Design & Technology

dehisceforkElectronics - Devices

Nov 2, 2013 (3 years and 7 months ago)

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Design & Technology

Materials and components

There are three types of materials used in
electronics

[electronics:
making devices which operate using the flow of electrons through
integrated circuits
-
such as TVs, radios and computers
]
: electrical
conductors, electrical insulators and semi
-
conductors.

Electronic components are divided into two groups: discrete
electronic components
-

such as diodes, transistors, capacitors
and resistors
-

and integrated circuits. You need to know what
the common discrete components are used for, and to
understand ohms and resistance values. You also need to know
how to represent

components using symbols when drawing
circuit diagrams.

Materials

There are three types of materials used in electronic components:

1.

Electrical conductors

are materials that allow electricity to flow
through them easily.

2.


An electrical insulator

Electrical insulators

are materials that prevent electrical flow. In
the diagram below the insulating material (plastic) surrounds the
conducting material (copper wires)

3.

Semi
-
conducting materials

exhibit both conducting and
insulating properties. The way i
n which the material is connected
to a power supply determines whether it will conduct an electrical
current or prevent it from flowing.

The most common semi
-
conducting material is
silicon
. Silicon needs
to have very small amounts of other elements such as

boron and
phosphorous added to it in order to become a semi
-
conductor. This is
called doping. Doped silicon is used to make components such as:



Transistors



Diodes



Integrated circuits

The simplest kind of semiconductor device is a diode. In a diode the
e
lectrical current can be made to flow in one direction only (see
diagram below). If the diode is reversed the flow of current is stopped.
This behaviour is due to the semi
-
conducting property of the doped
silicon.


A diode

Another semi
-
conducting material

is
germanium
, but this material is
used less widely than silicon.

The ease with which electricity flows through a material is called its
resistivity
. The value of resistivity is measured in
ohms/meter
. The
higher a material's resistivity, the more
difficult it will be for electricity
to flow through it:



Insulators

have very high resistivity values.



Conductors

have low resistivity values.

Components

Electronic components can be divided into two groups:

1. Discrete electronic components

These are separate components
that you can combine together to make a circuit on a breadboard,
printed circuit board or veroboard (discrete means separate).
Examples are
resistors

[
resistors
:
components which resists or 'slows
down' the current in a circuit by acting against the flow of electrons.
Resistance is measured in ohms.

]
,
transistors

[
transistors
:
components which do not conduct electricity, unless they are turned
on by a (different) electrical current. This means they can be used as
switches, amplifiers and in other ways.

]
,
capacitors

[
capacitors
:
circuit components which store and discharge electrical current. It is
made from two parallel metal plates separated by
an insulator (called
a dielectric).

]
,
relays

[
relays
:
type of switch which which uses an
electromagnetic coil to connect two or more contacts,
which close
when the coil is energised. Use of relays enables a very small voltage
in the relay circuit to control a much larger one in a separate circuit.

]

and
light emitting diodes or
[
LEDs
:
stands for Light
-
Emitting Diodes.
LEDs glow when current passes through them.

]
.


Discreet components

These components are called discrete because you can select them
individually and combine them to make up the circuit you require.
Discrete components can also be used as external components of an
integrated circuit system. For example a 555
astable

[
astable
:
(a
circuit) having two states, neither of which is stable. An astable circuit
therefore oscillates between the two states, giving a constant

on/off
digital output. Used, for example, to make LEDs flash continuously.

]

integrated circuit requires two discrete resistors and a discrete
capacitor to make it work.

Integrated circuits (ICs)

These are miniature circuits etched on to a piece of silicon or
chip
.
These chips are encapsulated inside a protective plastic package, and
nowadays are manufactured in vast numbers. The circuits inside the
package are arranged in different configurations
for particular
purposes, but the most common type of configuration is called the
dual
-
in
-
line or DIL

package, which has two rows of connecting 'legs',
one on each side.


A Surface Mount package integrated circuit
-

far smaller than a finger
-
tip

You don't
need to understand how the circuit inside a silicon chip
works
-

there's some quite complicated physics involved. It's best to
think of ICs simply as input
-
output process blocks, as shown below:


Input
-
output process block

When using ICs you need to know which pins need to be connected,
the function of each pin and how the IC is connected to the power
supply. A circuit diagram that includes one or more ICs should show
the pin numbers and how the pins are connected to the res
t of the
circuit.

Diodes

A diode is the simplest form of semiconductor. Diodes are a discrete
component that allows current to flow in one direction only. The
direction that current is allowed to pass is called the
forward bias
.
The direction that current is not allowed to pass is called the
reverse
bias
. A diode has two leads: for forward bias, the current comes in at
the
anode

(positive lead) and out at the
cathode

(negative lead).

Rectification

A common use for diodes is rect
ification
-

that is, the changing of
alternating current into direct current. (An alternating current (AC) is
one which flows alternately in opposite directions around a circuit,
while a direct current (DC) is one that always flows in one direction
only.)

A rectifying circuit can be found in the transformers used with many
types of equipment that require a mains alternating current to be
converted into a smaller direct current
-

eg electronic keyboards or
mobile phone chargers.

The circuit diagrams show the

two methods of rectification.


Full wave rectification:


Half wave rectification:

Light emitting diode


Light emiting diode (LED)

A light
-
emitting diode or LED is a special kind of diode that glows
when electricity passes through it. The LED is made from a semi
-
conducting material called gallium arsenide phosphide. LEDs can be
bought in a range of colours. In common with all diodes,
the LED will
only allow current to pass in one direction. The current required to
power an LED is usually 25 mA.

Seven
-
segment LED displays


Seven
-
segment LED display

A seven
-
segment LED is a special type of LED display used in digital
clocks, video recorders and microwave ovens.

Transistors

Electronics began with the development of the transistor in the 1950s.
Transistors are essentially tiny semiconductor amplifiers
and/or
switches, several thousands of which can be put on a 1mm
2

piece of
silicon.

Transistors have three leads: the
emitter
,
collector

and
base
. The
base lead controls the transistor: applying an electrical current to the
base lead switches the transistor

on. When the transistor is on,
current flows from the collector to the emitter
-

but when it is off no
current will flow.


A transistor and its three leads

Transistors are easily damaged, so it is important not to mix up the
three leads. To help
identify the leads some transistors have a dot
near the collector, and/or a tab near the emitter. Each type of
transistor is identified by a code printed on the side.

Darlington pairs

Some transistors can take a very low current flowing in the base and
amp
lify it to give a much higher current in the collector (called
gain

[
gain
:
the amount of amplification of the input voltage
-

ie

how much
bigger the output voltage is compared to the input voltage

]
). Other
transistors can output a large current at the collector
-

but without very
much gain. Single transistors cannot have both high gain and high
collector current.

To overcome this
problem, a high
-
gain transistor is paired up with a
high
-
current transistor in what is called a Darlington pair. The
combined transistors allows both a higher gain and a larger amount of
current to flow than would be possible with a single transistor.
Darl
ington pairs are often used to drive motors.


Circuit diagram for a Darlington pair

Transistors

are often used as interface devices
-

that is, devices
which ensure that the right amount of current is supplied to power
another device, such as an output co
mponent. Examples of output
components that might require transistors are direct current motors,
solenoids and meters.

Capacitors

A
capacitor

is a discrete component which can store an electrical
charge for a period of time. The larger the capacitance the more
charge it can store.

The unit of measurement of a capacitor is the
farad
. Often you will
see capacitors of much less than a farad. These

will be measured in
microfarads

(one millionth of a farad or 1/1,000,000) or
picofarads

(one million
-
millionth of a farad or 1/1,000,000,000,000).

There are two types of capacitor:



polarised or electrolytic capacitors, and



non
-
polarised or
non
-
electrolytic capacitors

Polarised capacitors




Polarised (electrolytic) capacitor

These generally have larger capacitance values. Polarised capacitors
have a positive pole and a negative pole, so you have to connect
them to a circuit the correct way
round. The polarity and value of a
capacitor are usually shown like this:



Mounting of polarised capacitors


Image shows two electrolytic capacitors. One is axially mounted, one
is radially mounted.

Electrolytic capacitors may be either axially mounted (on their side,
connected at each end) or radially mounted (upright with both
connections at the bottom).

Non
-
polarised capacitors

These are usually much smaller than the polarised type, and have
smaller capacitance values ranging from a few picofarads to a few
microfarads. Because they have no positive or negative poles these
capacitors can be connected to a circuit either way round.

There are
four types of non
-
polarised capacitor, each named after the material
they are made from:



Polyester



Polystyrene



Mica



Ceramic

Applications of capacitors



Smoothing

rectified alternating current voltages into steady direct
current voltages



Blocking

direct current signals whilst allowing alternating signals
to pass



Filtering out

unwanted portions of a fluctuating signal



Timing

applications



Storing charge

to keep a transistor turned on or off

Resistors

Resistors are components which restric
t or resist the flow of current.
The ability of a material or component to resist current flow is
measured in
ohms

[
ohms
:
units of electrical
resistance, usually
shown by the symbol R. 1 volt will force a current of 1 amp through a
resistance of 1 ohm.

]
. There are three main types of resistor:



Fixed resistors



Variable resistors, and



Special resistors, such as thermistors

and light
-
dependant
resistors (LDRs)

Fixed resistors

These are the most common type of resistor. They are found in nearly
every electronic circuit. Their three most important uses are:




A light
-
emitting diode (LED) protected by a fixed resistor

Protecting other components (such as an LED) from damage by too
much current.




A circuit diagram for a temperature detector

As
potential divid
ers

[
potential dividers
:
components which split a
circuit's voltage into two. Potential dividers consist of two resistors in
series.

]

(or voltage dividers).

A fixed resistor is used to split voltage
between different parts of the circuit. Potential dividers are used, for
example, with LDRs in circuits which detect changes in light.




A circuit diagram for a timing application

In timing applications. In this
role a fixed resistor is used with a
capacitor in series.

Variable resistors or potentiometers

There are two types of variable resistor:



The first type of variable resistor can be altered continually as they
work. For example the volume control in a radio.



The second type is called a pre
-
set potentiometer. It has a
resistance control that is adjusted and then fixed. These resistors
would n
ormally be adjusted once only.

The main difference between the two types of Potentiometers is their
size. The pre
-
set potentiometers tend to be smaller and are usually
adjusted with a screwdriver. A variable resistor is generally provided
with a long spind
le onto which an operating knob is attached.

Special resistors

Thermistors

change resistance as temperatures change. Most
thermistors have a negative temperature coefficient
-

meaning their
resistance falls as temperature increases. Thermistors are used i
n
temperature
-
sensing circuits.

Light
-
dependent resistors (LDRs)

have a resistance which changes
in response to changes in light levels, as detected by a photo
-
sensitive plate on the resistor. Most LDRs have a negative light
coefficient
-

meaning that thei
r resistance falls as the amount of light
falling on them increases. LDRs are used in light
-
detection circuits.

Ohms and resistance values

Ohm

The ohm is the unit of resistance. Larger values are measured in kilo
-
ohms (1000 ohms) and mega
-
ohms (1,000,000
ohms). Resistors are
marked, using a code specified in British Standard 1852, as follows:



The letter
R

means
ohm
. Numbers coming before the R indicate a
value more than one. So 1R (or 1R0) = 1 ohm; 47R = 47 ohms;
and 4R7 = 4.7 ohms. Numbers coming after the R indicate a value
less than one
-

so R56 = 0.56 ohms.



The letter
k

means
kilo
-
ohm
. Numbers coming before the

k
indicate a value more than one, while numbers coming after the k
indicate a value less than one. So 1k8 = 1.8 kilo
-
ohms and 5k6 =
5.6 kilo
-
ohms.



The letter
M

means
mega
-
ohm
. Numbers coming before the M
indicate a value of more than one, while numbers a
fter the M
indicate a value less than one. So 2M = 2 mega
-
ohms, and 2M2 =
2.2 mega
-
ohms

Resistance values

The resistance value of a resistor is shown by a series of coloured
bands.



The first band denotes tens, and the second band units. Each
colour stands for a different unit: black is zero, brown is one, red is
two; orange is three; yellow is four; green is five; blue is six; violet
is seven; grey is eight; white is nine. So the seq
uence red
-

red
denotes the value 22.



The third band is the multiplier. Black denotes a multiplier of one;
brown 10; red 100; orange 1000 and so on. So the sequence red
-

red
-

red denotes a value of 22 x 100, or 2.2 kilo
-
ohms.



The fourth band is the tol
erance. Manufacturers of resistors
cannot guarantee the exact resistance figure shown by the first 3
bands, so they give a percentage value by which the resistance
may be higher or lower than the resistance quoted. A red band
denotes a tolerance of 2 perce
nt; gold a tolerance of 5 percent;
and silver a tolerance of 10 percent. Thus a 100 ohm resistor of
10 per cent tolerance has an exact resistance value falling
somewhere between 90 ohm and 100 ohm.


Graphical table summarises the colour coding found on
the four
bands on a resistor.

Potential dividers

Potential dividers are used for dividing up the voltage, so that a part or
parts of a circuit only receive the voltage they require. Potential
dividers consist of two or more components (usually
resistors

[
resistors
:
components which resists

or 'slows down' the current in a
circuit by acting against the flow of electrons. Resistance is measured
in ohms.

]
) arranged
in series

[
in ser
ies
:
connected to a circuit in such
a way that the same current flows through each component in turn.
Opposite of in parallel

]

across a power supply.

The circuit diagrams below show three common types of potential
divider: two
fixed resistors

[
fixed resistors
:
type of resistor whose
resistance remains constant. Opposite of a variable resistor

]

in
series, a fixed resistor and
LDR

[
LDR
:
Light Dependent Resistor, or
LDR, is a type of resistor which is affected by changes in light levels.
A cadmium sulphide layer causes a decrease in
resistance in the light
and increase in the dark.

]

in series, and a
thermistor

[
thermistor
:
type of resistor that changes resistance with tempe
rature
-

also called
a Temperature
-
Dependent Resistor. Usually the resistance decreases
with an increase in temperature (and vice versa)

]

and
v
ariable
resistor

[
variable resistor
:
type of resistor whose resistance can be
varied to change the amount of current flowing through it. Opposite of
a fixed resistor

]

in series. (Note that the resistors are usually drawn
vertically on a circuit diagram.)


a circuit diagram showing two fixed resistors arranged one above the
other


circuit diagram shows an LDR and fixed resistor arranged one above
the other


A circuit diagram, a Thermistor and variable resistor

Common uses of potential dividers

Potential dividers are important in both transistor
-
switching circuits
and op
-
amp
comparator circuits

[
comparator circuits
:
circuit with a
comparator
-

a component which compares two voltages or currents,
amplifies the difference between them, and changes its output
depending on the result of the comparison

]
. The diagram shows a
darkness sensor circuit with a
transistor

[
transistor
:
components
which do not conduct electricity unless they are turned on by a
(different) electrical current. This means they can be used as
switches, ampl
ifiers and in other ways.

]

used as a switch. When the
LDR senses a drop in light, the LED is switched on.

When the LDR has light falling on it, its resistance is low
-

usually
around 400 Ohms. When the LDR is covered up the resistance
increases, (often t
o many kilo
-
Ohms). When the resistance of the LDR
is small its share of the voltage supply is small too, so Vout from the
voltage divider is small, and the transistor is switched off.

In the dark the large resistance of the LDR takes a large share of the
v
oltage supply, so Vout is large and the transistor and LED both
switch on.

Standard symbols guide

The standard symbols for the key components used in electronic
circuits are shown in the tables below. You may find it useful to print
off copies of these
tables to use for reference.


Electronic circuit diagram components

Some more common symbols, including output components and logic
gates, are shown in the table below.


Graphical table showing some more standard symbols for the key
components used in el
ectronic circuits

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