NIDA SERIES 130E
Block 1
INTRODUCTION TO ELECTRICITY
BASIC ELECTRICITY
UNIT I - DC CIRCUITS
LESSON 5A
RESISTORS
OBJECTIVES
OVERVIEW
On completion of this lesson, the student
will have learned to:
This lesson introduces students to
resistors. Students first learn what
resistors are and why we use resistors in
electronic circuits.
1. Recognize a resistor and define its
purpose.
2. Identify different types of fixed
resistors and explain their use in
electronic circuits.
3. Define tolerance, accuracy, and
precision as they relate to resistors.
4. Define power dissipation of resistors
and explain its relation to type and
size of resistor.
5. Read resistor color codes and
determine the value of a resistor from
its color codes and other identifying
marks.
6. Identify different types of variable
resistors and explain their use in
electronic circuits.
PREREQUISITES
The discussion then describes the
different types of resistors in use and
explains their advantages and
disadvantages.
Next, the lesson discusses the terms
tolerance, accuracy, and precision as
they pertain to resistors.
After explaining power dissipation, the
lesson discusses how power dissipation
relates to the type and size of a resistor.
Students learn to determine resistor
values by reading color codes and other
identifying marks on the resistors.
The lesson then discusses variable
resistors, explaining their power-handling
capabilities and uses.
None
EQUIPMENT REQUIRED
Nida Series 130 Experiment Card
PC130-4A
(If your Experiment Card is
PC130-1A, use Lesson 5.)
Copyright © 2002 by Nida Corporation
1-5A-1
LESSON 5A
RESISTORS
UNIT I
Block 1
Introduction to Electricity
INTRODUCTION
You just learned about conductors, insulators, and semiconductors in the previous lesson.
You now know that a conductor is a material, usually a length of wire, which delivers
current. Voltage at any two points along a conductor is always the same. Thus, the
potential difference between any two points along a conductor is equal to zero.
You also learned that insulators are the opposite of conductors--they are nonconductors.
Insulators prevent the flow of current between two conductors. Thus, we cover electrical
wires and cables with insulating material.
Semiconductors are neither conductors nor insulators. Semiconductor behavior is
between that of a conductor and that of an insulator. Semiconductors are resistive
materials which offer some opposition to current and, at the same time, allow reduced
flow.
What you learned in the last lesson about semiconductor materials and current flow is
basic to learning about resistors and understanding their use in electronic circuits.
WHAT IS A RESISTOR?
A resistor is a device made of a semiconductor (resistive) material such as carbon. It can
be constructed to provide whatever resistivity is desired.
DEFINITION
RESISTOR: An electronic component used as a limiting device to
adjust and set voltage and current levels in an electronic circuit.
How do resistors control the flow of current? Resistors act a lot like pipes in a water
delivery system. Large diameter pipes offer little resistance and deliver a large volume of
water. When pipe diameter is reduced, resistance increases and, thus, water flow is
restricted.
Like water pipes, resistors control the flow of electrical current. The larger the resistive
value, the greater the reduction in electrical current. Conversely, the smaller the resistive
value, the less restriction to current.
A common textbook circuit is one that has a flashlight bulb as a load. Current flows from
the negative potential to the positive potential through the lamp (resistor) connected in the
circuit. The resistor limits the amount of current in the circuit.
In a circuit, we represent the resistor with a
zig-zag line, as shown in Figure 1. We also
often label the resistor on the schematic
diagram with the letter R.
Figure 1. Resistor Symbol
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Copyright © 2002 by Nida Corporation
Block 1
Introduction to Electricity
UNIT I
LESSON 5A
RESISTORS
If we have more than one resistor in
a circuit, we label the resistors R1,
R2, R3, and so on, depending on
how many resistors are in the circuit.
The schematic diagram in Figure 2
shows a basic electric circuit with
three resistors.
The unit of measure for resistance is
the ohm, represented by the Greek
letter omega: Ω.
Figure 2. Basic Electric Circuit with Three Resistors
Resistance values can vary from a
fraction of an ohm to tens of millions
of ohms.
We are fortunate to have metric notation which we can use to help refer to such large
variations in resistor values. Resistor values up to 1000 are stated in ohms. Resistor
values over 1000 ohms are kilohms. Resistor values over 1,000,000 ohms are megohms.
For an example, look at the list below.
100 ohms
2,200 ohms
1,500,000 ohms
= 100 Ω
= 2.2 kilohms = 2.2 kΩ
= 1.5 megohms = 1.5 MΩ
Now that you have a general idea of what a resistor is and what it does, let's go on to
specifics about resistors. We classify resistors into two categories according to their use:
 Fixed resistors
 Variable resistors
Each category contains several types of resistors. The type is determined by the resistor's
construction. Let's start with the fixed resistors.
FIXED RESISTORS
Fixed resistors are those which have a specific, stationary ohmic value. The three types
of fixed resistors are:
• General Purpose Resistors
• Power Resistors
• Precision Resistors
You will learn to identify the different types just by looking at the resistors.
GENERAL PURPOSE RESISTORS
General purpose resistors, the first type of fixed resistor, are the most common. They are
typically carbon resistors, which are inexpensive and meet the basic needs of circuit
applications.
Copyright © 2002 by Nida Corporation
1-5A-3
LESSON 5A
RESISTORS
UNIT I
Block 1
Introduction to Electricity
Resistor Identification
General purpose resistors are identified by bands of colors printed on their bodies. Look at
your experiment card and examine the bands, as shown in Figure 3.
Color Bands
Figure 3. PC130-4A Experiment Card
There are two types of carbon resistors: carbon composition resistors and carbon film
resistors. Carbon film resistors are the most widely used.
Carbon Composition Resistors
Look at the carbon composition resistor
pictured in Figure 4. Carbon composition
resistors have molded, cylindrical bodies.
Figure 4. Carbon Composition Resistor
Carbon composition resistors are a mixture of carbon and other inert materials. Their
resistance is determined by the percentage of carbon they contain. The more carbon, the
less resistance. They are slightly larger than carbon film resistors.
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Copyright © 2002 by Nida Corporation
Block 1
Introduction to Electricity
UNIT I
LESSON 5A
RESISTORS
Carbon Film Resistors
Carbon film resistors are illustrated in Figure 5. These resistors are not cylindrical;
instead, they are bone shaped.
Figure 5. Carbon Film Resistors
Inside the carbon film resistor is a strip of carbon which is wound around a core. The
length of the strip of carbon is what determines the resistance.
The carbon film terminates on each end with metal caps or cups to which the resistor
leads are molded. The resistor is then dipped in an epoxy insulating material.
Look at your experiment card or Figure 3 and identify the carbon film resistors (R4 through
R12).
Power Rating
While resistors are rated by their ohmic value, they are also rated according to their power
handling ability, which is directly related to their ability to dissipate heat. Power is the
rate of doing work. The unit of measure for power is the watt. The power rating equals
the amount of heat the resistor generates, which is dependent upon the resistance of the
resistor and upon how much current is flowing through the resistor. Thus, all resistors
have both an ohmic value and a power handling (watt) rating. The physical size of a
resistor is a reflection of how much power it can handle. More heat disperses into a large
resistor than it does into a small resistor. A large resistor also has more surface area from
which the heat can escape into the surrounding air. Because of this, the larger the
resistor, the more power it can handle. If more heat is generated than the resistor can
dissipate, the resistor will burn up.
Resistor wattage ratings extend from an eighth of a watt to about 25 watts. Although
there really isn’t any upward limit, resistors are most often seen with wattage ratings of
one eighth watt to 2 watts.
Carbon film resistors commonly come in three power ratings or sizes: 1/8 watt, 1/4 watt,
and 1/2 watt. Larger sizes, however, are available, as you will see on PC130-4A. Look at
the experiment card in Figure 3. Two sizes of carbon film resistors are on this card. R4,
R5, and R6 are 2 watt resistors. R7 through R12 are 1/2 watt resistors.
Carbon composition resistors typically come in five sizes, ranging from 1/8 watt to
2 watts, as shown in Figure 6.
Copyright © 2002 by Nida Corporation
1-5A-5
LESSON 5A
RESISTORS
UNIT I
Block 1
Introduction to Electricity
1/8 WATT
1/4 WATT
1/2 WATT
1 WATT
2 WATT
Figure 6. Carbon Composition Resistors -- 5 Sizes
There are no carbon composition resistors on your experiment card.
When an engineer designs a circuit, he calculates how much power the resistor must
dissipate under worst conditions. He then adds a 100 percent safety factor. Thus, if the
calculated maximum power dissipation for a resistor is 1 watt, he usually uses a 2 watt
resistor. If it is 1/2 watt, he uses a 1 watt resistor, and so on.
Most circuits, however, dissipate very little power, so the resistors in the circuit need to
dissipate only a small fraction of a watt. Thus, most circuits contain 1/4 watt resistors.
Resistor Color Codes
As we stated at the beginning of this discussion, all fixed resistors have a specified ohmic
value. This value can be anywhere from a fraction of an ohm to tens of megohms. Since
the value is fixed, the ohmic value is marked on each resistor after its manufacture.
All carbon film and carbon composition
resistors have color bands. By reading
these color bands, you can identify the
value of the resistors. Look closely at
the resistors and their color bands on
your experiment card.
The drawing in Figure 7 shows a
typical carbon composition resistor
with four color bands. Each
band indicates some numerical
value of the resistor color code.
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First Significant Digit
Color
Bands
Second Significant Digit
Multiplier
Tolerance
Figure 7. Reading Resistor Color Codes
Copyright © 2002 by Nida Corporation
Block 1
Introduction to Electricity
UNIT I
LESSON 5A
RESISTORS
The band colors and their corresponding values are listed in Table 1.
Table 1. Resistor Color Bands and Their Values
COLOR
COLOR
VALUE
MULTIPLIER
RESISTANCE
TOLERANCE
Black
0
1
Brown
1
10
Red
2
100
Orange
3
1,000
Yellow
4
10,000
Green
5
100,000
Blue
6
1,000,000
Violet
7
10,000,000
Gray
8
100,000,000
White
9
1,000,000,000
Gold
t
0.1
± 5 percent
Silver
t
0.01
± 10 percent
No Color
± 20 percent
To read the color bands, you always start with the band which is closest to the end of the
resistor body and/or is not the metallic band (gold or silver).
–
The first band represents the first of two significant digits in the resistor's value.
–
The second band represents the second significant digit in the resistor's value.
–
The third band indicates the multiplier. The color's value represents the number of
zeros to be added to the two significant digits determined by the first and second
bands. (Gold and Silver as third bands are exceptions to this rule.)
Another method of interpreting the third band's color value is as a power of ten.
The two significant digits determined by the first and second bands are multiplied
by 10 to the power indicated by the third band. As you can see, the Multiplier
column number is the equivalent of 10 raised to the power of the Color Value
column number.
For example, to get the value of the resistor:
If the first band is brown, the first significant digit is 1.
If the second band is green, the second significant digit is 5.
If the third band is red:
you add 2 zeroes after the significant digits (1500)
or alternately:
you multiply the 1 and 5 (15) by 102 (that is, by 100) 15 x 102 = 1500
The value of the resistor is 1500 ohms (1500 Ω).
Copyright © 2002 by Nida Corporation
1-5A-7
LESSON 5A
RESISTORS
UNIT I
Block 1
Introduction to Electricity
The fourth band on the resistor indicates the tolerance of the resistor. Tolerance, which is
stated as a percentage, indicates how much the designated ohmic value of a resistor can
differ from the actual resistance value the device provides. Table 1 shows the fourth
band (tolerance) is either gold, silver, or no color (no fourth band on the resistor).
Let's return to the example resistor. The first band is brown, the second band is green,
and the third band is red. This resistor has an ohmic value of 1500 ohms. If this
resistor's fourth band is silver, the tolerance is 10% of its actual value, as listed in
Table 1. This means our 1500 ohm resistor's actual value could be 1500 ohms ± 10%,
or 1500 ohms ± 150 ohms. Thus, the resistor's actual value could be anywhere from
1350 to 1650 ohms.
Most resistors today have a tolerance of ± 5%, indicated by a gold colored fourth band.
Carbon resistors, both carbon film and carbon composition, come in many values or power
ratings. Table 2, Standard Nominal Resistance Values, lists the values of all carbon
resistors that are available. Notice the footnote to the table, which states that all values
are available with ± 5% tolerance, and the shaded values are available with ± 10%
tolerance as well.
Table 2. Standard Nominal Resistance Values*
OHMS
MEGAOHMS
1.0
4.7
22
100
470
2200
10000
47000
0.24 0.43
0.75
1.3
2.4
4.3
1.1
5.1
24
110
510
2400
11000
51000
0.27 0.47
0.82
1.5
2.7
4.7
7.5
13.0
1.2
5.6
27
120
560
2700
12000
56000
0.30 0.51
0.91
1.6
3.0
5.1
9.1 16.0
1.3
6.2
30
130
620
3000
13000
66000
0.33 0.56
1.0
1.8
3.3
5.6
10.0 18.0
1.5
6.8
33
150
680
3300
15000
68000
0.36 0.62
1.1
2.0
3.6
6.2
11.0 20.0
1.6
7.5
36
160
750
3600
16000
75000
0.39 0.68
1.2
2.2
3.9
6.8
12.0 22.0
1.8
8.2
39
180
820
3900
18000
82000
2.0
9.1
43
200
910
4300
20000
91000
2.2
10
47
220
1000
4700
22000 100000
2.4
11
51
240
1100
5100
25000 110000
2.7
12
56
270
1200
5600
27000 120000
3.0
13
62
300
1300
6200
30000 130000
3.3
15
68
330
1500
6800
33000 150000
3.6
16
75
360
1600
7500
36000 160000
3.9
18
82
390
1800
8200
39000 180000
4.3
20
91
430
2000
9100
43000 200000
8.2 15.0
* All values are available in ± 5% tolerances.
Shaded rows indicate ± 10% tolerance.
Often, when troubleshooting an electrical circuit, you will have to identify the resistors by
their color codes. You should learn the color code number values in Table 1 as quickly as
you can. Many people use tricks to remember such things. One of the most familiar
tricks is to create a ten-word sentence where the first letter of each word in the sentence
matches, consecutively, the first letter of each color in the table. (The first word of the
sentence would start with B for black and the last word would start with W for white.)
With some imagination, you can create an infallible – and invaluable – memory tool.
1-5A-8
Copyright © 2002 by Nida Corporation
Block 1
Introduction to Electricity
UNIT I
LESSON 5A
RESISTORS
Exercise 1: Carbon Resistors.
Table 3 lists all the resistors on experiment card PC130-4A. Fill in the blanks in the
table with information on the carbon film resistors only, using the abbreviation CF to fill in
the Type column. You'll fill in the remaining blanks later in the lesson.
The blanks for R4 have already been filled in as an example for you to follow.
Table 3. Resistors Mounted on Experiment Card PC130-4A
COLOR BANDS
RESISTOR
TYPE
POWER
CF
2
First
Second
Third
Fourth
BLUE
GRAY
BROWN
GOLD
VALUE
TOLERANCE
680 Ω
5%
R1
R2
R3
R4
R5
R6
R7
R8
R9
R10
R11
R12
Copyright © 2002 by Nida Corporation
1-5A-9
LESSON 5A
RESISTORS
UNIT I
Block 1
Introduction to Electricity
POWER RESISTORS
The second type of fixed resistor is the power resistor. While general purpose carbon
resistors are the most widely used, sometimes, such as in situations requiring higher
power dissipation and higher accuracy, they are not sufficient. In that case, a power
resistor would be used.
Power resistors are wire-wound resistors,
which means, of course, that they are made
of wire. Look at the drawing of a
wire-wound resistor in Figure 8.
Figure 8. Wire-Wound Resistor
To make the resistor, the wire is wound on
a core. Each end of the wire is connected
to the end caps or to cups to which the
resistor leads are soldered.
The ohmic value and the tolerance are usually
marked on the body of the power resistor, as
shown in Figure 9.
Other information, such as the manufacturer's name and/or part number as well
as the power rating, are also often marked on
the body of a wire-wound resistor.
Figure 9. Wire-Wound Resistor
Wire-wound resistors range in size from 1/2 watt to tens or even hundreds of watts.
The smallest wire-wound resistors have a power rating of 1/2 watt. Resistors up to 7
or 10 watts are typically the largest ones you find mounted on printed circuit cards.
Anything larger requires special mounting and would, therefore, most likely be mounted
separately on a chassis. Chassis mounting provides better heat dissipation. The
wire-wound resistors you will study in this course are mounted on assembled PC boards.
These resistors will normally not exceed 10 watts.
In many cases, the power handling capability of the wire-wound resistor is printed on the
resistor body; often, it is not. When the latter is the case, a data book must be consulted
to ascertain the wattage rating.
We use wire-wound resistors because they dissipate more heat than the carbon
composition resistors can handle. You would need at least three carbon composition
resistors to do the work of one 5-watt wire-wound resistor.
We also use wire-wound resistors because we can get them with a specific, very precise,
ohmic value. Carbon resistors, on the other hand, are available only in the nominal
standard resistance values.
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Copyright © 2002 by Nida Corporation
Block 1
Introduction to Electricity
UNIT I
LESSON 5A
RESISTORS
Let's say, for example, that an engineer needs a 6000 ohm resistor, accurate to ± 1%.
Thus, the value of the resistor he needs cannot be less than 5940 ohms and cannot
exceed 6060 ohms.
The closest the engineer could come to this with a carbon resistor is 6200 ohms with
a 5% tolerance. (See Table 2.) Thus, the value of the carbon resistor could be anywhere
from 5890 to 6510 ohms, a variation which is too wide to be acceptable to the engineer.
The 6000 ohm resistor he needs, therefore, would have to be made to order.
To make a wire wound resistor is easy since the resistance depends on the diameter of
the wire used and the length of the wire. The smaller the diameter of the wire, the higher
the resistance per inch of wire. If you know the resistance of the wire, you can easily
calculate the length you need. You then wrap the wire on the core of the resistor and cap
it on both ends. Then you dip it in insulating material to protect the resistor from damage
due to handling.
Wire-wound resistors come in various shapes, sizes, and mounting configurations.
Experiment card PC130-4A has three of them. Look at your experiment card or Figure 21.
Examine resistors R1, R2, and R3 on PC130-4A. These resistors are wire-wound. R1 is a
100 Ω, 3% tolerance resistor. The power rating on R1 is 7 watts; it is not evident and
must be determined from a manufacturer's data book. R2 and R3 appear identical.
However, close examination will reveal that R2 is a 250 Ω resistor while R3 is a 200 Ω
resistor. Both R2 and R3 have a 5-watt rating with a 5% tolerance. These specifications
are clearly printed on the resistor bodies.
A wire-wound resistor can heat up to a level where it even glows red hot without
suffering damage. A carbon composition resistor, however, would most likely start
smoking, turn black, and crack open. From this, you can see that a smaller wire-wound
resistor can dissipate more heat than a larger size carbon resistor.
Since wire-wound resistors are more expensive than carbon resistors, they are used only
when necessary. One of the typical applications of a power resistor is in the design of
power supply circuits for simulation of electrical loads.
For instance, if an engineer designs a power supply circuit to produce power to drive a
motor or a radio, the engineer might want to test the circuit before turning on the motor
or radio. To test the circuit, he must simulate the load that the motor or radio will
represent. He can simulate the load with a power resistor. In other words, the power
that will be supplied to the motor or radio is now supplied, instead, to the wire-wound
resistor.
Copyright © 2002 by Nida Corporation
1-5A-11
LESSON 5A
RESISTORS
UNIT I
Block 1
Introduction to Electricity
Exercise 2: Power Resistors.
Go back to Table 3 in Exercise 1. In the Type column, fill in WW for wire-wound
resistors R1, R2, and R3. Since these resistors do not have color bands, ignore the
Color Bands columns; however, fill in their wattage, Ω value, and % tolerance.
PRECISION RESISTORS
The third type of fixed resistor is the precision resistor. Precision resistors are usually
made of metal film material and have a 1% tolerance.
Before discussing precision resistors, let's make sure you understand the words accuracy
and precision. These two words are often used interchangeably, without any regard for
their very real differences in meaning.
ACCURACY means free from error or mistake; correct.
PRECISION means exactly or sharply defined; strictly
conforming to a pattern, standard, or convention.
As you can see, these two words do not have exactly the same meaning. For instance,
imagine that you are a marksman practicing with your rifle on a firing range. You have
just fired several shots at your target.
Look at the drawing of your target in Figure 10.
Notice that your shots, although bunched closely
together, are far away from the bull’s eye.
From the pattern of your shots, you know that you
were very precise in your shooting because your shot
group is tight.
Figure 10. Target
You were not very accurate, however, since all your
shots are in the outer ring of the target.
Reading a meter, such as the voltage meter on the Model 130 Test Console, can provide
another example of the difference in these two words. You can read the voltage very
precisely on the scale, but if the meter is incorrectly calibrated, you will not read the
voltage very accurately at all.
Keeping in mind the meanings of these two words, let's get back to precision resistors.
A precision resistor, quite simply, is a resistor with a tolerance that is very tight, ± 1% or
better. To measure a precision resistor, you obviously need very accurate instruments.
Precision resistors vary in size according to their power dissipation. In general, a precision
resistor is larger than an equivalent wattage carbon resistor. That's because temperature
affects the ohmic value of any resistor, so as a resistor heats up, its ohmic value can
change.
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Copyright © 2002 by Nida Corporation
Block 1
Introduction to Electricity
UNIT I
LESSON 5A
RESISTORS
Since precision resistors have such tight tolerances, their ohmic value cannot change
much. Precision resistors, therefore, are large resistors, since the larger the resistor, the
less the body temperature rises. Thus, the chance of a change in ohmic values is less.
Table 4 lists the standard resistance values for precision resistors. The decade multipliers
of these values are also available. For instance, the value in the fourth row of the ninth
column is 4.99. This means precision resistors are available in the following values:
4.99 Ω f 49.9 Ω f 499 Ω f 4.99 kΩ f 49.9 kΩ f 499 kΩ f 4.99 MΩ f 49.9 MΩ f 499 MΩ
Table 4. Precision Resistor Standard Resistance Values
1.00 1.21 1.47 1.78 2.15 2.61 3.16 3.83 4.64 5.62 6.81 8.25
1.02 1.24 1.50 1.82 2.21 2.67 3.24 3.92 4.75 5.76 6.98 8.45
1.05 1.27 1.54 1.87 2.26 2.74 3.32 4.02 4.87 5.90 7.15 8.66
1.07 1.30 1.58 1.91 2.32 2.80 3.40 4.12 4.99 6.04 7.32 8.87
1.10 1.33 1.62 1.96 2.37 2.87 3.48 4.22 5.11 6.19 6.50 9.09
1.13 1.37 1.65 2.00 2.43 2.94 3.57 4.32 5.23 6.34 7.68 9.31
1.15 1.40 1.69 2.05 2.49 3.01 3.65 4.42 5.36 6.49 7.87 9.53
1.18 1.43 1.74 2.10 2.55 3.09 3.74 4.53 5.49 6.65 8.06 9.76
Sometimes, especially if you are working on equipment designed for aerospace or military
applications, you will find resistors marked in accordance to military specifications. In that
case, a 2430 ohm resistor would be marked the military way, which is 2431F, rather than
the standard way, which is 2.43 kΩ ± 1%.
You know how to read the markings when they are written in the standard way. The
military way, however, reads as follows:
1st digit (2):
2nd digit (4):
3rd digit (3):
4th digit (1):
Letter (F):
the
the
the
the
the
first significant digit of the ohmic value
second significant digit of the ohmic value
third significant digit of the ohmic value
multiplier (number of zeros following the ohmic value)
tolerance of ± 1%
A resistor labeled 2431F, the military way, is a 2430 Ω resistor with a tolerance of ± 1%.
The labels RN60D or RN65D designate style, power dissipation, and temperature
characteristics. The first part of this indicates the size of the resistor:
RN55 means the resistor is an 1/8 watt resistor
RN60 means the resistor is a 1/4 watt resistor
RN65 means the resistor is a 1/2 watt resistor
Copyright © 2002 by Nida Corporation
1-5A-13
LESSON 5A
RESISTORS
UNIT I
Block 1
Introduction to Electricity
The letter D designates the temperature coefficient. This is a manufacturer's code and
may vary from one manufacturer to another. The temperature coefficient tells the user of
this resistor by how much it can or may change in ohmic value as a result of a change in
its body temperature.
The appearance of precision resistors differs depending on who manufactured the
resistors. Each manufacturer has its own unique way of making and coating resistors,
with the result that each manufacturer makes his resistors in his own distinctive color.
RESISTOR NETWORKS
Some manufacturers preassemble multiple resistors into a single resistive package at the
factory. This reduces the time required to assemble resistors on PC boards. It also
reduces the space these resistors take up on a PC board.
Resistor networks are packaged in single in-line packages (Figure 11A) or in dual in-line
packages (Figure 11B). Some networks have the resistors individually mounted
(Figure 11C) or interconnected in combinations (Figure 11D). In the future, more and
more resistive networks will be seen in use.
11A. Single In-Line Package
Resistor Network
11C.
Individually Mounted
Resistor Network
11B. Dual In-Line Package
Resistor Network
11D. Interconnected Combination
Resistor Network
Figure 11. Resistor Networks
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Copyright © 2002 by Nida Corporation
Block 1
Introduction to Electricity
UNIT I
LESSON 5A
RESISTORS
VARIABLE RESISTORS
Up to now, we have discussed fixed value resistors only. These resistors are two-terminal
devices, that is, components with two wires or leads attached to them. The value of the
resistance between these two leads is fixed at the factory.
Sometimes, however, equipment requirements dictate that the resistor value be adjusted
after the circuit is assembled. In that case, a variable resistor is required. Variable
resistors are of two different types:
•
potentiometers (pots)
•
rheostats
Potentiometers and Rheostats
One use of a variable resistor is as a potentiometer with a value that can be adjusted by
the technician or engineer during the manufacture of the circuit or during maintenance
procedures and alignments. Such a variable resistor is commonly called a trimming
potentiometer (trimmer or trim pot, for short). Trimming potentiometers are normally
adjusted by means of a small screw or thumb-wheel that is turned.
Potentiometers are often used as control devices. For instance, the volume and tone
controls on your home or car stereo use this kind of resistor.
Physically, a rheostat is fundamentally the same thing as a potentiometer except that
rheostats have two electronically segregated legs while potentiometers have three.
Potentiometers are used to control voltage while rheostats are used to control current.
Due to the difference in application, rheostats are frequently larger than potentiometers.
In reality, a potentiometer can be used in a rheostat application assuming that the circuit
parameters do not exceed the rating of the device. When a potentiometer is used in such
an application, one leg is left open or tied to the wiper.
The drawing in Figure 12 is a schematic representation of a potentiometer.
Figure 12. Schematic of a Potentiometer
The resistive element has some given maximum value. Measurements across the element
will indicate a fixed value regardless of wiper position. Measurements taken from the
wiper to either side of the element will vary with the position of the wiper.
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LESSON 5A
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UNIT I
Block 1
Introduction to Electricity
For instance, in the equivalent schematic of Figure 13, if the resistive element is 1000 Ω,
you would read 1000 Ω when you measure resistance between Terminals 1 and 3. If the
wiper (Terminal 2) is placed exactly in the center and a measurement is made between the
wiper and either end of the resistive element, the resistor would read 500 Ω. If you turn
the wiper control, the resistive element would then increase or decrease in value.
You can connect a variable resistor three different ways. The first way, shown in the
schematic diagram of Figure 13, shows the device as a potentiometer being used as a
voltage divider. In this case, the voltage source is applied to Terminals 1 and 3 across the
resistive element.
Figure 13. Potentiometer as a Voltage Divider
When you look at the wiper at Terminal 2, you see only a portion of the applied voltage.
This voltage depends on the position of the wiper on the resistive element.
Figure 14. Potentiometers
Examine Figure 14. In this example, the same variable resistor is shown with the wiper
terminal in various positions. Notice that when the wiper is placed fully to either end of
the resistor, the voltage is maximum at one end and minimum at the other. When the
wiper is positioned somewhere in between, the amount of resistance varies. The summed
amount of resistance between the wiper and both ends of the resistor will equal the total
resistance of the device.
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Copyright © 2002 by Nida Corporation
Block 1
Introduction to Electricity
UNIT I
LESSON 5A
RESISTORS
Figure 15 shows a second way to connect potentiometers. This configuration
demonstrates the use of a potentiometer as a rheostat.
Figure 15. Potentiometer as a Rheostat, Method One
This potentiometer has the wiper shorted to one of the other leads. In this case,
Terminal 2 is connected to Terminal 3, resulting in a resistor which has only two leads.
This is a rheostat configuration.
Here, resistor RB is shorted out, and only the RA portion is in the circuit. As the control
knob is turned, RA either increases or decreases.
The third way to connect potentiometers is shown in the drawing of Figure 16.
Figure 16. Potentiometer as a Rheostat, Method Two
This time you connect only two leads of the potentiometer: the wiper terminal and one of
the other terminals. The third terminal is not connected.
When you connect a potentiometer this way, it is effectively configured as a rheostat, the
same as the device in Figure 15.
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LESSON 5A
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UNIT I
Block 1
Introduction to Electricity
TRIMMER POTENTIOMETERS
The trimmer potentiometer or "trim pot" is a very popular component in electronics. It is
used more often in analog circuits than in digital circuits. You will see trim pots
throughout this course.
Trim pots are available in many sizes and shapes. Several of these are shown in
Figure 17. Most of them are designed to mount on PC cards.
Figure 17. Trimmer Potentiometers
A typical potentiometer rotates just short of one complete revolution, about 330o. Some
trim pots, however, are what we call multi-turn pots. A multi-turn pot has built-in gears
so the user can rotate the shaft 5, 10, or even 25 times around. For every revolution of
the shaft, the wiper will move 1/5th, 1/10th, or 1/25th of the distance of the resistive
element. Multi-turn pots are needed when very precise adjustments must be made.
The material from which trim pots are made varies. The elements of some trim pots are
wire wound; others are made of carbon, cement, or other materials. Power dissipation of
a trim pot is very low, usually not greater than 1/2 watt.
CONTROL POTENTIOMETERS
Control potentiometers are generally larger in size than trim pots. They range from
1/2 watt to about 5 watts in power dissipation. Control pots also have provisions for
panel and control knob mounting. Typical control pots are shown in Figure 18.
Figure 18. Control Potentiometers
Notice that one of the control pots in Figure 18 has a second potentiometer attached.
This one is a dual potentiometer. A dual potentiometer is used when two independent
variable resistors have to be changed simultaneously.
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Copyright © 2002 by Nida Corporation
Block 1
Introduction to Electricity
UNIT I
LESSON 5A
RESISTORS
RHEOSTATS
A rheostat, as shown in Figure 19, is a high-wattage variable resistor.
Figure 19. Rheostat
This variable resistor is a large, wire-wound power potentiometer. It is used in power
control applications.
Rheostats must be mounted on panels or stand-offs because of their high power
dissipation. Panel mounting permits air to circulate around them, which reduces the
heating effect of the rheostat.
Figure 20. Adjustable Wire Resistor
Another kind of adjustable power resistor is shown in Figure 20. It consists of a typical
fixed wire-wound resistor, rated 12 watts and above.
What makes this resistor an adjustable resistor is the metal ring around its body. By
sliding this ring up and down the body of the resistor, you adjust the RA and RB resistors.
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Block 1
Introduction to Electricity
UNIT I
LESSON 5A
RESISTORS
SUMMARY
You have learned about resistors in this lesson. Here's a summary of the important points
you should remember.
 A resistor is an electronic component used as a limiting device to adjust and set
voltage and current levels in an electronic circuit.
 Resistors are either fixed value resistors or variable (adjustable) value resistors.
 The three general types of fixed resistors are:
– General purpose resistors (carbon resistors)
– Power resistors (wire-wound resistors)
– Precision resistors (metal film resistors)
 The two general types of variable resistors are:
– Potentiometers (PC board mounted)
– Rheostats (chassis mounted)
 Carbon resistors, the most popular type of resistor in electronics, have color bands
marked on their bodies to indicate their ohmic values.
 Wire-wound resistors are used in applications where excessive heat must be
dissipated.
 Precision resistors, made with tolerances of ± 1% or better, are used in circuits
where voltage or current levels are very critical.
 Trimmer potentiometers are used when the ohmic value of a resistor is set at the
time a circuit is manufactured and tested.
 Control potentiometers, usually panel mounted, are used for manual control of
voltage or current levels.
 Rheostats are variable resistors used to control or set current.
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