Switch types
An electrical switch is any device used to interrupt the flow of electrons
in a circuit. Switches are essentially binary devices: they are either
completely
on ("closed") or completely off ("open"). There are many different types of
switches, and we will explore some of these types in this chapter.
The simplest type of switch is one where two electrical conductors are
brought in contact with each other by the motion of an actuating mechanism.
Other
switches are more complex, containing electronic circuits able to turn on or
off depending on some physical stimulus (such as light or magnetic field)
sensed. In any case, the final output of any switch will be (at least) a
pair of wire-connection terminals that will either be connected together by
the
switch's internal contact mechanism ("closed"), or not connected together
("open").
Any switch designed to be operated by a person is generally called a hand
switch, and they are manufactured in several varieties:
Toggle switches are actuated by a lever angled in one of two or more
positions. The common light switch used in household wiring is an example of
a toggle
switch. Most toggle switches will come to rest in any of their lever
positions, while others have an internal spring mechanism returning the
lever to a
certain normal position, allowing for what is called "momentary" operation.
Pushbutton switches are two-position devices actuated with a button that is
pressed and released. Most pushbutton switches have an internal spring
mechanism
returning the button to its "out," or "unpressed," position, for momentary
operation. Some pushbutton switches will latch alternately on or off with
every
push of the button. Other pushbutton switches will stay in their "in," or
"pressed," position until the button is pulled back out. This last type of
pushbutton
switches usually have a mushroom-shaped button for easy push-pull action.
Selector switches are actuated with a rotary knob or lever of some sort to
select one of two or more positions. Like the toggle switch, selector
switches
can either rest in any of their positions or contain spring-return
mechanisms for momentary operation.
A joystick switch is actuated by a lever free to move in more than one axis
of motion. One or more of several switch contact mechanisms are actuated
depending
on which way the lever is pushed, and sometimes by how far it is pushed. The
circle-and-dot notation on the switch symbol represents the direction of
joystick
lever motion required to actuate the contact. Joystick hand switches are
commonly used for crane and robot control.
Some switches are specifically designed to be operated by the motion of a
machine rather than by the hand of a human operator. These motion-operated
switches
are commonly called limit switches, because they are often used to limit the
motion of a machine by turning off the actuating power to a component if it
moves too far. As with hand switches, limit switches come in several
varieties:
These limit switches closely resemble rugged toggle or selector hand
switches fitted with a lever pushed by the machine part. Often, the levers
are tipped
with a small roller bearing, preventing the lever from being worn off by
repeated contact with the machine part.
Proximity switches sense the approach of a metallic machine part either by a
magnetic or high-frequency electromagnetic field. Simple proximity switches
use a permanent magnet to actuate a sealed switch mechanism whenever the
machine part gets close (typically 1 inch or less). More complex proximity
switches
work like a metal detector, energizing a coil of wire with a high-frequency
current, and electronically monitoring the magnitude of that current. If a
metallic part (not necessarily magnetic) gets close enough to the coil, the
current will increase, and trip the monitoring circuit. The symbol shown
here
for the proximity switch is of the electronic variety, as indicated by the
diamond-shaped box surrounding the switch. A non-electronic proximity switch
would use the same symbol as the lever-actuated limit switch.
Another form of proximity switch is the optical switch, comprised of a light
source and photocell. Machine position is detected by either the
interruption
or reflection of a light beam. Optical switches are also useful in safety
applications, where beams of light can be used to detect personnel entry
into
a dangerous area.
In many industrial processes, it is necessary to monitor various physical
quantities with switches. Such switches can be used to sound alarms,
indicating
that a process variable has exceeded normal parameters, or they can be used
to shut down processes or equipment if those variables have reached
dangerous
or destructive levels. There are many different types of process switches:
These switches sense the rotary speed of a shaft either by a centrifugal
weight mechanism mounted on the shaft, or by some kind of non-contact
detection
of shaft motion such as optical or magnetic.
Gas or liquid pressure can be used to actuate a switch mechanism if that
pressure is applied to a piston, diaphragm, or bellows, which converts
pressure
to mechanical force.
An inexpensive temperature-sensing mechanism is the "bimetallic strip:" a
thin strip of two metals, joined back-to-back, each metal having a different
rate
of thermal expansion. When the strip heats or cools, differing rates of
thermal expansion between the two metals causes it to bend. The bending of
the
strip can then be used to actuate a switch contact mechanism. Other
temperature switches use a brass bulb filled with either a liquid or gas,
with a tiny
tube connecting the bulb to a pressure-sensing switch. As the bulb is
heated, the gas or liquid expands, generating a pressure increase which then
actuates
the switch mechanism.
A floating object can be used to actuate a switch mechanism when the liquid
level in an tank rises past a certain point. If the liquid is electrically
conductive,
the liquid itself can be used as a conductor to bridge between two metal
probes inserted into the tank at the required depth. The conductivity
technique
is usually implemented with a special design of relay triggered by a small
amount of current through the conductive liquid. In most cases it is
impractical
and dangerous to switch the full load current of the circuit through a
liquid.
Level switches can also be designed to detect the level of solid materials
such as wood chips, grain, coal, or animal feed in a storage silo, bin, or
hopper.
A common design for this application is a small paddle wheel, inserted into
the bin at the desired height, which is slowly turned by a small electric
motor.
When the solid material fills the bin to that height, the material prevents
the paddle wheel from turning. The torque response of the small motor than
trips the switch mechanism. Another design uses a "tuning fork" shaped metal
prong, inserted into the bin from the outside at the desired height. The
fork
is vibrated at its resonant frequency by an electronic circuit and
magnet/electromagnet coil assembly. When the bin fills to that height, the
solid material
dampens the vibration of the fork, the change in vibration amplitude and/or
frequency detected by the electronic circuit.
Inserted into a pipe, a flow switch will detect any gas or liquid flow rate
in excess of a certain threshold, usually with a small paddle or vane which
is pushed by the flow. Other flow switches are constructed as differential
pressure switches, measuring the pressure drop across a restriction built
into
the pipe.
Another type of level switch, suitable for liquid or solid material
detection, is the nuclear switch. Composed of a radioactive source material
and a radiation
detector, the two are mounted across the diameter of a storage vessel for
either solid or liquid material. Any height of material beyond the level of
the
source/detector arrangement will attenuate the strength of radiation
reaching the detector. This decrease in radiation at the detector can be
used to trigger
a relay mechanism to provide a switch contact for measurement, alarm point,
or even control of the vessel level.
Both source and detector are outside of the vessel, with no intrusion at all
except the radiation flux itself. The radioactive sources used are fairly
weak
and pose no immediate health threat to operations or maintenance personnel.
As usual, there is usually more than one way to implement a switch to
monitor a physical process or serve as an operator control. There is usually
no single
"perfect" switch for any application, although some obviously exhibit
certain advantages over others. Switches must be intelligently matched to
the task
for efficient and reliable operation.
. A switch is an electrical device, usually electromechanical, used to
control continuity between two points.
. Hand switches are actuated by human touch.
. Limit switches are actuated by machine motion.
. Process switches are actuated by changes in some physical process
(temperature, level, flow, etc.).
