Relay

Where radio transmitters and receivers share one antenna, often 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 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 the transmission line impedance of the system, for example, 50 ohms.^[20]

A contactor is a heavy-duty relay with higher current ratings,^[21] 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.^[22] Contactors with overload protection devices are often used to start motors.^[23]

A force-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 force-guided contacts is to enable the safety circuit 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 "safety relays".

These safety relays have to follow design rules and manufacturing rules that are defined in one main machinery standard EN 50205 : Relays with forcibly guided (mechanically linked) contacts. These rules for the safety design are the one defined in type B standards such as EN 13849-2 as Basic 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 same state, however, they do guarantee, subject to no gross mechanical fault, that no contacts are in opposite 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 set of contacts is broken, with a marginal gap, while the other remains closed. By introducing both NO and NC contacts, or more commonly, changeover contacts, on the same relay, it then becomes possible to guarantee that if any NC contact is closed, all NO contacts are open, and conversely, if any NO contact is closed, all NC contacts are open. It is not possible to reliably ensure that any particular contact is closed, except by potentially intrusive and safety-degrading sensing of its circuit conditions, however in safety systems it is usually the NO state that is most important, and as explained above, this is reliably verifiable by 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 auxiliary contact sets, often of reduced current or voltage rating, used for the monitoring system. Contacts may be all NO, all NC, changeover, or a mixture of these, for the monitoring contacts, so that the safety system designer can select the correct configuration for the particular application. Safety relays are used as part of an engineered safety system.

A latching relay, also called impulse, bistable, keep, or stay relay, or simply latch, maintains either contact position indefinitely without power applied to the coil. The advantage 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 power outage. A latching relay allows remote control of building lighting without the hum that may be produced from a continuously (AC) energized coil.

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

Otro tipo de enclavamiento tiene un núcleo remanente que mantiene los contactos en la posición de operación mediante el magnetismo remanente en el núcleo. Este tipo requiere un pulso de corriente de polaridad opuesta para liberar los contactos. Una variante utiliza un imán permanente que produce parte de la fuerza necesaria para cerrar el contacto; la bobina proporciona la fuerza suficiente para mover el contacto, ya sea abriéndolo o cerrándolo, al favorecer u oponerse al campo del imán permanente. ^{[ 24 ]} Un relé controlado por polaridad necesita interruptores de conmutación o un circuito de accionamiento en puente H para su control. El relé puede ser menos costoso que otros tipos, pero esto se compensa parcialmente con el aumento de los costos en el circuito externo.

En otro tipo, un relé de trinquete posee un mecanismo que mantiene los contactos cerrados tras la activación momentánea de la bobina. Un segundo impulso, en la misma bobina o en una diferente, libera los contactos. ^{[ 24 ]} Este tipo se puede encontrar en ciertos automóviles, para el control de luces bajas y otras funciones que requieren un funcionamiento alterno con cada activación del interruptor.

Un relé de pasos es un tipo especializado de relé de enclavamiento multidireccional diseñado para las primeras centrales telefónicas automáticas .

Un interruptor diferencial incluye un relé de enclavamiento especializado.

Los primeros ordenadores solían almacenar bits en un relé de enclavamiento magnético, como el ferreed o el posterior remreed del conmutador 1ESS .

Algunas computadoras antiguas utilizaban relés comunes como una especie de pestillo : almacenaban bits en relés de resorte de alambre o relés de láminas mediante la realimentación de un cable de salida como entrada, lo que resultaba en un bucle de retroalimentación o circuito secuencial . Este tipo de relé de enclavamiento eléctrico requiere alimentación continua para mantener su estado, a diferencia de los relés de enclavamiento magnético o los relés de trinquete mecánico. Si bien los circuitos de auto-retención suelen implementarse con relés, también pueden implementarse por otros medios.

En las memorias de las computadoras, los relés de enclavamiento y otros relés fueron reemplazados por memorias de línea de retardo , que a su vez fueron reemplazadas por una serie de tecnologías de memoria cada vez más rápidas y cada vez más pequeñas.

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.

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 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.

A mercury relay is a relay that uses mercury as the switching element. They are used where contact erosion would be a problem for conventional relay contacts. Owing to environmental considerations about significant amount of mercury used and modern alternatives, they are now comparatively uncommon.

A mercury-wetted reed relay is a form 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 result in poor conductivity for low-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 be 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 coalesce, and the current rise time through the contacts is generally considered to be a few picoseconds. However, in a practical circuit it may be limited by the inductance of the contacts and wiring. It was quite common, before restrictions on the use of mercury, to use a mercury-wetted relay in the laboratory as a convenient means of generating fast rise time pulses, however 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 variations.

The same coalescence process causes another effect, which is a nuisance in some applications. The contact resistance is not stable immediately after contact closure, and drifts, mostly downwards, for several seconds after closure, the change perhaps being 0.5 ohm.

Multi-voltage relays are devices designed to work for wide voltage ranges such as 24 to 240 VAC and VDC and wide frequency ranges such as 0 to 300 Hz. They are indicated for use in installations that do not have stable supply voltages.

Electric motors need 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.^[25] The overload sensing devices are a form 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 coil, so they turn off the motor when it overheats.^[26]

This thermal protection operates relatively slowly allowing the motor to draw higher starting currents before the protection relay will trip. Where the overload relay is exposed to the same ambient temperature as the motor, a useful though crude compensation for motor ambient temperature is provided.^[27]

The other common overload protection system uses an electromagnet coil in series with the motor circuit that directly operates contacts. This is similar to a control relay but requires a rather high fault current to operate the contacts. To prevent short 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.

Los relés electrónicos de protección contra sobrecarga miden la corriente del motor y pueden estimar la temperatura del bobinado mediante un "modelo térmico" del sistema de armadura del motor, que se puede configurar para proporcionar una protección más precisa. Algunos relés de protección de motor incluyen entradas para detectores de temperatura que permiten la medición directa mediante un termopar o un sensor de resistencia integrado en el bobinado. ^{[ 28 ]}

Un relé polarizado coloca la armadura entre los polos de un imán permanente para aumentar la sensibilidad. Los relés polarizados se utilizaban en las centrales telefónicas de mediados del siglo XX para detectar pulsos débiles y corregir la distorsión telegráfica .

Un relé de láminas es un interruptor de láminas encerrado en un solenoide. El interruptor tiene un conjunto de contactos dentro de un tubo de vidrio al vacío o lleno de gas inerte que protege los contactos contra la corrosión atmosférica ; los contactos están hechos de material magnético que los hace moverse bajo la influencia del campo del solenoide que lo contiene o de un imán externo.

Los relés de láminas pueden conmutar más rápido que los relés de mayor tamaño y requieren muy poca potencia del circuito de control. Sin embargo, tienen valores nominales de corriente y voltaje de conmutación relativamente bajos. Aunque es poco común, las láminas pueden magnetizarse con el tiempo, lo que provoca que permanezcan activadas incluso cuando no hay corriente; cambiar la orientación de las láminas o desmagnetizar el interruptor con respecto al campo magnético del solenoide puede solucionar este problema.

Los contactos sellados con contactos humedecidos con mercurio tienen una vida útil más larga y menos vibraciones de contacto que cualquier otro tipo de relé. ^{[ 29 ]}

Los relés de seguridad son dispositivos que generalmente implementan funciones de protección. En caso de peligro, la tarea de dicha función de seguridad es utilizar las medidas adecuadas para reducir el riesgo existente a un nivel aceptable. ^{[ 30 ]}

Un contactor de estado sólido es un relé de estado sólido de alta resistencia, que incluye el disipador de calor necesario, utilizado en aplicaciones que requieren ciclos frecuentes de encendido y apagado, como en calentadores eléctricos, pequeños motores eléctricos y sistemas de iluminación. No tiene piezas móviles que se desgasten ni presenta rebotes de contacto debido a vibraciones. Se activa mediante señales de control de CA o CC provenientes de controladores lógicos programables (PLC), PC, fuentes de lógica TTL (transistor-transistor ) u otros sistemas de control de microprocesadores y microcontroladores.

A solid-state relay (SSR) is a solid state electronic component that provides a function 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 be used to isolate control and controlled circuits.^[31]

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

Timing relays are arranged for an intentional delay in operating their 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 a 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; both air-filled and oil-filled dashpots are used. The time period can be varied by increasing or decreasing the flow rate. 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 panel. Modern microprocessor-based timing relays provide precision timing over a great range.

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

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

En primer lugar, tenemos el contacto normalmente abierto con retardo de cierre (NOTC). Este tipo de contacto está normalmente abierto cuando la bobina no recibe alimentación (está desenergizada). El contacto se cierra al aplicar alimentación a la bobina del relé, pero solo después de que la bobina haya recibido alimentación continua durante el tiempo especificado. En otras palabras, la dirección del movimiento del contacto (ya sea para cerrar o abrir) es idéntica a la de un contacto normalmente abierto (NO), pero con un retardo en el cierre. Debido a que el retardo se produce en la dirección de la energización de la bobina, este tipo de contacto también se conoce como contacto normalmente abierto con retardo de encendido.

Un relé de vacío es un relé sensible cuyos contactos están montados en una carcasa de vidrio al vacío, lo que permite manejar voltajes de radiofrecuencia de hasta 20.000 voltios sin descargas disruptivas entre los contactos, incluso cuando la separación entre ellos es de tan solo unas centésimas de pulgada cuando están abiertos.

Para la protección de aparatos eléctricos y líneas de transmisión, se utilizaban relés electromecánicos con características operativas precisas para detectar sobrecargas, cortocircuitos y otras fallas. Si bien muchos de estos relés aún se utilizan, los relés de protección digitales actuales ofrecen funciones de protección equivalentes y más complejas.

Los relés de señalización ferroviaria son de gran tamaño si se tienen en cuenta los bajos voltajes (menos de 120  V) y corrientes (quizás 100  mA) que conmutan. Los contactos están ampliamente espaciados para evitar descargas disruptivas y cortocircuitos durante una vida útil que puede superar los cincuenta años.

Dado que los circuitos de señalización ferroviaria deben ser altamente fiables, se emplean técnicas especiales para detectar y prevenir fallos en el sistema de relés. Para proteger contra falsas alimentaciones, se suelen utilizar contactos de relé de doble conmutación tanto en el lado positivo como en el negativo del circuito, de modo que se necesitan dos falsas alimentaciones para generar una señal errónea. No todos los circuitos de relés pueden ser verificados, por lo que se recurre a características constructivas como los contactos de carbono a plata para resistir la soldadura de contactos inducida por rayos y proporcionar inmunidad a la corriente alterna.

Opto-isolators are also used in some instances with railway signalling, especially where only a single contact is to be switched.

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 did not get dropped while dialing the number.
• Contact current rating — small relays switch a few amperes, large contactors are rated for up to 3000 amperes, alternating 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 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 affected by the type of load switched. Breaking load current causes undesired arcing between the contacts, eventually leading to contacts that weld shut or contacts that fail due to erosion by the arc.^[34]
• Coil voltage — machine-tool relays usually 24 VDC, 120 or 250 VAC, relays for switchgear may have 125 V or 250 VDC coils,
• Coil current — Minimum current required for reliable operation and minimum holding current, as well as effects of power dissipation on coil temperature at various duty cycles. "Sensitive" relays operate on a few milliamperes.
• 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 temperature 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 equipment
• Switching 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 50 g, or more.
• 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.^[29] Larger relays often handle higher voltage and current than smaller relays.
• Accessories such as timers, auxiliary contacts, pilot lamps, and 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, including 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 μs are available. Reed relays which are actuated by low currents and switch fast are suitable for controlling small currents.

As with any switch, the contact current (unrelated to the coil current) must not exceed a given value to avoid damage. In high-inductance circuits such as motors, other issues 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 current exists. When the circuit is broken, the current cannot change instantaneously, which creates a potentially damaging arc across the separating contacts.

Consequently, for relays used to control inductive loads, we must specify the maximum 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.