Transcript No Slide Title

Chapter 4
Resistance
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Resistance
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Opposition to electron flow in a circuit.
 Expressed by the symbol R.
 Measured in ohms.
 Abbreviated with the Greek symbol .
 Varies from material to material.
 Silver is best.
 Copper is most common.
 Gold doesn’t tarnish.
 Affected by temperature.
 Affected by the size (diameter) of the conductor.
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Resistivity
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The resistance of a material to current flow.
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Resistivity is different for different materials.
Even good conductors have different levels of resistivity.
In electric circuit, the larger the diameter of the
wire, the lower the electrical resistance to current
flow,
One ohm is the resistance of a circuit, or circuit
element, that permits a steady current flow of one
amp when one volt is applied to the circuit.
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Conductance

The ability of a material to pass electrons.
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Unit known as Mho (ohm backwards).
Abbreviated with the inverted Greek symbol Ω.
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Resistors
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Components manufactured to possess a
specific value of resistance to the flow of
current.
Come in two classifications:
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Fixed value
Variable
Variety of shapes and sizes to meet specific
circuit, space, and operating requirements.
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Tolerance
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The amount that the resistor may vary and
still be acceptable.
The larger the tolerance, the cheaper it is
to manufacture.
Resistors are available with tolerances of
±20%, 10%, 5%, 2%, and 1%.
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Molded carbon resistor
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The most commonly used.
Inexpensive
Manufactured in standard resistor values.
Wire wound
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Used in high-current circuits.
Resistance varies from a fraction of an ohm
to several thousand ohms.
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Film resistors
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Becoming increasingly popular.
Three types: carbon film, metal film, and
tin oxide film.
Surface mount resistors
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Ideal for small circuit applications.
Available in both thick and thin films.
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Variable resistors*
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Allow the resistance to vary.
Vary linearly or logarithmically.
Called a potentiometer when used to
control voltage.
Called a rheostat when used to control
current.
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Resistor identification
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Alphanumeric
EIA (Electronic Industries Association)
Color Code
The color bands are read from left to right,
Resistors – Color Codes
Reproduced by permission of Tony van Roon, 2002 http://www.uoguelph.ca/~antoon
EIA Color Code
1st Band
2nd Band
3rd Band
4th Band
1st Digit
2nd Digit
Number of Zero’s
Tolerance
BLACK
0
0
N/A
N/A
BROWN
1
1
0
1%
RED
2
2
00
2%
ORANGE
3
3
000
N/A
YELLOW
4
4
0,000
N/A
GREEN
5
5
00,000
0.5%
BLUE
6
6
000,000
0.25%
VIOLET
7
7
N/A
0.10%
GRAY
8
8
N/A
0.05%
GOLD
N/A
N/A
X.1
5%
SILVER
N/A
N/A
X.01
10%
NO COLOR
N/A
N/A
N/A
20%
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Example of EIA color code:*
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Orange = 3
White = 9
Red
= 2 zeros
Gold = 5%
= 3900 ohms or,
Resistor is a 3.9K Ohm 5% tolerance
fixed resistor.
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Resistors in Circuits
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Resistors are typically configured in a
circuit in one of three different ways:
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Series circuit configuration.
Parallel circuit configuration.
Compound circuit configuration.
Lab 1-4
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Go to the classroom website and
complete the Labs for Resistance.
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You can find Lab 1-4-A and 1-4-B in the
Handouts tab in the dashboard on the
classroom website.
Type your answers in Microsoft Word and
upload to the classroom website when
completed.
Resistors in a Series Circuit
• A series circuit contains tow or more resistors
and provides on path for current to flow.
• Current runs from the negative side to the
positive side,
• The more resistors in the circuit, the more
resistance to current flow,
• The total Resistance in a series circuit is the
sum of the individual resistors in the circuit,
• Series circuit comprised of resistors:
• To calculate out the total resistance we
use the formula RT = R1 + R2 + R3 + etc.
• The total resistance for the above figure
is:
• 60 OHMS