How to Make a Board Relay Work Again Using Compressed Gas

This article is nigh the electric component. For other uses, see Relay (disambiguation).

Electrically-operated switch

Electromechanical relay schematic showing a control coil, iv pairs of ordinarily open and one pair of normally airtight contacts

An automotive-mode miniature relay with the dust cover taken off

A relay is an electrically operated switch. It consists of a prepare of input terminals for a unmarried or multiple control signals, and a set of operating contact terminals. The switch may have any number of contacts in multiple contact forms, such as make contacts, break contacts, or combinations thereof.

Relays are used where it is necessary to command a circuit by an independent low-ability signal, or where several circuits must be controlled by 1 signal. Relays were first used in long-altitude telegraph circuits every bit signal repeaters: they refresh the point coming in from 1 circuit by transmitting it on some other excursion. Relays were used extensively in telephone exchanges and early computers to perform logical operations.

The traditional form of a relay uses an electromagnet to close or open up the contacts, but other operating principles have been invented, such every bit in solid-state relays which employ semiconductor backdrop for control without relying on moving parts. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electric circuits from overload or faults; in modernistic electrical power systems these functions are performed past digital instruments still called protective relays.

Latching relays require only a single pulse of command power to operate the switch persistently. Another pulse applied to a 2nd set up of control terminals, or a pulse with opposite polarity, resets the switch, while repeated pulses of the same kind accept no effects. Magnetic latching relays are useful in applications when interrupted power should non bear upon the circuits that the relay is controlling.

History [edit]

This section appears to contradict itself. Delight run into the talk page for more information. (September 2021)

Telegraph relay contacts and bound

In 1809 Samuel Thomas von Sömmerring designed an electrolytic relay as part of his electro-chemical telegraph.^[1]

Solely electric relays got their start as a further improvement to telegraphs, with American scientist Joseph Henry who is ofttimes cited to have invented a relay in 1835 in gild to improve his version of the electric telegraph, adult earlier in 1831.^{[2] [3] [4] [five]}

However, an official patent wasn't issued until 1840 to Samuel Morse for his telegraph, which is now called a relay. The machinery described acted equally a digital amplifier, repeating the telegraph signal, and thus allowing signals to exist propagated as far equally desired.^[6]

The give-and-take relay appears in the context of electromagnetic operations from 1860 onwards.^[seven]

Bones pattern and functioning [edit]

Elementary electromechanical relay

Operation without flyback diode, arcing causes degradation of the switch contacts

Operation with flyback diode, arcing in the control circuit is avoided

A simple electromagnetic relay consists of a coil of wire wrapped around a soft iron cadre (a solenoid), an iron yoke which provides a low reluctance path for magnetic flux, a movable atomic number 26 armature, and one or more sets of contacts (at that place are ii contacts in the relay pictured). The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts. The armature is held in place by a jump and then that when the relay is de-energized in that location is an air gap in the magnetic circuit. In this status, one of the two sets of contacts in the relay pictured is closed, and the other set is open. Other relays may accept more than or fewer sets of contacts depending on their part. The relay in the picture too has a wire connecting the armature to the yoke. This ensures continuity of the excursion 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 current is passed through the curlicue it generates a magnetic field that activates the armature, and the consequent motion 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 breaks the connectedness, and vice versa if the contacts were open. When the current to the coil is switched off, the armature is returned by a strength, approximately half equally strong equally the magnetic strength, to its relaxed position. Usually this force is provided by a spring, but gravity is also used commonly in industrial motor starters. Virtually relays are manufactured to operate rapidly. In a depression-voltage application this reduces racket; in a high voltage or current awarding it reduces arcing.

Performance of a 12 A relay

When the coil is energized with direct current, a diode or resistor is often placed beyond the gyre to misemploy the free energy from the collapsing magnetic field at deactivation, which would otherwise generate a voltage spike dangerous to semiconductor excursion components. Such diodes were not widely used earlier the application of transistors every bit relay drivers, but presently became ubiquitous as early on germanium transistors were easily destroyed by this surge. Some automotive relays include a diode inside the relay case. Resistors, while more durable than diodes, are less efficient at eliminating voltage spikes generated past relays^[8] and therefore not as commonly used.

A small cradle relay often used in electronics. The "cradle" term refers to the shape of the relay'due south armature

If the relay is driving a large, or especially a reactive load, there may be a like problem of surge currents around the relay output contacts. In this case a snubber circuit (a capacitor and resistor in series) beyond the contacts may absorb the surge. Suitably rated capacitors and the associated resistor are sold as a unmarried packaged component for this commonplace use.

If the coil is designed to be energized with alternate current (Air-conditioning), some method is used to split the flux into two out-of-stage components which add together together, increasing the minimum pull on the armature during the Air-conditioning bike. Typically this is done with a pocket-sized copper "shading ring" crimped around a portion of the core that creates the delayed, out-of-stage component,^[9] which holds the contacts during the zero crossings of the control voltage.^[ten]

Contact materials for relays vary past application. Materials with low contact resistance may be oxidized by the air, or may tend to "stick" instead of cleanly parting when opening. Contact material may be optimized for low electric resistance, high strength to withstand repeated operations, or high chapters to withstand the heat of an arc. Where very depression resistance is required, or low thermally-induced voltages are desired, gold-plated contacts may be used, along with palladium and other not-oxidizing, semi-precious metals. Silverish or silver-plated contacts are used for point switching. Mercury-wetted relays make and break circuits using a sparse, cocky-renewing film of liquid mercury. For higher-ability relays switching many amperes, such as motor circuit contactors, contacts are made with a mixtures of silver and cadmium oxide, providing depression contact resistance and high resistance to the heat of arcing. Contacts used in circuits conveying scores or hundreds of amperes may include boosted structures for heat dissipation and management of the arc produced when interrupting the circuit.^[xi] Some relays have field-replaceable contacts, such equally certain machine tool relays; these may be replaced when worn out, or inverse between commonly open and unremarkably closed state, to allow for changes in the controlled excursion.^[12]

Terminology [edit]

Circuit symbols of relays (C denotes the common terminal in SPDT and DPDT types.)

Since relays are switches, the terminology applied to switches is as well applied to relays; a relay switches i or more than poles, each of whose contacts can be thrown past energizing the coil. Usually open (NO) contacts connect the circuit when the relay is activated; the circuit is disconnected when the relay is inactive. Commonly closed (NC) contacts disconnect the excursion when the relay is activated; the circuit is connected when the relay is inactive. All of the contact forms involve combinations of NO and NC connections.

The National Clan of Relay Manufacturers and its successor, the Relay and Switch Industry Association define 23 distinct electrical contact forms found in relays and switches.^[13] Of these, the post-obit are normally encountered:

• SPST-NO (Single-Pole Single-Throw, Unremarkably-Open up) relays have a unmarried Form A contact or make contact. These take two terminals which can exist connected or asunder. Including ii for the whorl, such a relay has four terminals in total.
• SPST-NC (Unmarried-Pole Unmarried-Throw, Normally-Closed) relays have a single Grade B or suspension contact. As with an SPST-NO relay, such a relay has four terminals in total.
• SPDT (Single-Pole Double-Throw) relays take a single prepare of Form C, suspension before make or transfer contacts. That is, a mutual terminal connects to either of two others, never connecting to both at the same time. Including two for the coil, such a relay has a total of five terminals.
• DPST – Double-Pole Single-Throw relays are equivalent to a pair of SPST switches or relays actuated by a unmarried coil. Including 2 for the coil, such a relay has a full of 6 terminals. The poles may be Grade A or Course B (or one of each; the designations NO and NC should be used to resolve the ambivalence).
• DPDT – Double-Pole Double-Throw relays take two sets of Class C contacts. These are equivalent to two SPDT switches or relays actuated by a single coil. Such a relay has eight terminals, including the coil
• Form D – make earlier intermission^[14]
• Form E – combination of D and B^[14]

The S (single) or D (double) designator for the pole count may be replaced with a number, indicating multiple contacts continued to a single actuator. For example, 4PDT indicates a 4-pole double-throw relay that has 12 switching terminals.

EN 50005 are among applicable standards for relay terminal numbering; a typical EN 50005-compliant SPDT relay's terminals would be numbered eleven, 12, 14, A1 and A2 for the C, NC, NO, and coil connections, respectively.^[xv]

DIN 72552 defines contact numbers in relays for automotive use:

• 85 = relay ringlet -
• 86 = relay coil +
• 87 = common contact
• 87a = normally closed contact
• 87b = normally open up contact

Types [edit]

Coaxial relay [edit]

Where radio transmitters and receivers share ane antenna, oftentimes a coaxial relay is used as a TR (transmit-receive) relay, which switches the antenna from the receiver to the transmitter. This protects the receiver from the loftier power of the transmitter. Such relays are frequently used in transceivers which combine transmitter and receiver in 1 unit of measurement. The relay contacts are designed not to reverberate any radio frequency ability dorsum toward the source, and to provide very loftier isolation between receiver and transmitter terminals. The characteristic impedance of the relay is matched to the manual line impedance of the system, for instance, fifty ohms.^[16]

Contactor [edit]

A contactor is a heavy-duty relay with higher current ratings,^[17] used for switching electrical 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 argent. The unavoidable arcing causes the contacts to oxidize; nevertheless, silverish oxide is still a skilful conductor.^[18] Contactors with overload protection devices are often used to start motors.^[19]

Forcefulness-guided contacts relay [edit]

A forcefulness-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 motility together. If i set up of contacts in the relay becomes immobilized, no other contact of the aforementioned relay will be able to move. The function of strength-guided contacts is to enable the safety excursion to check the status of the relay. Force-guided contacts are also known as "positive-guided contacts", "captive contacts", "locked contacts", "mechanically linked contacts", or "prophylactic relays".

These rubber relays take to follow pattern rules and manufacturing rules that are defined in ane main machinery standard EN 50205 : Relays with forcibly guided (mechanically linked) contacts. These rules for the safety pattern are the ane defined in type B standards such as EN 13849-ii equally Bones safety principles and Well-tried safety principles for machinery that applies to all machines.

Force-guided contacts by themselves can not guarantee that all contacts are in the aforementioned state, all the same, they do guarantee, discipline to no gross mechanical error, that no contacts are in contrary states. Otherwise, a relay with several normally open (NO) contacts may stick when energized, with some contacts closed and others still slightly open, due to mechanical tolerances. Similarly, a relay with several normally closed (NC) contacts may stick to the unenergized position, so that when energized, the circuit through one gear up of contacts is broken, with a marginal gap, while the other remains closed. By introducing both NO and NC contacts, or more than commonly, changeover contacts, on the same relay, it and then becomes possible to guarantee that if any NC contact is closed, all NO contacts are open, and conversely, if any NO contact is airtight, all NC contacts are open. It is non possible to reliably ensure that any particular contact is airtight, except by potentially intrusive and rubber-degrading sensing of its circuit weather, however in condom systems information technology is commonly the NO state that is most important, and as explained higher up, this is reliably verifiable past detecting the closure of a contact of opposite sense.

Force-guided contact relays are made with different main contact sets, either NO, NC or changeover, and one or more than auxiliary contact sets, often of reduced current or voltage rating, used for the monitoring organization. Contacts may be all NO, all NC, changeover, or a mixture of these, for the monitoring contacts, so that the prophylactic system designer can select the right configuration for the particular application. Safe relays are used equally part of an engineered condom system.

Latching relay [edit]

Latching relay with permanent magnet

A latching relay, also called impulse, bistable, proceed, or stay relay, or simply latch, maintains either contact position indefinitely without power applied to the coil. The reward is that one coil consumes power only for an instant while the relay is being switched, and the relay contacts retain this setting across a ability outage. A latching relay allows remote control of building lighting without the hum that may be produced from a continuously (Air conditioning) energized coil.

In 1 mechanism, two opposing coils with an over-center jump or permanent magnet hold the contacts in position later the gyre is de-energized. A pulse to one ringlet turns the relay on, and a pulse to the opposite coil turns the relay off. This type is widely used where command is from unproblematic switches or single-ended outputs of a control system, and such relays are constitute in avionics and numerous industrial applications.

Another latching type has a remanent core that retains the contacts in the operated position past the remanent magnetism in the core. This blazon requires a current pulse of reverse polarity to release the contacts. A variation uses a permanent magnet that produces role of the force required to close the contact; the coil supplies sufficient force to motility the contact open or closed by aiding or opposing the field of the permanent magnet.^[20] A polarity controlled relay needs changeover switches or an H-bridge drive circuit to control it. The relay may exist less expensive than other types, but this is partly offset by the increased costs in the external circuit.

In another blazon, a ratchet relay has a ratchet machinery that holds the contacts airtight after the coil is momentarily energized. A second impulse, in the same or a split ringlet, releases the contacts.^[20] This type may exist found in certain cars, for headlamp dipping and other functions where alternating operation on each switch actuation is needed.

A stepping relay is a specialized kind of multi-mode latching relay designed for early automated telephone exchanges.

An globe-leakage excursion breaker includes a specialized latching relay.

Very early computers often stored $.25 in a magnetically latching relay, such every bit ferreed or the later remreed in the 1ESS switch.

Some early computers used ordinary relays equally a kind of latch—they shop bits in ordinary wire-leap relays or reed relays by feeding an output wire back as an input, resulting in a feedback loop or sequential excursion. Such an electrically latching relay requires continuous ability to maintain country, unlike magnetically latching relays or mechanically ratcheting relays.

In computer memories, latching relays and other relays were replaced past delay-line memory, which in plow was replaced by a series of ever faster and e'er smaller memory technologies.

Auto tool relay [edit]

A machine tool relay is a type standardized for industrial command of automobile tools, transfer machines, and other sequential command. They are characterized by a large number of contacts (sometimes extendable in the field) which are easily converted from normally open to normally closed condition, easily replaceable coils, and a grade factor that allows compactly installing many relays in a command panel. Although such relays once were the backbone of automation in such industries as machine assembly, the programmable logic controller (PLC) more often than not displaced the auto tool relay from sequential control applications.

A relay allows circuits to exist switched by electric 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 systems. 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.

Mercury relay [edit]

A mercury relay is a relay that uses mercury as the switching chemical element. They are used where contact erosion would be a problem for conventional relay contacts. Attributable to ecology considerations virtually meaning amount of mercury used and modern alternatives, they are now insufficiently uncommon.

Mercury-wetted relay [edit]

A mercury-wetted reed relay

A mercury-wetted reed relay is a class of reed relay that employs a mercury switch, in which the contacts are wetted with mercury. Mercury reduces the contact resistance and mitigates the associated voltage drop. Surface contamination may consequence in poor conductivity for depression-current signals. For high-speed applications, the mercury eliminates contact bounce, and provides virtually instantaneous circuit closure. Mercury wetted relays are position-sensitive and must exist mounted according to the manufacturer's specifications. Because of the toxicity and expense of liquid mercury, these relays have increasingly fallen into disuse.

The high speed of switching action of the mercury-wetted relay is a notable advantage. The mercury globules on each contact coagulate, and the current rise time through the contacts is generally considered to be a few picoseconds. All the same, in a practical circuit it may be express by the inductance of the contacts and wiring. Information technology was quite common, earlier restrictions on the apply of mercury, to use a mercury-wetted relay in the laboratory as a convenient ways of generating fast rise time pulses, nevertheless although the rise time may be picoseconds, the exact timing of the event is, like all other types of relay, subject to considerable jitter, possibly milliseconds, due to mechanical imperfections.

The same coalescence process causes another outcome, which is a nuisance in some applications. The contact resistance is not stable immediately after contact closure, and drifts, generally downwards, for several seconds later on closure, the alter perhaps beingness 0.5 ohm.

Multi-voltage relays [edit]

Multi-voltage relays are devices designed to work for wide voltage ranges such equally 24 to 240 VAC and VDC and wide frequency ranges such as 0 to 300 Hz. They are indicated for apply in installations that do non have stable supply voltages.

Overload protection relay [edit]

Electric motors demand overcurrent protection to prevent damage from over-loading the motor, or to protect against short circuits in connecting cables or internal faults in the motor windings.^[21] The overload sensing devices are a class of heat operated relay where a coil heats a bimetallic strip, or where a solder pot melts, to operate auxiliary contacts. These auxiliary contacts are in series with the motor's contactor scroll, so they turn off the motor when it overheats.^[22]

This thermal protection operates relatively slowly allowing the motor to describe college starting currents earlier the protection relay will trip. Where the overload relay is exposed to the aforementioned ambience temperature equally the motor, a useful though crude compensation for motor ambient temperature is provided.^[23]

The other common overload protection system uses an electromagnet coil in series with the motor circuit that direct operates contacts. This is like to a control relay merely requires a rather loftier fault current to operate the contacts. To prevent brusque over current spikes from causing nuisance triggering the armature movement is damped with a dashpot. The thermal and magnetic overload detections are typically used together in a motor protection relay.^{[ commendation needed ]}

Electronic overload protection relays measure motor current and can estimate motor winding temperature using a "thermal model" of the motor armature organisation that tin can be set to provide more accurate motor protection. Some motor protection relays include temperature detector inputs for direct measurement from a thermocouple or resistance thermometer sensor embedded in the winding.^{[ citation needed ]}

Polarized relay [edit]

A polarized relay places the armature betwixt the poles of a permanent magnet to increase sensitivity. Polarized relays were used in middle 20th Century telephone exchanges to detect faint pulses and right telegraphic distortion.

Reed relay [edit]

(from tiptop) Unmarried-pole reed switch, four-pole reed switch and unmarried-pole reed relay. Scale in centimeters

A reed relay is a reed switch enclosed in a solenoid. The switch has a set up of contacts within an evacuated or inert gas-filled glass tube that protects the contacts against atmospheric corrosion; the contacts are made of magnetic fabric that makes them move under the influence of the field of the enclosing solenoid or an external magnet.

Reed relays can switch faster than larger relays and require very picayune power from the command circuit. However, they have relatively low switching current and voltage ratings. Though rare, the reeds tin become magnetized over time, which makes them stick "on", even when no current is present; changing the orientation of the reeds or degaussing the switch with respect to the solenoid'south magnetic field can resolve this problem.

Sealed contacts with mercury-wetted contacts take longer operating lives and less contact churr than whatever other kind of relay.^[24]

Safety relays [edit]

Safety relays are devices which mostly implement protection functions. In the event of a risk, the task of such a prophylactic part is to employ appropriate measures to reduce the existing run a risk to an acceptable level.^[25]

Solid-state contactor [edit]

A solid-state contactor is a heavy-duty solid land relay, including the necessary oestrus sink, used where frequent on-off cycles are required, such equally with electric heaters, small electric motors, and lighting loads. There are no moving parts to article of clothing out and there is no contact bounce due to vibration. They are activated by Ac control signals or DC control signals from programmable logic controllers (PLCs), PCs, transistor-transistor logic (TTL) sources, or other microprocessor and microcontroller controls.

Solid-land relay [edit]

25 A and 40 A solid state contactors

A solid-state relay (SSR) is a solid land electronic component that provides a office similar to an electromechanical relay but does not have any moving components, increasing long-term reliability. A solid-state relay uses a thyristor, TRIAC 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 exist used to isolate control and controlled circuits.^[26]

Static relay [edit]

A static relay consists of electronic circuitry to emulate all those characteristics which are accomplished by moving parts in an electro-magnetic relay.

Time-delay relay [edit]

Timing relays are bundled for an intentional delay in operating their contacts. A very brusk (a fraction of a second) delay would utilize a copper disk betwixt the armature and moving bract assembly. Electric current flowing in the disk maintains a magnetic field for a short time, lengthening release fourth dimension. For a slightly longer (up to a minute) filibuster, a dashpot is used. A dashpot is a piston filled with fluid that is allowed to escape slowly; both air-filled and oil-filled dashpots are used. The fourth dimension period can be varied by increasing or decreasing the flow charge per unit. For longer time periods, a mechanical clockwork timer is installed. Relays may be arranged for a fixed timing period, or may be field-adjustable, or remotely set from a control console. Modern microprocessor-based timing relays provide precision timing over a great range.

Some relays are constructed with a kind of "daze absorber" mechanism attached to the armature, which prevents immediate, full motility when the scroll is either energized or de-energized. This addition gives the relay the property of time-filibuster actuation. Time-filibuster relays can be constructed to delay armature motion on coil energization, de-energization, or both.

Time-delay relay contacts must be specified not only every bit either normally open or normally closed, just whether the delay operates in the management of closing or in the direction of opening. The following is a description of the four basic types of time-filibuster relay contacts.

Showtime, we have the normally open, timed-closed (NOTC) contact. This type of contact is unremarkably open up when the coil is unpowered (de-energized). The contact is closed by the application of power to the relay coil, but only afterwards the coil has been continuously powered for the specified amount of time. In other words, the direction of the contact'due south motion (either to shut or to open up) is identical to a regular NO contact, but in that location is a delay in closing direction. Because the filibuster occurs in the direction of coil energization, this type of contact is alternatively known as a normally open, on-filibuster.

Vacuum relays [edit]

A vacuum relay is a sensitive relay having its contacts mounted in an evacuated drinking glass housing, to allow handling radio-frequency voltages as loftier every bit twenty,000 volts without flashover between contacts even though contact spacing is every bit low every bit a few hundredths of an inch when open up.

Applications [edit]

A DPDT AC coil relay with "water ice cube" packaging

Relays are used wherever it is necessary to control a loftier power or loftier voltage circuit with a low power circuit, especially when galvanic isolation is desirable. The showtime application of relays was in long telegraph lines, where the weak signal received at an intermediate station could control a contact, regenerating the signal for further transmission. Loftier-voltage or high-current devices tin exist controlled with minor, low voltage wiring and pilots switches. Operators can be isolated from the high voltage circuit. Low power devices such every bit microprocessors can drive relays to control electric loads beyond their direct drive adequacy. In an automobile, a starter relay allows the high current of the cranking motor to be controlled with small wiring and contacts in the ignition key.

Electromechanical switching systems including Strowger and Crossbar telephone exchanges made extensive use of relays in coincident control circuits. The Relay Automatic Telephone Company too manufactured telephone exchanges based solely on relay switching techniques designed by Gotthilf Ansgarius Betulander. The kickoff public relay based telephone exchange in the UK was installed in Fleetwood on 15 July 1922 and remained in service until 1959.^{[27] [28]}

The use of relays for the logical control of complex switching systems like telephone exchanges was studied by Claude Shannon, who formalized the application of Boolean algebra to relay circuit blueprint in A Symbolic Analysis of Relay and Switching Circuits. Relays can perform the bones operations of Boolean combinatorial logic. For case, the boolean AND function is realised by connecting normally open relay contacts in series, the OR office by connecting normally open contacts in parallel. Inversion of a logical input can exist washed with a unremarkably closed contact. Relays were used for control of automated systems for auto tools and product lines and in lift controllers from the 1960s and 1970s. The Ladder programming linguistic communication is often used for designing relay logic networks.

Early electro-mechanical computers such as the ARRA, Harvard Marker II, Zuse Z2, and Zuse Z3 used relays for logic and working registers. However, electronic devices proved faster and easier to use.

Considering relays are much more resistant than semiconductors to nuclear radiations, they are widely used in prophylactic-disquisitional logic, such as the control panels of nuclear waste-handling machinery. Electromechanical protective relays are used to detect overload and other faults on electrical lines by opening and endmost circuit breakers.

Protective relays [edit]

For protection of electrical apparatus and transmission lines, electromechanical relays with accurate operating characteristics were used to discover overload, short-circuits, and other faults. While many such relays remain in use, digital protective relays at present provide equivalent and more complex protective functions.

Railway signalling [edit]

Part of a relay interlocking using Britain Q-fashion miniature plug-in relays

Railway signalling relays are large because the mostly small voltages (less than 120 5) and currents (perhaps 100 mA) that they switch. Contacts are widely spaced to forbid flashovers and short circuits over a lifetime that may exceed l years.

Since rails signal circuits must be highly reliable, special techniques are used to detect and forbid failures in the relay system. To protect against false feeds, double switching relay contacts are often used on both the positive and negative side of a circuit, and so that two faux feeds are needed to cause a false signal. Not all relay circuits tin be proved and then there is reliance on construction features such as carbon to silverish 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 unmarried contact is to be switched.

Choice considerations [edit]

Several thirty-contact relays in "Connector" circuits in mid-20th century 1XB switch and 5XB switch telephone exchanges; cover removed on one.

Selection of an appropriate relay for a item application requires evaluation of many dissimilar factors:

• Number and blazon of contacts — ordinarily open, unremarkably airtight, (double-throw)
• Contact sequence — "make earlier break" or "interruption earlier make". For example, the old mode telephone exchanges required make-earlier-interruption then that the connectedness didn't get dropped while dialing the number.
• Contact electric current rating — small relays switch a few amperes, large contactors are rated for up to 3000 amperes, alternate or direct current
• Contact voltage rating — typical control relays rated 300 VAC or 600 VAC, automotive types to 50 VDC, special high-voltage relays to almost 15,000 5
• Operating lifetime, useful life — the number of times the relay tin be expected to operate reliably. In that location is both a mechanical life and a contact life. The contact life is affected by the type of load switched. Breaking load current causes undesired arcing between the contacts, somewhen leading to contacts that weld shut or contacts that neglect due to erosion by the arc.^[29]
• Coil voltage — auto-tool relays usually 24 VDC, 120 or 250 VAC, relays for switchgear may have 125 V or 250 VDC coils,
• Coil current — Minimum electric current required for reliable operation and minimum holding electric current, too every bit effects of power dissipation on coil temperature at diverse duty cycles. "Sensitive" relays operate on a few milliamperes.
• Package/enclosure — open, touch-condom, double-voltage for isolation between circuits, explosion proof, outdoor, oil and splash resistant, washable for printed circuit lath assembly
• Operating environment — minimum and maximum operating temperature and other environmental considerations, such as effects of humidity and table salt
• Associates — Some relays feature a sticker that keeps the enclosure sealed to permit PCB post soldering cleaning, which is removed once assembly is consummate.
• Mounting — sockets, plug board, rail mountain, console mount, through-panel mount, enclosure for mounting on walls or equipment
• Switching time — where loftier speed is required
• "Dry out" contacts — when switching very depression level signals, special contact materials may be needed such equally golden-plated contacts
• Contact protection — suppress arcing in very inductive circuits
• Scroll protection — suppress the surge voltage produced when switching the curlicue current
• Isolation between coil contacts
• Aerospace or radiations-resistant testing, special quality assurance
• Expected mechanical loads due to dispatch — some relays used in aerospace applications are designed to function in shock loads of 50 m, or more than.
• Size — smaller relays often resist mechanical vibration and shock better than larger relays, because of the lower inertia of the moving parts and the higher natural frequencies of smaller parts.^[thirty] Larger relays often handle college voltage and current than smaller relays.
• Accessories such as timers, auxiliary contacts, pilot lamps, and examination buttons.
• Regulatory approvals.
• Devious magnetic linkage between coils of adjacent relays on a printed excursion lath.

There are many considerations involved in the correct choice of a command relay for a item awarding, including factors such every bit speed of operation, sensitivity, and hysteresis. Although typical command relays operate in the 5 ms to 20 ms range, relays with switching speeds as fast as 100 μs are bachelor. Reed relays which are actuated by low currents and switch fast are suitable for decision-making small currents.

Every bit with any switch, the contact current (unrelated to the curlicue current) must not exceed a given value to avoid damage. In loftier-inductance circuits such every bit motors, other problems must be addressed. When an inductance is connected to a power source, an input surge current or electromotor starting current larger than the steady-state electric current exists. When the circuit is cleaved, the current cannot change instantaneously, which creates a potentially damaging arc across the separating contacts.

Consequently, for relays used to control anterior loads, nosotros must specify the maximum electric current that may flow through the relay contacts when it actuates, the make rating; the continuous rating; and the break rating. The make rating may be several times larger than the continuous rating, which is larger than the break rating.

Condom and reliability [edit]

Switching while "wet" (under load) causes undesired arcing between the contacts, eventually leading to contacts that weld shut or contacts that fail due to a buildup of surface harm acquired by the destructive arc energy.^[29]

Inside the Number One Electronic Switching System (1ESS) batten switch and certain other high-reliability designs, the reed switches are always switched "dry" (without load) to avoid that trouble, leading to much longer contact life.^[31]

Without adequate contact protection, the occurrence of electrical electric current arcing causes significant deposition of the contacts, which suffer pregnant and visible impairment. Every time the relay contacts open or close under load, an electric arc tin can occur between the contacts of the relay, either a break arc (when opening), or a make / bounce arc (when closing). In many situations, the suspension arc is more energetic and thus more than destructive, in particular with inductive loads, simply this can be mitigated by bridging the contacts with a snubber circuit. The inrush electric current of tungsten filament incandescent lamps is typically 10 times the normal operating electric current. Thus, relays intended for tungsten loads may apply special contact composition, or the relay may have lower contact ratings for tungsten loads than for purely resistive loads.

An electric arc across relay contacts tin can exist very hot — thousands of degrees Fahrenheit — causing the metallic on the contact surfaces to cook, puddle, and migrate with the current. The extremely high temperature of the arc splits the surrounding gas molecules, creating ozone, carbon monoxide, and other compounds. Over time, the arc energy slowly destroys the contact metal, causing some material to escape into the air equally fine particulate affair. This activeness causes the material in the contacts to degrade, resulting in device failure. This contact degradation drastically limits the overall life of a relay to a range of about ten,000 to 100,000 operations, a level far below the mechanical life of the device, which can exist in excess of 20 million operations.^[32]

See too [edit]

• Analogue switch
• Buchholz relay
• Dry contact
• Flyback diode
• Nanoelectromechanical relay
• Race condition
• Stepping switch
• Wire jump relay

References [edit]

1. ^ https://mysite.du.edu/~jcalvert/tel/morse/morse.htm#H1
2. ^ Icons of Invention: The Makers of the Modern World from Gutenberg to Gates. ABC-CLIO. 2009. p. 153. ISBN9780313347436.
3. ^ "The electromechanical relay of Joseph Henry". Georgi Dalakov. Archived from the original on 2012-06-18. Retrieved 2012-06-21 .
4. ^ Scientific American Inventions and Discoveries: All the Milestones in Ingenuity--From the Discovery of Burn down to the Invention of the Microwave Oven. John Wiley & Sons. 2005-01-28. p. 311. ISBN9780471660248.
5. ^ Thomas Coulson (1950). Joseph Henry: His Life and Work . Princeton: Princeton University Printing.
6. ^ U.s.a. 1647, Morse, Samuel E.B., "Comeback in the Mode of Communicating Data past Signals past the Application of Electromagnetism", published June 20, 1840 Archived May 24, 2012, at the Wayback Machine
7. ^ "Relay". EtymOnline.com.
8. ^ "Understanding Relays & Wiring Diagrams". Swe-Check. Swe-Bank check. Retrieved 16 Dec 2020.
9. ^ Mason, C. R. "Art & Science of Protective Relaying, Chapter 2, GE Consumer & Electrical". Retrieved October 9, 2011.
10. ^ Riba, J.R.; Espinosa, A.Thousand.; Cusidó, J.; Ortega, J.A.; Romeral, Fifty. (November 2008). Design of Shading Coils for Minimizing the Contact Bouncing of AC Contactors. Electrical Contacts. p. 130. Retrieved 2018-01-07 .
11. ^ Ian Sinclair, Passive Components for Circuit Design, Elsevier, 2000 ISBN 008051359X,pp. 161-164
12. ^ Fleckenstein, Joseph Due east. (2017). Three-Stage Electric Ability. CRC Press. p. 321. ISBN978-1498737784.
13. ^ Section 1.half dozen, Engineers' Relay Handbook, 5th ed, Relay and Switch Industry Association, Arlington, VA; 3rd ed, National Association of Relay Manufacturers, Elkhart Ind., 1980; second Ed. Hayden, New York, 1966; big parts of the 5th edition are on line here Archived 2017-07-05 at the Wayback Machine.
14. ^ ^{a b} Alexandrovich, George. "The Audio Engineer'due south Handbook" (PDF). Db: The Sound Engineering Magazine. September 1968: ten.
15. ^ EN 50005:1976 "Specification for low voltage switchgear and controlgear for industrial use. Concluding marking and distinctive number. General rules." (1976). In the Great britain published past BSI as BS 5472:1977.
16. ^ Ian Sinclair, Passive Components for Circuit Design, Newnes, 2000 ISBN 008051359X, page 170
17. ^ Croft, Terrell; Summers, Wilford, eds. (1987). American Electricians' Handbook (Eleventh ed.). New York: McGraw Hill. p. vii-124. ISBN978-0-07-013932-9.
18. ^ Rexford, Kenneth B.; Giuliani, Peter R. (2002). Electrical control for machines (6th ed.). Cengage Learning. p. 58. ISBN978-0-7668-6198-five.
19. ^ "Contactor or Motor Starter – What is the Divergence?". EECOOnline.com. 2015-01-xiii. Retrieved 2018-04-19 .
20. ^ ^{a b} Sinclair, Ian R. (2001), Sensors and Transducers (3rd ed.), Elsevier, p. 262, ISBN978-0-7506-4932-2
21. ^ Zocholl, Stan (2003). Ac Motor Protection. Schweitzer Engineering Laboratories. ISBN978-0972502610.
22. ^ Edvard (2013-03-09). "Working Principle of Thermal Motor Protection Relay". Electrical-Engineering-Portal.com. Electrical Engineering Portal. Retrieved 2017-12-30 .
23. ^ "Coordinated Ability Systems Protection". Department of the Army Technical Manual. United states of america Department of the Regular army (811–814): 3–1. 1991.
24. ^ "Contempo Developments in Bong Systems Relays, 1964" (PDF).
25. ^ "Condom Compendium, Chapter 4 Safe control technology" (PDF). p. 115.
26. ^ "Optocoupler Tutorial".
27. ^ "Relay Automatic Phone Company". Retrieved October half dozen, 2014.
28. ^ "British Telecom History 1912-1968". Archived from the original on October 14, 2014. Retrieved October eight, 2014.
29. ^ ^{a b} "Arc Suppression to Protect Relays From Destructive Arc Energy". Retrieved December half dozen, 2013.
30. ^ A. C. Keller. "Recent Developments in Bong System Relays -- Especially Sealed Contact and Miniature Relays" ^{[ permanent expressionless link ]} . The Bell System Technical Journal. 1964.
31. ^ Varney, Al L. (1991). "Questions About The No. ane ESS Switch".
32. ^ "Lab Note #105: Contact Life — Unsuppressed vs. Suppressed Arcing". Arc Suppression Technologies. April 2011. Retrieved Oct 9, 2011.

External links [edit]

• Media related to Relay at Wikimedia Commons

garnernothat.blogspot.com

Source: https://en.wikipedia.org/wiki/Relay

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