An electrical or solid-state device, operated by changes in input, which is used to control or operate other devices connected to the output. Most relays are used in electrical circuits, though they may have mechanical input or output. They have a range of applications in telephone exchanges, switches, and automation systems.
A relay is an electrical switch that opens and closes under control of another electrical circuit. Because a relay is able to control an output circuit of higher power than the input circuit, it can be considered, in a broad sense, to be a form of electrical amplifier.
Operation
When a current flows through the coil, the resulting magnetic field attracts an armature that is mechanically linked to a moving contact. Relays are manufactured to operate quickly.
The contacts can be either Normally Open (NO), Normally Closed (NC), or change-over contacts.
Normally-open contacts connect the circuit when the relay is activated; the circuit is disconnected when the relay is inactive. It is also called Form A contact or "make" contact. Normally-closed contacts disconnect the circuit when the relay is activated; the circuit is connected when the relay is inactive. Form B contact is ideal for applications that require the circuit to remain closed until the relay is activated. Change-over contacts control two circuits: one normally-open contact and one normally-closed contact with a common terminal.By analogy with the functions of the original electromagnetic device, a solid-state relay is made with a thyristor or other solid-state switching device.
Types of relay
latching relays are available that have two relaxed states (bistable). When the current is switched off, the relay remains in its last state. This is achieved with a solenoid operating a ratchet and cam mechanism, or by having two opposing coils with an over-center spring or permanent magnet to hold the armature and contacts in position while the coil is relaxed, or with a remnant core. In the ratchet and cam example, the first pulse to the coil turns the relay on and the second pulse turns it off. In the two coil example, a pulse to one coil turns the relay on and a pulse to the opposite coil turns the relay off. This type of relay has the advantage that it consumes power only for an instant, while it is being switched, and it retains its last setting across a power outage. A mercury wetted relay is a form of reed relay in which the contacts are wetted with mercury. Such relays are used to switch low-voltage signals (one volt or less) because of its low contact resistance, or for high-speed counting and timing applications where the mercury eliminated contact bounce. 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 mostly displaced the machine tool relay from sequential control applications. A contactor is a very heavy-duty relay used for switching electric motors and lighting loads. The overload sensing devices are a form of heat operated relay where a coil heats a bi-metal strip, or where a solder pot melts, releasing a spring to operate auxiliary contacts. A forced-guided contacts relay has relay contacts that are mechanically linked together, so that when the relay coil is energized or de-energized, all of the linked contacts move together. If one set of contacts in the relay becomes immobilized, no other contact of the same relay will be able to move. The function of forced-guided contacts is to enable the safety circuit to check the status of the relay. Forced-guided contacts are also known as "positive-guided contacts", "captive contacts", "locked contacts", or "safety relays". 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. One type of motor overload protection relay is operated by a heating element in series with the motor. Where the overload relay is exposed to the same environment as the motor, a useful though crude compensation for motor ambient temperature is provided.Since relays are switches, the terminology applied to switches is also applied to relays. According to this classification, relays can be of the following types:
SPST - Single Pole Single Throw. Equivalent to two SPST switches or relays actuated by a single coil. Equivalent to two SPDT switches or relays actuated by a single coil. Equivalent to four SPDT switches or relays actuated by a single coil or two DPDT relays.Applications
Relays are used:
to control a high-voltage circuit with a low-voltage signal, as in some types of modems, to control a high-current circuit with a low-current signal, as in the starter solenoid of an automobile, to detect and isolate faults on transmission and distribution lines by opening and closing circuit breakers (protection relays), to isolate the controlling circuit from the controlled circuit when the two are at different potentials, for example when controlling a mains-powered device from a low-voltage switch. For example, the boolean AND function is realised by connecting NO relay contacts in series, the OR function by connecting NO contacts in parallel. Due to the failure modes of a relay compared with a semiconductor, they are widely used in safety critical logic, such as the control panels of radioactive waste handling machinery. Relays can be modified to delay opening or delay closing a set of contacts.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, changeover (double-throw) In the case of changeover, there are two types. 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 relays to about 15,000 V Coil voltage - machine-tool relays usually 24 VAC or 120 VAC, relays for switchgear may have 125 V or 250 VDC coils, "sensitive" relays operate on a few milliamperes Package/enclosure - open, touch-safe, double-voltage for isolation between circuits, explosion proof, outdoor, oil-splashresistant 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 circuit and contacts Aerospace or radiation-resistant testing, special quality assurance 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.Protection relay
A protection relay is a complex electromechanical apparatus, often with more than one coil, designed to calculate operating conditions on an electrical circuit and trip circuit breakers when a fault was found. Unlike switching type relays with fixed and usually ill-defined operating voltage thresholds and operating times, protection relays had well-established, selectable, time/current (or other operating parameter) curves. Such relays were very elaborate, using arrays of induction disks, shaded-pole magnets, operating and restraint coils, solenoid-type operators, telephone-relay style contacts, and phase-shifting networks to allow the relay to respond to such conditions as over-current, over-voltage, reverse power flow, over- and under- frequency, and even distance relays that would trip for faults up to a certain distance away from a substation but not beyond that point. Each of the protective functions available on a given relay are denoted by standard ANSI Device Numbers. For example, a relay including function 51 would be a timed overcurrent protection relay. Today these devices are nearly entirely replaced (in new designs) with microprocessor-based instruments (numerical relays) that emulate their electromechanical ancestors with great precision and convenience in application. By combining several functions in one case, numerical relays also save capital cost and maintenance cost over electromechanical relays.
Overcurrent relay
An "Overcurrent Relay" is a type of protective relay. In a typical application the overcurrent relay is used for overcurrent protection, connected to a current transformer and calibrated to operate at or above a specific current level. When the relay operates, one or more contacts will operate and energize a trip coil in a Circuit Breaker and trip (open) the Circuit Breaker.
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