what is relay?

Question

in electrical use

Answer 1

Relay - An electrically controlled mechanical device that opens and closes electrical contacts when a voltage (or current) is applied to a coil. A relay provides isolation of control signals from switched signals.

An electromagnetic switching device

Electromechanical device that opens or closes contacts when a current is passed through a coil.

An electromechanical switch that uses a small amount of incoming electricity to charge an electromagnet, which physically pulls down a connecting switch to complete a circuit. This allows a low-power circuit to divert the electricity in a high-power circuit.

an electromechanical switch, operated by passing current through a coil of wire wound around a steel core, which acts as an electromagnet, pulling the switch contact down to make or break a circuit. These are available in several types, including SPST (single-pole, single-throw), SPDT (single-pole, double throw), DPST (double-pole, single throw), and DPDT (double-pole, double-throw), and not as commonly, in multi-circuit configurations such as 3PDT or 4PDT (three and four poles, double-throw).

An electronic switch used to transfer power from the batteries to the track. Each time you pay the raceway owner to buy some track time, he turns a switch which applies current to the coil of the relay, which closes the contacts of the relay, allowing the current to pass from the battery to your car.

These electronic components control other devices in a circuit. A set of contacts, the switching mechanism, open or close when the relay’s magnetic coil becomes energized. Resistance The opposition to current flow through a conductor in a closed circuit.

Answer 2

A relay is an electrically operated switch. Many relays use an electromagnet to mechanically operate a switch, but other operating principles are also used, such as solid-state relays. Relays are used where it is necessary to control a circuit by a low-power signal, or where several circuits must be controlled by one signal. The first relays were used in long distance telegraph circuits as amplifiers: they repeated the signal coming in from one circuit and re-transmitted it on another circuit. 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 or other 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 from overload or faults; in modern electric power systems these functions are performed by digital instruments still called "protective relays".

https://www.electrikals.com/products/r/relays?cid=690&page=1&pagesize=15

Answer 3

relay, electromechanical switch operated by a flow of electricity in one circuit and controlling the flow of electricity in another circuit. A relay consists basically of an electromagnet with a soft iron bar, called an armature, held close to it. A movable contact is connected to the armature in such a way that the contact is held in its normal position by a spring. When the electromagnet is energized, it exerts a force on the armature that overcomes the pull of the spring and moves the contact so as to either complete or break a circuit. When the electromagnet is de-energized, the contact returns to its original position. Variations on this mechanism are possible: some relays have multiple contacts; some are encapsulated; some have built-in circuits that delay contact closure after actuation; some, as in early telephone circuits, advance through a series of positions step by step as they are energized and de-energized

wikipedia

relay
Relay is also another name for a Repeater and a series of medium-altitude satellites; the first of which was launched in 1962. "Relay" can also refer to relay races.

Automotive style miniature relayA relay is an electrical switch that opens and closes under control of another electrical circuit. In the original form, the switch is operated by an electromagnet to open or close one or many sets of contacts. It was invented by Joseph Henry in 1835. 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.

These 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. Form A contact is ideal for applications that require to switch a high-current power source from a remote device.
Normally-closed contacts disconnect the circuit when the relay is activated; the circuit is connected when the relay is inactive. It is also called Form B contact or "break" contact. 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. It is also called Form C contact.

Operation
When a current flows through the coil, the resulting magnetic field attracts an armature that is mechanically linked to a moving contact. The movement either makes or breaks a connection with a fixed contact. When the current to the coil is switched off, the armature is returned by a force that is half as strong as the magnetic force to its relaxed position. Usually this is a spring, but gravity is also used commonly in industrial motor starters. The magnetic flux in the armature induces a current in opposition to the current provided to the coil called 'back emf'. There is a rush of current to operate the coil and move the contacts, but once the armature is closed, the current required to hold the armature closed is a small fraction of that, typically a tenth. Relays are manufactured to operate quickly. In a low voltage application, this is to reduce noise. In a high voltage or high current application, this is to reduce arcing.

If the coil is energized with DC current, regardless of the current through the contacts, a diode is generally installed across the coil. When the coil is energized, a magnetic field is established. When the coil is de-energized, the collapsing magnetic field will generate a spike of current that could damage the rest of the circuit. If the coil is energized with AC current, a small copper ring is crimped to the end of the solenoid. Alternating current is at zero volts 120 times a second. At zero volts, there's no magnetic force holding the contacts closed. The small copper ring provides a small out of phase current called a shadow pole. The sum of the AC current and the shadow pole keeps the armature engaged at all times.

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. To achieve electrical isolation, a light emitting diode LED is used with a photo transitor.


Types of relay

Small relay as used in electronics
A solid state relay, which has no moving partslatching relays are available that have two relaxed states (bistable). These are also called 'keep' relays. When the current is switched off, the relay remains in its last state. This is achieved either 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. 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 reed relay has a set of, usually normally open, contacts inside a vacuum or inert gas filled glass tube. This protects the contacts against atmospheric corrosion. The two contacts are closed by magnetism from a coil around the glass tube. See also reed switch.
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. Mercury wetted relays are position-sensitive and must be mounted vertically to work properly. Because of the toxicity and expense of liquid mercury, these relays are rarely specified for new equipment. See also mercury switch.
A machine tool relay is a type standardized for industrial control of machine tools, transfer machines, and other seqential 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 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. With high current, the contacts are made with pure silver. The unavoidable arcing causes the contacts to oxidize and silver oxide is still a good conductor. Such devices are often used for motor starters. A motor starter is a contactor with an overload protection devices attached. The overload sensing devices are a form of heat operated relay where a coil heats a bi-metal strip, or where a solder pot melts, releasing a spring to operate auxiliary contacts. These auxiliary contacts are in series with the coil. If the overload senses exess current in the load, the coil is de-energized.
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.
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. The tradeoff comes from the fact that every transistor has a small voltage drop across it, typically 0.7 vdc. If the SSR has a load of 10 amps, there is 7 watts of heat to dissipate. A hundred amp load would require fans to be mounted to shed 70 watts of heat.
One type of motor overload protection relay is operated by a heating element in series with the motor. The heat generated by the motor current operates a bi-metal strip or melts solder, releasing a spring to operate contacts. Where the overload relay is exposed to the same environment as the motor, a useful though crude compensation for motor ambient temperature is provided.

Circuit symbols of relays. "C" denotes the common terminal in SPDT and DPDT types.
The diagram on the package of a DPDT AC coil relaySince 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. These have two terminals which can be switched on/off. In total, four terminals when the coil is also included.
SPDT - Single Pole Double Throw. These have one row of three terminals. One terminal (common) switches between the other two poles. It is the same as a single change-over switch. In total, five terminals when the coil is also included.
DPST - Double Pole Single Throw. These have two pairs of terminals. Equivalent to two SPST switches or relays actuated by a single coil. In total, six terminals when the coil is also included.
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. In total, eight terminals when the coil is also included.

Applications

A DPDT AC coil relay with "ice cube" packagingRelays 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. 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 in an effort to conserve energy,
to perform logic functions. For example, the boolean AND function is realised by connecting NO relay contacts in series, the OR function by connecting NO 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 NC contacts. 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.
to perform 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.

Relay application considerations

A large relay with two coils and many sets of contacts, used in an old telephone switching system.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. This style of relay can be manufactured two different ways. "Make before Break" and "Break before Make". The old style telephone switch required Make-before-break so that the connection didn't get dropped while dialing the number. The railroad still uses them to control railroad crossings.
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, 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. An important transmission line or generator unit would have had cubicles dedicated to protection, with a score of individual electromechanical devices.

Design and theory of these protective devices is an important part of the education of an electrical engineer who specializes in power systems. 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. However, due to their very long life span, tens of thousands of these "silent sentinels" are still protecting transmission lines and electrical apparatus all over the world.


Reed relay and reed switches
Current relay
A current relay is a relay connected in series with a current loop. Its contacts change state if a specified current is exceeded.