, R2 and R3 are all 90 Ohms: Divide the resistance value by the number of resistances: 90 3
total resistance = 30 Ohms
73
< + R2
12V R1
+ 12V
74
Two resistances with different values
If there are only two resistances, each with a different value - e.g. R1 = 5 Ohms, R2 = 15 Ohms: Use product over sum: R1 x R2 R1 + R2
5 x 15 5 + 15
75 20
Answer 3.75 Ohms
75
< + R2
12V
R3
R1
+ 12V
76
More than two resistances with different values
If there are more than two resistances, each with a different value - e.g. R1 = 5 Ohms, R2 = 15 Ohms, R3 = 25 Ohms: Use:
1 1 1 R1 + R2 + R3 1 5 +
1 15 +
1 25
0.20 + 0.06 + 0.04
1 0.20 + 0.06 + 0.04 1 0.3
Answer = 3.3 Ohms
77
78
Components 79
Conductor: Diam - mm Cross sectional area - mm2
80
Wires
In a vehicle, wires are the main conductors of electric current, connecting one component to another. Wires act as a paths or pipes through which free electrons with their charge - can flow. Wires are full of movable charge - always. Wires distribute power from the battery to many devices located all over the vehicle. Wires also transmit signals to and from electronic control units and sensors. This means that there are many different types and thicknesses of wire in a vehicle. The diameter of a wire (mm), or cross sectional area (mm2) is an indication of how much current it carries - the larger the diameter, the more current it can carry e.g. the wires connected to the battery are very thick.
81
Twisted pair
Co-axial shielding
Multi-strand copper wires
82
The thickness of the insulation is an indication of the voltage in the wire - thick insulation means high voltage e.g. spark plug wires. Sensor signals
Multi-strand
Signals to and from sensors - often digital in form - are extremely susceptable to interference and corruption from external forces. Wires carrying such signals are normally shielded from interference by being arranged as a twisted pair, or by having a co-axial braided shield between the outer insulation and the insulation around the inner conductor. Almost all vehicle wires are made of copper, and are multistranded. Generally, the finer the strands, the lower the resistance and the more current the wire can carry. Multi-stranded wire is also more flexible and easier to handle.
83
_ - -- ---
+ ----- -
_ + 84
Batteries Circuit symbol
+
-
This symbol indicates a single battery cell of, nominally, 2V. A 12V battery symbol will have six such cells or, may be shown by two cells linked by dotted line.
A battery does not store electricity. It stores chemical energy and converts this to electrical energy when required. A battery does not make electrical charge, electrons or any other particle. The electrons - with their charge - are there by nature in the conductors. All conductors are full of movable charge - always. A battery acts as a pump which causes electric charge to flow. In the simple single cell above, there is a positive plate cathode - and a negative plate - anode. They are immersed in electrolyte. Electrolyte is a solution which is capable of conducting an electric charge.
85
>
>
_ --
+ ---
-
-
Electron flow in external circuit - battery supplying current
86
The chemical reactions between the different composition of the plates, and the electrolyte causes electrons to leave the cathode and move to the anode. The cathode now has an excess positive charge, and the anode has an excess negative charge. This results in an electrical difference - or potential difference - between the anode and cathode. This difference can be considered an unstable distribution of electrons, which can be rebalanced only by the excess electrons at the anode returning to the cathode. Note that electrons cannot move from one pole to another within the battery without a chemical reaction occuring. The only way they can return - from negative to positive - is via an outside circuit formed when a wire is attached to both terminals. In doing so, an electrical current - which can light a lamp - is created.
87
G
--
>
_
< +
---
-
-
Electron flow in external circuit - battery receiving current
88
However, as the battery discharges current, the plates become more chemically alike, and the electrolyte is diluted to mostly water. The transfer of electrons stops. There is now no potential difference to cause a current flow. Fortunately, in the type of battery used on vehicles, the chemical process can be reversed - it can be recharged.
Recharging
Whenever the engine is running, the alternator is recharging the battery by driving current into the battery in the opposite direction to the discharge flow. This charging continues until the plate coatings have been converted back to their original composition, and the electrolyte has been converted back to a solution of acid and water.
89
12V
-
+ 2V 2V
2V
2V
2V 2V
+ - + - + - + - + - + -
90
Lead acid battery
Automotive batteries are a type called lead acid. The plates are made of lead. In a charged battery, the positive plate is coated with lead peroxide, and the negative plate is coated with sponge lead. The electrolyte is a mixture of approx. 35% sulphuric acid and 65 % water The chemical reaction between these materials and the electrolyte can produce approx. 2 V per cell. To obtain a nominal 12V, six cells are connected in series.
Battery capacity
Battery capacity can be expressed in several ways, but either way it depends on the construction. Capacity is roughly proportional to the total surface area of the plates - that’s why batteries have several thin plates instead of a less number of thick plates. Note that temperature affects capacity. Because the chemical reactions are faster,capacity increases with temperature. 91
92
Cold cranking Amps
The function of an automotive battery is to provide a high current, to operate the starter motor, for a short time. The capacity measure of cold cranking Amps is, therefore one of the most important ratings. Cold cranking Amps is defined as the current that a battery will give at - 18° C for a given time and end voltage; Standard BS DIN SAE
Reserve capacity
Time 60 sec. 30 sec. 30 sec.
End voltage 8.4 9.0 7.2
This is defined as the time in minutes a fully charged battery at 25° C can supply 25A before the terminal voltage falls below 10.5 V .
93
94
Ampere hour rating
This is defined as the amount of current, in Ampere hours, a fully charged battery at 27° C can supply for 20 hours before the terminal voltage falls below 10.5 V. Example: a battery labelled as 50Ah should deliver 2.5 Amps for 20 hours. Note that when two batteries are connected in series, the voltage doubles, but the Ah capacity remains the same. Conversely, when two batteries are connected in parallel, the Ah capacity doubles, but the voltage remains the same.
95
6
4
3 1
5 2
96
Alternator
An alternator is a machine that converts mechanical energy into electrical energy. The alternator does not create electrons: it simply sets up the conditions for electrons, already in the conductors, to be made to flow. All conductors are full of movable charge - always. The main components are:
Rotor - 1
Two claw shaped iron pole pieces, each with six poles, enclose a copper wire coil (2) wound on a former. The coil is fed with a magnetising (exitation) current via the slip rings. The assembly is mounted on shaft, and acts as rotating electromagnet.
Slip-rings - 3
Collect the magnetising current from the carbon brushes (5). One slip ring is connected to one end of the rotor coil, and the other to the other end of the coil. 97
6
4
3 1
5 2
98
Stator - 4
Rectifier/ regulator - 5
Voltage output control
Three sets of copper wire coils are wound around a laminated iron core. The stator is attached to the alternator main housing.
The rectifier is a series of diodes. The main diodes are connected so that the AC current, produced by the alternator, is converted - rectified - to DC for the vehicle systems. Three auxilliary diodes convert AC to DC for the rotor coil magnetising current. The regulator senses the battery voltage, and adjusts the amount of magnetising current applied to the rotor coil. This varying current controls the output voltage, and keeps it at fairly constant level. Maximum charging voltage is controlled at approx. 28.5 V. Without regulation, the output would rise and fall with engine speed.
99
N S
N S
S N
N S
N S
N S
+ _ C
< 0
90
180
> 270
360
450
100
Alternating current
Frequency
Mechanical energy is converted to electrical energy using the principle of magnetic induction. This principle is based on the fact that whenever a conductor is moved within a magnetic field, so that the conductor cuts across the magnetic lines of force, voltage is generated in the conductor. The illustration above shows a simple coil which is cut by the magnetic north and south poles of a rotating magnet. The voltage generated in the coil changes direction as the poles by - N > S > N > S. If the coil is connected to a complete external circuit, the current will change direction at the same rate, or frequency, at which the poles rotate. This is alternating current - AC. The number of revolutions per second of the magnet corresponds to the frequency of cycles (C) of the current wave. Frequency is expressed in Hertz (Hz). E.g. 50 rps = 50 Hz. 101
R
B Y
+ Vo _ 0O
R Y B
360O
102
Electromagnet
In an alternator, the simple permanent magnet is replaced by an electromagnet - the rotor. The rotor is magnetised using a portion of the rectified DC output, which is collected by the slip rings from the brushes.
Three-phase output
To improve efficiency, and output, most alternators use three sets of six stator coils. Because the coils overlap, at least two coils are generating current at any instant. The three AC outputs are called phases, and generally referred to as red, blue and green. An 18-pole three-phase alternator and output wave is shown above. Each phase is generated separately from every third stator coil. The three phase voltages are plotted against time, or rotor angle.
103
D+
R B
+
R Y
B+
0 -
Y
B
D-
104
Rectification
Alternator drive
In a three-phase alternator, a pair of diodes for each phase rectifies both the positive and negative half cycles. This does not produce a perfectly flat DC line, as would be obtained from a battery. However, because the wave forms of the three phases overlap, the rectified DC current has very small pulsations, with a high average voltage level, and this is acceptable for most vehicle applications.
The mechanical rotational energy of the engine is transferred to the alternator by a belt driven from the crankshaft pulley - so the ultimate energy source - on the vehicle - is the diesel fuel.
105
5
2
1
2
2
2
2 4
3
106
Starter motor
The starter motor is a high power sliding gear motor. This type of motor operates in two stages to ensure that full power is not applied until the drive pinion is fully meshed with the flywheel ring gear. The main components are:
Field windings
Armature
There are two sets of field windings (1). The main winding is connected in series with the armature winding. The shunt winding connection with the armature can be switched from parallel connection to series connection. The armature (2) has a hollow shaft, through which es an engagement rod (3). The rod is actuated by a solenoid, and pushes the drive spindle (4) and pinion (5) forward during the first stage of motor actuation.
107
5
2
1
2
2
2
2 4
3
108
Control relay
The control relay (6) controls current supply to the field winding and solenoid windings.
Solenoid
When energised, the solenoid (7) armature moves forward and pushes the engagement rod forward.
Multi-disc clutch
The multi-disc clutch (8) provides the connection between the armature and the drive spindle which carries the drive pinion.
Actuation stage 1
When the starter switch is closed, the control relay is energised and directs current to the shunt field winding and to the solenoid windings. The shunt winding is now in series with the armature winding. The weak magnetic field established by the shunt winding causes the armature to rotate slowly. At the same time, the solenoid pushes the pinion forward via the engagement rod.
109
110
The slowly rotating pinion eventually meshes with the ring gear. Just before the pinion is fully in mesh: Actuation stage 2
- a mechanical trip lever allows closure of another in the control relay, and current is fed to the armature main field winding. - a finger on the solenoid actuates a microswitch, and switches the shunt winding from a series connection with the armature to a parallel connection. Full current now flows through the shunt winding, and through the main filed winding, and the the torque developed by both windings is applied to the pinion to crank the engine. The pressure of the pinion, pushing against the ring gear, presses the clutch plate together, allowing full torque to be applied. 111
Spade or Blade fuse
Glass fuse
112
Fuses Circuit symbol Rating
Used for
Fuse rating is designated in Amps, and marked on the fuse. Fuses are used to protect wiring and components from damage due to current overload. Fuses are a special type of wire conductor in a self contained connector. The conductor is a metal similar to solder, which has a melting point lower than the attached wires. The diameter of the conductor is selected very carefully so that when the rated current is reached, enough heat is rapidly generated to melt the conductor and break the circuit.
113
Colour
Rating - A
Purple Pink Orange
3 4 5
Brown Red Blue Yellow White Green
7.5 10 15 20 25 30
114
Because different circuits carry widely differing amounts of current, several different fuse ratings are needed.
Like for like
!WARNING! A replacement fuse must be exactly the same rating as the old one.
115
86 30
87 85 85 87a 30
86
87a numbering
87
116
Relays
A relay is a simple electromechanical, remote control switch. Relays have the following parts: - electromagnet - armature, that can be attracted by the electromagnet - spring - set of electrical s
numbering
Note that there is a strict convention for numbering the s: 86 Electromagnet winding supply 85 Electromagnet winding ground 30 Load power supply - input 87 Load power supply output, electromagnet energised 30 and 87 have continuity 87a Load power supply output, electromagnet deenergised - 30 and 87a have continuity
117
86 30
87 85 85 87a 30
86
87a numbering
87
118
When the electromagnet is de-energised, the return spring in the relay ensures that s 30 > 87a are normally closed (NC). Therefore, s 30 > 87 are normally open (NO). Normally, the load supply circuit is to be closed when the electromagnet is energised, so the load is connected to 87. Because current will be flowing whenever the relay is deenergised, it is not normal for the load to be connected to 87a. However, a load may be connected to 87a - and to 87 when the relay is being used as a changeover relay, to switch power from one circuit to another.
119
87a
20 - 50 Amps
87 30 86
0.25 amps
85
120
Application
Operation
In general, the use of a relay is to allow a small amount of current to switch a much larger amount of current. For example: a switch - and the wires - to handle the current needed for say, a heater element, would be quite large and heavy. It would be impractical to have such switches on the dashboard. So, a small lightweight switch, mounted on the dashboard, controls a remote relay. A relay has two separate and independent circuits. One circuit energises the electromagnet when the lightweight switch is closed. Once energised, the armature is attracted to the electromagnet, and acts as a heavy duty switch to close the load circuit. The lightweight switch, controlling the electromagnet, may be switching e.g. 0.25 Amps. The armature, controlling the load, may be switching 20 50 Amps. 121
122
Resistors Circuit symbol Rating
Used for
Resistors are rated by two main characteristics: - Resistance in Ohms - Power disipation in Watts Resistors are used to limit current or voltage. The resistor is the most basic electronic component. In a circuit, the resistor opposes the flow of current through itself. It does this by absorbing some of the electrical energy applied to it, and disipating that energy as heat. Therefore, the resistor provides a way of limiting, or controlling, the amount of current that can through the circuit. A resistor is a piece of material deliberately put into a circuit to produce a resistance of a specific value.
123
RESISTORS Fixed
Carbon/metal oxide
Variable
Wire wound
Rheostats
Potentiometers
Thermisters
>
>
>
T
Symbol
124
Fixed resistors
Carbon resistors are generally in two forms: Carbon film resistors are formed by coating a ceramic slug with a carbon film. The resistance value is adjusted by cutting a helical groove in the film - the width of cut determines the resistance. In carbon composition resistors, carbon is mixed with a binder - the more carbon, the lower the resistance. typical resistors have a power rating of 0.125 - 2 W For metal oxide resistors, a chemical like tin-chloride is applied to a ceramic slug. These resistors often have better temperature stability, and are typically used for the 1 - 2 W range.
125
7W
1W
10W
0.5W
2W
0.25W
50W for heat sink mounting
0.125W
126
Fixed resistors cont.
Power handling
In wire wound resistors, a continuous length of resistance wire is either wound around a former, or encased in ceramic. These resistors are accurate and heat stable, and are generally used for higher power circuits, typically 2 - 50 W. Resistors absorb power and generate heat. The power handling of a resistor is generally determined by its ability to disipate that heat. Maximum operating temperatures are determined by the materials used to make the resistor. To avoid damage, the temperature must be kept below the maximum value by heat disipation. If the resistor is not mounted on a heat sink, its physical size generally determines the power rating. Larger resistors have more surface area, and can disipate more heat than smaller resistors. Even large size resistors sometimes need heat disipation improving by mounting them on a large heat sink.
127
< + R1
12V
+
LOAD
12V
128
Varying current and voltage Resistors as current limiters
The vehicle batteries deliver a fixed voltage and current, but not all components or circuits require - or can withstand - full voltage or current. Resistors can be used to limit current flow, and match it to the load requirement. As we have seen - Ohms law - increasing resistance reduces current flow. Example: battery voltage is 24 V, load requires a current of 1.5 Amps: R = V/ I
V I
R
R = 24/1.5
R = 16
To limit the current through the load to 1.5 A, R1 needs to have a resistance of 16 Ohms.
129
< + R1
12V
LOAD
+
R2
12V
130
Resistors as voltage droppers
Some circuits or components may require a voltage different from the battery output. In most cases the voltage required is less than battery voltage. A single resistor in series with the supply voltage will cause a volt drop. However, because resistors are available in specific resistance values and tolerances, a single resistor of the right value to obtain the required volt drop may not be available. A solution to this problem is to use two resistors in series R1 and R2, and use the voltage appearing across one of them. Because this arrangement divides the battery voltage into two parts, it is called a voltage divider.
131
< + R1 1000Ohms
12V
A
+ 12V
LOAD
R2 1000 Ohms
B
132
Voltage drop
The voltage drop across all resistors in a series circuit will always be equal to the supply voltage. If R1 and R2 both have a value of 1000 Ohms, the voltage drop across each is the same. Each resistor drops half the supply voltage - 12V. If there were three resistors of equal value, each would drop one third of the supply voltage - 8V. So, a load connected between A - the junction of the two resistors - and B would receive a voltage of 12V. In this case the resistors are being used to divide the supply voltage. By selecting the correct value resistors, voltages, different from the supply voltage, can be obtained at the required point in the circuit.
133
Colour Black Brown Red Orange Yellow Green Blue Violet Grey White Gold Silver
1st digit 1 2 3 4 5 6 7 8 9
2nd digit 0 1 2 3 4 5 6 7 8 9
Zeros 1 10 100 1,000 10,000 100,000 1,000,000 10,000,000 100,000,000 1,000,000,000 x 0.1 x 0.01
Tolerance % +/-1% +/-2%
+/-5% +/-10%
134
Fixed resistor colour codes
The reistance value of fixed resistors is indicated by a series of coloured bands around the resistor. There are commonly four bands, but some high precision resistors may have five bands. Bands one and two give the digit or number value. Band three gives the number of zeros to be put behind the number. Band four gives the tolerance value as a percentage of the given resistance.
Example
Yellow Violet 4 7
Orange 000
Gold 5%
A resisitor with this colour banding has a resistance of: 47,000 Ohms or 47k, and a tolerance of 5%. The resisitance range is, therefore: 44.6k - 49.3k.
135
2 1
136
Variable resistors
Variable resistor is common term used to cover a variety of devices whose resistance can be varied, usually by mechanical rotary or linear movement. They have a resistance track (1), with a connection at both ends, and a wiper (2) which moves along the track, and also has a connection. The track may made from carbon, a ceramic /metal compound, a plastics material or a coil of wire. Such devices are called rheostats or potentiometers (pots). The are sometimes mixed, and the construction and appearance may be the same, but they are used in different ways.
Rheostats
Although all mechanical variable resistors may have three terminals, rheostats use only two - the wiper and one end of the track. Rheostats control resistance and, therefore, control current flow.
137
BRIGHT
DIM 1
2
138
Variable resistors
Before solid state electronic circuits became the norm, rheostats were typically used for lamp dimming. Terminal - (1) is connected to the fixed end of a resistor, and (2) to a sliding on the resistor. Moving the slider towards BRIGHT places a short length of the resistor in series with the lamp, and the lamp is bright. Moving the slider towards DIM places a longer length of the resistor in series with the lamp, and the lamp is dimmer.
139
5V REF SIGNAL
R
GROUND G Throttle position sender Engine control module
140
Variable resistors Potentiometers
Potentiometers use all three terminals. One end of the track is fed a reference voltage, and the other end is connected to ground. The wiper connection carries the signal voltage collected from the track. Unlike a rheostat, a pot is used to vary the voltage in a circuit. Pots are used to measure changes in position - e.g. the position of the throttle pedal in an electronic engine management system. Current flows from R, along the track to G. The wiper measures the changing signal voltage as it moves, and transmits this voltage to the ECM. The closer to R the wiper is, the higher is the measured voltage.
141
Thermister
142
Variable resistors Thermisters
Thermisters are resistors that change resistance as the temperature changes. Typical uses are - measuring coolant/oil/ air temperature.
Negative temperature coefficient
NTC thermisters: as temperature goes up, resistance goes down. NTC thermisters are commonly used in automotive applications.
Positive temperature coefficient
PTC thermisters: as temperature goes up, resistance goes up.
143
5V R2 VM
R1
Temperature sender
Engine control module
144
In the example circuit above, a regulated 5V is fed to one connection of thermister pellet R1, via R2. As the engine warms up, the resistance of NTC thermister R1 goes down. This results in a decreased volt drop across R1, and an increased volt drop across R2 - R2 now uses more voltage. The increased volt drop across R2 results in a lower voltage - VM - measured by the monitor circuit, which is seen by the ECM as an increase in engine temperature.
145
Polarity sensitive capacitors - large values, 1uF + + + +
10uF 63V
220uF 25V
Electrolytic
-
10u
+
+
Tantalum bead
Non polarity sensitive capacitors - small values , up to 1uF 0.1 M
0.1 uF
102
1st number = 1st digit 2nd number = 2nd digit 3rd number = number of 0’s
1000 pF
146
Capacitors Circuit symbol
+
Polarity sensitive
Non polarity sensitive
Rating
Capacitors are rated by capacitance and voltage. Capacitance is expressed in Farads, but a farad is a very large value of capacitance, so most common values are expressed as fractions of a farad: - e.g. one millionth of farad is a micro-farad shown as 1 uF
Used for
Capacitors are used: - to smooth varying DC supplies - voltage peaks are absorbed, and voltage dips are filled. - with resistors in timing circuits. - in filter circuits because capacitors easily AC signals, but block DC signals. - to store charge for instantaneous release - e.g. a camera flash.
147
+
+
148
In a way, a capacitor is similar to a small battery - they both store energy. A battery stores chemical energy: a capacitor stores electrical energy. Like a battery, a capacitor has two terminals. Inside the capacitor, the terminals are connected to two metal plates. The plates are separated by an insulating material called a dielectric. The dielectric can be air, plastic, paper or anything that does not conduct electricity. In the circuit above, a capacitor is connected to a battery via a switch. At the instant the switch is closed, electrons are attracted from the positive plate to the battery positive. The same number are pushed into the negative plate from the battery negative terminal.
149
DANGER
Although not usually found in automotive applications, large capacitors can store a potentially lethal charge, which can be dumped instantaneously if certain components or connections are touched. Such large capacitors are found in flash units and TV sets, and warning labels usually warn against inexpert handling.
150
The initial charging rate is very fast, but slows down the more the capacitor is charged. In small capacitors, the complete charging process happens almost instantaneously. Enough electrons move into and out of the capacitor to make the voltage equal to the battery voltage. The positive plate now has a shortage of electrons, and the negative plated has an excess of electrons - the capacitor is now charged. Because there is no current path between the plates, the charge remains on the plates even after the switch is opened. If the capacitor is now removed from the circuit, and the terminals connected together, the stored charge will be released in a fraction of a second.
151
+
+ C1
C2
C3
+
Four capacitors in parallel
C1
C2
C3
C4
C4
+
Four capacitors in series
152
The total capacitance of capacitors in parallel and series circuits is the opposite of that for resistors.
Capacitors in parallel
The total capacitance of capacitors in parallel is the sum of all capacitors: C total = C1 + C2 + C3 + C4
Capacitors in series
The total capacitance of capacitors in series is the sum of the reciprocals of all capacitors: C total =
1 + C1
1 + 1 + C2 C3
1 C4
153
154
Diodes
>
Circuit symbol <
Used for
permitted direction of < electron flow
Note - that the arrow head points against the direction of electron flow, but in the direction of conventional current flow.
Diodes are used for: - protection against incorrect polarity connections. - changing - rectifying - AC current to DC current, e.g. in an alternator. - protection against voltage spikes, e.g. connected to a relay to prevent arcing when s open. The most common type of diode is the semiconductor diode, made from silicon.
155
> +
>
< permitted direction < of electron flow
< permitted direction < of electron flow
+ +
+
Current flows - diode is forward biased
Current cannot flow - diode is reverse biased
156
Forward/ reverse bias
One way valve
A diode in a simple battery/lamp circuit will either allow or prevent current flow - depending on the polarity of supply voltage. When the supply voltage polarity is arranged so that electrons can flow from the negative terminal of the battery through the diode in the permitted direction, the diode is forward biased, and allows current flow. When the supply voltage polarity is reversed, the diode is reverse biased, and no current can flow. Diodes can be regarded as one-way valves or check valves. Like check valves, diodes are essentially pressure operated devices - voltage being the operating pressure.
157
0.7 V
>
>
>
>
>
>
24 V
+ 12V
12V
<
<
+ 0.0 V
23.3 V
+
<
<
12V
12V
+
Forward biased
Reverse biased
158
The essential difference between forward bias and reverse bias is the polarity of the voltage dropped across the diode. When the diode is forward biased and conducting current, a small amount of voltage is dropped across it - this being the voltage expended to push the current through the diode - approx 0.7 V. So, most of the voltage is dropped across the lamp.
Diode marking
When the diode is reverse biased, it drops all of the voltage and leaves none for the lamp. Cathode Anode
>
The cathode end of the diode is marked by a stripe, spot or raised marker. Some diodes may have code of coloured stripes, which can be used to identify the device type from an identification system for semiconductors.
159
160
Measurements & Tests 161
Multimeter Display
Push buttons
Rotary switch
Input terminals
162
Using a multimeter
Display
Push buttons
Rotary switch
Input terminals
Multimeters - like the model used and recommended by Volvo Technical Training - can be set to measure several types of electrical signal. The result of the measurement is shown on the display.
Push buttons select different functions.
The rotary switch is used to select the type of signal to be measured. The probe connecting leads are plugged into the input terminals, according to the signal to be measured.
163
164
Wire sizing chart
165
Multipliers Tera Giga Mega Kilo
T G M K
-----
(Unit)
milli micro nano pico
m u n p
1,000,000,000,000 1,000,000,000 1,000,000 1,000
1/1,000 1/1,000,000 1/1,000,000,000 1/1,000,000,000,000
166
Formula wheel
VxI 2
V R I2 x R IxR
PxR
V R
P V P R
Watts Amps P I R V Volts Ohms
P I
P I2
V2 P V I 167
168
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