Relay - Wikipedia, The Free Encyclopedia

7/29/2019 Relay - Wikipedia, The Free Encyclopedia

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3/20/13 Relay - Wikipedia, the free encyclopedia

en.wikipedia.org/wiki/Relay

Automotive-style miniature relay,

dust cover is taken off

RelayFrom Wikipedia, the free encyclopedia

A relay is an electrically operated switch. Many relays use an electromagnet

to operate a switching mechanism mechanically, but other operating

principles are also used. Relays are used where it is necessary to control a

circuit by a low-power signal (with complete electrical isolation between

control and controlled circuits), or where several circuits must be controlledby 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 were used extensively in telephone exchanges and early

computers to perform logical operations.

A type of relay that can handle the high power required to directly control an

electric motor orother loads is called a contactor. Solid-state relays control

power circuits with no moving parts, instead using a semiconductor device to

perform switching. Relays with calibrated operating characteristics and

sometimes multiple operating coils are used to protect electrical circuits fromoverload orfaults; in modern electric power systems these functions are

performed by digital instruments still called "protective relays".

Contents

1 Basic design and operation

2 Types

2.1 Latching relay2.2 Reed relay

2.3 Mercury-wetted relay

2.4 Polarized relay

2.5 Machine tool relay

2.6 Ratchet relay

2.7 Coaxial relay

2.8 Contactor relay

2.9 Solid-state relay

2.10 Solid state contactor relay

2.11 Buchholz relay

2.12 Forced-guided contacts relay

2.13 Overload protection relays

3 Pole and throw

4 Applications

5 Relay application considerations

5.1 Derating factors

5.2 Undesired arcing

6 Protective relays
http://en.wikipedia.org/wiki/Relay#Protective_relayshttp://en.wikipedia.org/wiki/Relay#Relay_application_considerationshttp://en.wikipedia.org/wiki/Relay#Applicationshttp://en.wikipedia.org/wiki/Relay#Pole_and_throwhttp://en.wikipedia.org/wiki/Relay#Overload_protection_relayshttp://en.wikipedia.org/wiki/Relay#Forced-guided_contacts_relayhttp://en.wikipedia.org/wiki/Relay#Buchholz_relayhttp://en.wikipedia.org/wiki/Relay#Solid_state_contactor_relayhttp://en.wikipedia.org/wiki/Relay#Solid-state_relayhttp://en.wikipedia.org/wiki/Relay#Contactor_relayhttp://en.wikipedia.org/wiki/Relay#Coaxial_relayhttp://en.wikipedia.org/wiki/Relay#Machine_tool_relayhttp://en.wikipedia.org/wiki/Relay#Polarized_relayhttp://en.wikipedia.org/wiki/Relay#Reed_relayhttp://en.wikipedia.org/wiki/Relay#Latching_relayhttp://en.wikipedia.org/wiki/Relay#Typeshttp://en.wikipedia.org/wiki/Relay#Basic_design_and_operationhttp://en.wikipedia.org/wiki/Electrichttp://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Relay#Protective_relayshttp://en.wikipedia.org/wiki/Relay#Undesired_arcinghttp://en.wikipedia.org/wiki/Relay#Derating_factorshttp://en.wikipedia.org/wiki/Relay#Relay_application_considerationshttp://en.wikipedia.org/wiki/Relay#Applicationshttp://en.wikipedia.org/wiki/Relay#Pole_and_throwhttp://en.wikipedia.org/wiki/Relay#Overload_protection_relayshttp://en.wikipedia.org/wiki/Relay#Forced-guided_contacts_relayhttp://en.wikipedia.org/wiki/Relay#Buchholz_relayhttp://en.wikipedia.org/wiki/Relay#Solid_state_contactor_relayhttp://en.wikipedia.org/wiki/Relay#Solid-state_relayhttp://en.wikipedia.org/wiki/Relay#Contactor_relayhttp://en.wikipedia.org/wiki/Relay#Coaxial_relayhttp://en.wikipedia.org/wiki/Relay#Ratchet_relayhttp://en.wikipedia.org/wiki/Relay#Machine_tool_relayhttp://en.wikipedia.org/wiki/Relay#Polarized_relayhttp://en.wikipedia.org/wiki/Relay#Mercury-wetted_relayhttp://en.wikipedia.org/wiki/Relay#Reed_relayhttp://en.wikipedia.org/wiki/Relay#Latching_relayhttp://en.wikipedia.org/wiki/Relay#Typeshttp://en.wikipedia.org/wiki/Relay#Basic_design_and_operationhttp://en.wikipedia.org/wiki/Protective_relayhttp://en.wikipedia.org/wiki/Moving_partshttp://en.wikipedia.org/wiki/Solid-state_relayhttp://en.wikipedia.org/wiki/Contactorhttp://en.wikipedia.org/wiki/Electromagnethttp://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Electrichttp://en.wikipedia.org/wiki/File:Relay.jpg


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en.wikipedia.org/wiki/Relay 2

Simple electromechanical relay.

Small "cradle" relay often used in

electronics. The "cradle" term

refers to the shape of the relay's

armature.

7 Railway signalling

8 History

9 See also

10 References

11 External links

Basic design and operation

A simple electromagnetic relay consists of a coil of wire wrapped around

a soft iron core, an iron yoke which provides a low reluctance path for

magnetic flux, a movable iron armature, and one or more sets of contacts

(there are two in the relay pictured). The armature is hinged to the yoke

and mechanically linked to one or more sets of moving contacts. It is held

in place by a spring so that when the relay is de-energized there is an air

gap in the magnetic circuit. In this condition, one of the two sets of

contacts in the relay pictured is closed, and the other set is open. Other

relays may have more or fewer sets of contacts depending on their

function. The relay in the picture also has a wire connecting the armature

to the yoke. This ensures continuity of the circuit between the moving

contacts on the armature, and the circuit track on the printed circuit board

(PCB) via the yoke, which is soldered to the PCB.

When an electric current is passed through the coil it generates a magnetic

field that activates the armature, and the consequent movement of the

movable contact(s) either makes or breaks (depending upon construction) a

connection with a fixed contact. If the set of contacts was closed when the

relay was de-energized, then the movement opens the contacts and breaksthe connection, and vice versa if the contacts were open. When the current

to the coil is switched off, the armature is returned by a force, approximately

half as strong as the magnetic force, to its relaxed position. Usually this force

is provided by a spring, but gravity is also used commonly in industrial motor

starters. Most relays are manufactured to operate quickly. In a low-voltage

application this reduces noise; in a high voltage or current application it

reduces arcing.

When the coil is energized with direct current, a diode is often placed across

the coil to dissipate the energy from the collapsing magnetic field at deactivation, which would otherwise generate voltage spike dangerous to semiconductor circuit components. Some automotive relays include a diode inside the

relay case. Alternatively, a contact protection network consisting of a capacitor and resistor in series (snubber

circuit) may absorb the surge. If the coil is designed to be energized with alternating current (AC), a small copper

"shading ring" can be crimped to the end of the solenoid, creating a small out-of-phase current which increases the

minimum pull on the armature during the AC cycle.[1]

A solid-state relay uses a thyristor or other solid-state switching device, activated by the control signal, to switch

the controlled load, instead of a solenoid. An optocoupler (a light-emitting diode (LED) coupled with a photo

transistor) can be used to isolate control and controlled circuits.
http://en.wikipedia.org/wiki/Photo_transistorhttp://en.wikipedia.org/wiki/Light-emitting_diodehttp://en.wikipedia.org/wiki/Optocouplerhttp://en.wikipedia.org/wiki/Thyristorhttp://en.wikipedia.org/wiki/Relay#cite_note-1http://en.wikipedia.org/wiki/Alternating_currenthttp://en.wikipedia.org/wiki/Snubberhttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Voltage_spikehttp://en.wikipedia.org/wiki/Diodehttp://en.wikipedia.org/wiki/Direct_currenthttp://en.wikipedia.org/wiki/Arcinghttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Printed_circuit_boardhttp://en.wikipedia.org/wiki/Spring_(device)http://en.wikipedia.org/wiki/Armature_(electrical_engineering)http://en.wikipedia.org/wiki/Magnetic_reluctancehttp://en.wikipedia.org/wiki/Magnetic_corehttp://en.wikipedia.org/wiki/Coilhttp://en.wikipedia.org/wiki/Relay#External_linkshttp://en.wikipedia.org/wiki/Relay#Referenceshttp://en.wikipedia.org/wiki/Relay#See_alsohttp://en.wikipedia.org/wiki/Relay#Historyhttp://en.wikipedia.org/wiki/Relay#Railway_signallinghttp://en.wikipedia.org/wiki/File:Relay2.jpghttp://en.wikipedia.org/wiki/File:Relay_Parts.jpg


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Latching relay with permanent

magnet

Types

Latching relay

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 anover-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 one coil consumes power only

for an instant, while it is being switched, and the relay contacts retain this

setting across a power outage. A remanent core latching relay requires a

current pulse of opposite polarity to make it change state.

A stepping relay is a specialized kind of multi-way latching relay designed for early automatic telephone exchanges

An earth leakage circuit breaker includes a specialized latching relay.

Very early computers often stored bits in a magnetically latching relay, such as ferreed or the later memreed in the

1ESS switch.

Some early computers used ordinary relays as a kind of latch -- they store bits in ordinary wire spring relays or

reed relays by feeding an output wire back as an input, resulting in a feedback loop or sequential circuit. Such an

electrically-latching relay requires continuous power to maintain state, unlike magnetically latching relays or

mechanically racheting relays.

In computer memories, latching relays and other relays were replaced by delay line memory, which in turn was

replaced by a series of ever-faster and ever-smaller memory technologies.

Reed relay

Main article: reed relay

A reed relay is a reed switch enclosed in a solenoid. The switch has a set of contacts inside an evacuated or inert

gas-filled glass tube which protects the contacts against atmospheric corrosion; the contacts are made of magneticmaterial that makes them move under the influence of the field of the enclosing solenoid. Reed relays can switch

faster than larger relays, require very little power from the control circuit. However they have relatively low

switching current and voltage ratings. Though rare, the reeds can become magnetized over time, which makes them

stick 'on' even when no current is present; changing the orientation of the reeds with respect to the solenoid's

magnetic field can resolve this problem.

Mercury-wetted relay

See also: mercury switch
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Top, middle: reed switches, bottom:

reed 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) where the mercury reduces the contact resistance and

associated voltage drop, for low-current signals where surface contamination may make for a poor contact, or for

high-speed 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 now rarely used.

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.

External links

Schematic diagram

(http://ia600200.us.archive.org/5/items/TTYBulletin1044Issue4PartsRelays/teletype-bulletin-1044-

issue-4-1940-11-RY-20-WECO-215A-and-RY-28-WECO215H-and-RY-30-WECO255A.pdf

of a polarized relay used in a teletype machine.

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) whichare 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.

A relay allows circuits to be switched by electrical equipment: for example, a timer circuit with a relay could switch

power at a preset time. For many years relays were the standard method of controlling industrial electronic system

A number of relays could be used together to carry out complex functions (relay logic). The principle of relay logic

is based on relays which energize and de-energize associated contacts. Relay logic is the predecessor of ladder

logic, which is commonly used in programmable logic controllers.

Ratchet relay

This is again a clapper type relay which does not need continuous current through its coil to retain its operation. A

ratchet holds the contacts closed after the coil is momentarily energized. A second impulse, in the same or a

separate coil, releases the contacts.

Coaxial relay
http://en.wikipedia.org/wiki/Programmable_logic_controllerhttp://en.wikipedia.org/wiki/Ladder_logichttp://en.wikipedia.org/wiki/Relay_logichttp://en.wikipedia.org/wiki/Programmable_logic_controllerhttp://en.wikipedia.org/wiki/Designhttp://en.wikipedia.org/wiki/Teletype_machinehttp://ia600200.us.archive.org/5/items/TTYBulletin1044Issue4PartsRelays/teletype-bulletin-1044-issue-4-1940-11-RY-20-WECO-215A-and-RY-28-WECO215H-and-RY-30-WECO255A.pdfhttp://en.wikipedia.org/wiki/Distortion#Teletypewriter_or_modem_signalinghttp://en.wikipedia.org/wiki/Crossbar_switchhttp://en.wikipedia.org/wiki/Mercury_(element)http://en.wikipedia.org/wiki/File:Reedrelay.jpg


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Solid state relay with no moving parts

25 A or 40 A solid state contactors

Where radio transmitters and receivers share a common antenna, often a coaxial relay is used as a TR (transmit-

recieve) relay, which switches the antenna from the receiver to the transmitter. This protects the receiver from the

high power of the transmitter. Such relays are often used in transceivers which combine transmitter and receiver in

one unit. The relay contacts are designed not to reflect any radio frequency power back toward the source, and to

provide very high isolation between receiver and transmitter terminals. The characteristic impedance of the relay is

matched to thetransmission line impedance of the system, for example, 50 ohms. [2]

Contactor relay

Main article: Contactor

A contactor is a very heavy-duty relay used for switching electric motors and lighting loads, although contactors

are not generally called relays. 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.[3] Such devices are often used for motor

starters. A motor starter is a contactor with overload protection devices attached. 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. Every

solid-state device has a small voltage drop across it. This voltage drop

limits the amount of current a given SSR can handle. The minimum

voltage drop for such a relay is a function of the material used to make

the device. Solid-state relays rated to handle as much as 1,200 amperes

have become commercially available. Compared to electromagneticrelays, they may be falsely triggered by transients.

Solid state contactor relay

A solid state contactor is a heavy-duty solid state relay, including the

necessary heat sink, used where frequent on/off cycles are required, such

as with electric heaters, small electric motors, and lighting loads. 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-transistorlogic (TTL) sources, or other microprocessor and microcontroller

controls.

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.
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Circuit symbols of relays. (C

denotes the common terminal in

SPDT and DPDT types.)

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 normall

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 fo

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 o

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 12 terminals).

EN 50005 are among applicable standards for relay terminal numbering; a

typical EN 50005-compliant SPDT relay's terminals would be numbered

11, 12, 14, A1 and A2 for the C, NC, NO, and coil connections,

respectively.

Applications

Relays are used for:

Amplifying a digital signal, switching a large amount of power with a

small operating power. Some special cases are:

A telegraph relay, repeating a weak signal received at the end

of a long wire

Controlling a high-voltage circuit with a low-voltage signal, as

in some types of modems or audio amplifiers,

Controlling a high-current circuit with a low-current signal, as in

the starter solenoid of an automobile,

Detecting and isolating faults on transmission and distribution lines by opening and closing circuit breakers

(protection relays),

Isolating the controlling circuit from the controlled circuit when the two are at different potentials, for examp

when controlling a mains-powered device from a low-voltage switch. The latter is often applied to control

office lighting as the low voltage wires are easily installed in partitions, which may be often moved as needs

change. They may also be controlled by room occupancy detectors to conserve energy,

Logic functions. For example, the boolean AND function is realised by connecting normally open relay

contacts in series, the OR function by connecting normally open contacts in parallel. The change-over or

Form C contacts perform the XOR (exclusive or) function. Similar functions for NAND and NOR are

accomplished using normally closed contacts. The Ladder programming language is often used for designing
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A DPDT AC coil relay

with "ice cube"

packaging

A large relay with two coils and

many sets of contacts, used in an

old telephone switching system.

relay logic networks.

The application of Boolean Algebra to relay circuit design was formalized by Claude Shannon in A

Symbolic Analysis of Relay and Switching Circuits

Early computing. Before vacuum tubes and transistors, relays were used as logical elements in digital

computers. See electro-mechanical computers such as ARRA (computer), Harvard Mark II, Zuse

Z2, and Zuse Z3.

Safety-critical logic. Because relays are much more resistant than semiconductors to nuclear radiation

they are widely used in safety-critical logic, such as the control panelsof radioactive waste-handling machinery.

Time delay functions. Relays can be modified to delay opening or delay

closing a set of contacts. A very short (a fraction of a second) delay would

use a copper disk between the armature and moving blade assembly. Current

flowing in the disk maintains magnetic field for a short time, lengthening release

time. For a slightly longer (up to a minute) delay, a dashpot is used. A dashpot

is a piston filled with fluid that is allowed to escape slowly. The time period

can be varied by increasing or decreasing the flow rate. For longer time

periods, a mechanical clockwork timer is installed.

Vehicle battery isolation. A 12v relay is often used to isolate any second

battery in cars, 4WDs, RVs and boats.

Switching to a standby power supply.

Relay application considerations

Selection of an appropriate relay for a particular application requires

evaluation of many different factors:

Number and type of contacts normally open, normally closed,(double-throw)

Contact sequence "Make before Break" or "Break before Make".

For example, the old style telephone exchanges required Make-

before-break so that the connection didn't get dropped while dialing

the number.

Rating of contacts small relays switch a few amperes, large

contactors are rated for up to 3000 amperes, alternating or direct

current

Voltage rating of contacts typical control relays rated 300 VAC or

600 VAC, automotive types to 50 VDC, special high-voltage relaysto about 15 000 V

Operating lifetime, useful life - the number of times the relay can be expected to operate reliably. There is

both a mechanical life and a contact life; the contact life is naturally affected by the kind of load being

switched.

Coil voltage machine-tool relays usually 24 VDC, 120 or 250 VAC, relays for switchgear may have 125

V or 250 VDC coils, "sensitive" relays operate on a few milliamperes

Coil current - including minimum current required to operate reliably and minimum current to hold. Also

effects of power dissipation on coil temperature at various duty cycles.
http://en.wikipedia.org/wiki/Dashpothttp://en.wikipedia.org/wiki/Zuse_Z3http://en.wikipedia.org/wiki/Zuse_Z2http://en.wikipedia.org/wiki/Harvard_Mark_IIhttp://en.wikipedia.org/wiki/ARRA_(computer)http://en.wikipedia.org/wiki/Mechanical_computer#Electro-mechanical_computershttp://en.wikipedia.org/wiki/Digital_computerhttp://en.wikipedia.org/wiki/Transistorhttp://en.wikipedia.org/wiki/Vacuum_tubehttp://en.wikipedia.org/wiki/A_Symbolic_Analysis_of_Relay_and_Switching_Circuitshttp://en.wikipedia.org/wiki/Claude_Shannonhttp://en.wikipedia.org/wiki/Boolean_Algebrahttp://en.wikipedia.org/wiki/Relay_logichttp://en.wikipedia.org/wiki/File:Phonerelay.pnghttp://en.wikipedia.org/wiki/File:ACRelay.jpg


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Several 30-contact relays in

"Connector" circuits in mid 20th

century 1XB switch and 5XB switch

telephone exchanges; cover removed

on one

Package/enclosure open, touch-safe, double-voltage for isolation between circuits, explosion proof,

outdoor, oil and splash resistant, washable for printed circuit board assembly

Operating environment - minimum and maximum operating temperatures and other environmental

considerations such as effects of humidity and salt

Assembly Some relays feature a sticker that keeps the enclosure sealed to allow PCB post soldering

cleaning, which is removed once assembly is complete.

Mounting sockets, plug board, rail mount, panel mount, through-panel mount, enclosure for mounting on

walls or equipmentSwitching time where high speed is required

"Dry" contacts when switching very low level signals, special

contact materials may be needed such as gold-plated contacts

Contact protection suppress arcing in very inductive circuits

Coil protection suppress the surge voltage produced when

switching the coil current

Isolation between coil contacts

Aerospace or radiation-resistant testing, special quality assurance

Expected mechanical loads due to acceleration some relays used

in aerospace applications are designed to function in shock loads

of 50gor more

Accessories such as timers, auxiliary contacts, pilot lamps, test

buttons

Regulatory approvals

Stray magnetic linkage between coils of adjacent relays on a

printed circuit board.

There are many considerations involved in the correct selection of a

control relay for a particular application. These considerations include factors such as speed of operation,

sensitivity, and hysteresis. Although typical control relays operate in the 5 ms to 20 ms range, relays with switching

speeds as fast as 100 us are available. Reed relays which are actuated by low currents and switch fast are suitable

for controlling small currents.

As for any switch, the current through the relay contacts (unrelated to the current through the coil) must not exceed

a certain value to avoid damage. In the particular case of high-inductance circuits such as motors other issues mus

be addressed. When a power source is connected to an inductance, an input surge current which may be several

times larger than the steady current exists. When the circuit is broken, the current cannot change instantaneously,

which creates a potentially damaging spark across the separating contacts.

Consequently for relays which may be used to control inductive loads we must specify the maximum current thatmay flow through the relay contacts when it actuates, the make rating; the continuous rating; and the break ratin

The make rating may be several times larger than the continuous rating, which is itself larger than the break rating.

Derating factors

Control relays should not be operated above rated temperature because of resulting increased degradation and

fatigue. Common practice is to derate 20 degrees Celsius from the maximum rated temperature limit. Relays

operating at rated load are also affected by their environment. Oil vapors may greatly decrease the contact tip life,

and dust or dirt may cause the tips to burn before their normal life expectancy. Control relay life cycle varies from
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UK Q-style signalling relay and base.

positive and negative side of a circuit, so that two false feeds are needed

to cause a false signal. Not all relay circuits can be proved so there is

reliance on construction features such as carbon to silver contacts to

resist lightning induced contact welding and to provide AC immunity.

Opto-isolators are also used in some instances with railway signalling,

especially where only a single contact is to be switched.

Signalling relays and their circuits come in a number of schools, including:

United Kingdom

American

German

France

American signaling relays are the origin of the 19 inch rack.

History

The relay was invented in 1835 by American scientist Joseph Henry in order to improve his version of the electrica

telegraph, developed earlier in 1831.[6][7][8][9]

It is claimed that the English inventor Edward Davy "certainly invented the electric relay"[10] in his electric

telegraph c.1835.

A simple device, which we now call a relay, was included in the original 1840 telegraph patent [11] of Samuel

Morse. The mechanism described acted as a digital amplifier, repeating the telegraph signal, and thus allowing

signals to be propagated as far as desired. This overcame the problem of limited range of earlier telegraphy

schemes.[citation needed]

See also

Contactor

Digital protective relay

Dry contact

Race condition

Stepping switch - a kind of multi-position relayWire spring relay

References

1. ^ Mason, C. R.. "Art & Science of Protective Relaying, Chapter 2, GE Consumer & Electrical"

(http://www.gedigitalenergy.com/multilin/notes/artsci/) . http://www.gedigitalenergy.com/multilin/notes/artsci/.

Retrieved October 9, 2011.

2. ^ Ian Sinclair,Passive Components for Circuit Design, Newnes, 2000 ISBN 008051359X, page 170

3. ^ Kenneth B. Rexford and Peter R. Giuliani (2002). Electrical control for machines
http://books.google.com/books?id=5RkbwbYq1joC&pg=PA58&lpg=PA58&f=falsehttp://books.google.com/books?id=5RkbwbYq1joC&pg=PA58&lpg=PA58&f=falsehttp://en.wikipedia.org/wiki/Relay#cite_ref-3http://en.wikipedia.org/wiki/Special:BookSources/008051359Xhttp://en.wikipedia.org/wiki/Relay#cite_ref-2http://www.gedigitalenergy.com/multilin/notes/artsci/http://www.gedigitalenergy.com/multilin/notes/artsci/http://en.wikipedia.org/wiki/Relay#cite_ref-1http://en.wikipedia.org/wiki/Wire_spring_relayhttp://en.wikipedia.org/wiki/Stepping_switchhttp://en.wikipedia.org/wiki/Race_conditionhttp://en.wikipedia.org/wiki/Dry_contacthttp://en.wikipedia.org/wiki/Digital_protective_relayhttp://en.wikipedia.org/wiki/Contactorhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Samuel_Morsehttp://en.wikipedia.org/wiki/Relay#cite_note-11http://en.wikipedia.org/wiki/Telegraphyhttp://en.wikipedia.org/wiki/Electric_telegraphhttp://en.wikipedia.org/wiki/Relay#cite_note-adb-10http://en.wikipedia.org/wiki/Edward_Davyhttp://en.wikipedia.org/wiki/Relay#cite_note-9http://en.wikipedia.org/wiki/Relay#cite_note-8http://en.wikipedia.org/wiki/Relay#cite_note-7http://en.wikipedia.org/wiki/Relay#cite_note-6http://en.wikipedia.org/wiki/Electrical_telegraphhttp://en.wikipedia.org/wiki/Joseph_Henryhttp://en.wikipedia.org/wiki/19_inch_rackhttp://en.wikipedia.org/wiki/Francehttp://en.wikipedia.org/wiki/Germanyhttp://en.wikipedia.org/wiki/United_Stateshttp://en.wikipedia.org/wiki/United_Kingdomhttp://en.wikipedia.org/wiki/Opto-isolatorhttp://en.wikipedia.org/wiki/File:Rail_Signalling_Relay_Q-style.JPG


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(http://books.google.com/books?id=5RkbwbYq1joC&pg=PA58&lpg=PA58&f=false) (6th ed.). Cengage Learning

p. 58. ISBN 978-0-7668-6198-5. http://books.google.com/books?

id=5RkbwbYq1joC&pg=PA58&lpg=PA58&f=false.

4. ^ Zocholl, Stan (2003).AC Motor Protection. Schweitzer Engineering Laboratories, Inc. . ISBN 0-9725026-1-0,

978-0972502610.

5. ^ "Lab Note #105 Contact Life - Unsuppressed vs. Suppressed Arcing"

(http://arcsuppressiontechnologies.com/Documents/Lab%20Note%20105%20-%20APR2011%20-

%20Contact%20Life%20100K.pdf) . Arc Suppression Technologies. April 2011.

http://arcsuppressiontechnologies.com/Documents/Lab%20Note%20105%20-%20APR2011%20-

%20Contact%20Life%20100K.pdf. Retrieved October 9, 2011.

6. ^Icons of Invention: The Makers of the Modern World from Gutenberg to Gates (http://books.google.com/book

id=WKuG-VIwID8C&pg=PA153&dq=Invention+of+the+relay&hl=en&sa=X&ei=-

mnmT5bJK4no9AT8l4mMAQ&ved=0CF8Q6AEwCA#v=onepage&q=Invention%20of%20the%20relay&f=false)

ABC-CLIO. p. 153. http://books.google.com/books?id=WKuG-

VIwID8C&pg=PA153&dq=Invention+of+the+relay&hl=en&sa=X&ei=-

mnmT5bJK4no9AT8l4mMAQ&ved=0CF8Q6AEwCA#v=onepage&q=Invention%20of%20the%20relay&f=false.

7. ^ "The electromechanical relay of Joseph Henry" (http://history-computer.com/ModernComputer/Basis/relay.htm

. Georgi Dalakov. http://history-computer.com/ModernComputer/Basis/relay.html.

8. ^Scientific American Inventions and Discoveries: All the Milestones in Ingenuity--From the Discovery of Fire to

the Invention of the Microwave Oven (http://books.google.com/books?

id=pDbQVE3IdTcC&pg=PA311&dq=relay+Joseph+Henry+1835&hl=en&sa=X&ei=GNSIT6PyMJP02wXO3K3G

CQ&ved=0CFoQ6AEwBg#v=onepage&q=relay%20Joseph%20Henry%201835&f=false) . John Wiley & Sons.

p. 311. http://books.google.com/books?

id=pDbQVE3IdTcC&pg=PA311&dq=relay+Joseph+Henry+1835&hl=en&sa=X&ei=GNSIT6PyMJP02wXO3K3G

CQ&ved=0CFoQ6AEwBg#v=onepage&q=relay%20Joseph%20Henry%201835&f=false.

9. ^ Thomas Coulson (1950).Joseph Henry: His Life and Work. Princeton: Princeton University Press .

10. ^ Gibberd, William (1966). "Edward Davy" (http://adb.anu.edu.au/biography/davy-edward-1966) . Australian

Dictionary of Biography. Canberra: Australian National University. Retrieved 7 June 2012.

11. ^ US Patent 1,647, Improvement in the mode of communicating information by signals by the application of

electro-magnetism, June 20, 1840 (http://www.google.com/patents?

id=Xx5AAAAAEBAJ&printsec=abstract&zoom=4&dq=1647)

Gurevich, Vladimir (2005).Electrical Relays: Principles and Applications. London - New York: CRC Press.

Westinghouse Corporation (1976).Applied Protective Relaying. Westinghouse Corporation. Library of Congress

card no. 76-8060.

Terrell Croft and Wilford Summers (ed) (1987).American Electricians' Handbook, Eleventh Edition. New York

McGraw Hill. ISBN 978-0-07-013932-9.

Walter A. Elmore.Protective Relaying Theory and Applications. Marcel Dekker kana, Inc.. ISBN 978-0-8247-

9152-0.

Vladimir Gurevich (2008).Electronic Devices on Discrete Components for Industrial and Power Engineering.

London - New York: CRC Press. p. 418.

Vladimir Gurevich (2003).Protection Devices and Systems for High-Voltage Applications. London - New York:

CRC Press. p. 292.Vladimir Gurevich (2010).Digital Protective Relays: Problems and Solutions. London - New York: CRC Press.

p. 422.

Colin Simpson, Principles of Electronics, Prentice-Hall, 2002, ISBN 0-06-868603-6

External links

Information about relays and the Latching Relay circuit (http://www.eleinmec.com/article.asp?24)

"Harry Porter's Relay Computer", a computer made out of relays.

(http://web.cecs.pdx.edu/~harry/Relay/index.html)
http://books.google.com/books?id=5RkbwbYq1joC&pg=PA58&lpg=PA58&f=falsehttp://web.cecs.pdx.edu/~harry/Relay/index.htmlhttp://www.eleinmec.com/article.asp?24http://en.wikipedia.org/wiki/Special:BookSources/0068686036http://en.wikipedia.org/wiki/Principles_of_Electronicshttp://en.wikipedia.org/wiki/Colin_Simpson_(electronics_professor)http://en.wikipedia.org/wiki/Special:BookSources/978-0-8247-9152-0http://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://en.wikipedia.org/wiki/Special:BookSources/978-0-07-013932-9http://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://www.google.com/patents?id=Xx5AAAAAEBAJ&printsec=abstract&zoom=4&dq=1647http://en.wikipedia.org/wiki/Relay#cite_ref-11http://en.wikipedia.org/wiki/Australian_National_Universityhttp://en.wikipedia.org/wiki/Australian_Dictionary_of_Biographyhttp://adb.anu.edu.au/biography/davy-edward-1966http://en.wikipedia.org/wiki/Relay#cite_ref-adb_10-0http://en.wikipedia.org/wiki/Relay#cite_ref-9http://books.google.com/books?id=pDbQVE3IdTcC&pg=PA311&dq=relay+Joseph+Henry+1835&hl=en&sa=X&ei=GNSIT6PyMJP02wXO3K3GCQ&ved=0CFoQ6AEwBg#v=onepage&q=relay%20Joseph%20Henry%201835&f=falsehttp://books.google.com/books?id=pDbQVE3IdTcC&pg=PA311&dq=relay+Joseph+Henry+1835&hl=en&sa=X&ei=GNSIT6PyMJP02wXO3K3GCQ&ved=0CFoQ6AEwBg#v=onepage&q=relay%20Joseph%20Henry%201835&f=falsehttp://en.wikipedia.org/wiki/Relay#cite_ref-8http://history-computer.com/ModernComputer/Basis/relay.htmlhttp://history-computer.com/ModernComputer/Basis/relay.htmlhttp://en.wikipedia.org/wiki/Relay#cite_ref-7http://books.google.com/books?id=WKuG-VIwID8C&pg=PA153&dq=Invention+of+the+relay&hl=en&sa=X&ei=-mnmT5bJK4no9AT8l4mMAQ&ved=0CF8Q6AEwCA#v=onepage&q=Invention%20of%20the%20relay&f=falsehttp://books.google.com/books?id=WKuG-VIwID8C&pg=PA153&dq=Invention+of+the+relay&hl=en&sa=X&ei=-mnmT5bJK4no9AT8l4mMAQ&ved=0CF8Q6AEwCA#v=onepage&q=Invention%20of%20the%20relay&f=falsehttp://en.wikipedia.org/wiki/Relay#cite_ref-6http://arcsuppressiontechnologies.com/Documents/Lab%20Note%20105%20-%20APR2011%20-%20Contact%20Life%20100K.pdfhttp://arcsuppressiontechnologies.com/Documents/Lab%20Note%20105%20-%20APR2011%20-%20Contact%20Life%20100K.pdfhttp://en.wikipedia.org/wiki/Relay#cite_ref-5http://en.wikipedia.org/wiki/Special:BookSources/0-9725026-1-0,_978-0972502610http://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://en.wikipedia.org/wiki/Relay#cite_ref-4http://books.google.com/books?id=5RkbwbYq1joC&pg=PA58&lpg=PA58&f=falsehttp://en.wikipedia.org/wiki/Special:BookSources/978-0-7668-6198-5http://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://books.google.com/books?id=5RkbwbYq1joC&pg=PA58&lpg=PA58&f=false


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"Relay Computer Two", by Jon Stanley.

(http://www.electronixandmore.com/project/relaycomputertwo/index.html)

Interfacing Relay To Microcontroller. (http://www.dnatechindia.com/Tutorial/8051-Tutorial/Interfacing-

Relay-to-Microcontroller.html)

Relays Technical Write (http://www.oenindia.com/pdf/relays_Technical_Write_up.pdf)

http://relays.te.com/schrack/pdf/C0_v4bg_5.pdf

Retrieved from "http://en.wikipedia.org/w/index.php?title=Relay&oldid=545433968"

Categories: Electromagnetic components American inventions Relay switches Transducers

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