Biomedical Engineering Reference
In-Depth Information
FIGURE 2-1
Examples of some
panel switches.
(a) Toggle. (b) Push
button. (c) Rotary.
FIGURE 2-2
Fundamental
switching
configurations.
(a) Single pole,
single throw.
(b) Single pole,
double throw.
(c) Double pole,
double throw.
2.2.1 Toggle Switches
Toggle switches are available in a number of different configurations depending on the
application and are characterized by the number of switching contacts (or poles) and the
number of terminals. For example, a single on-off switch shown in Figure 2-2a is referred
to as a single pole single throw (SPST) device. In the case where the switch can connect
to a pair of terminals, as shown in Figure 2-2b, it is referred to as a single pole double
throw (SPDT) device. Also common are multipole switches, in which individual switch-
ing elements are ganged together to form double, triple, and even quadruple devices. The
nomenclature follows that of the previous cases, so a double pole double throw switch
would be referred to as DPDT.
All double throw switches are break-before-make in that the pole never contacts both
of the terminals simultaneously. This is important to ensure proper isolation and to avoid
short circuits.
2.2.2 Push-Button Switches
Push-button switches are momentary contact devices because they operate only when
force is applied. They come in two varieties, with a normally open (NO) or a normally
closed (NC) contact for single throw devices, but are often also configured as double throw
devices as shown in Figure 2-3c.
2.2.3 Limit Switches
Mechanical limit switches are a variation of the standard push-button configuration fitted
with a lever arm. These are often known as microswitches because of their small size.
