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ELECTRONIC AND ELECTRICAL
COMPONENTS
Electronic and Electrical Components can control the Current in various ways if they are connected in an
Organised manner in a Circuit.
RESISTOR
Although all Components restrict the flow of Electrons ( Current ) to a certain degree, Resistors are
specifically made to cut down the Current in a Circuit. They are used to control the Voltage and the Current
for other Components, or to prevent a delicate Component from being damaged by too much Current.
Resistors are the most commonly used Components in Electronic Circuits.
Resistance of a Resistor is measured in ohms ( Ω ). Ω is the Greek letter omega. A Resistor's Value
can be from a few ohms to millions of ohms.
There are four different types of Resistor.
1.
2.
3.
4.
Fixed Resistor
Variable Resistor
Light Dependent Resistor ( L.D.R. )
Thermistor ( or Thermal Resistor )
Fixed Resistor
Fixed Resistors are very cheap and easy to use. A Fixed Resistor's Value is shown by Coloured Bands
Painted on it ( please refer to Electronic Components - Resistor Colour Code ). Apart from the Resistor's
Value, three other factors have to be taken into account when choosing a Resistor.
i. The Power Rating - When the Current flows through a Resistor, some Heat is produced which is
measured in watts ( W ). If the rate at which a Resistor changes Electrical Energy into Heat is more than
the Power Rating, it gets Overheated, its Resistance Value will change and it could be damaged. A larger
size Resistor has greater Power Rating. Resistors can have Power Rating of 1 /4 W, 1 /2 W, 1 W, 2 W and
up to 10 W for unmarked Resistors.
Resistors with Power Rating of 1 /4 W and 1 /2 W are normally used in most Electronic Circuits.
ii. The Tolerance - Since Resistors are made by Mass Production Method, their Exact Value is Not
guaranteed. The Tolerance gives the Minimum and Maximum Values a Resistor can have. For example a
Resistor with a Value of 1000 ohms ( Ω ) and a Tolerance of +10% will have a Value between 900 ohms
and 1100 ohms. In most Electronic Circuits, the Exact Value of Resistors is Not important.
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iii. The Stability - The ability of a Resistor to keep the same Value after a long Time with Temperature
changes and other physical conditions is called Stability. This is quite important in some Electronic Circuits.
Types of Fixed Resistors
There are different types of Fixed Resistors; Carbon Film, Carbon Composite, Wire Wound and
Metal Oxide. The most common type is the Carbon Film Resistor in which a thin film of Carbon is
deposited over a Ceramic Rod and protected by a tough insulating coating with each end having a Wire
Leg connection. The Resistance depends on the type of Carbon used. Their Values vary from just a few
ohms, to 10 megohms (10 million ohms). Carbon Film Resistors have ±5% Tolerance with a very good
Stability and their Ratings are from 0.125 W to 1 W. Carbon Composite Resistors are made of a mixture
of Carbon ( a Conductor ) and Clay ( a Non-Conductor ) pressed and moulded into Rods by Heating. Their
Values, Ratings and Cost are similar to Carbon Film Resistors but Tolerance is ±10% with Poor Stability.
Metal Oxide Resistors have similar appearance and construction to Carbon Film Resistors with Carbon
being replaced by Tin Oxide. Their Ratings are 0.5 W and Tolerance is ±3% with High Stability over a long
Time. Wire-Wound Resistors are made of three Alloys Manganin, Nichrome and Constantan Wires
wound on a tube with a protective coating. These Alloys have higher Resistance than Copper. Wire Wound
Resistors have High Stability, Low Tolerance with Large Power Ratings. Their Values are from a fraction of
an ohm to about 25 kilohms ( kΩ ) depending on the Diameter and Length of Wire used.
Fixed Resistors have two Leads which can be connected either way round to the Circuit.
Fixed Resistors
Circuit Symbol for a Fixed Resistor
© Jila Yousefzadeh, June 2001
RESISTOR COLOUR CODE
A Fixed Resistor's Value and Tolerance is determined from the Colour Stripes painted on it. The three
Stripes closest together show the Value of the Resistor and the fourth Stripe is the Tolerance. The first
two Colour Stripes give the first two Numbers of the Resistor's Value. The Colour of the third Stripe shows
number of Zeros to be added after this Number.
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VALUE CODE
TOLERANCE CODE
Number Colour
0
1
2
3
4
5
6
7
8
9
Black
Brown
Red
Orange
Yellow
Green
Blue
Violet
Grey
White
Percentage
Colour
± 1%
± 2%
± 5%
± 10%
± 20%
Brown
Red
Gold
Silver
No Fourth Band
1st Number 2nd Number
Number of 0s
Tolerance
Blue
Grey
Brown
Silver
For example a Resistor with Blue - Grey - Brown and fourth Silver Band is 680 Ω ± 10% i.e. it may have a
Value between
680 + 680 x 10/100 = 680 + 68 = 748 Ω
and
680 - 680 x 10/100 = 612 Ω.
The exact Value of Fixed Resistors is Not crucial in most Electronic Circuits.
Resistor Conversion Table
1 kΩ ( kilohms ) = 1000 Ω = 103 Ω
1 MΩ (megohms ) = 1000,000 Ω = 106 Ω
Variable Resistor or Potentiometer
A Variable Resistor is useful when a change of Resistance is needed while a Circuit is working. They can
be used to control the Current and Voltage in a Circuit by changing the Value of the Resistor. Volume
Controls on Radios, CD Players and TVs are Variable Resistors. Variable Resistors are of different Sizes
and Shapes. Their Values are from a few ohms to a few megohms. The common types have three
Terminals. They consist of an Incomplete Circular Resistive Track which runs between the two outside
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Terminals. The Resistive Track is either a Fixed Carbon Resistor for low Power and high Values or a Fixed
Wire Wound Resistor for high Power. A Slider which makes contact with the Track and moves along it, is
connected to the middle Terminal. The moving Slider changes the Resistance between middle and the
end Terminals. Variable Resistors can be used in two ways. They can be used in a Circuit to control the
Current by connecting to the middle and one end Terminal. By turning the Slider, as the Resistance
increases the Current decreases. It can also be used as Voltage or Potential Divider by connecting to the
three Terminals. A very small Variable Resistor called a Preset Resistor has Tracks of Carbon or Cermet
(Ceramic and Metal Oxide ) and its Resistance can Only be adjusted with a very small Screwdriver inserted
in the Slot at the Centre of the Resistor and turned to the left or to the right for a Minimum or a Maximum
Value. The Resistance of another type of Variable Resistor which is much larger in size than the Preset
type and of different shape can be changed by turning the Knob on the Resistor with fingers. There is
also another type called Slide Potentiometer ( Variable Resistor ) which has a Straight Track and its Value
can be changed by moving the Slider forwards or backwards.
In Variable Resistors with Linear Tracks, when the Slider is turned through equal angles, change of
Resistance is equal. In Variable Resistors with Log Tracks for equal angular rotation of the Slider, the
change of Resistance at one end of the Track is less than the other end.
Variable Resistors ( Presets )
© Jila Yousefzadeh, June 2001
Variable Resistor
Circuit Symbols for a Potentiometer
and a Variable Resistor
© Jila Yousefzadeh, June 2001
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LIGHT DEPENDENT RESISTOR ( L.D.R. )
A Light Dependent Resistor ( L.D.R. ) is a light sensitive device which is suitable for detecting Light levels.
A L.D.R. is made of a Semiconductor material called Cadmium Sulphide. Its Resistance decreases as
the amount of light falling on it increases. ORP12 is a popular L.D.R. with a thin layer of Cadmium
Sulphide situated behind the clear window. Its Resistance increases from about 1 kilohms ( 1000 ohms )
in Daylight to 10 megohms (10,000,000 ohms ) in the Dark. L.D.R.s have two Leads which can be
connected either way round to the Circuit like most Resistors. L.D.R.s are used in Street Lights, in
Cameras to detect the amount of Light around, in Security Lights to detect intruder or to draw Curtains and
Blinds in daylight and at night.
10MΩ
Intensity of Light
1kΩ
L.D.R. Symbol
Dark
Daylight
Graph of L.D.R. Resistance against Intensity of Light
Light Dependent Resistor - ORP12
Light Dependent
Resistor ( L.D.R. )
© Jila Yousefzadeh, June 2001
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THERMISTOR ( or Thermal Resistor )
A Thermistor is a heat sensitive device. It detects changes in Temperature. A Thermistor is made of
Semiconducting materials. Its resistance depends on how Hot or Cold it is. Most Thermistors have Low
Resistance at High Temperature and a High Resistance at Low Temperature. These are called negative
temperature coefficient ( n.t.c. ) type Thermistors and are made of Copper, Manganese, Cobalt,
Nickel and other materials. The n.t.c. type Thermistors are used for Measurement and Temperature
Control and are heated either internally by the Current flowing through them or externally from the
surroundings. Positive temperature coefficient ( p.t.c. ) type Thermistors have High Resistance
at High Temperature and Low Resistance at Low Temperature. They are made of Barium Titanate and are
mainly used to prevent damage to the Components in Circuits which may experience a large rise in
Temperature e.g., to an overloaded Electric Motor. Thermistors can be used in Central Heating
Thermostats, Fire Alarms or Freezers ( as an Ice Detector ). There are different Shapes and Sizes of
Thermistors. They are called Rod, Bead and Disc. Thermistors have two Leads which can be
connected either way round to the Circuit.
Thermistors
-to
Circuit Symbol for a
n.t.c. Type Thermistor
© Jila Yousefzadeh, June 2001
CAPACITOR
A Capacitor stores Electrical Charge. In its simplest form it is made of two parallel metal plates
( Conductors ) separated by an insulating material called the Dielectric.
NOTE: A Dielectric is an insulating medium which has the capacity of sustaining an Electric Field.
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Many different types of Insulating Material are used for Capacitors, like Polyester, Mica, Paper, Ceramic,
Polystyrene and Polypropylene materials.
Capacitors are used in Televisions to store high Voltages needed to make them work. Capacitors can
remain highly charged for a while even after the equipment has been disconnected from the Power
Supply. This is one of the reasons why it is very dangerous to touch inside an Electrical Appliance that
uses Mains Electricity even when it has been unplugged.
The Charge-Storing ability of a Capacitor is called its Capacitance ( C ). Capacitance is measured in
farads ( F ) and it is named after Michael Faraday, a British physicist responsible for most of the
pioneering work in Electricity. Since a farad is a very large unit, most Capacitors have Values in
microfarads ( µF ). One farad is equal to 1,000,000 µF.
The Capacitance of a Capacitor depends on the Material of the Dielectric, Separation of the Plates
d ( large Separation gives small C ) and the Area A of the Plates ( large Area gives large C ).
Dielectric
d
C = ε x A/ d
where
C is the Capacitance, measured in farads ( F )
A is the Area of the Plates overlap, measured in square metres ( m2 )
Area
A
d is the Distance between the Plates, measured in metres ( m )
ε
is the Permittivity, measured in farads per metre ( F/m )
ε is called the Permittivity of the Dielectric material i.e. it is a measure of the ability of the material to
Store Electrical Energy in a given Electrical Field Strength.
There are two types of Capacitor: Polarised and Non-polarised. Polarised Capacitors have a Positive
and a Negative leg which can be identified by the marking on the Capacitor and must be connected the
correct way round in a Circuit so that Conventional Current enters their Positive Terminal if they are not to
be damaged. A Capacitor larger than 1 µF is normally Polarised. Electrolytic Capacitors are
Polarised. Non-polarised Capacitors can be connected either way round in a Circuit and are less than 1
µF. Ceramic, Polyester and Mica Capacitors are Non-polarised.
Apart from the Capacitor's Value, which is Not crucial in most Electronic Circuits, four other factors have
to be taken into account; Stability, Tolerance ( as mentioned in Resistors ) and two more factors,
Leakage Current and Working Voltage.
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i. The Leakage Current - The properties of Fixed Capacitors depend on the Dielectric used.
Since no Dielectric is a perfect insulator, the loss of Charge through the Dielectric is called Leakage
Current which should be Small.
ii. The Working Voltage - The maximum Voltage that can be applied across a Capacitor is called its
Working Voltage and it is written on the Capacitor. If the Working Voltage is exceeded, the Capacitor gets
damaged.
Polarised Capacitors
Non-polarised Capacitors
-
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+
Circuit Symbols for
Non-polarised and
Polarised Capacitors
Variable Capacitors
or
Circuit Symbols for a
Variable Capacitor
© Jila Yousefzadeh, June 2001
Variable Capacitor
A Variable Capacitor is made of two sets of parallel metal plates. One set is fixed and the other moves on a
Spindle. The Plates are separated by a Dielectric, normally Air but in some cases thin flexible Sheets.
Turning Spindle varies the Area of overlap ( A in the equation ) and therefore varies the Capacitance.
Preset Capacitors or Trimmers are small Variable Capacitors and they are used to make fine,
infrequent adjustments to the Capacitance of a Circuit. In Mica Compression type Variable Capacitors, the
mica sheets and the metal foil plates are compressed more or less by turning a Screw which changes the
Capacitance.
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Variable Capacitors are used in Radios to tune into stations. They are also used for filtering out unwanted
Electrical Signals.
Capacitor Conversion Table
1 farad ( F )
= 1,000,000 µF ( microfarads ) = 106 µF
1 farad ( F ) = 1,000,000,000 nF ( nanofarads ) = 109 nF
1 farad ( F ) = 1,000,000,000,000 pF ( picofarads ) = 101 2 pF
Charging a Capacitor
Direct Current can Not flow across the Plates of a Capacitor because they are separated by an Insulating
Material and the Capacitor behaves as if Alternating Current flows through it. Electric Current is flow of
Electrons and Electrons have Negative Charge. Therefore when Electrons flow, Negative Electric Charge
flows.
By Connecting a Capacitor across a Power Supply or a Battery, a Current will flow and the Capacitor fills up
with its Charge, so that the Plate connected to the Positive Terminal of the Power Supply becomes short
of Electrons because the Electrons are attracted to the Positive Terminal leaving this Plate with a Positive
Charge and the Plate connected to the Negative Terminal gains an equal number of Electrons because
the Electrons are repelled by the Negative Terminal giving this Plate a Negative Charge. When the
Capacitor is fully Charged the flow of Current stops and the Voltage between the two Plates is equal and
opposite Polarity to that of the Battery or Power Supply. If the Plate connected to the Positive Terminal
has Charge +
Q, then the Plate connected to the Negative Terminal has Charge - Q and the Capacitor
will have Charge Q.
The amount of Electric Charge Q is measured in coulombs ( C ). To provide a total Charge of one
Coulomb, about six million million million electrons are required. The flow of one Coulomb of
Charge through one point in a Circuit in one second, gives one ampere of Electric Current.
1 ampere = 1 coulomb per second
The Capacitance of a Capacitor C, is the property which determines the quantity of Charge Q that can be
stored for a given Voltage V, applied to the Plates.
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C = Q/V
where
C is measured in farads ( F )
Q is measured in coulombs ( C )
V is measured in volts ( V )
Discharging a Capacitor
If a charged Capacitor is disconnected from the Power Source and its Leads touched together, the
Capacitor will be discharged quickly. If a Lamp is connected across the Capacitor, it will light up for a short
Time while the Capacitor is being discharged.
DIODE
A Diode is a Semiconductor Component and consists of two pieces of pure and unidentical
Semiconductor materials either of Silicon ( Si ) or of Germanium ( Ge ) with very small amounts of
different impurities ( about one part in ten millions ) called Dopants added. There are two types of Doped
Semiconductor material, called P-type ( Positive ) and N-type ( Negative ) which refer to the type of
Charge Carriers in the material. The Dopants usually used are Boron to produce P-type Semiconductor
material and Arsenic or Phosphorus to produce N-type material. In a Diode pieces of N-type and
P-type materials are fused together. This forms a layer called the Depletion Layer at their junction
being a narrow layer of material that has a very high Resistance and there is a shortage of free Charge
Carriers needed for Conduction.
N-type
Depletion Region
+
+
+
+
Positive Charge
P-type
-
Forward Voltage
Negative Charge
A Diode acts like a one way street for Electricity and allows the Conventional Current to flow in one
direction Only i.e. from the Anode, the Positive Terminal, to the Cathode, the Negative Terminal.
Therefore a Diode must be connected the right way round in a Circuit otherwise Current will be stopped.
The Band round the Diode is near the Negative Terminal.
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To force the Current through the Diode, a small Voltage called Forward Voltage (
VF ) or Turn-on
Voltage is needed. The Forward Voltage for a Diode number 1N4001, which is a Silicon type, is
0.7 volts and for a Diode number OA91, which is a Germanium type, is 0.15 volts. The Value of
VF
is almost the same for a wide range of Forward Currents ( IF ). Too much Forward Current will damage
a Diode, therefore IF must be kept below a certain Value. The maximum average Forward Current
I F ( av ) for 1N4001 is 1 amp ( A ) and for OA91 is 50 milliamps ( mA ).
If a Voltage is applied to a Diode in a reverse direction, an extremely small Current called Reverse
Current ( IR ) will flow, which is negligible, but if too much Reverse Voltage is applied to a Diode, it will
Break Down and will let a destructive Current pass. To avoid this, Reverse Voltage must be less than the
maximum Reverse Voltage allowed i.e. less than the Peak Inverse Volts ( PIV ) of the Diode. PIV for
1N4001 is 50 V and for OA91 is 100 V.
Two identification Codes are used for Diodes. In the Continental System, the first letter represents the
Semiconductor Material ( B = Silicon and A = Germanium ) and the second letter gives the Use
( Y = Rectifier Diode, Z = Zener Diode, A = Signal Diode ), e.g., AA119 is a Germanium
Signal Diode. In the American System, the Code always starts with 1N followed by a Serial Number,
e.g., 1N4001. Some Manufacturers have their own Codes.
There are different types of Diodes.
Junction Diode
Junction Diodes are made of Silicon or Germanium. They consist of P-N junction with one connection to
the N side ( Negative or the Cathode K - normally marked with a Band ) and another to the P side
( positive or the anode A ).
Diode
Anode
a
k
Cathode
Circuit Symbol for a Diode
© Jila Yousefzadeh, June 2001
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Junction Diodes are used to convert Alternating Current ( A.C. ) to Direct Current ( D.C. ) which is called
Rectification. They are also used to prevent damage to a Circuit caused by a Reversed Voltage e.g.,
by connecting it reversed across the Terminals of a Relay.
A Silicon type Junction Diode is preferred to Germanium because it provides a better conversion of A.C.
to D.C. by having a much lower Reverse Current. Also Germanium type is more vulnerable to Heat than
Silicon type.
Zener Diode
A Zener Diode is a Silicon Junction Diode with an accurate Break Down Voltage. Since this type of
Diode is used to provide a Standard or Reference Voltage it is often referred to as a Reference Diode.
A Zener Diode has Anode and Cathode ( often marked with a Band ) Terminals. It normally has its
Reference Voltage value written on it. A Zener Diode conducts at about 0.6 volts when Forward Biased
like an ordinary Silicon Diode but since the Current increases sharply, it is usually used in Reverse Bias i.e.
the Cathode ( Negative ) end is connected towards the Positive Terminal of the Battery or Power Supply.
The Reverse Current ( IR ) is extremely small until the Reverse Voltage ( VR ) reaches its Break Down
Voltage, when IR increases suddenly and rapidly.
The Zener Diode may be damaged by overheating unless IR is limited by a Resistor placed in Series with
the Zener Diode to make sure that the Power dissipated by it is within its designed limits. By connecting a
Resistor in Series with the Diode, the Voltage across the Diode will remain Constant at Break Down
Voltage over a wide range of IR. This characteristic of Zener Diode makes it useful for Stabilising
( keeping steady ) the D.C. Output Voltage of a Power Supply ( please refer to Essential Equipment in
Electronics - Power Supply.
Zener Diodes
k
Cathode
a
Anode
Circuit Symbol for a
Zener Diode
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PHOTO DIODE
A Photo Diode is a Semiconductor Junction Diode that responds to the amount of Light falling on it. It has
two Terminals, the Anode and the Cathode. The p-n junction is in a case with a transparent window
through which Light can enter. A Photo Diode is operated in Reverse Bias i.e. the D.C. Voltage is
reversed and therefore the Diode acts like a High Resistance which allows a small Leakage ( Reverse )
Current to flow. The Light reduces the effective resistance by releasing more Electric Charge, therefore
the Reverse Current increases in proportion to the intensity of light falling on the Junction.
A Light Dependent Resistor ( L.D.R. ) can be used in the same way as a Photo Diode but it takes a
relatively long time to respond to changes of light level. Although in some cases a time of one-tenth of a
second could be acceptable but in many Electronic Switching Circuits the response time might need to
be much less than a micro second. Due to the fast response of the Photo Diode to a change of Light
intensity, it is used as a Detector of Infra-red Light in an Optical Communications System and as a Fast
Counter that generates a Current Pulse every time a Light beam is interrupted for example to read Holes
in Punched Cards. They can also be used to measure the intensity of light such as Light Meters for
Photography.
Photo Diode
Cathode
Anode
Circuit Symbol for a Photo Diode
© Jila Yousefzadeh, June 2001
L.E.D. ( LIGHT EMITTING DIODE )
A L.E.D. is an Opto-Semiconductor. It is a Junction Diode but looks quite different from an ordinary Diode.
By using specially selected N-type and P-type Semiconductor materials like Gallium Arsenide or
Gallium Arsenide Phosphide, it is possible to make the P-N junction emit Coloured Light from a
L.E.D. ( Light Emitting Diode ). A L.E.D. lights up like a Lamp, but unlike a Lamp will Only function
properly if it is connected the correct way round i.e. the Cathode ( Negative ) Lead is connected towards
Negative Terminal of the Battery and the Anode ( Positive ) Lead is connected towards the Positive
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Terminal of the Battery or Power Supply and a small amount of Conventional Current passes through it
from the Positive to the Negative. The longer Lead is normally the Anode and the shorter Lead is the
Cathode. The Negative Lead can also be Identified by a Flattened side of the Body of the L.E.D. A L.E.D.
is often used as an Indicator Light in equipment, such as CD Players, Computers and Hi-Fi systems to
show that Power is On or Off.
A L.E.D. emits light when Forward Biased i.e. when it conducts Electricity. It can easily be damaged if too
much Current is allowed to pass through it. Therefore a Current Limiting Resistor, with a Correct Value
depending on the Supply Voltage in use, must always be used in Series with the L.E.D. to control the
Current and prevent the L.E.D. from being damaged due to overheating unless the L.E.D. is of a
Constant Current Type that have a built-in constant Current generator and can be used over a wide
range of Supply Voltage without the need for an external Current Limiting Resistor. The maximum Current
for a Standard L.E.D. is about 20 mA ( milliamps ). The Voltage across the L.E.D. is usually about 1.8
volts to 2 volts and the Power consumed by a L.E.D. is about 20 milliwatts. This is one the great
advantages of L.E.D.s over Lamps which enables them to be used in small Battery Powered equipment.
Unlike Lamps, L.E.D.s do not operate by having any form of hot Filament and normally work at quite cool
Temperatures. Therefore they do not have the limited operating lives like Filament Lamps. They have
long life, high operating speed and are reliable.
A L.E.D. does not behave quite like Silicon or Germanium Diode from the Electrical point of view. Its
maximum Forward Voltage is quite high, and is normally about 2 volts and its Reverse Break
Down Voltage is quite low, and is normally about 5 to 7 volts. The Energy dissipated by ordinary
Diodes ends up as Heat, but since L.E.D.s are made of Gallium Arsenide or Gallium Arsenide Phosphide,
some of the Energy is emitted as Light. The cheapest visible light L.E.D. gives mainly red light.
L.E.D.s ( Light Emitting Diodes )
Anode
Cathode
Circuit Symbol for a
L.E.D.
© Jila Yousefzadeh, June 2001
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In
Infra-red L.E.D., used for Remote Control Application, the light Output is in the Infra-red
region, and there is No Output at Frequencies that the Human eyes can detect.
Standard L.E.D.s normally have low light Output levels and under Bright Ambient Light, it is
sometimes difficult to know if they are On or Off.
High intensity, Super bright and Ultra bright L.E.D.s are much Brighter than a
Standard type with the same amount of Current flowing through them. They are used in situations
where a Standard L.E.D. is Not adequate such as Alarms and Warning Devices.
Flashing L.E.D. Consists of a Standard L.E.D. with a small Integrated Circuit ( I.C. ) i.e. a Micro
Chip inside it that makes the L.E.D. flash On and Off. The small black dot inside the plastic bulb of the
Flashing L.E.D. is the I.C. Flashing L.E.D.s can work at a slightly higher Current than Standard L.E.D.s
i.e. about 40 to 50 milliamps ( mA ). Normally 9 Volts Voltage can be used across them and Series
Resistor is Not required. Flashing L.E.D.s are suitable for Alarm Systems and Warning Devices.
Flashing L.E.D.s
Anode
Cathode
Circuit Symbol for a
Flashing L.E.D.
© Jila Yousefzadeh, June 2001
Bi-polar L.E.D.s Consist of two L.E.D.s of the same Colour connected in Inverse Parallel in the
Same Package with two Lead out wires. They are similar in appearance to Standard L.E.D.s. A Bi-polar
L.E.D. needs a Current Limiting Resistor in Series to protect It from being damaged. The L.E.D. will glow
if connected either way to the Circuit.
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L.E.D.s are available in different Colours; Red, Yellow, Green, Amber, Orange, Blue, Bi-colour L.E.D.s,
Tri-colour L.E.D.s and Multi-colour L.E.D.s depending on the impurity content and composition of the
compound.
Bi-colour L.E.D.s are similar in appearance to Standard L.E.D.s. They consist of a plastic bulb
with two Lead out wires. Unlike the Standard L.E.D., the package contains two L.E.D.s of different
Colours connected in Parallel but in opposite directions with two Lead out wires. When the Current flows
in one direction through the L.E.D., it will Light with one Colour. When the Current flows in the opposite
direction, it will Light with its other Colour. Like the Standard L.E.D., a Bi-colour L.E.D. needs a Current
Limiting Resistor in Series to protect it from being damaged. Bi-colour L.E.D.s are available in Red-Yellow,
Red-Green and Green-Yellow.
Bi-polar and Bi-colour L.E.D.s
Circuit Symbol for Bi-polar
or Bi-colour L.E.D.s
© Jila Yousefzadeh, June 2001
Tri-colour L.E.D.s, have two L.E.D.s of different Colours in the same encapsulation but with
three Lead out wires. The two L.E.D.s are connected together with a shared Negative Terminal i.e.
Common Cathode method of connection. The two shorter Leads are each separate L.E.D. Positives
( Anodes ) and the longer Lead is the Negative ( Common Cathode ). Since there are Two Standard
L.E.D.s in the package, therefore a Tri-colour L.E.D. needs two Resistors in Series, Each connected to
the Positive Lead of each L.E.D. for protection. Tri-colour L.E.D.s are available in Red-Yellow, GreenRed and Green-Yellow. For example in a Green-Red L.E.D., by switching on one section or the other,
one L.E.D. lights Green, and the other Red. By connecting both L.E.D.s together at the Positive end
and varying the relative L.E.D. Currents, it is possible to obtain a range of Oranges and Yellows too.
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Components
Anodes
Tri-colour L.E.D.s
Cathode
Circuit Symbol for a
Tri-colour L.E.D.
© Jila Yousefzadeh, June 2001
Multi-colour L.E.D.s offer a wide combination of Colours. The L.E.D. contains Red, Green and
two Blue L.E.D. Chips housed in a white diffused or water clear package with six Lead out wires i.e. two
Common Cathodes and four Anodes. The L.E.D.s are terminated at connections enabling the user to
achieve a wide range of Colour variations.
Anodes
Multi-colour L.E.D.s
Cathodes
© Jila Yousefzadeh, June 2001
© 2014 NOVICE ELECTRONICS
Circuit Symbol for a
Multi-colour L.E.D.
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Components
L.E.D.s are also available in a variety of Sizes and Shapes; L.E.D. Cluster, Surface Mount L.E.D.s,
Lighthouse L.E.D.s, Cylindrical L.E.D.s, P.C.B. Mounting L.E.D.s, Rectangular L.E.D.s,
Rectangular Legend L.E.D.s, Triangular L.E.D.s, Square L.E.D.s, Tombstone L.E.D.s,
5 Segment L.E.D. Array, 12 Segment L.E.D. Array, L.E.D. Light Bars, L.E.D. Displays, Dot
Matrix L.E.D. Displays and Intelligent Dot Matrix L.E.D. Displays.
A
L.E.D. Cluster array contains many Ultra Bright L.E.D.s connected in Series or Series - Parallel
combinations. The Cluster could be used to illuminate Sign Systems.
L.E.D. Cluster
© Jila Yousefzadeh, June 2001
Circuit Diagram for L.E.D. Cluster Array in Series-Parallel
L.E.D. Display. An important type of L.E.D. Display is the Seven
Segment Display. They
have Seven rectangular Segments in a Figure of 8 pattern and an eighth Segment being used for
Decimal Point. It is possible to produce any Digit from 0 to 9, if the appropriate Segments are switched
On. The Segments have identification Letters from a to g plus dp for the Decimal Point. In Seven
Segment Display either all the Cathodes ( - ) or all the Anodes ( + ) are connected to a Common Terminal.
It is important to choose the correct type Display because Circuits are designed to work with one type or
the other. Seven Segment Displays are available in different Sizes and Packages and have more than
one kind of pinout arrangement.
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Components
Seven Segment Displays
a
f
b
g
c
d
dp
e
Seven Segment Display
© Jila Yousefzadeh, June 2001
a
b
c
d
e
f
g
dp
Common Anode Configuration of Seven Segment Display
a
b
c
d
e
f
g
dp
Common Cathode Configuration of Seven Segment Display
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Components
Dot Matrix L.E.D. Display
is a small panel having 35 L.E.D.s in a 5 x 7 L.E.D.s Matrix
Display with the L.E.D.s connected together in the Rows - Columns ( X-Y ) configuration i.e. in Row
Anode or Cathode Configurations. If a suitable Driver Circuit is used with this type of Display, a full range
of Alpha - Numeric Characters can be displayed. The 5 x 7 Dot Matrix L.E.D.s were used in early and
inexpensive Dot Matrix Printers.
Dot Matrix L.E.D. Displays
© Jila Yousefzadeh, June 2001
THYRISTOR
A Thyristor is one of a range of multi layer Diodes. It consists of a four Layer P-N-P-N structure with three
Terminals. The Terminal connected to the N region at one end is called the Cathode ( C ), the Terminal
connected to the P region at the opposite end is called the Anode ( A ) and the third Terminal
connected to the other P region is called the Gate ( G ).
Thyristors
A
A
P
N
P
N
G
G
C
C
The Structure and Circuit Symbol for a
Thyristor
© Jila Yousefzadeh, June 2001
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Components
The word Thyristor is derived from Greek Thyra ( Door ). A Thyristor does Not conduct when it is
Forward Biased until a Positive Voltage is applied to its Gate Terminal which lets a small amount of Current
to flow into the Gate and the Thyristor will switch On ( Fire ), allowing a larger Current to flow through the
Cathode and the Anode. Conduction continues even after removing the Positive Gate Voltage. The
Thyristor will stay On, or Latch. The Only way to Switch Off the Thyristor is when the Supply Voltage is
Reversed or switched Off.
A Thyristor used to be called a SCR ( Silicon Controlled Rectifier ) Since the Semiconductor
material used is Silicon. A Thyristor is a Controlled Diode. It Only conducts when a short Positive
Pulse is applied to the Gate and with a sufficiently Positive Anode Voltage i.e. 1 to 2 volts. The
Thyristor is a Rectifier Diode because it Only allows Current to flow in one way in a D.C. Power Control
and is a Half-Wave device in an A.C. Power Control, i.e. it allows the Current to be supplied to the Load
during Only part of each Cycle. During each Positive Half-Cycle of Input, a Gate Pulse is applied at a
selected stage which will allow the Thyristor to switch On and therefore Half the Power to the Load will be
achieved. The Thyristor switches Off during the Negative Half-Cycles.
TRIAC
A Triac consists of two Thyristors connected in Parallel but in opposite direction and controlled by the
same Gate. It is a two-directional Thyristor which allows Current to flow through it in either direction. A Triac
is triggered on both Positive and Negative Half Cycles of A.C. Input i.e. it allows full A.C. flow. A Triac has
three connections called Gate (
G ), Main Terminal 1 ( MT1 ) and Main Terminal 2 ( M T2 ).
The triggering Pulses are applied between Gate and Main Terminal 1. A Gate Current of about 50 mA
(milliamps ) may be adequate for a Triac handling up to 100 A ( amps ).
Triacs are used in Lamp Dimmer Circuits, Electric Drills and in Food Processors.
Triac
MT1
G
MT2
Circuit Symbol for a Triac
© Jila Yousefzadeh, June 2001
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© 2014 Jila Yousefzadeh