3-5
It would not be possible to describe all the different switches used. This chapter will describe the
most common types of switches.
MANUAL SWITCHES
A manual switch is a switch that is controlled by a person. In other words, a manual switch is a
switch that you turn on or off. Examples of common manual switches are a light switch, the ignition
switch on a motor vehicle, or the channel selector on a television. You may not think of the channel
selector as a switch that you use to turn something on or off, but that is what it does. The channel selector
is used to turn on the proper circuit and allows the television to receive the channel you have selected.
AUTOMATIC SWITCH
An automatic switch is a switch that is controlled by a mechanical or electrical device. You do not
have to turn an automatic switch on or off. Two examples of automatic switches are a thermostat and the
distributor in a motor vehicle. The thermostat will turn a furnace or air conditioner on or off by
responding to the temperature in a room. The distributor electrically turns on the spark plug circuit at the
proper time by responding to the mechanical rotation of a shaft. Even the switch that turns on the light in
a refrigerator when the door is opened is an automatic switch.
Automatic switches are not always as simple as the examples given above. Limit switches, which
sense some limit such as fluid level, mechanical movement, pressure (altitude or depth under water), or an
electrical quantity, are automatic switches. Computers use and control automatic switches that are
sometimes quite complicated.
Basically, any switch that will turn a circuit on or off without human action is an automatic switch.
MULTICONTACT SWITCHES
Switches are sometimes used to control more than one circuit or to select one of several possible
circuits. An example of a switch controlling more than one circuit is the AM/FM selector on a radio. This
switch enables you to control either the AM or FM portion of the radio with a single switch. An example
of a switch that selects one of several circuits is the channel selector of a television set. These switches are
called MULTICONTACT switches because they have more than one contact or MULTI(ple)
CONTACTS.
Number of Poles and Number of Throws
Multicontact switches (other than rotary switches, which will be covered later) are usually classified
by the number of POLES and number of THROWS. Poles are shown in schematics as those contacts
through which current enters the switch; they are connected to the movable contacts. Each pole may be
connected to another part of the circuit through the switch by "throwing" the switch (movable contacts) to
another position. This action provides an individual conduction path through the switch for each pole
connection. The number of THROWS indicates the number of different circuits that can be controlled by
each pole. By counting the number of points where current enters the switch (from the schematic symbol
or the switch itself), you can determine the number of poles. By counting the number of different points
each pole can connect with, you can determine the number of throws.
Figure 3-3 will help you understand this concept by showing illustrations of various multicontact
switches and their schematic symbols.
