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Instructor: Prof. Dr. Atalay BARKANA
ANADOLU UNIVERSITY
DEPT. OF ELECTRICAL AND ELECTRONICS ENGINEERING
EEM401Professional Aspects of Electrical Engineering
30.12.2009
Homework6: Switches & Relays
16169230356 Osman GÜLERCAN
I-Introduction:
"A switch is an electrical component that can break an electrical circuit, interrupting the
current or diverting it from one conductor to another. The most familiar form of switch is a
manually operated electromechanical device with one or more sets of electrical contacts. Each
set of contacts can be in one of two states: either 'closed' meaning the contacts are touching
and electricity can flow between them, or 'open', meaning the contacts are separated and
nonconducting."1
"A relay is an electrically operated switch. Many relays use an electromagnet to operate a
switching mechanism, but other operating principles are also used. Relays find applications
where it is necessary to control a circuit by a low-power signal, or where several circuits must
be controlled by one signal. The first relays were used in long distance telegraph circuits,
repeating the signal coming in from one circuit and re-transmitting it to another. Relays found
extensive use in telephone exchanges and early computers to perform logical operations. A
type of relay that can handle the high power required to directly drive an electric motor is
called a contactor. Solid-state relays control power circuits with no moving parts, instead
using a semiconductor device triggered by light to perform switching. Relays with calibrated
operating characteristics and sometimes multiple operating coils are used to protect electrical
circuits from overload or faults; in modern electric power systems these functions are
performed by digital instruments still called 'protection relays'."2
II-Main Text
Relays & Switches
"Electrical switches are devices utilized to manually control electric current flow. Manually
operated switches include an actuator used to cause making/breaking action of switch contacts
to energize or de-energize one or more electrical circuits associated with the contacts. In
addition to switching electrical signals responsive to physical actuation of the switch,
electrical switches may include actuation indication including tactile feedback or audible
feedback. Switches are available that are equipped with lamps used for helping people find
the switches in the dark. There are several different types of light switches. Common switch
types include rocker, toggle, push button, rotary, keylock, slide, snap action and reed
1 http://en.wikipedia.org/wiki/Switch2 http://en.wikipedia.org/wiki/Relay
switches. Switches are highly varied based on their intended utility. In electromechanical
systems, switches provide the function of making and breaking electrical contacts and
consequently electrical circuits. Switches responsive to fluid levels have utilized a variety of
approaches for opening and closing an electrical circuit. Switches have found particular use in
industrial control systems, where it is often desirable to monitor conditions so that appropriate
actions may be taken in response to the monitored conditions. A relay is used to perform
automatic electrical switching. An electromechanical relay is opened or closed by energizing
an electromagnet to either attract or repel a metal contact on a movable strip of metal. A solid
state relay has all its components made from solid state devices and involves no mechanical
movement. They are compatible with digital circuitry and have a wide variety of uses with
such circuits." 3
"A switch may be directly manipulated by a human as a control signal to a system, such as a
computer keyboard button, or to control power flow in a circuit, such as a light switch.
Automatically-operated switches can be used to control the motions of machines, for
example, to indicate that a garage door has reached its full open position or that a machine
tool is in a position to accept another workpiece. Switches may be operated by process
variables such as pressure, temperature, flow, current, voltage, and force, acting as sensors in
a process and used to automatically control a system. For example, a thermostat is an
automatically-operated switch used to control a heating process. A switch that is operated by
another electrical circuit is called a relay. Large switches may be remotely operated by a
motor drive mechanism. Some switches are used to isolate electric power from a system,
providing a visible point of isolation that can be pad-locked if necessary to prevent accidental
operation of a machine during maintenance, or to prevent electric shock.
Switches:
Switches are classified according to the arrangement of their contacts in electronics.
Electricians installing building wiring use different nomenclature, such as "one-way", "two-
way", "three-way" and "four-way" switches, which have different meanings in North
American and British cultural regions as described in the table below.
Some contacts are normally open (Abbreviated "n.o." or "no") until closed by operation of the
switch, while others are normally closed ("n.c. or "nc") and opened by the switch action.
3 http://www.electronics-manufacturers.com/products/relays-switches/
A switch with both types of contact is called a changeover switch. These may be "make-
before-break" which momentarily connect both circuits, or may be "break-before-make"
which interrupts one circuit before closing the other.
The terms pole and throw are also used to describe switch contact variations. A pole is a set of
contacts and terminals that are connected to a single circuit. A throw is one of two or more
positions that the switch can adopt. A single-throw switch has one position that closes
contacts, a double-throw switch has two position, and so on.
These terms give rise to abbreviations for the types of switch which are used in the electronics
industry such as "single-pole, single-throw" (SPST) (the simplest type, "on or off") or "single-
pole, double-throw" (SPDT), connecting either of two terminals to the common terminal. In
electrical power wiring (i.e. House and building wiring by electricians) names generally
involving the suffixed word "-way" are used; however, these terms differ between British and
American English and the terms two way and three way are used in both with different
meanings. "4
DIP Switch
Modern electronic circuits that have elements mounted on printed circuit boards often provide
switch selectable options to provide different functions. Such electronic circuits require a
miniature electrical switch having leads that fit into the openings of a printed circuit board
arranged at the same standard spacing used for integrated circuit packages, or which are
surface-mountable on printed circuit boards. Switches configured as dual inline packages
(DIP switches) are utilized within electronic equipment to change the configuration of the
equipment. DIP switches are typically found on electronic circuits such as motherboards
within computers or on expansion cards or auxiliary cards which mount within the computer.
DIP switches are small rectangular electronic components, usually packaged in a plastic
housing which contains the electrical and mechanical components of the switch and a means
to connect and solder the package to a mounting surface. The DIP switches allow easy
changes in configuration in the system, for example, the configuration of a system can be
varied to change the output location or printer or plotter model by merely changing the switch
settings. There are many different kinds of DIP switches which include SPST (single-pole,
4 http://en.wikipedia.org/wiki/Switch
single-throw), MPST (multiple-pole, single-throw), MTSP (multiple-throw, single-pole),
rotary contacting, and other variations. "5
Disconnect Switch
"Disconnect switches are used to isolate a component of an electrical system from the power
source. Electrical power distribution systems require switching for many reasons, including
fault isolation, transfer loads from one source to another, isolation of line segments for
purpose of maintenance or new construction, and in some instances for shedding loads.
Disconnect switches are frequently used to switch transformer magnetizing current, low level
line or cable charging current, and parallel load currents. Disconnect switches are manually
operated switches which are used for isolating high-voltage equipment, so that this equipment
may be worked on safely. AC disconnect switches are often used for connecting AC current
to and disconnecting AC current and operate between on and off conditions by inserting a
pullout handle assembly into and removing the pullout handle assembly from a stationary
terminal block assembly mounted within the enclosure of the AC disconnect switch.
Emergency Stop Switch
In factories and the like where industrial machinery is installed, in order to ensure the safety
of an operator in cases such as where a fault occurs during operation of machinery, an
emergency stop switch for emergency stop of the machinery is necessarily provided. A
machine is typically powered by an electrical power source and typically has an on/off switch
for use during normal operating conditions. For safety reasons, a machine will usually also
include an emergency stop switch for terminating electrical power to the machine in an
emergency situation. The emergency switch is activated under circumstances demanding an
immediate cessation of operation of the machine. Most of the conventional exercise apparatus
driven by electric motor include an emergency stop switch in the circuit controller for an
immediate cutoff of the power supply to stop the motor, thereby ensuring the safety of the
operator. An emergency stop safety switch as commonly used in machinery or exercise
equipment utilizes either a push-button or pull configuration.
5 http://www.electronics-manufacturers.com/products/relays-switches
Emergency Stop Button
E-Stop button uses a push-to-stop, twist-to-release mechanism to provide emergency stop
actuation. This button provides a rapid way to disconnect the energy source of the device to
protect the operator. On large, industrial machines, an emergency stop button is typically
located on the control panel, and possibly needed in several other areas of the machine.
Flow Switch
A flow switch is used to sense the flow of a fluid passing through its valve body and to send
an electrical control signal to control the switching unit. In many applications, it is essential to
be able to determine whether fluid is flowing in a pipeline, duct or other conduit and to
respond accordingly to such a determination. For example, flow responsive devices for
producing a control signal which is used to de-energize a pump when the flow rate falls below
a preselected minimum are commonly employed in systems for transferring fluid between
reservoirs. Fluid flow switch sensing devices have been developed for monitoring fluid flow
in pipelines, ducts, and other conduits. A flow switch produces an electrical signal which is
commensurate with a preselected rate of flow of a fluid in a conduit. Various flow switches
have been developed to be responsive to the flow rate of a fluid within a flow line. Usually the
flow switch is connected into the flow line so that the flow path of the fluid passes through the
flow switch. Several such devices rely on the pivotal movement of a rod supported blade
which is deflected depending on the amount of fluid flow.
Keylock Switch
Electrical switches typically operate to open and close an electrical circuit by moving one or
more contacts between contact positions. Keylock switches are employed to actuate electrical
devices in applications requiring security measures where it is desired to limit control of an
electrical circuit to a keyholder. Key operated switches provide a means to conveniently and
affirmatively switch electrical current. Many variations of key operated switches exist that
utilize a key which acts upon a mechanism to connect a load or device to an electrical current.
Keylock switches are frequently employed in circuits for controlling installations or machines
in order to perform on-off control of all or part of their functions. Key operated switches
provide added security by permitting only those individuals with a key to operate the switch.
A key lock switch requires that a key be inserted into a lock mechanism and rotated at some
degrees in a particular direction in order to switch power on. To switch the power to a load
off, the actuation process is reversed. Rotary switches may also be provided as a key lock
rotary switch having a lock body and a lock cylinder."6
Light Switch
"Light switches are commonly used in the home and at the office. Light switches and their
cover plates are available in a range of sizes, types and designs. Typically switches are
mounted onto walls for easy access by those using electrically powered devices. Such
switches are often mounted in the wall near a doorway, allowing a person to flip the switch to
turn on the room lights. Light switches may also control other electrical appliances that are
plugged into wall plugs or receptacles that are controlled by the switch. Two main types of
switches are presently used in homes, offices, or other places to activate electrical appliances
such as interior lights. They're toggle type switch and rocker-type switch. The function of
these switches is essentially to provide ON and OFF positions and consequently to turn on or
turn off the appliances so controlled. Dimmer switches are regulating devices used for varying
the electric power applied to an electric load such as an electric light. More advanced
electronic switches may include a motion sensor or infrared sensor to detect when a person is
moving in a room. Thus the room lights appear to turn on automatically when a person enters
a room. A timer may also be used to turn off the lights after a period of time without motion.
Push Button Switch
A push button switch is used to either close or open an electrical circuit depending on the
application. Push button switches are used in various applications such as industrial
equipment control handles, outdoor controls, mobile communication terminals, and medical
equipment, and etc. Push button switches generally include a push button disposed within a
housing. The push button may be depressed to cause movement of the push button relative to
the housing for directly or indirectly changing the state of an electrical contact to open or
close the contact. Also included in a pushbutton switch may be an actuator, driver, or plunger
of some type that is situated within a switch housing having at least two contacts in
communication with an electrical circuit within which the switch is incorporated. Typical
6 http://www.electronics-manufacturers.com/products/relays-switches
actuators used for contact switches include spring loaded force cap actuators that reciprocate
within a sleeve disposed within the canister. The actuator is typically coupled to the
movement of the cap assembly, such that the actuator translates in a direction that is parallel
with the cap. A push button switch for a data input unit for a mobile communication device
such as a cellular phone, a key board for a personal computer or the like is generally
constructed by mounting a cover member directly on a circuit board. Printed circuit board
(PCB) mounted pushbutton switches are an inexpensive means of providing an operator
interface on industrial control products. In such push button switches, a substrate which
includes a plurality of movable sections is formed of a rubber elastomer. The key top is
formed on a top surface thereof with a figure, a character or the like by printing, to thereby
provide a cover member. Push button switches incorporating lighted displays have been used
in a variety of applications. Such switches are typically comprised of a pushbutton, an opaque
legend plate, and a back light to illuminate the legend plate."7
Pole and Throw Configurations of Switches:
- SPST : Single pole, single throw One-way Two-way A simple on-off switch, The two
terminals are either connected together or not connected to anything. An example is a
light switch.
- SPDT :Single pole, double throw Two-way Three-way A simple changeover switch, C
(COM, Common) is connected to L1 or to L2.
- SPCO; SPTT, c.o. : Single pole changeover or Single pole, centre off or Single Pole,
Triple Throw Similar to SPDT. Some suppliers use SPCO/SPTT for switches with a
stable off position in the centre and SPDT for those without.
- DPST : Double pole, single throw Double pole Double pole, Equivalent to two SPST
switches controlled by a single mechanism.
- DPDT : Double pole, double throw, Equivalent to two SPDT switches controlled by a
single mechanism: A is connected to B and D to E, or A is connected to C and D to F.
- DPCO : Double pole changeover or Double pole, centre off, Equivalent to DPDT.
Some suppliers use DPCO for switches with a stable off position in the centre and
DPDT for those without.
Relays:
7 http://www.electronics-manufacturers.com/products/relays-switches/
"A relay is an electrically operated switch. Current flowing through the coil of the relay
creates a magnetic field which attracts a lever and changes the switch contacts. The coil
current can be on or off so relays have two switch positions and they are double throw
(changeover) switches.
Relays allow one circuit to switch a second circuit which can be completely separate from the
first. For example a low voltage battery circuit can use a relay to switch a 230V AC mains
circuit. There is no electrical connection inside the relay between the two circuits, the link is
magnetic and mechanical.
The coil of a relay passes a relatively large current, typically 30mA for a 12V relay, but it can
be as much as 100mA for relays designed to operate from lower voltages. Most ICs (chips)
cannot provide this current and a transistor is usually used to amplify the small IC current to
the larger value required for the relay coil. The maximum output current for the popular 555
timer IC is 200mA so these devices can supply relay coils directly without amplification.
Relays are usuallly SPDT or DPDT but they can have many more sets of switch contacts, for
example relays with 4 sets of changeover contacts are readily available. For further
information about switch contacts and the terms used to describe them please see the page on
switches.
Most relays are designed for PCB mounting but you can solder wires directly to the pins
providing you take care to avoid melting the plastic case of the relay.
The supplier's catalogue should show you the relay's connections. The coil will be obvious
and it may be connected either way round. Relay coils produce brief high voltage 'spikes'
when they are switched off and this can destroy transistors and ICs in the circuit. To prevent
damage you must connect a protection diode across the relay coil.
The animated picture shows a working relay with its coil and switch contacts. You can see a
lever on the left being attracted by magnetism when the coil is switched on. This lever moves
the switch contacts. There is one set of contacts (SPDT) in the foreground and another behind
them, making the relay DPDT. "
Choosing a Relay
You need to consider several features when choosing a relay:
1. Physical size and pin arrangement
If you are choosing a relay for an existing PCB you will need to ensure that its
dimensions and pin arrangement are suitable. You should find this information in the
supplier's catalogue.
2. Coil voltage
The relay's coil voltage rating and resistance must suit the circuit powering the relay
coil. Many relays have a coil rated for a 12V supply but 5V and 24V relays are also
readily available. Some relays operate perfectly well with a supply voltage which is a
little lower than their rated value.
3. Coil resistance
The circuit must be able to supply the current required by the relay coil.
4. Switch ratings (voltage and current)
The relay's switch contacts must be suitable for the circuit they are to control. You will
need to check the voltage and current ratings. Note that the voltage rating is usually
higher for AC, for example: "5A at 24V DC or 125V AC".
5. Switch contact arrangement (SPDT, DPDT etc.)
Most relays are SPDT or DPDT which are often described as "single pole changeover"
(SPCO) or 'double pole changeover' (DPCO). For further information please see the
page on switches." 8
Types of Electrical Relay
There are different types of relay available in the market depending on the end use. These are:
Latching Relay
"Latching relay, dust cover removed, showing pawl and ratchet mechanism. The ratchet
operates a cam, which raises and lowers the moving contact arm, seen edge-on just below it.
The moving and fixed contacts are visible at the left side of the image.
A latching relay has two relaxed states (bistable). These are also called "impulse", "keep", or
"stay" relays. When the current is switched off, the relay remains in its last state. This is
achieved with a solenoid operating a ratchet and cam mechanism, or by having two opposing
coils with an over-center spring or permanent magnet to hold the armature and contacts in
position while the coil is relaxed, or with a remanent core. In the ratchet and cam example, the
first pulse to the coil turns the relay on and the second pulse turns it off. In the two coil
example, a pulse to one coil turns the relay on and a pulse to the opposite coil turns the relay
off. This type of relay has the advantage that it consumes power only for an instant, while it is
8 http://www.kpsec.freeuk.com/components/relay.htm
being switched, and it retains its last setting across a power outage. A remanent core latching
relay requires a current pulse of opposite polarity to make it change state.
Reed Relay
A reed relay has a set of contacts inside a vacuum or inert gas filled glass tube, which protects
the contacts against atmospheric corrosion. The contacts are closed by a magnetic field
generated when current passes through a coil around the glass tube. Reed relays are capable of
faster switching speeds than larger types of relays, but have low switch current and voltage
ratings. See also reed switch.
Mercury-wetted Relay
A mercury-wetted reed relay is a form of reed relay in which the contacts are wetted with
mercury. Such relays are used to switch low-voltage signals (one volt or less) because of their
low contact resistance, or for high-speed counting and timing applications where the mercury
eliminates contact bounce. Mercury wetted relays are position-sensitive and must be mounted
vertically to work properly. Because of the toxicity and expense of liquid mercury, these
relays are rarely specified for new equipment. See also mercury switch.
Polarized Relay
A polarized relay placed the armature between the poles of a permanent magnet to increase
sensitivity. Polarized relays were used in middle 20th Century telephone exchanges to detect
faint pulses and correct telegraphic distortion. The poles were on screws, so a technician
could first adjust them for maximum sensitivity and then apply a bias spring to set the critical
current that would operate the relay.
Machine Tool Relay
A machine tool relay is a type standardized for industrial control of machine tools, transfer
machines, and other sequential control. They are characterized by a large number of contacts
(sometimes extendable in the field) which are easily converted from normally-open to
normally-closed status, easily replaceable coils, and a form factor that allows compactly
installing many relays in a control panel. Although such relays once were the backbone of
automation in such industries as automobile assembly, the programmable logic controller
(PLC) mostly displaced the machine tool relay from sequential control applications.
Contactor Relay
A contactor is a very heavy-duty relay used for switching electric motors and lighting loads.
Continuous current ratings for common contactors range from 10 amps to several hundred
amps. High-current contacts are made with alloys containing silver. The unavoidable arcing
causes the contacts to oxidize; however, silver oxide is still a good conductor.[2] Such devices
are often used for motor starters. A motor starter is a contactor with overload protection
devices attached. The overload sensing devices are a form of heat operated relay where a coil
heats a bi-metal strip, or where a solder pot melts, releasing a spring to operate auxiliary
contacts. These auxiliary contacts are in series with the coil. If the overload senses excess
current in the load, the coil is de-energized. Contactor relays can be extremely loud to operate,
making them unfit for use where noise is a chief concern.
Solid-State Relay
A solid state relay (SSR) is a solid state electronic component that provides a similar function
to an electromechanical relay but does not have any moving components, increasing long-
term reliability. With early SSR's, the tradeoff came from the fact that every transistor has a
small voltage drop across it. This voltage drop limited the amount of current a given SSR
could handle. As transistors improved, higher current SSR's, able to handle 100 to 1,200
Amperes, have become commercially available. Compared to electromagnetic relays, they
may be falsely triggered by transients.
Solid State Contactor Relay
A solid state contactor is a very heavy-duty solid state relay, including the necessary heat
sink, used for switching electric heaters, small electric motors and lighting loads; where
frequent on/off cycles are required. There are no moving parts to wear out and there is no
contact bounce due to vibration. They are activated by AC control signals or DC control
signals from Programmable logic controller (PLCs), PCs, Transistor-transistor logic (TTL)
sources, or other microprocessor and microcontroller controls."9
Buchholz Relay
"A Buchholz relay is a safety device sensing the accumulation of gas in large oil-filled
transformers, which will alarm on slow accumulation of gas or shut down the transformer if
gas is produced rapidly in the transformer oil.
9 http://en.wikipedia.org/wiki/Relay
Forced-Guided Contacts Relay
A forced-guided contacts relay has relay contacts that are mechanically linked together, so
that when the relay coil is energized or de-energized, all of the linked contacts move together.
If one set of contacts in the relay becomes immobilized, no other contact of the same relay
will be able to move. The function of forced-guided contacts is to enable the safety circuit to
check the status of the relay. Forced-guided contacts are also known as "positive-guided
contacts", "captive contacts", "locked contacts", or "safety relays".
Overload Protection Relay
One type of electric motor overload protection relay is operated by a heating element in series
with the electric motor . The heat generated by the motor current operates a bi-metal strip or
melts solder, releasing a spring to operate contacts. Where the overload relay is exposed to the
same environment as the motor, a useful though crude compensation for motor ambient
temperature is provided.
Pole and Throw Configurations of Relays
Since relays are switches, the terminology applied to switches is also applied to relays. A
relay will switch one or more poles, each of whose contacts can be thrown by energizing the
coil in one of three ways:
Normally-open ( NO ) contacts connect the circuit when the relay is activated; the
circuit is disconnected when the relay is inactive. It is also called a Form A contact or
"make" contact.
Normally-closed ( NC ) contacts disconnect the circuit when the relay is activated; the
circuit is connected when the relay is inactive. It is also called a Form B contact or
"break" contact.
Change-over ( CO ), or double-throw (DT), contacts control two circuits: one normally-
open contact and one normally-closed contact with a common terminal. It is also
called a Form C contact or "transfer" contact ("break before make"). If this type of
contact utilizes a "make before break" functionality, then it is called a Form D contact.
The following designations are commonly encountered:
SPST – Single Pole Single Throw : These have two terminals which can be connected
or disconnected. Including two for the coil, such a relay has four terminals in total. It
is ambiguous whether the pole is normally open or normally closed. The terminology
"SPNO" and "SPNC" is sometimes used to resolve the ambiguity.
SPDT – Single Pole Double Throw: A common terminal connects to either of two
others. Including two for the coil, such a relay has five terminals in total.
DPST – Double Pole Single Throw: These have two pairs of terminals. Equivalent to
two SPST switches or relays actuated by a single coil. Including two for the coil, such
a relay has six terminals in total. The poles may be Form A or Form B (or one of
each).
DPDT – Double Pole Double Throw: These have two rows of change-over terminals.
Equivalent to two SPDT switches or relays actuated by a single coil. Such a relay has
eight terminals, including the coil.
The "S" or "D" may be replaced with a number, indicating multiple switches connected to a
single actuator. For example 4PDT indicates a four pole double throw relay (with 14
terminals)."10
Specifications to be Considered:
"There are certain important specifications that must be considered while selecting an
appropriate relay for a particular application. These are: Selection of an appropriate relay for a
particular application requires evaluation of many different factors:
Number and type of contacts: These can be normally open, normally closed, or
changeover (double-throw).
Rating of contacts: While small relays switch a few amperes, there are large contactors
that are rated for up to 3000 amperes.
Voltage rating of contacts: This can vary depending on the type of relay. While control
relays are rated 300 VAC or 600 VAC, automotive types to 50 VDC, special high-
voltage relays to about 15,000 V.
Package/enclosure: The packing or enclosures of relay can be open, touch-safe,
double-voltage for isolation between circuits, outdoor, oil-splash resistant and
explosion proof.
Mounting: This can be plug board, rail mount, panel mount, through-panel mount,
enclosure for mounting on walls or equipment.
Regulatory approvals: Relays approved from agencies assure high quality to
customers. " 11
Electrical Relays Applications
10 http://en.wikipedia.org/wiki/Relay11 http://www.allelectricalproducts.com/electrical-relays.html
Relays are used in numerous applications including:
Modem
Automobiles
Aerospace
Telecommunications
III-Conclusion:
Switches and relays are electrical components to control current flow on the circuit. Switches
are controlled manually, whereas relays are controlled with small electrical current. They are
commonly used in many sectors covering home automotion systems to telecommunication
systems. The main idea to operate them are to open or close one or many sets of contacts.
Today, there are lots of switch and relay types that even some of time delay functions can be
performed. As transistors impoved, relays are lost their popularity; however, many sefety-
critical logic circuit prefer relays because relays are more resistance than semiconductors. To
sum up, in many sectors, switches and relays are widely used devices to interrupt the flow of
electrons in the circuit.
IV-References:
1) Wikipedia
http://en.wikipedia.org/
2) Electronics Information Online
http://www.electronics-manufacturers.com/
3) John Hewes' Electronics Club
http://www.kpsec.freeuk.com/components/relay.htm/
