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.
Though it may seem strange to cover this elementary electrical topic at
such a late stage in this book series, I do so because the chapters that
follow explore an older realm of digital technology based on mechanical
switch contacts rather than solid-state gate circuits, and a thorough
understanding of switch types is necessary for the undertaking. Learning the
function of switch-based circuits at the same time that you learn about
solid-state logic gates makes both topics easier to grasp, and sets the
stage for an enhanced learning experience in Boolean algebra, the
mathematics behind digital logic circuits.
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.
REVIEW:
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.).
|