Any device which could be used to distrupt/interrupt
electrons circulating within a circuit is known as electrical switch.
Essentially, switches are binary devices: they are either entirely on
(“closed”) or totally off (“open”). There are a lot of
different kinds of switches and in this chapter we will discuss some of those
Although it may seem odd to discuss this basic electrical
subject at such a late stage in this book series, I do so because the following
chapters address an older field of digital technology focused on mechanical
switch contacts rather than solid-state gate circuits, and the undertaking
needs a thorough understanding of switch types. Understanding the function of
switch-based circuits while also mastering solid-state logic gates helps make
both topics easier to understand and it sets the stage for an improved learning
experience in Boolean algebra, the digital logic circuit mathematics.
The simplest type of switch is referred to as contact
block, where the action of an actuating mechanism brings two electrical
conductors into contact with one another. Some switches are more complicated,
comprising electronic circuits that are capable of turning on or off based on some
perceived physical stimuli (such as magnetic field or light). In any scenario,
the final output of any kind of switch will be (at least) a pair of
wire-connection terminals which are either bound by the internal communication
mechanism (“closed”) of the switch, or not linked together
Any switch intended to be operated by a human is generally
referred to as a hand switch, and it is made in several varieties:
A lever which is angled in one of two or more positions
actuates the toggle switches. A toggle switch is one example of a popular light
switch which is used in household wiring. Most toggle switches are
designed come to rest at any of their lever positions, and some will have an
internal spring mechanism which will return the lever to some kind of normal
position allowing what is termed “momentary” operation.
The push-button switches are two-position devices that are
controlled with a push and release button. For momentary action,
majority of push-button switches have a spring mechanism internally
which returns the button to its “unpressed,” or
“out,” position. With every press of the button, several
push-button switches will turn on or off alternately. Some push-button switches
will remain in their “pressed” or “in” position
till the button is pulled back out. Normally, this type of
push-button switches does have a mushroom-shaped button for quick push-pull
This type of switches are actuated with some sort
of rotary knob or lever to select one of two positions or more. Compared to the
toggle switch, selector switches can either stop at any of their positions or
have spring-return mechanisms for momentary activity.
A joystick switch is controlled by a lever which can move
in more than one direction. One or more of the mechanisms for switch contact
are actuated based on how far and which way the lever is pushed.
On the switch symbol is the circle-and-dot notation which shows
the direction of the joystick lever motion needed to actuate the
contact. Joystick hand switches are widely used for operating cranes and
Many switches are designed specifically to be
controlled by a machine’s action, rather than by a human operator’s hand. Such
motion controlled switches are commonly referred to as limit switches, as they
are often used to restrict a machine’s motion by shutting off the actuating
control to a device if it goes too far. Limit switches are of several types, as
with hand switches:
actuator limit switch
These limit switches closely resemble selector hand or
rugged toggle switches equipped with a lever driven by the system component.
The levers are often tipped with a small roller bearing which prevents the
lever from getting worn off by repeated contact with the part of the
Proximity switches detect either a magnetic or high
frequency electromagnetic field approaching a metallic system component. Simple
proximity switches use a permanent magnet to control a sealed switch mechanism
while closing the system component (usually 1 inch or less). More sophisticated
proximity switches operate like a metal detector, energizing a wire coil with a
high-frequency current, and measuring the amplitude of that current
electronically. If a metallic (not inherently magnetic) part gets close enough
to the wire, the current increases, and the control circuit goes off. The
proximity switch symbol shown here is of the electronic type, as shown by the
diamond-shaped box circling the switch. A non-electronic
limit switch and the lever-actuated limit switch
have the same symbol.
The optical switch, containing a photocell and a light
source, is another type of proximity switch. The position of the machine is
sensed either by a light beam interruption or by reflection. Often, optical
switches are effective in protection systems, where light beams can be used to
identify workers approaching a dangerous area.
The monitoring of various physical quantities with
switches is necessary in many industrial processes. These switches may be used
to sound warnings, signaling that a process variable has surpassed standard
thresholds, or they may be used to shut down systems or facilities when those
variables have reached unsafe or harmful rates. There are different types of
These switches detect a shaft’s rotational speed
either through a centrifugal weight device installed on the shaft, or through
some kind of non-contact sensing of shaft motion such as magnetic or optical.
Liquid pressure or Pressure switch Gas may be used to
actuate a switch function if that pressure is applied to a bellow, diaphragm,
or piston, which transforms pressure to mechanical force.
The “bimetallic strip” is an inexpensive
temperature sensing mechanism: a thin strip of two metals, joined back to back,
with each metal having a different thermal expansion rate. Differing levels of
thermal expansion between the two metals allow it to bend when the strip
heats or cools. Then, a switch contact mechanism can then be actuated by
the bending of the strip. Many temperature controls use a brass bulb
packed with either a gas or liquid, with a tiny tube linking the bulb to a
switch that detects heat. The gas or liquid expands as the filament is heated,
creating an increase in pressure which then stimulates the switching mechanism.
A floating body may be used to actuate a
switch mechanism when the volume of liquid in a tank increases past a
certain point. If the liquid is electrically conductive, the liquid itself may
be used as a conductor for bridging at the appropriate depth between two metal
probes embedded in the tank. The conductivity strategy is typically applied
through the conductive material, with a specific relay configuration caused by
a small amount of current. In most situations, moving the circuit’s full load
current through a liquid is inefficient and risky.
The amount of solid materials such as animal feed,
coal, wood chips or grain in a hopper, bin, or a storage silo can also be
detected by the use of specially designed level switches. A typical design
for this usage is a small paddle wheel, placed at the desired height into the
bin which is turned gradually by a small electric motor. The material stops the
paddle wheel from spinning once the solid material fills the bin to this
height. The small engine’s torque response then trips the switch mechanism. One
other design utilizes a metal prong shaped “tuning fork,” placed into
the bin at the desired height from the outside. The fork is vibrated by an
electronic circuit and the magnet / electromagnet coil assembly at its resonant
frequency. When the bin fills up to that height, the solid material dampens the
fork force, the vibration intensity and/or frequency shift that the electronic
Inserted in a pipe, a flow switch will detect
any liquid or gas flow rate that exceeds a certain level, usually
with a small vane or paddle that is pushed by the flow. Other flow switches are
designed as differential pressure switches, measuring the pressure drop over a
Another kind of level switch is the nuclear
switch, ideal for detecting solid or liquid materials. Composed of a radiation
detector and a radioactive source material, both are mounted for either liquid
or solid material over the diameter of a storage vessel. Any material height
above the level of the detector/source arrangement will reduce the radiation
strength that reaches the detector. This decrease in detector radiation may be
used to activate a relay mechanism to provide a switch contact for vessel level
measurement, alarm point, or even control.
Both source and detector are outside the vessel, and there
is no intrusion except for the radiation flux itself. The radioactive sources
used do not pose any immediate threat to maintenance or operations staff
as they are fairly weak.
Usually, there is more than a way of implementing the
switch to serve as an operator control or monitor a physical process. For any
application, there is usually no single “perfect” switch, although
some obviously show some advantages over others. Switches have to be
intelligently adapted to the task for reliable and efficient operation.
• The switch is an electrical device. It is usually electromechanical
and is used to control the continuity between two points.
• Hand switches are powered with human touch.
• Limit switches are machine movement driven.
• Process switches are activated by some physical process
changes (temperature, flow, level, etc.).
A switch may be built with any mechanism putting two
conductors into contact with each other in a regulated manner. This can be as
easy as having two copper wires to meet each other through the action of a
lever or by moving two metal strips into direct contact. Nevertheless, a proper
switch design must be reliable and rugged, and avoid the possibility of
electrical shock posed to the user. Industrial switch designs therefore are
never that basic.
The conductive components used for making and
breaking the electrical connection in a switch are called
contacts. Contacts are made primarily from silver or silver cadmium alloy, the
conductive properties of which are not significantly impaired by oxidation or
surface corrosion. Gold contacts have the greatest resistance to corrosion, but
are reduced in current-carrying capacity and may be “cold welded”
when combined with high mechanical force. The switch contacts are driven by a
system that guarantees even and square contact for optimum reliability and
minimal resistance, whatever the metal preference.
Contacts like these can be built to accommodate extremely
large quantities of electric current, in some cases up to thousands of volts.
The limiting factors for the ampacity of the switch contact are:
• Sparking triggered by opening or closing contacts;
• Voltage over open switch contacts (potential of jumping
current across the gap).
• Heat produced by the current from metal contacts (while
The exposure of the contacts in the standard switch
contacts to the surrounding atmosphere is a major disadvantage. This is
usually not a problem in a good, tidy, control-room setting. But most
industrial conditions aren’t so innocuous. The existence of corrosive
chemicals in the air will cause the contacts to prematurely deteriorate and
fail. The risk of regular contact sparking which causes flammable or explosive
chemicals to ignite is even more worrying.
When there are such environmental concerns, certain forms
of contacts for small switches may be considered. These other contact types are
sealed from outside air contact and therefore do not experience the same
exposure problems as standard contacts do.
The mercury switch is one common type of sealed-contact
switch. Mercury is a metallic element and it is liquid
at room-temperature. Being a metal, it has outstanding conductive
characteristics. It can be put into contact within a sealed chamber with metal
probes (to close a circuit) as it a liquid simply by inclining the
chamber so that the probes are on the bottom Most commercial switches use
small mercury-containing glass tubes that are tilted one way to open the
contact, and tilted another way to close it. Such systems are an excellent
alternative to open-air transfer connections anywhere environmental exposure
issues are concerned, apart from the problems of tube breakage and spilled
mercury (which however is a toxic material), and sensitivity to vibration.
There, in the open position, a mercury switch (often
referred to as tilt switch) is shown where the mercury is seen to
be out of contact with the two metal contacts and also at the other
end of the glass bulb: the same switch is shown in the closed position. Gravity
also holds the liquid mercury in contact with the two metal contacts, ensuring
electrical stability from one to the other: Mercury switch contacts are
inefficient to create in large sizes, so you will typically find these contacts
rated at no more than 120 volts and no more than a few amps. Of course, there
are variations but these are some common limits.
Another type of sealed-contact switch is the magnetic
reed switch. As with the mercury switch, the contacts of a reed switch are
found within a sealed tube. Unlike the mercury switch that utilizes liquid
metal as the contact medium, the reed switch is essentially a pair of very
small, magnetic, metal strips (hence the term “reed”) that are
brought into contact with each other by adding a strong magnetic field outside
the sealed loop. The magnetic field source in this type of switch is typically
a permanent magnet, which is pushed by the actuating mechanism closer or
further away from the tube. This form of contact is usually measured at lower
voltages and currents than the normal mercury switch, due to the small size
of the reeds. Usually, reed switches tolerate vibration better than mercury
contacts though, because there is no liquid inside the conduit to splash about.
General-purpose switch contact voltage and current
levels are commonly found to be higher on any specified switch or relay if the
electrical power being transferred is AC instead of DC. The explanation for
this is the tendency of an alternating-current arc over an air gap to
self-extinguish. Due to the fact that 60 Hz power line current ceases and
reverses direction 120 times per second, there are many ways for an arc’s
ionized air to lose adequate temperature to avoid conducting current, to the
extent where the arc will not restart at the next voltage level. However, DC is
a steady, constant movement of electrons that tends to keep an arc even
smoother over an air gap. Therefore, by switching a specified value of the
direct current, switch contacts of any kind cause more wear than for the
same amount of alternating current. When the load has a significant amount of
inductance, the issue of switching DC is amplified, as there will be very
strong voltages produced through the contacts of the switch when the circuit is
opened (the inducer will do its best to maintain circuit current at the same
magnitude as when the switch is closed).
Contact arcing can be reduced with both AC and DC by
inserting a “snubber” circuit (a resistor and capacitor wired in
series) in parallel with the contact, such as this: a sudden rise in
voltage across the switch contact induced by the contact opening will be
tempered by the charging operation of the capacitor (the capacitor
countering the rise in voltage through drawing current). The amount of current
discharged through the contact whenever it closes again is limited by
the resistor. If the resistor was not present, the capacitor could actually
make the arcing worse during contact closure than the arcing during
contact opening without a capacitor! While this extension to the circuit tends
to reduce contact arcing, it is not without disadvantage: a prime factor
is the probability of a broken (shortened) resistor / capacitor
combination supplying electrons with a route to pass through the circuit at all
times, even if the contact is open and the current is not needed. The threat
of this malfunction, and the extent of the resulting consequences, must be
weighed against the enhanced contact wear without the snubber circuit (and
eventual failure of the contact).
One thing which is not new is the use of snubbers in DC
switch circuits: this has been practiced by automobile manufacturers on engine
ignition systems for years, reducing arcing across the switch contact
“points” in the supplier with a tiny capacitor called a condenser. As
any technician will tell you: the service life of the “points” of the
distributor directly depends on the operation of the condenser.
With all this talk about minimizing switch contact arcing,
one might assume that for a mechanical switch, less current is always better.
This isn’t always so, however. A small amount of intermittent arcing will work
out great for the switch contacts, because it protects the contact faces from
corrosion and small amounts of debris. The contacts may tend to accumulate
undue resistance if a mechanical switch contact is worked with too little
current and may fail prematurely! This minimum amount of electrical current
that is required to sustain mechanical switch contact in good health is
called the wetting current.
In a properly designed system, the wetting current rating
of a switch is normally well below its maximum current rating, and also
well below its normal operating current load. Nonetheless, there are
situations where the contact a mechanical switch may be needed to
manage currents regularly below usual wetting current limits (for example, when
a mechanical selector switch has to open or close an analog
electronic or a digital logic circuit where there is an extremely
small current value). Gold-plated switch contacts are highly recommended
to be listed in those applications. Gold will not corrode like other
metals as it is a “noble” metal. As a consequence,
such contacts have extremely low current wetting criteria. Regular
contacts with the silver or copper alloy will not provide effective
operation when used in this low-current service!
• Reed switches are another form of sealed-contact system,
the contact being created by two thin metal “reeds” inside a glass
tube, coupled by an external magnetic field effect.
• Mercury switches use the liquid mercury metal slug as a
moving contact. Shielded in a glass tube, the spark of the mercury
contact is shielded from the outside world, rendering this form of
switch ideally suited for atmospheres that host potentially explosive
• The parts of the switch responsible for making and
breaking electrical synchronization are considered the “contacts.”
Usually made of corrosion-resistant metal alloy, contacts are designed to meet
each other by a process that helps to maintain proper alignment and spacing.
• Switch contacts suffer from DC switching more than AC.
This is due primarily to the self-extinguishing nature of an AC arc.
• To reduce contact arcing, a resistor-capacitor network
called a “snubber” may be connected in parallel to a switch contact.
• The minimum amount of electric current needed to
carry a switch contact to be self-cleaned is called wetting current. This value
is normally far below the maximum current rating for the switch
make/break sequence and “normal” state of contact
Any kind of switch contact can be configured such that the
contacts “close” (establish continuity) when actuated, or
“open” (interrupt continuity) when actuated. In switches that have a
spring-return function in them, the spring returns it to a direction with no
applied force. This direction is known as the normal position Then contacts
open in this position are normally called open, and contacts closed in this
position are normally called closed.
The normal position, or condition, for process switches is
what the switch is in when there is no process control over it. One easy way to
figure out a process switch’s usual state is to find the switch status as it
lies uninstalled on a storage shelf. Definitions of “normal”
process switch situations are as follows:
• Pressure switch: When the pressure applied is zero
• Speed switch: shaft not spinning
• Level switch: empty bin or tank
• Flow switch: Zero flow of liquid
• Temperature switch: ambient temperature
One important thing to do is to distinguish between the
“normal” state of the switch and its “normal” use in the
operational process Consider the example of a liquid flow switch that acts as a
low-flow alarm in a system of cooling water. The cooling water system’s
natural, or properly functioning state is to have a fairly constant
coolant flow through this pipe. If we want to close the contact of the
flow switch in case of a lack of coolant flow (for example, to complete an
electrical circuit that triggers an alarm siren), we would like to rather than
use a flow switch with normally-open contacts use one with
normally-closed contacts. The contacts of the switch are forced open when there
is sufficient flow through the pipe; when the flow rate decreases to an
abnormally low point the contacts revert to their usual (closed) state. Thinking
of “normal” as the process’s usual state could be confusing, so
be careful to always think of the “normal” state of a switch as that
in which it is as it is sitting on a shelf.
The symbol for switches varies depending on the intent and
actuation of the switch. A normally open switch contain contact is designed in
such a way that it indicates an open connection ready to shut when it is
actuated. In comparison, a switch that is normally closed is designed as a
closed connection that when actuated, opens. Remember the following
symbols: Each switch contact also has a common symbol, representing the
contact points within a switch with a pair of vertical lines.
Normally open contacts are marked by non-touching lines while normally closed
contacts are defined with a bridging diagonal line between the two lines.
Compare the two:
When actuated, the switch seen on the left will close
and open while in the normal state (unactuated). Once actuated, the switch on
the right opens and is closed in the normal (non-actuated) position. When
switches are described with these common symbols, the type of switch should
typically be indicated in text next to the symbol immediately Please note the
icon on the left should not be mistaken with a capacitor’s. If a capacitor
requires to be depicted in a diagram of control logic, it will be shown as
In regular electronic symbology, the above figure is
intended for polarity-sensitive capacitors. The capacitor symbol is used for
every type of capacitor in control logic symbology, even when the capacitor is
not prone to polarity, so as to clearly differentiate it from a normal-open
A further design aspect must be addressed for
multiple-position selector switches: that is, the process of breaking old
connections and making new connections as the switch is pushed from position to
position, the moving contact reaching numerous stationary contacts in sequence
The selector switch shown above moves the common contact
lever to one out of five different positions, to connect wires
numbered 1 to 5. The most popular design of a multi-position switch like this
is one such that contact is interrupted with one place before contact
is made with the next position. This design is known as break-before-make. For
example, if the switch was placed at position number 3 and turned slowly in the
clockwise direction, the contact lever would shift off the position number 3,
getting the circuit to open, going to a position between numbers 3
and 4 (both circuit paths open), and then touching position number 4, and
getting the circuit to close.
There are situations where full opening of the circuit
attached to the common wire at any point in time is unacceptable. A
make-before-breakswitch mechanism can be designed for such an operation, in
which the movable contact lever essentially bridges between two contact
positions (between number 3 and number 4, in the scenario above) as it
moves between positions. The concession here is that, as the selector knob is
rotated from position to position, the circuit must be able to manage switch
closures between neighboring position contacts (1 and 2, 2 and 3, 3, and 4, 4
and 5). Here is such switch:
If movable contacts with stationary contacts can be
put into one of many positions, such positions are sometimes referred to as
throws. Often, the number of movable contacts is called poles. The selector
switches seen above would be defined as “single-pole, five-throw”
switches with five stationary contacts and one moving contact.
If two identical five-throw, single-pole switches were
mechanically coupled in such a way that they were operated by the same
mechanism, the whole assembly will be called a “double-pole,
With process switches, this is the uninstalled state it is
in while sitting on a shelf.
Some common configurations of switch and their abbreviated
open unactuated switch is called normally-open switch. An
unactuated closed switch is referred to as normally-closed. The
terms “normally-closed” and “normally-open” are
sometimes abbreviated as N.C and N.O., respectively.
- The general symbol for N.C.
- Multi-position switches
can be either make-before-break or the very common one, break-before-make.
switch “poles” refers to the number of moving contacts whereas the
switch “throws” refers to the amount of stationary contacts per
In a single, clean instant, when a switch is triggered and
contacts touch each other under the actuating force they are expected to
maintain continuity. However, though, switches do not achieve exactly this aim.
any inherent elasticity in the mechanism and/or contact materials, and
mass of the moving contact, contacts will “bounce” for a period of
milliseconds upon closure before coming to full rest and providing unbroken
contact. Switch bounce is of no concern in many applications: it matters little
if a switch regulating an incandescent lamp “bounces” for a few
cycles whenever it is actuated. Since the warm-up time of the lamp exceeds
considerably the bounce duration, there will be no irregularity in the activity
of the lamp.
Nevertheless, if the switch is used to send a signal with a
quick response time to an electronic amplifier or to some other circuit,
contact bounce may cause very visible and undesired effects:
A closer look at the oscilloscope monitor shows a rather
hideous set of makes and breaks when the switch is triggered one time:
When, for example, this switch is used to transmit
a “clock” signal to a digital counter circuit, so that each
pushbutton switch is intended to raise the counter by a value of 1, what will
happen then is that the counter increments (by several counts) each time
the switch is actuated. Since mechanical switches also interface in modern
systems with digital electronic circuits, switch contact bounce is a frequent
feature of the architecture. Somehow, it is necessary to eliminate the
“chattering” created by bouncing contacts, so that the receiving
circuit has a smooth, crisp off / on transition:
Switch contacts can be debounced in several different ways.
The most straightforward method, is to address the problem at its core: the
switch itself. Here are some ideas for the design of minimal bounce switch
- Reducing the
kinetic energy of the moving contact. This reduces the impact force when it
comes to rest on the stationary contact, thus reducing the bounce.
the stationary contact(s) with “buffer springs” so that they are able to
rebound and softly absorb the impact force from the moving contact
the “wipe” or “slide” contact switch rather than the direct
impact. Sliding contacts are used in “knife” switch designs.
the action of the switch mechanism using the shock absorber” device
for air or oil.
sets of contacts in parallel, each slightly different in mass or contact
gap such that when one is rebounding off the stationary contact at least one of
the others is still in firm contact.
the contacts “wet” in a sealed
environment with liquid mercury. When initial contact is made,
mercury “surface tension should maintain continuity of the circuit although the
moving contact may bounce off the stationary contact multiple times.
Each of these solutions compromises some element of switch
efficiency for limited bounce, and designing all switches with limited contact
bounce in mind is unpractical. Attempts made to minimize the contact’s kinetic
energy may result in a small open-contact distance or a slow-moving contact,
which reduces the voltage that the switch can accommodate and the amount of
current it can interrupt Although non-bouncing, sliding contacts often create
“noise” (abnormal current induced by irregular contact resistance
during movement) and suffer from more mechanical damage than normal contacts.
Multiple parallel contacts provide fewer bounce, but only
more cost and switch complexity. The use of mercury to
“wet” the contacts is a very effective means of reducing the bounce,
but it is sadly restricted to low-ampacity switch contacts. Also in mounting
position mercury-wetted contacts are usually limited, because gravity may
trigger the contacts to bridge inadvertently if they are positioned the wrong
If re-designing the switch system is not a solution,
mechanical switch contacts can be externally debounced, utilizing supplementary
circuit components to modify the signal. For example, a low-pass filter
circuit connected to the switch output can minimize the voltage / current
fluctuations produced by the contact bounce:
Switch contacts can be electronically debounced using
hysteretic transistor circuits (high or low-state “latch” circuits)
with built-in time delays (known as “one-shot” circuits), or two
inputs that are controlled by a double-throw switch.