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Chapter 4

Chapter 4. Resistance. Objectives After completing this chapter, the student should be able to: Define resistance and explain its effect in a circuit. Determine the tolerance range of a resistor. Identify carbon composition, wirewound, and film resistors.

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Chapter 4

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  1. Chapter 4 Resistance

  2. Objectives • After completing this chapter, the student should be able to: • Define resistance and explain its effect in a circuit. • Determine the tolerance range of a resistor. • Identify carbon composition, wirewound, and film resistors. • Identify potentiometers and rheostats. • Describe how a variable resistor operates.

  3. Decode a resistor’s value using the color code or alphanumerical code. • Identify the three types of resistor circuits. • Calculate total resistance in series, parallel, and series-parallel circuits.

  4. 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.

  5. Affected by temperature. • Affected by the size (diameter) of the conductor.

  6. Resistivity • The resistance of a material to current flow. • Resistivity is different for different materials. • Even good conductors have different levels of resistivity.

  7. Conductance • The ability of a material to pass electrons. • Expressed as G. • Unit known as Mho (ohm backwards). • Abbreviated with the inverted Greek symbol omega • Is derived by: R = 1/G or G = 1/R (Ω)

  8. 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.

  9. 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%.

  10. Molded carbon resistor • The most commonly used. • Inexpensive. • Manufactured in standard resistor values. • Wirewound • Used in high-current circuits. • Resistance varies from a fraction of an ohm to several thousand ohms.

  11. 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.

  12. 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.

  13. Resistor identification • Alphanumeric • EIA (Electronic Industries Association) Color Code

  14. Example of an alphanumeric code: RN60D5112F RN60 = Resistor style (composition, wirewound, film) D = Characteristics (effects of temperature) 5112 = Resistance value (2 represents number of zeros) F = Tolerance

  15. 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%

  16. Example of EIA color code: • Orange = 3 • White = 9 • Red = 2 zeros • Gold = 5% • Resistor is a 3.9K Ohm 5% tolerance fixed resistor.

  17. 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.

  18. Series circuit comprised of resistors: • To calculate out the total resistance we use the formula RT + R1 + R2 + R3 + etc. • The total resistance for Figure 4-19 is: • 60 OHMS

  19. Parallel circuit comprised of resistors: • Formula to calculate the total resistance • The total resistance for Figure 4-21 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 Figure 4-24 is: • 66.7 ohms

  21. In Summary: • Resistance • Resistors • Resistivity • Conductivity • Resistor identification • Fixed • Variable

  22. Circuit configurations • Series • Parallel • Compound

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