Chapter 4

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# Chapter 4 - PowerPoint PPT Presentation

Chapter 4 . Resistance. Resistance 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.

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### Chapter 4

Resistance

Resistance
• 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.
Resistivity
• The resistance of a material to current flow.
• 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.
Conductance
• The ability of a material to pass electrons.
• Unit known as Mho (ohm backwards).
• Abbreviated with the inverted Greek symbol Ω.
Resistors
• Components manufactured to possess a specific value of resistance to the flow of current.
• Come in two classifications:
• Fixed value
• Variable
• Variety of shapes and sizes to meet specific circuit, space, and operating requirements.
Tolerance
• 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%.
Molded carbon resistor
• The most commonly used.
• Inexpensive
• Manufactured in standard resistor values.
• Wire wound
• Used in high-current circuits.
• Resistance varies from a fraction of an ohm to several thousand ohms.
Film resistors
• Becoming increasingly popular.
• Three types: carbon film, metal film, and tin oxide film.
• Surface mount resistors
• Ideal for small circuit applications.
• Available in both thick and thin films.
Variable resistors*
• 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.
Resistor identification
• 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, 2002http://www.uoguelph.ca/~antoon

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%

EIA Color Code

Example of EIA color code:*
• Orange = 3
• White = 9
• Red = 2 zeros
• Gold = 5%

= 3900 ohms or,

• Resistor is a 3.9K Ohm 5% tolerance fixed resistor.
Resistors in Circuits
• Resistors are typically configured in a circuit in one of three different ways:
• Series circuit configuration.
• Parallel circuit configuration.
• Compound circuit configuration.
Lab 1-4
• Go to the classroom website and complete the Labs for Resistance.
• You can find Lab 1-4-A and 1-4-B in the Handouts tab in the dashboard on the classroom website.
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
Resistors in Parallel
• A parallel circuit contains two or more resistors and provides two or more paths for current to flow,
• Each current path is called a “branch”,
• The more “branches”, or paths, the less opposition there is to current flow, (less resistance),
• When a resistor is added in parallel to a circuit, the total resistance in the circuit decreases.
Parallel circuit comprised of resistors:
• Formula to calculate the total resistance
• The total resistance for the above figure is:
• 5.45 OHMS
Compound circuit comprised of resistors:
• To calculate the total resistance we use both the series and parallel formulas.
• Total resistance for the above figure is:
• 66.7 OHMS
In Summary:
• Resistance
• Resistors
• Resistivity
• Conductivity
• Resistor identification
• Fixed
• Variable
• Circuit configurations
• Series
• Parallel
• Compound