1. Chapter 4
Resistance

2. Objectives After completing this chapter, you will 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

3. Objectives (cont’d.) Describe how a variable resistor operates
Decode a resistor’s value using the color code or alphanumeric code
Identify the three types of resistor circuits
Calculate total resistance in series, parallel, and series-parallel circuits

4. Resistance Figure 4-1. Resistance of several conductors
of the same length and cross-section area.

5. Conductance Conductance (G) Ability of a material to pass electrons
Unit of conductance is Mho
Measured in Siemens (S)

6. Resistors
Resistors
Possess a specific value of resistance to current flow
Either fixed or variable
Resistor’s tolerance
Amount the resistor can vary and still be acceptable
10% tolerance is satisfactory

7. Resistors (cont’d.) Figure 4-6. Carbon composition resistors were the
most widely used resistors in electronic circuits.

8. Resistors (cont’d.)
Figure 4-7. Wirewound resistors are available in many different styles.

9. Resistors (cont’d.)
Figure 4-8. The film resistor offers the size of the carbon resistor
with the accuracy of the wirewound resistor.

10. Figure 4-10. Variable resistors allow the resistance to increase or decrease at random.

11. Resistors (cont’d.) Potentiometer (or pot) Rheostat
A variable resistor used to control voltage
Rheostat
A variable resistor used to control current

12. Figure 4-12. The Electronic Industries Association (EIA) color code.

13. Resistor Identification (cont’d.)
Figure Meaning of the colored bands on a carbon composition resistor.

14. Resistor Identification (cont’d.)
Figure This resistor has a resistance value of 1500 ohms.

15. Resistor Identification (cont’d.)
Figure The fifth band on a resistor indicates the resistor’s reliability.

16. Resistor Identification (cont’d.)
Figure Resistors may also be identified by a letter-and-number system.

17. Figure 4-18. Three types of resistive circuits: (A) series circuit,
(B) parallel circuit, (C) series-parallel circuit.

18. Connecting Resistors in Series
Series circuit
Provides one path for current to flow
Total resistance formula:
RT = R1 + R2 + R3 … + Rn

19. Connecting Resistors in Parallel
Parallel circuit
Provides two or more paths for current to flow
Total resistance formula:
1/RT = 1/R1 + 1/R2 + 1/R3 … + 1/Rn
Total resistance is always less than smallest resistor

20. Connecting Resistors in Series and Parallel
Series-parallel circuit
Combination of a series and a parallel circuit
To calculate total resistance, use the series and parallel formulas

21. Summary Resistance is opposition to flow of current
Several factors, such as size and type of wire, affect electrical resistance
Resistors are either fixed or variable
Three major categories of resistors: molded carbon composition, wirewound, or film

22. Summary (cont’d.) Variable resistors allow resistance to vary
Resistor values may be identified by colored bands or by an alphanumeric system
Three types of resistive circuits: series, parallel, series-parallel