INDICATED VALUE

Red Violet Brown Gold —

Violet Green Orange — —

Orange Orange Silver Gold —

Brown Red Orange Brown Silver

Gray Red Yellow Gold Orange

Table 2-5. Determine the Value of These Resistors

11. The end-to-end resistance of a potentiometer changes when the potentiometer is adjusted. (True or False)

12. A rheostat is a terminal device, but a potentiometer is a _________-terminal device.

2.8 Switches

A switch is another basic, but important, component used in electronic circuits. Its purpose is to break (open) or make (close) circuit connections. You are familiar with the switches in your home that are used to control the lighting. When you operate the switch to turn the lights on, you are closing the circuit and connecting the lightbulb to the power line. When the switch is off, the lightbulb is isolated from the power line.

Basic Switch Operation

Figure 2-36 shows a basic switch circuit. The type of switch shown in the picture is called a knife switch. Although it is not often used in electronic circuits, this type of switch is easy to understand, and its operation is representative of other, less obvious, switch types.

When the switch is in the upper position, as in Figure 2-36(a), the circuit is open and the lamp remains dark. When the switch closes the circuit, as in Figure 2-36(b), then electrons can travel from the battery, through the lamp, through the switch, and return to the posi-tive side of the battery. This simple switch represents a general class of switches called single-pole single-throw (SPST). The schematic symbol for a SPST switch is shown in Figure 2-37 along with the symbols for several other types of switches.

The term pole refers to the movable portion of the switch. For example, a double-pole switch has two movable arms; it acts as two electrically separate switches that are mechani-cally linked so they operate simultaneously. The pole of a switch is often identified with an arrowhead on the switch symbol. The term throw identifies the number of circuits that are

Switches are used to open or close the path for current flow in an electrical or electronic circuit.

KEY POINTS

+ –

(a)

+ –

(b) Figure 2-36. A basic switch circuit.

In a multiple-pole switch, one mechanical link con-trols more than one elec-trical switch.

KEY POINTS

opened or closed by each pole when the switch is operated. For example, a double-throw switch opens or closes two circuits on each pole.

Figure 2-38 illustrates how to connect a single-pole double-throw (SPDT) switch into a lamp circuit. In Figure 2-38(a), the red lamp is lit while the blue lamp is off. In the switch position shown in Figure 2-38(b), the circuit for the blue lamp is closed and the red lamp is extinguished.

Single-pole single-throw Push button (normally open)

Push button (normally closed) Double-pole single-throw

Single-pole double-throw

Double-pole double-throw Three-pole four-position rotary

Figure 2-37. Schematic symbols for several types of switches.

V_T

S₁

Blue Red

(a)

V_T

S₁

Blue Red

(b)

Figure 2-38. A SPDT switch can alternately connect power to two different circuits.

Switching Mechanism

Any of the basic switch types (e.g., SPST, SPDT, DPST, DPDT, and so on) can be made with different mechanical means for operating the switch. Some of the more common means for switch operation are listed:

• toggle

• rocker

• push button

• rotary

• slide

There is a tremendous range of switch forms based on these basic classes of mechanisms.

Figure 2-39 shows a sampling of switch types.

Momentary-Contact Switches

Some switches open and/or close a circuit when you operate the switch, but return to the original state as soon as the switch button is released. This type of switch is called a momentary-contact switch. Doorbell and car horn switches are two examples of momen-tary-contact switches.

Technicians must know which contacts are closed and which ones are open when the switch is in its normal state. There are two ways this is indicated on schematic diagrams.

First, the poles for all switch contacts are drawn in the normal (not operated) position.

Second, the letters NC and NO are sometimes written near the contacts to indicate normally closed and normally open contacts, respectively.

Ganged Switches

When switches have multiple poles, the individual poles are mechanically linked, or ganged, together so that they operate simultaneously. This mechanical link is shown on

Figure 2-39. There many types of switching mechanisms available from switch manufacturers.

Switches may be con-structed with momentary contacts, which change states when activated but return to their normal state when the switch is released.

KEY POINTS

The normal state of the switch is shown on the schematic symbol.

KEY POINTS

schematic diagrams as a dotted line. The multiple-pole switches shown in Figure 2-37 have dotted lines to show the mechanical link between sections of the switch.

Switch Specifications

Switch manufacturers provide extensive catalogs that detail the physical and electrical characteristics of their switches. A technician or engineer needs this information to select a particular switch for a specific application. Some of the factors to be considered in switch selection are cited:

• contact form (i.e., SPDT, DPDT, and so on)

• switching mechanism (e.g., push button, toggle, rocker, and so on)

• voltage rating

• current rating

• environmental performance (e.g., moisture immunity, ruggedness, and so on)

1. How many electrical connections are there on a SPST switch?

2. How many electrical terminals are there on a DPDT switch?

3. How many electrical connections are there on a two-pole, six-position rotary switch?

4. Describe the symptoms if an electrician accidentally installed a momentary-contact switch as a light switch in your home.

5. Figure 2-40 shows an electrical circuit with three switches and three lamps. Each switch has its positions labeled as “A” and “B.” Analyze the circuit and determine which lamps are on for each of the conditions listed in Table 2-6.

Exercise Problems 2.8 Exercise Problems 2.8

V_T

L₁ L₂ L₃

SW₁

SW₂

SW₃ A

B

A

B A

B

Figure 2-40. A switch circuit for Exercise Problem 5.

2.9 Fuses and Circuit Breakers

Most electrical and electronic circuits are susceptible to damage by excessive current flow.

The increased current normally flows as a result of a defective component or an accidental short circuit. When a normal path for current flow is bypassed by a much lower resistance path, we call the low-resistance path a short circuit. Short circuits are generally character-ized by increased current flow. Fuses and circuit breakers provide protection against damage from excessive current flow.

Fuses

Fuses are connected such that the current flowing through the protected circuit also flows through the fuse. Figure 2-41 shows the operation of a fuse. There is a resistive link inside the fuse body that heats up when current flows through it. If the current is sufficiently high, the resistive link burns open, which stops all current flow in the circuit. When a fuse has burned open, we say it is blown.