Download Variable resistors

Contents
Introduction
3
Linear resistors
4
Specification of resistors
5
Low power resistors
6
Medium and high power resistors
7
Non-linear resistors
Thermistors
9
9
Voltage-dependent resistors (VDR)
10
Light-dependent resistors (LDR)
11
Variable resistors
12
Identification of resistors
14
Labelled resistors
14
Colour coded resistors
14
Preferred values
16
Summary
24
Answers
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Introduction
This section aims to help you understand different resistor types and their
characteristics.
We will describe how resistor components are specified for a particular
application, the differences between low and high power resistors, and how
resistance values are marked on the component.
We introduce a variety of special purpose resistors including variable
resistors and resistors that change resistance in response to temperature,
light, and voltage.
After completing this topic, you should be able to:

describe the features of fixed and variable resistor types and typical
applications

list the characteristics of temperature dependent, voltage dependent,
and light dependent resistors giving applications of each

specify a resistor for a particular application

determine the resistance of a colour-coded resistor using a colour code
table and to confirm the value by measurement.
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Linear resistors
A resistor is obviously a component designed to offer resistance, or
opposition to an electric current. Resistors may be placed in a circuit to
produce a voltage drop, or to limit the circuit current to a desired value.
Symbols for some of the different types of resistor are shown in Figure 1.
Figure 1: Resistor symbols
The linear resistor is by far the most common type of resistor in electrical
and electronic circuits. Linear simply means that the resistance is designed
to remain constant. Non-linear resistors have a resistance which varies in
response to changes in temperature or voltage for example, and these will be
discussed later.
Look at the graph in Figure 2, and note that it is a straight line or linear
graph commencing at zero current and voltage. This type of graph for a
device is known as its ‘characteristic’, which describes how the current
through the device changes with the applied voltage. These graphs are
drawn from results taken when an increasing voltage is applied to a resistor
and the current is read off for each voltage step.
This graph is really just an expression of Ohm’s Law, which describes a
linear relationship between voltage and current caused by a constant
resistance (I = V/R). (Note that only linear resistors obey Ohm’s Law.)
Any constant resistance will have a linear characteristic like this. Larger
resistances will have a smaller slope (not as steep), because less current will
flow at the same voltage. Smaller resistances will have a steeper slope.
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Figure 2: Linear resistance
We can calculate the resistance from any point on the graph because it is
linear. Note that any point on the graph will give the same resistance:
(a) At 2 volts:
R
V
I
2
0.11 10 3
 18181 

(b) At 6 volts:
R
V
I
6
0.33 103
 18181 

Specification of resistors
There are two important values to mention when specifying a resistor for a
particular job. These are:

the resistance value itself (in ohms)

the power rating of the resistor (in watts).
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For example, a 10 K, 1 watt resistor has an ohmic value of 10 000 Ω and can
safely dissipate 1 watt of power.
The range of common linear resistor components varies from a fraction of
an ohm to millions of ohms. Power ratings vary mainly from about 0.1 W to
10 W, with exceptions above this. Some high-power resistors have power
ratings well in excess of 10 watts.
Low power resistors
The materials and construction details of resistor components depend upon:

the resistance value needed

the tolerance required

the power to be dissipated
Low power resistors do not have to dissipate much power, and so can be
made physically small. These resistors are used in electronic circuitry where
the tolerance, linearity and stability over time are important factors.
Different types include carbon film, metal film, and metal oxide resistors.
Carbon film resistors
These resistors are made by spraying a film of carbon on a glass or ceramic
rod. The thickness of the film dictates the resistance obtained. Small metal
end caps provide connection to the film and leads are connected to the end
caps to provide circuit connection. A final coating of insulation and colour
coding bands is then applied.
The resistance of carbon film resistors ranges from a few ohm up to several
million ohm with power ratings varying from one quarter watt up to two
watt.
Metal film resistors
These are manufactured in a similar manner to the carbon film type with
a nickel-chromium film being deposited on the ceramic rod. Though the
resistance of the carbon film type varies only by about 1 per cent over its
life span, the metal film resistor varies even less. They are more costly to
manufacture than the carbon film type. However, they have a more accurate
and consistent resistance and lower temperature co-efficient. This makes
them preferable to the carbon type. Metal file resistors are the most common
type of low power resistor used today in electronics.
Resistances available vary from about 10 Ω to 1M Ω with power ratings
from 0.1 watt to 1 watt.
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Metal oxide resistors
These involve a coating of oxides of tin and antimony on the ceramic rod.
Like the metal film type their resistance is very stable, varying over their
life by less than 0.5 per cent.
Their resistance range varies from about 10 Ω to 100 kΩ with power ratings
from 0.25 W to 10 W.
Medium and high power resistors
Medium and high power resistors are called upon to dissipate more power,
and so must be constructed robustly and designed to dissipate heat
effectively. They therefore must be physically larger that the low-power
types.
There are two major divisions within this category:

wire-wound resistors

grid resistors.
Wire wound resistors
These consist of a length of resistance wire (often nickel-chromium) wound
on a heat-resistant bobbin. The wire is welded to terminals at either end after
which the device is coated with an insulating material. The end result is a
resistor larger than the film or oxide types, capable of dissipating substantial
amounts of power.
The resistance of wire-wound resistors ranges from a fraction of an ohm to
about 200 kΩ, while their power rating ranges from 0.5 W to 25 W
for electronic applications and to several kilowatts for power engineering
applications.
Grid resistors
For high power applications, wire wound resistors become impractical. In
this case, the resistors are stamped out of stainless steel alloy sheet or made
from cast iron. These are known as grid resistors.
If you have Hampson, read the section called ‘Resistors’ on pages 53 to 56,
looking at the illustrations for details.
If you have Jenneson, refer to Section 2.12 on page 42 for more information
on resistors. Particularly note Figures 2.22 and 2.23 for the physical
identification of various resistors.
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Activity 1
1
Describe the characteristic of linear resistors.
_____________________________________________________________________
_____________________________________________________________________
2
Draw the symbols of four different types of resistor.
3
Describe the difference between carbon-film and metal film resistors.
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
4
Describe the difference between wire-wound and grid resistors.
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
Check your answers with those given at the end of the section.
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Non-linear resistors
Remember that linear resistors are designed to maintain a constant
resistance, regardless of the voltage and temperature. These resistors obey
Ohm’s Law and are the most common type.
For some particular applications, we can make use of materials that are
sensitive to changes in heat, light, or voltage. These are known as non-linear
resistors, because changes in resistance will result in a non-linear
characteristic.
Thermistors
Resistors whose resistance changes with temperature are called thermistors
and are either:

positive temperature co-efficient thermistors

negative temperature co-efficient thermistors.
Positive temperature co-efficient (PTC) thermistors
These are designed so that an increase in temperature causes an increase in
resistance. A decrease in temperature would cause a decrease in resistance.
This relationship is a ‘positive’ one, hence the term PTC thermistor.
PTC thermistors are used to protect the windings of electric motors from
burning out through excessive temperatures caused by overloads. Due to
their small physical size, PTC thermistors can be inserted directly into motor
windings. They can quickly sense temperature rises and, through the use of
relays, disconnect the windings from mains supply before damage occurs.
Negative temperature co-efficient (NTC) thermistors
These are designed so that an increase in temperature causes a decrease
in resistance. A decrease in temperature would cause an increase in
resistance. This relationship is a ‘negative’ one, therefore the term NTC
thermistor.
NTC thermistors can be used to suppress high in-rush currents that occur
when projector lamps are switched on. They can also be used for
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temperature measurement, and temperature dependent biasing of electronic
devices.
The characteristics curves of both PTC and NTC types are shown in
Figure 3.
Figure 3: Characteristics curves of PTC and NTC thermistors
Voltage-dependent resistors (VDR)
Voltage-dependent resistors have a characteristic curve as shown in Figure
6. The circuit symbol used for it is shown below in Figure 7.
Figure 6: Characteristic of voltage-dependent resistor
Figure 7: Symbol for voltage-dependent resistor
In viewing the characteristic curve you will note that at very low voltages
almost no current flows, indicating that the device has a very high resistance
at low voltage. At higher voltages you will see the current increasing rapidly
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indicating that the resistance of the device has fallen substantially. It is truly
then a voltage-dependent resistor.
VDRs are used extensively to suppress voltage surges. In the electrical
supply industry they are connected between overhead transmission lines and
earth. At normal voltages their resistance is very high and almost no current
from the line passes through them. When lightning strikes the line, the high
voltage created causes their resistance to drop substantially and the surge
currents in the line, instead of continuing through to the substation and
causing damage, are diverted to earth through the VDRs.
Light-dependent resistors (LDR)
This type of resistor is sensitive to light. In darkness, the LDR has a high
resistance that quickly falls to a low value when light falls on it. It is made
from cadmium sulphide.
A major use of the LDR is in a circuit that turns the street lights on as it gets
dark. The characteristic and circuit symbol for the device are shown in
Figures 8 and 9.
Figure 8: Characteristic of light-dependent resistor
Figure 9: Circuit symbol for light dependent resistor
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Variable resistors
The resistance of a variable resistor can be mechanically varied between two
values. Variable resistors are most commonly adjusted by turning a shaft,
which changes the position of a wiper blade moving across the surface of
the resistive material.
Carbon types are used for low power applications while the wire-wound
type is used for medium and high power applications.
Variable resistors can be connected as ‘potentiometers’ or ‘rheostats’. We
use a potentiometer to vary voltage and a rheostat to vary current. See
Figures 4 and 5.
Figure 4: Variable resistor connected as rheostat
When the variable resistor is connected as a rheostat, the resistance of the
variable resistor is placed in the current path. With a constant voltage
source, the load current will vary with the position of movable contact.
Figure 5: Variable resistor connected as a potentiometer
When the variable resistor is connected as a potentiometer as shown is
figure 5, the voltage between A and B is variable depending on the position
of moveable arm A. V0 represents the variable output voltage.
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Some variable resistors are designed to vary non-linearly as they are
adjusted. An example of this is the logarithmic variable resistor, which is
used for the volume control of radios or other audio devices.
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Identification of resistors
Two methods used to identify the resistance of resistors. Either the value can
be written on the resistor in text, or a colour code is used.
Labelled resistors
If the resistor is physically large enough the value can be printed on it, for
example “1 ohm”. Medium and high power resistors are commonly
identified in this way. Commonly the multiplier character is placed where
the decimal point would go. Here are some examples:
‘2k2’ means 2.2 kΩ
‘4M7’ means 4.7 MΩ
The letter ‘R’ is used where there is no multiplier, for example.
‘1R5’ means 1.5 Ω
This style of labelling eliminates the uncertainty caused by the small
decimal point being difficult to read reliably. Figure 10 illustrates this style.
Figure 10: Resistor identification
Colour coded resistors
Low power resistors which are too small to have the resistance written on
them are identified by a colour code. During manufacture coloured bands
are printed on the resistor in a specific order. Table 1 outlines what the
bands represent.
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Table 1: Resistor band meanings
Position of band
What it represents
closest to the end
the first significant figure
second band
second significant figure
third band
the multiplier
fourth band (if there is one)
the tolerance (see the section on ‘preferred values’ below).
Figure 11: Method of reading the value of a colour-coded resistor
The figures represented by band colours are shown in Table 2.
Table 2: Resistor colour code
Colour
Significant
figures
Multiplier
black
0
1 (100)
brown
1
10 (101)
1%
red
2
100 (102)
2%
orange
3
1 000 (103)
yellow
4
10 000 (104)
green
5
100 000 (105)
blue
6
1 000 000 (106)
violet
7
–
grey
8
–
white
9
–
gold
–
0.1
5%
silver
–
0.01
10%
none
–
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Tolerance
20%
15
Example
If the colour bands are brown, black, red and silver, state the value of the
resistance?
Solution

Brown = 1 (first digit)

Black = 0 (second digit)

Red = 2 (multiplier = 102 = 100)

Silver (representing 10% tolerance.)
Therefore the resistance is 10 × 102 = 1000 Ω or 1 kΩ, with a tolerance of
10%.
Example
What is the resistance of a resistor with bands of red, red, green, and gold ?
Solution

red = 2 (first digit)

red = 2 (second digit)

green = 5 (multiplier = 105 = 100 000)
Therefore the resistance is 22 × 100 000 = 2.2 MΩ
If you have Hampson, read the section ‘Resistor colour code’ on page 56
checking out Figure 2.25 and completing questions 11 and 13.
If you have Jenneson, refer to Section 2.12.4 and 2.12.5 for more examples
in using the resistor colour code and the idea of preferred values.
Preferred values
Resistors for electronics cover a wide range of values, from fractions of an
ohm up to many megohm. The number of different values is limited to a
standard set of values called the ‘preferred value’ shown in Table 3.
These are the basic figures only. Resistors are made larger and also smaller
than the tabled values in steps of 10 up to 106. In other words, a 15 ohm
resistor is part of a range including 150, 1500, 15 K, 150 K and so on.
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20% tolerance
10
10% tolerance
5% tolerance
10
10
11
12
12
13
15
15
15
16
18
18
20
22
22
22
24
27
27
30
33
33
33
36
39
39
43
47
47
47
51
56
56
62
68
68
68
75
82
82
91
Table 3: Preferred value resistors
Activity 2
1
Describe the characteristics of both NTC and PTC thermistors.
_____________________________________________________________________
_____________________________________________________________________
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2
Draw diagrams showing how a variable resistor can be used as a normal rheostat and
as a potentiometer.
3
Explain how a VDR can be used to prevent lightning damage to transmission lines and
associated equipment.
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
4
Explain how an LDR works.
_____________________________________________________________________
_____________________________________________________________________
5
State why colour coding is used on resistors, and nominate how it is used.
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
6
Why are resistors manufactured in ‘preferred values’?
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
Check your answers with those given at the end of the section.
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Check your progress
In Questions 1–10 write your answer in the brackets provided.
1
The best ohmmeter range to measure a resistance of 7 ohm is:
(a) ohm × 1
(b) ohm × 7
(c) ohm × 10
(d) ohm × 100
2
(
)
(
)
(
)
(
)
The colour code of a 27 ohm resistor is:
(a) red, violet, brown
(b) red, blue, brown
(c) red, violet, black
(d) red, blue, black
3
A resistor with colour bands of orange, white, yellow has a value of:
(a) 390 
(b) 39k 
(c) 3.9 
(d) 390 k
4
A resistor with a tolerance band of gold, has a tolerance of:
(a) 1%
(b) 2%
(c) 5%
(d) 10%
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5
A power resistor has 68R stamped on its body, indicating a resistance of:
(a) 0.68 
(b) 6.8 
(c) 68 
(d) 680 
6
(
)
An ohmmeter, set to its ‘Ohm × 100’ range is indicating 500 on the scale. The actual
resistance is:
(a) 5 
(b) 500 
(c) 50 000 
(d) 500 000 
7
(
)
(
)
(
)
A 2.2 kΩ resistor has tolerance of 10%. Its acceptable resistance range is from:
(a) 1100 to 3300 
(b) 1980 to 2420 
(c) 2090 to 2310 
(d) 2178 to 2222 
8
The resistance of a thermistor varies with changes in:
(a) humidity
(b) voltage
(c) light
(d) temperature
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9
The symbol shown below is for a:
(a) thermistor
(b) varistor
(c) light dependent resistor
(d) linear resistor
(
)
(
)
10 The symbol shown in Figure 13 is for a:
(a) thermistor
(b) varistor
(c) light dependent resistor
(d) linear resistor
11 (a) What is the tolerance range for a 12 kΩ 10% resistor?
__________________________________________________________________
__________________________________________________________________
(b) A resistor is known to have a range between 4794 Ω and 4606 .
(i) What is the nominal value of the resistor?
______________________________________________________________
______________________________________________________________
(ii)
What is its percentage tolerance?
______________________________________________________________
______________________________________________________________
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12 Complete Table 4 by filling in the blank spaces.
Table 4
Band colour
Resistance

First
Second
Third
Fourth
Red
Red
Brown
Gold
Yellow
Violet
Orange
Silver
Brown
Orange
Black
White
Blue
Brown
Tolerance
%
12k
5
0.33
5
680
10
1k
2
2.7M
5
Gold
Red
13 A resistor value of approximately 6.0 M is required.
(a) What preferred value of resistor should be chosen?
__________________________________________________________________
(b) If the tolerance of the chosen resistor were 10%, what would be its resistance
range?
__________________________________________________________________
__________________________________________________________________
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14 It is required that a resistor chosen for a particular job has a value between 1.43 and
1.52 k.
(a) What preferred value of the resistor should be chosen?
__________________________________________________________________
(b) What should be its minimum tolerance?
__________________________________________________________________
15 What coloured bands would resistors have if they had the following resistance?
(a) 470 10%
___________________________________________________________________
(b) 1k2 5%
___________________________________________________________________
(c) 68k 10%
___________________________________________________________________
(d) 2M7 5%
___________________________________________________________________
Check your answers with those given at the end of the section.
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Summary
24

A resistor is a device used in an electrical circuit to limit circuit current,
produce a specific potential difference or produce heat.

Resistors can be linear or non-linear. A linear resistor has constant
resistance (within tolerance) over a prescribed temperature range.

A non-linear resistor is designed to vary its resistance under the
influence of light, heat, voltage or current.

Resistors are classified according to their Ohmic value and the power
capable of being dissipated.

Low power resistors made from carbon film, metal film or metal oxide
are produced in set values called the ‘preferred value’ range.

Medium or high power resistors are either wire-wound or made from
pressed steel or cast iron grids.

A variable resistor connected so as to vary current is called a rheostat.

A thermistor is a non-linear resistor whose resistance changes under the
influence of heat.

NTC thermistors are used to suppress high in-rush currents, measure
temperature and for automatic temperature-dependent biasing.

PTC thermistors are connected in electrical machines to isolate the
machines under overload conditions.

VDRs vary their resistance inversely with the applied voltage. They are
used to suppress or divert ‘surge’ voltages.

LDRs have high resistance when not subjected to light and the resistance
falls quickly when light is applied.

High power resistors are identified by having the value of resistance
printed on them.

Low power resistors are identified by a resistor colour code.
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Answers
Activity 1
1
The resistance of a linear resistor remains constant (within a given
tolerance) over a prescribed temperature range.
A non-linear resistor is designed so that its resistance varies at a
controlled under the influence of light, heat, voltage or current.
2
3
Carbon-film resistors are made by spraying a film of carbon on rods of
glass or ceramic materials. They vary in range from a few to millions of
ohm and are made up to 2 watt in power rating.
Metal-film resistors are made similar to the carbon film type except that
the carbon film is replaced by one of nickel-chromium. They are more
accurate than the carbon-film type and range from about 10 ohm to one
megohm, with power ratings up to 1 watt.
4
Wire-wound resistors consist of a definite length of resistance wire,
wound on a heat-resistant bobbin. They are capable of dissipating up
to about 25 W of power.
Grid resistors are either stamped out of stainless steel alloy sheet or cast
from a suitable cast iron. They are capable of dissipating some
thousands of watts of power.
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Activity 2
1
An NTC thermistor is a non-linear resistor whose resistance decreases
with an increase of temperature. A PTC thermistor is a non-linear
resistor whose resistance increases with an increase in temperature.
2
See the diagram below.
3
A VDR is connected between the line and earth. At normal line voltage
its resistance is exceedingly high and no current passes through it.
The very high voltage of a lightning strike, however, makes its
resistance drop substantially, causing the ‘strike’ to go to earth and not
continue through the line and inflict damage on associated equipment.
4
In darkness, an LDR has a very high resistance, limiting current to
zero. When light is applied, its resistance drops substantially, allowing
current to flow. It can, therefore, be used to control lighting, in
conjunction with a relay, acting as an on-off switch.
5
Colour coding is used on resistors, rather than writing the resistance
value on the resistor, because with time any decimal point on the
resistance value might disappear leaving digits only in what would
then be an incorrect resistance indication. As an example 2.2 K might,
after some time in use, look like 22 K.
Colour coding uses bands or rings of colour, each band colour
representing a particular digit. Three or four bands may be used and
an experienced operator, having memorised band colour values, can
quickly identify the resistance value.
6
To attempt to manufacture resistors covering every value from, say,
1 ohm to 1 000 000 ohm would not be a good idea. Think of the stock
you would have to keep to cover a wide range. Preferred values
represent those resistors that cover a wide range using a tolerance.
As an example, take a 33 ohm resistor with a 10% tolerance. Ten per
cent of 33 is 3.3 ohm. Thus that resistor covers a range from
33–3.3 (29.7 Ω) to 33 + 3.3 (36.3 Ω). The next resistor needed above
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33 Ω is 39 Ω, as it covers down to 39–3.9 (35.1 Ω) which is within the
tolerance allowed. Table 2 identifies these preferred values.
Check your progress
1
(a)
2
(c)
3
(d)
4
(c)
5
(c)
6
(c)
7
(d)
8
(d)
9
(b)
10 (c)
11 (a) 10.8 k to 13.2 k.
(b) (i) 4700 
(ii) 2%
12
Band colour
Resistance

Tolerance
%
First
Second
Third
Fourth
Red
Red
Brown
Gold
220
5
Yellow
Violet
Orange
Silver
47 k
10
Brown
Red
Orange
Gold
12 k
5
Orange
Orange
Silver
Gold
0.33
5
Brown
Black
Blue
Gold
10 M
5
Blue
Grey
Brown
Silver
680
10
Orange
White
Brown
Red
390
2
Brown
Black
Red
Red
1k
2
Red
Violet
Green
Green
2.7 M
5
13 (a) 5.6 M
(b) 5.04 to 6.16 M.
14 (a) 1.5 k
(b) 5%.
EEE042A: 8 Select and identify resistors
 NSW DET 2017 2006/060/05/2017 LRR 3663
27
15 (a) yellow violet brown silver
(b) brown red red gold
(c) blue grey orange silver
(d) red violet green gold.
28
EEE042A: 8 Select and identify resistors
 NSW DET 2017 2006/060/05/2017 LRR 3663