Basic electronics 651 Electronics 652 Electronic semiconductors 653 Electronic components – diodes and transistors 654 Other electronic components 656 Use of electronic components 658 Electronic systems 660 Micro-computer 662 Body electronic module 663 Technical terms 665 Review questions 665
Trang 1Basic electronics
Chapter 37
Electronics
Electronic semiconductors
Electronic components – diodes and transistors
Other electronic components
Use of electronic components
Electronic systems
Micro-computer
Body electronic module
Technical terms
Review questions
Trang 2This chapter has two objectives: the first is to provide a
basic understanding of how electronic components
operate, and the second is to gain an appreciation of
how electronic systems can function.
The basic operation of semiconductors and
semi-conductor components will be treated as simply as
possible, and then a systems approach will be used to
gain an understanding of how electronic systems
operate When using this approach, only the functions
of the main parts of a system are considered and not
the operation of the individual electronic components
which make up the system.
Electronics
Electronics is that aspect of electrics which deals with
the behaviour of electrons However, with the use of
semiconductor materials in so-called solid-state
devices such as diodes and transistors, the term
electronics has been applied generally to units and
systems that use these components.
Electronic components are used in radios, television
sets, computers, industrial control systems and
automotive systems, to name just a few common
examples There are a number of automotive
appli-cations, such as electronic ignition, electronic fuel
injection, electronic instruments and electronic testing
equipment.
The expansion of electronics in so many fields has
only been possible because of the development of
semiconductor materials and the manufacture of the
silicon chip, which is the basis of most electronic units.
Some of the small electronic components that are used
are shown in Figure 37.1 The following sections
outline the use of diodes and transistors, which are the
two most common electronic components.
Diode action
A diode has two external connections When used in a
circuit, it acts as a one-way valve by allowing current
to flow in one direction only.
Figure 37.2 shows a diode in a simple circuit,
where the diode and a bulb are connected in series to a
battery In the circuit (Figure 37.2(a)), the diode is
conducting to allow current to flow and the bulb to
light In the circuit (Figure 37.2(b)), the diode has been
reversed and is now non-conducting to prevent current
flow and so the bulb does not light.
The circuit arrangement has a practical application.
Diodes that are used in alternators can be connected to
a bulb and battery as shown to check their operation, although an ohmmeter would usually be used.
Transistor action
Transistors are used as electronic switching devices In
a circuit, they act in the same way as an electro-magnetic relay, but are quite small by comparison and can be operated by very low currents Transistors have three external connections, known as the base (B), collector (C) and emitter (E).
Figure 37.3 shows the basic action of a transistor, which is being used as a switching device for a lamp.
figure 37.1 Small semiconductor components
figure 37.2 Action of a diode
(a) current passes through the diode to light the bulb (b) the diode blocks current flow
Trang 3With the manual switch SW closed, a small current
from the battery passes through the resistance R to the
base B This triggers the action of the transistor, which
becomes a conductor and completes the lamp circuit to
the battery and so lights the bulb With the switch SW
open, the transistor is non-conducting and the bulb
does not light.
Electronic semiconductors
Semiconductors, as the name suggests, fall between
conductors and insulators They act as conductors
under some conditions, but do not conduct under
others.
Electronic semiconductors are formed by doping.
The basic material, which is usually silicon but can be
germanium, is doped by adding a small quantity of
other material to it Both n-type and p-type electronic
semiconductors can be produced by different types of
doping These are shown in Figure 37.4, with
electrons being shown as dashes and protons shown as
holes.
■ In simple terms, one of these materials is a
negative-charge conductor and the other is
a positive-charge conductor.
n-type semiconductors
When silicon is doped with certain chemicals, there will be a predominance of electrons (negative charges) and an n-type semiconductor will result.
Conductivity in this type of semiconductor is based
on movement of electrons in a somewhat similar way
to the movement of electrons in a normal conductor When voltage is applied, the electrons will move towards the positive pole of the voltage source.
p-type semiconductors
When silicon is doped with different materials, then the electrons which carry a charge are reduced, and holes remain These holes represent a shortage of electrons, and so a hole can be considered as being equivalent to a positive charge.
Conductivity in this type of semiconductor is based
on movement of the positive holes When voltage is applied to a p-type semiconductor, the holes will move towards the negative pole of the voltage source so that, simply, a flow of positive charges will result.
Charge carriers
From the above, it can be seen that both electrons and holes act as charge carriers, with the electrons being negative-charge carriers and the holes being positive-charge carriers.
Electrons in semiconductor materials behave according to the electron theory of current flow, and the holes behave according to the conventional theory
of current flow.
Therefore, when a semiconductor is connected to a source of voltage, such as a battery, the electrons in the semiconductor will tend to move towards the positive pole (from negative to positive), and the positive holes will tend to move towards the negative pole (from positive to negative).
The electrons will carry a negative charge with them and the holes will carry a positive charge Because a flow of both negative and positive charges occurs, semiconductors are referred to as being bipolar, as distinct from current flow in metals, where only the electrons move.
While there is much more to electron and hole movement than has been described, the previous paragraphs provide a basic concept that enables the operation of electronic semiconductors to be understood.
■ The next step is to consider how semiconductor components function.
figure 37.3 Action of a transistor – a small current
operates the transistor as a switch
figure 37.4 Electronic semiconductors – p-type has a
predominance of holes, and n-type has
a predominance of electrons
Trang 4p–n junction
The semiconductor components that are used in
electronic circuits consist of a combination of n-type
and p-type semiconductors With these two types of
semiconductors joined together, an n-region and a
p-region are formed The junction of the two regions is
referred to as the p–n junction and this junction
performs an important function in the operation of
semiconductor components.
When the two types of semiconductor materials are
joined, electrons move away from the junction into
the n-region and the holes move away from the
junction into the p-region This leaves an area around
the p–n junction which is depleted of both electrons
and holes and so the area has no charges which are
effective.
The p–n junction therefore forms a layer, known as
the depletion layer, which is free of charges This acts
as an insulator between the two regions of the
semi-conductor component (Figure 37.5).
■ Both n-type and p-type conductors are used in
almost all electronic components, and these make
use of one or more p–n junctions.
Diodes
A diode consists of an n-region and a p-region, with a single p–n junction as previously described If the diode is connected to a source of voltage, with the p-region connected to the negative and the n-region connected to positive, as shown in Figure 37.6, the holes will move away from the p–n junction towards the negative pole, and the electrons will move away from the junction towards the positive pole.
This increases the width of the depletion layer at the p–n junction and a more effective barrier is set up
to prevent movement of the charge carriers across the junction Under these conditions, the diode acts as a non-conductor.
However, if the polarity of the battery is reversed,
as shown in Figure 37.7, then the charge carriers will move The electrons will travel from the n-region
figure 37.5 Diode
p-type and n-type semiconductors are joined
to form a diode
Electronic components –
diodes and transistors
There are a number of electronic components that use
semiconductor material of doped silicon These
include diodes, transistors, integrated circuits and
forms of electronic resistors.
Diodes and transistors are the two basic
compo-nents that are most commonly used The diode is the
simplest electronic component, and it forms the basis
of many other components.
figure 37.6 A battery connected to a diode in the reverse
direction increases the width of the depletion layer at the p–n junction
figure 37.7 A battery connected to a diode in the forward
direction causes both holes and electrons to move
Trang 5towards the positive pole and the holes will travel from
the p-region towards the negative pole The p–n
junction will become flooded with charge carriers and
the depletion layer at the junction will disappear.
Under these changed conditions, the diode acts as a
conductor and carries current.
■ Current flow in this instance consists of movement
of both positive charges (holes) and negative
charges (electrons).
Bias of diodes
With the polarity of the voltage source connected so
that the diode conducts, it is said to be forward-biased,
but if the polarity is such that the diode does not
conduct, then the diode is said to be reverse-biased
(Figure 37.8).
It resists current flow (when it is reverse-biased) in a similar way to a normal diode, but only up to a certain voltage, which is referred to as the breakdown voltage When this voltage is reached, the resistance of the Zener diode breaks down enabling current to flow through it.
A diode of this type can therefore be used as a voltage-related switch This has an application in transistorised voltage regulators, which are fitted to alternators The Zener diode is used as a device to limit the alternator output.
Zener diode as protection
An example of how a Zener diode can be used as a protection device is shown in Figure 37.9 It is wired in parallel with an electrical instrument and arranged to break down at the maximum safe voltage that the instrument can stand.
figure 37.8 Bias of diodes
(a) the diode is forward-biased and conducting (b) the diode is reverse-biased and non-conducting
figure 37.9 A Zener diode in parallel with a voltmeter is
used as a protective device
The diode, as a one-way valve, is used in rectifying
circuits where current is required to flow in a particular
direction.
In the charging system, alternating current
produced by the alternator is changed to direct current
(rectified) by the action of diodes The diodes conduct
when forward-biased, but do not conduct when
reverse-biased.
Diode symbol
The symbol used to represent a diode in circuit
diagrams is also shown in Figure 37.8 This consists of
a bar and an arrow In a circuit, it can be considered
that current can flow in the direction of the arrow, but
not in the opposite direction, being blocked by the bar.
Zener diodes
A Zener diode, also referred to as a unidirectional
breakdown diode, is doped differently to a normal diode.
Normally, the Zener diode would be non-conducting but, if breakdown voltage was reached, the Zener diode would act as a shunt and carry current This would decrease the current through the instrument and protect it from overload damage.
Should the voltage drop, the Zener diode will again become non-conducting, and the meter will operate normally.
Transistor operation
A transistor is similar to two diodes connected together, so that it has three semiconductor regions and two p–n junctions There are two general types of transistors: n–p–n transistors and p–n–p transistors Both types operate in the same way, except that the current flows in different directions.
Trang 6Diagrams and symbols for these are shown in
Figure 37.10 The arrow on the emitter indicates the
direction of forward current flow (These have been
shown for conventional current flow.)
An n–p–n transistor has a wafer of p-type
conductor material between two layers of n-type
semi-conductor material, while a p–n–p transistor has a wafer
of n-type material between two layers of p-type material.
There are three external connections, one for each
region The centre one is the base B, one side is the
collector C and the other side is the emitter E.
■ Simply, in operation C collects charges, E emits
charges and B operates the transistor.
n–p–n transistor action
In Figure 37.11, an n–p–n transistor is used as the
switching device for a lamp With the manual switch
SW off, the p–n junctions in the transistor prevent
current from flowing through the transistor, and the
lamp does not light.
When the manual switch is closed, a low voltage is
applied across the p–n junction between B and E The
voltage is low because there is a voltage drop across
the resistor R.
This makes this part of the transistor conductive so
that a small current flows from the positive pole of
the battery, through the transistor from B to E to the
negative pole of the battery.
The wafer of p-type material is very thin, and the
voltage applied between B and E will also influence
the p–n junction between B and C and so this part of
the transistor also becomes conductive.
Current can now flow from the positive battery terminal, through the lamp and right through the transistor from C to E, to reach the negative battery terminal As a result, the lamp will light.
While most transistors are small, larger ones are sometimes used A large transistor is shown in Figure 37.12 This is part of an electronic-ignition control unit and is used as a switching device It is attached to a metal container that encloses other parts, but the metal also acts as a heat sink.
The transistor generates heat that could cause damage and this is shed into the heat sink.
■ A heat sink has a relatively large metal surface that
is often provided with fins Heat from its surface area is dispersed into the surrounding air.
figure 37.10 Transistor diagrams and symbols
E emitter, C collector, B base
figure 37.11 Action of an n–p–n transistor – the transistor
symbol is shown at the top of the diagram
figure 37.12 A large transistor is mounted in the heat sink
of an electronic control unit MITSUBISHI
Other electronic components
There are a number of other electronic components with semiconductor materials Some of these are noted under the headings that follow.
Trang 7Darlington pair
This is a name given to two transistors that are
connected together to form a single component It has
three external connections, E, C and B just like a
single transistor, but has better switching and
amplification characteristics which make it suitable
for use in electronic timing circuits One of these units
forms part of the circuit of an electronic voltage
regulator.
Resistors
Resistors are diodes in which the semiconductor
materials have been arranged to retard the movement
of holes and electrons They act as a resistance to
provide a voltage drop or to reduce the flow of current
in a circuit.
Wire-wound resistors and carbon resistors that
carry higher loads were covered in the previous
chapter, but semiconductor resistors that are used in
electronic circuits are much smaller than those.
Capacitors
A capacitor is a form of diode that has its
semi-conductor material doped so that electrons and holes
are able to accumulate They are used in parallel with
other components to act as a buffer and absorb voltage
pulses.
The simplest way to consider a capacitor is as
a small reservoir for electric charges The charges
are temporarily stored and then fed back into the
system.
■ Most large capacitors that are used in electrical
circuits do not have semiconductor materials but
are made of insulated metal foil.
Integrated circuits
An integrated circuit (IC) is a composite electronic
component that is made up of a number of
semi-conductor devices It can include diodes and
transistors, resistors and capacitors.
The various devices are formed on a single silicon
chip to create a completely integrated circuit This is
quite small in size and ideal for use in the
micro-circuits which are used in electronics.
The alternative to an integrated circuit is a printed
circuit board with a number of components attached to
it Obviously, this would be much larger than an
integrated circuit.
Thermistors
The name thermistor is an abbreviation for thermally sensitive resistor As this indicates, thermistors are semiconductor resistors in which the resistance varies
in relation to its temperature Some thermistors have more resistance when hot than when cold, and so are better conductors at lower temperatures They are known as ptc resistors (positive temperature co-efficient resistors).
Other thermistors are designed to have less resistance when hot, and are known as ntc resistors (negative temperature coefficient resistors) These types of semiconductor devices have applications in temperature control where they can be used to sense changes in temperature, for example, in the cooling system of the engine.
Figure 37.13 shows how a temperature sensor for
a cooling system operates It consists of a thermistor
in a housing that is screwed into the water-jacket of the engine This senses the temperature of the coolant.
The sensor in the illustration is connected to an ohmmeter to check its resistance while it is being heated in a container of water The sensor should have less resistance when hot than when cold.
If the sensor is operating correctly, the pointer of the ohmmeter will move down the scale as the temperature of the water increases to show that there is less resistance when hot than when cold.
figure 37.13 The sensor for a temperature gauge of a
cooling system has less resistance when hot than when cold HOLDEN LTD
Thyristors
A thyristor is another form of transistor It has four semiconductor regions It is a switching device that can be operated by an electric pulse It also has rectifying properties.
Trang 8Voltage-dependent resistors (VDRs) have a high
resistance at low voltage, but this rapidly decreases as
the voltage increases They can be used for
voltage-overload protection and other forms of voltage control.
Light-dependent resistors (LDRs) are made of
semiconductor material that is sensitive to light The
conductivity will increase (and the resistance decrease)
as the LDR is exposed to brighter light.
This type of component could be used with a
system that is required to function according to
whether it is day or night.
Light-emitting diode (LED)
A light-emitting diode is made in such a way that light
is emitted from its p–n junction when a current is
passing through it Transparent material enables the
light to be seen as a small red or green light.
Light-emitting diodes are used singly as an
indicator or warning light, or a number of them can be
arranged to form numbers or letters These types of
indicators are made up of small dots or segments
which are illuminated whenever current is passed
through the segment.
Liquid-crystal display (LCD)
This uses a liquid crystal that is capable of polarising
light Numerals on the face of the instrument are
arranged in segments (as for an LED) and these are
made visible by the polarised light.
During polarisation, certain light rays are reflected
onto the face of the instrument and these are used to
illuminate segments and so form the numbers or letters
required Voltage applied to different parts of the
crystal produce the different displays.
Figure 37.14 shows how digital numerals consist of
a number of segments Altering the segments that are
illuminated enables different numbers or letters to be
displayed.
Vacuum fluorescent display
A vacuum fluorescent display uses electrons to bombard a small phosphorus-coated glass screen As the electrons strike the screen, they emit a green light The segments of the display are controlled by an electronic switching arrangement This operates in such a way that the appropriate segment is illuminated and a number or other symbol is displayed.
■ This display is produced in a somewhat similar way
to the image in a television picture tube.
Use of electronic components
Small electronic components are used together in circuits to form larger components, or units, such as control units, voltage regulators, electronic-ignition modules, electronic fuel-injection control units and electronic dashboard instruments.
To make up these larger components, a printed circuit board is used and the small electronic com-ponents are soldered to the conductors on the printed circuit Because the individual electronic components are so small, electronic units can be very compact Diodes, transistors, resistors, integrated circuits and capacitors can all be mounted on or built into the printed circuit board and connected together by the printed conductors.
The following headings cover some of the appli-cations of electronic components and provide an appreciation of how they are used.
Charging system
An automotive charging system can be represented by
a block diagram as shown in Figure 37.15 The arrangement operates in the following way:
1 The alternator is providing alternating current (ac) This is being rectified to direct current (dc) by the rectifying diodes which allow current to flow in one direction only.
2 The current leaving the diodes is now direct current (dc) and so can be used to charge the battery and operate the various electrical components of the vehicle.
3 The voltage regulator is a control unit which can be classed as a subsystem Its transistors and diodes act as control and switching devices to provide a feedback to the alternator.
4 The feedback to the alternator regulates the alter-nator output, so that it acts as a closed-loop system.
figure 37.14 Digital numbers are made up of a number of
segments formed by light-emitting diodes or liquid crystals
Trang 9In this illustration, block diagrams have been used
for simplicity and these are often sufficient to provide
an understanding of a system However, where more
detail is needed, the complete charging system can be
shown as a circuit diagram with the individual
components represented by symbols.
Alternator and regulator
An alternator and its electronic voltage regulator are
shown in Figure 37.16 Its function is to control the
alternator voltage and limit its maximum output The
regulator unit is mounted either on the back of the
alternator or inside the alternator It is sealed and
cannot be adjusted or repaired If it gives trouble, then
it is replaced with a new unit.
A basic circuit diagram for a battery charger is shown in Figure 37.17 This is a very simple charger that uses only one diode Large chargers use a number
of diodes.
The primary side of the transformer has 240 volts
ac, and this has to be reduced and rectified to a nominal 15 volts dc, which is a suitable voltage to charge an automotive battery This is accomplished by means of a step-down transformer that reduces the voltage, and a diode in the secondary circuit which rectifies the current.
The alternating current in both the primary and secondary windings is constantly changing The waveform for this is shown in Figure 37.18(a) as an
figure 37.15 Block diagram of the charging system
figure 37.16 An electronic voltage regulator is used with
an alternator MITSUBISHI
Diode as a rectifier
Apart from being used in motor vehicles to rectify
alternator current, diodes are used with many other
types of electrical equipment A battery charger is an
example of how diodes can be used A transformer is
used to reduce the mains voltage, and diodes act as
one-way valves to rectify the current.
figure 37.17 Circuit of a simple battery charger
C capacitor
figure 37.18 Current waveforms
(a) alternating current (b) half-wave rectifica-tion (c) peaks smoothed by use of a capacitor
Trang 10undulating curve, and this would be the effect if the
diode was not fitted.
However, the action of the diode, in allowing
current to flow in only one direction, produces a
waveform as shown in Figure 37.18(b) This is referred
to as half-wave rectification A reversal of current does
not now occur, but only half the output is being used
and this produces surges as shown.
Figure 37.18(c) shows the effect of the capacitor
(C) in the circuit The capacitor holds a charge
temporarily, and then discharges it back into the
circuit This has the effect of smoothing out the surges
to produce a more uniform flow of current.
Power diodes
The diodes in the alternator carry relatively large
currents and are sometimes referred to as power
diodes The currents generate heat, which is
detri-mental to the diode.
To prevent damage from overheating, power
diodes, such as those in an alternator, are enclosed in a
metal case and then fitted into a heat sink The heat
sink is an aluminium moulding or extrusion that is
large enough to absorb and dissipate heat from the
diodes and so prevent them from reaching too high a
temperature.
The rectifier of an alternator is shown in
Figure 37.19 This has six power diodes mounted
in heat sinks Six diodes are needed to provide
full-wave rectification of the ac produced by the alternator.
The diode as a protection device
Figure 37.20 shows a diode being used as a protection device It prevents an electrical component from being damaged due to incorrect battery polarity.
Normally, the diode would provide a circuit path to connect the component to the battery so that current flows from the positive battery terminal to the negative battery terminal However, if the battery is connected with its polarity reversed, then the diode would be reverse-biased and current would be prevented from flowing through it.
The arrow head of the diode indicates that current can flow in that direction, but is blocked off in the reverse direction by the bar across the point.
■ While diodes are used as protective devices in some circuits, installing the battery with reverse polarity could damage electronic components.
Electronic systems
There are two ways in which the operation of an electronic system can be considered The complete circuit diagram is used, with all the components shown
as symbols together with their connections, or block diagrams similar to Figure 37.15 are used.
The latter method is often referred to as a systems approach When using this, the system as a whole is considered but it is broken into basic blocks, which represent subsystems The blocks can also be referred
to as black boxes.
A block diagram for a basic electronic system
is shown in Figure 37.21 This has the following parts:
1 A sensor This collects some form of information and provides it as an electrical signal.
figure 37.19 Power diodes are mounted in a heat sink to
prevent overheating
figure 37.20 A diode being used to protect a component
against reverse polarity