1. Chapter 4 Resistance

2. 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. 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. 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. 5 –Affected by temperature. –Affected by the size (diameter) of the conductor.

6. 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. 7

8. 8 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)(Ω)

9. 9 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.

10. 10 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%.

11. 11 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.

12. 12 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.

13. 13 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.

14. 14 Resistor identification –Alphanumeric –EIA (Electronic Industries Association) Color Code

15. 15 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

16. 16 EIA Color Code 1st Band2nd Band3rd Band4th Band 1st Digit2nd DigitNumber of Zero’sTolerance BLACK00N/AN/A BROWN1101% RED22002% ORANGE33000N/A YELLOW440,000N/A GREEN5500,0000.5% BLUE66000,0000.25% VIOLET77N/A0.10% GRAY88N/A0.05% GOLDN/AN/AX.15% SILVERN/AN/AX.0110% NO COLORN/AN/AN/A20%

17. 17 Example of EIA color code: –Orange = 3 –White = 9 –Red = 2 zeros –Gold = 5% Resistor is a 3.9K Ohm 5% tolerance fixed resistor.

18. 18 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.

19. 19 Series circuit comprised of resistors: To calculate out the total resistance we use the formula R T + R 1 + R 2 + R 3 + etc. The total resistance for Figure 4-19 is: 60 OHMS

20. 20 Parallel circuit comprised of resistors: Formula to calculate the total resistance The total resistance for Figure 4-21 is: 5.45 OHMS

21. 21 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

22. 22 In Summary: –Resistance Resistors Resistivity Conductivity –Resistor identification Fixed Variable

23. 23 –Circuit configurations Series Parallel Compound