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Chapter 4

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

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  1. Chapter 4 Resistance

  2. 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.

  3. 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.

  4. Conductance • The ability of a material to pass electrons. • Unit known as Mho (ohm backwards). • Abbreviated with the inverted Greek symbol Ω.

  5. 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.

  6. 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%.

  7. 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.

  8. 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.

  9. 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.

  10. Resistor identification • Alphanumeric • EIA (Electronic Industries Association) Color Code • The color bands are read from left to right,

  11. Resistors – Color Codes Reproduced by permission of Tony van Roon, 2002http://www.uoguelph.ca/~antoon

  12. 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

  13. 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.

  14. 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.

  15. 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. • Type your answers in Microsoft Word and upload to the classroom website when completed.

  16. 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,

  17. 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

  18. 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.

  19. Parallel circuit comprised of resistors: • Formula to calculate the total resistance • The total resistance for the above figure is: • 5.45 OHMS

  20. 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

  21. In Summary: • Resistance • Resistors • Resistivity • Conductivity • Resistor identification • Fixed • Variable • Circuit configurations • Series • Parallel • Compound

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