Switches are used to control the flow
of electrical current. Either fully allowing or fully cutting off the
flow of electricity. A switch works like a valve that is always either
fully open or fully closed. There are many switch
arrangements, some of which are shown below.
The simplest switch
is the Single Pole Single Throw (SPST) switch. This switch is often
used as an "On/Off" switch. It controls the current flow in a single
circuit. The Single Pole Double Throw (SPDT) switch can control current
from, or to, either of two circuits. Sometimes a double throw switch is
referred to as an "On/On" switch. Some double throw switches have a
center position where there is no connection to either circuit. This is
sometimes referred to as a "center-off" or an "On/Off/On" switch.
pole switches are essentially two single pole switches that work in
unison. These are used where two circuits need to be controlled
simultaneously. The dashed lines shown with the double pole switches
indicate that the switches work together.
Some switches are
spring-loaded and return to the original position when released (either
"on" or "off", depending on the switch and how it is used). These are
called momentary-contact switches.
Basic Switch Types
Single Pole Single Throw (SPST)
Single Pole Double Throw (SPDT)
Double Pole Single Throw (DPST)
Double Pole Double Throw (DPDT)
Normally-open Pushbutton Switch
Normally-closed Pushbutton Switch
Styles of switches
Top row: circuit breaker, mercury switch,
rotary switch, dip switch (slide type), surface mount switch, magnetic
reed switch; Bottom row: wall switch, bat handle toggle switch, inline
switch, pushbutton switch, rocker switch, microswitch.
Ganging switches (and other devices)
that the double pole switches above are essentially two switches
connected by dashed lines. This means that the two switches work off
one handle or knob. This is called ganging and is also used for other
devices that need to work in unison. Ganged devices may be widely
separated on a schematic diagram but will be connected by a dashed line.