Relay

A relay ( electrical) [ ʁəlɛ ː ] ( Pl: Relay [ ʁəlɛ ː s] ) is a powered by electricity, usually electromagnetically acting, remote controlled switch with usually two switching positions. The relay is activated by a control circuit, and can switch more circuits.

2.1 Advantages and disadvantages

3.1 miniature relays
3.2 shooter
3.3 SSR
3.4 Bistable relay
3.5 Polarised relay
3.6 relay in motor vehicles 3.6.1 Special functions

5.1 Normalized identifying the connections
5.2 Usual description of the switch contacts

7.1 telegraphy
7.2 telephone switching
7.3 Relay at the beginning of computer development

Principle of operation

A mechanical relay generally operates on the principle of the electromagnet. A current in the exciting coil generates a magnetic flux through the ferromagnetic core and a thereon mounted movably also ferromagnetic armature. On an air gap, it is to the action of force on the armature, causing it to turn one or more contacts. The armature is moved back by spring force into the starting position when the coil is no longer energized.

Schematic structure

An example of this is shown with a normally open a hinged armature relay. The left picture shows the relay at rest; the coil is energized, the relay contact opens. On the right image is located on the coil to a voltage, whereby the armature is attracted by the iron core of the coil and the normally open contact is closed.

Terms

A contact is called contact or make contact when it is closed when de-energized open anchor or without current excitation coil and the armature is attracted or current-carrying coil. In closed or normally closed contact is referred to when he interrupts the circuit, in the tightened condition of the relay. A combination of NO and NC is called the changer or changeover. A relay may have one or more such contacts.

A relay is " normally on " when it is flowing through it and tightened at rest by the stream, for example for monitoring of power failure or wire breakage. In the other, and the predominant case in which it is normally in the idle state, it is referred to as " shunt relay".

In the circuit diagram of relay are always drawn in the fallen state, even if they work as idle current relay.

Use

Relays are electromechanical devices. They are mainly used for the following applications:

For simultaneous and electrically isolated switching of several load circuits with only one control circuit
For the switching of high electrical performance with low power ( amplifier )
To achieve electrical isolation between steuerndem and a switching circuit
To low switching contact resistance in the closed state of the contact to achieve at the same time a very large contact resistance in the open condition

Pros and Cons

Electromechanical relays have been replaced in many applications of electronic switches that use transistors. Relays have advantages over transistors After some, but also advantages, among others:

Cons:

Dependence of the insulation capacity of the air pressure or the height above sea level (except for hermetically sealed relay case )
Vibration and shock sensitivity
Noise when shifting
High attack and release time (milliseconds to micro -and nano- seconds in semiconductors, that is three to six orders of magnitude )
Depending on the contact material, the contact resistance with the lifetime may change depending on the switched load
Wear principle ( electrical and mechanical), that is, it must always be the maximum possible number of switching life of the assembly are compared with

Advantages:

Low contact resistance in the milliohm range combined with low capacity of the switching path
High switching or high overload
Relays require no refrigeration
Relays can switch slightest signals to high RF performance and show this little inclination to talk over
Relays can vary depending on the contact material and short-circuit current turn short circuits without losing their function
Switching condition is often seen with the naked eye
Immunity by pronounced hysteresis and robustness of the coil, it will surge a few times without prejudice towards (EMC and ESD)

Relay types

Among the relay there is a very large number of different types and designs. In addition, relays can be typed according to various aspects, such as the number of possible switching states in de-energized state, by design, size, area of operation, type or material of contacts Switching capacity or operating principle. A relay can therefore often be counted on different types.

The main types are:

Miniature relays

To the somewhat ambiguous term defined miniature relays include a variety most commonly used for low voltage relays, which are often designed for installation on printed circuit boards ( " PCB relay "). Other examples are DIL relays, comb- guided relay or miniature SMD relay.

Sagittarius

A relay for much greater achievements in power engineering is called the Schütz. The current and voltage in the load circuit may be a multiple greater than in the coil. Sagittarius have a tie rod, a slightly higher performance is required for its control, and they usually have several similar switch contacts as they are needed for switching AC loads. Furthermore, there are so-called auxiliary contactors, which in turn are used to control the aforementioned main contactors.

SSR

SSR (English solid state relay, SSR, therefore Germanized also solid-state relay called ) are not actual relay. Rather, it is to electronic components, the - simulate the effect of real relay - based on completely different physical processes. SSRs are implemented with transistors or thyristors or triacs. They work without moving parts, are very durable and suitable for high switching frequency and unfavorable environmental conditions (such as environments with explosive gas mixtures ).

With SSR, it is possible to switch AC voltage during the zero crossing, thus interfering pulses can be avoided. Galvanic isolation is at SSR achieved by integrated in the component optocoupler. SSRs have the advantage over mechanical relays higher losses in the load current path and therefore must often be mounted on a heatsink. There are also solid-state relays, which the apex of the mains voltage or immediately upon driving, so currently off. Vertex switches are used for switching inductances, which have no or only a small residual magnetization. They exhibit no hysteresis.

A special role called OptoMOS or PhotoMOS relay, because they are similar in structure optocouplers: You work on the control side as an optocoupler with an IR LED and have the load side in contrast to the SSR previously described no triacs or thyristors, but MOSFETs with which they can switch AC and DC voltages in typically rather low power. You do not need to be cooled, and have at low load current a lower voltage drop than SSRs, however, typically show a higher " contact resistance " as a mechanical signal relay. They work without bounce or wear-free and with high switching speeds (a few microseconds), which can reach up to 100 kHz with special designs switching frequencies.

Bistable relay

Latching relays are characterized by their property that they can occupy two different stable switching states in the de-energized state. Some of the bistable relay

Polarized relays

There are two types of relay, in which the polarity is provided for:

In polarized relay polarity to be applied excitation voltage is fixed. Polarized relays have an integrated permanent magnet whose field is superimposed on the excitation coil of the additive. Thus, the pull-in voltage is reduced, or the suit sensitivity is increased.
In relay with integrated free-wheeling diode, the voltage can be applied only in the reverse direction of the diode. This variant is found mainly in relay in the DIL package.

Relay in motor vehicles

Automotive relays are ruggedly built relays that can withstand the increased demands in motor vehicles for shock and temperature range. They work with the on-board voltage of 12 V or 24 V and can switch higher currents. They usually have connections with 6.3mm flat connectors. They often contained in the housing have components ( resistor, diode) for restricting the back-emf of the coil.

Special Functions

The " relay" that are installed as plug-in modules, inter alia, in the fuse box of motor vehicles, are often relays with other functions or electronic assemblies or small control devices.

Examples:

Fuel pump relay (time and speed- dependent control of the fuel pump )
Flasher relay ( clock for the turn signal )
EGR relay (control of air pressure and speed-dependent exhaust gas recirculation)
Glow Time for the glow plug for diesel engines ( timing and power path control)
Interval relay for the wipers ( electrical clock, some with adjustable or rain Controlled Interval Time)
Control for folding exterior mirrors and curbs function

In many of these small control devices is actually still, although a mechanical relay comprise the term relay for the whole unit is rather historical. In modern cars, most of the features in greater central control units are integrated - so today the typical sound of the flasher relay is often generated either by speakers or with a relay that switches no load.

Telecommunications relay

In the electro-mechanical switches and relays telephone systems have been widely used. They served the logical flow of control during assembly and disassembly of the dial-up connections. Here were the participants in the subscriber circuit and the switching matrix, which consisted mostly of voters assigned to relay fixed. Among the most important representatives of this type of relay, which are now found only occasionally encountered include the flat relay, the round relay and the relay - ESK.

AC relay

Electromagnetic relays can not be used readily with AC voltage as the magnetic field, which is to hold the armature, is constantly changing the polarity and therefore in the meantime is too weak or zero. Although the anchor pulls usually at voltages of mains frequency, but rattles and precise switching of the contacts is not guaranteed. The following relays can be operated with AC:

DC relay with a rectifier (which is sometimes built into the relay housing).
Phase relay is a relay having two coils on separate limbs iron, wherein the current flow is out of phase in one of the windings by means of a capacitor connected in series by approximately 90 degrees. Wherein a coil is always fully energized when the excitation current in the other passes through zero.
Spaltpolrelais with a shaded pole have a shorted turn. The current induced in the short circuit loop current that is phase-shifted with respect to the control current, the holding force maintains, while the control current has its zero crossing.

Drehspulrelais

The Drehspulrelais is polarized with a permanent magnet special relays for low power. The structure corresponds to a moving coil mechanism with a rotatably mounted coil, external permanent magnet and a return spring. Instead of a pointer before a Anzeigeskale contacts are triggered at certain angles of rotation of the rotating coil at the Drehspulrelais. Due to the principle of the permanent magnets can capture only direct variables such as DC voltage, which is why they are combined in AC applications with bridge rectifiers Drehspulrelais.

Application found the Drehspulrelais in various forms of electrical power protection in electrical power networks such as the distance relays. In particular, the pre- set limits are exceeded Drehspulrelais appropriate warning and cut-out contacts were automatically triggered which trigger the associated circuit breakers in substations.

Other types of relays

Bimetallic not work electromagnetically but use the thermal effect of the current flow. They are used for time-delayed switching. A wrapped with a heating conductor bimetallic strip is heated slowly and then switches a contact.

Differential relays have two coils with the same electromagnetic properties and speak at small current differences between the windings on. Following the principle of current differential evaluation example working leakage circuit breaker.

Coaxial relays are used for switching high frequency signals, and have a defined line impedance (e.g., 50 ohms) between the contact path and the shield.

Alarm relays have except the switch contacts a trapdoor, which maintains its position at the fall damper relay after triggering up to an acknowledgment. The mechanical and optical signaling function also has electrical contacts. Signal relays with current coil can save in this way once current overshoot events and show until acknowledged.

Polwenderelais have two coils and two contacts, which are connected internally to an H-bridge for reversing the direction of rotation of DC motors. Furthermore, these relays are used to control clock lines in clock systems.

Mercury relay contact used for switching the liquid at room temperature metal mercury, which is under protective gas in a glass tube. In early automatic stairway lighting such a glass tube ( mercury switch) is tilted electromagnetically. Plunger relay (including diving relay) having a floating on the mercury magnet armature, which is drawn as a pull magnet with a current-carrying coil in the mercury and thereby its level increased so that an above -mounted contact is achieved by the mercury level. There is also a construction in which a magnetic holding pin falls into the mercury, when the magnetic field decreases.

Another design is thermal mercury relay, in which a heater acts on a volume of gas by the thermal expansion thereof, a change in position of the mercury is achieved whereby it opens or closes contacts. They react naturally quite slow, but in the often customary use in thermostats no disadvantage was or was even used for time delay. Thermal mercury relays were often used together with contact thermometers. Mercury relays can be performed by means of an ingenious mechanics as a latching relay.

Relay in a broader sense

These relays are also provided with a more or less complex mechanics or electronics.

Stepping relays were used to control in historic telephone systems, traffic lights, or even washing machines.

Time relays are available in electronic or electromechanical design, they are used for time-control in machines and devices.

One form of the timing relay is the impulse relay. It switches on receipt of an enable pulse to the contact for a defined period, so operates analogously to a monostable multivibrator. A typical example is a staircase switch.

A wiper relay ( see also wiping contact) is a pulse or a bistable relay, which specifically addresses ( " wiped " ) even on very short pulses.

Report monitoring relays using a sensor, the over or fall below certain preset values . For example, temperatures, fluid levels, tensions, asymmetries can be monitored in three-phase systems by means of asymmetry relay or any other physically measurable quantities.

Characteristics

In the following table the main characteristics are listed, on the one relay is specified. In addition, of course, is still a scale drawing, pin assignment and so interesting. The examples relate to a typical 12V automotive relay.

Identification of a relay

Normalized identifying the connections

A1, A2: coil
11, 12: Opener
13, 14: Normally open (contact closes when voltage is applied to the coil )
15, 16, 18: changer ( 15 is the common contact, tongue, must always be present here voltage)

Have multiple relay actuating coil, the other coils with A3/A4 etc. However. The first number of the contact name is increased by a numerically relay with multiple contacts. The rear paragraph are of the type of the relay contact. So called, for example 53/54 the 5th contact, who is a closer.

In the ( German ) Telecommunications following rules is available (DIN 41220 ): You may refer to:

The arrangement in the contact set with Roman numerals
The associated contacts with the corresponding lowercase
Coils with capital letters

Usual description of the switch contacts

In data sheets and tables comparing relay is often found English abbreviations for the number of switch contacts and positions:

Number of contact points: The easiest switch has a contact point; one of the conductors is connected directly to the switch movable element: SM - Single Make (NO) SB - Love Break ( NC) SM -SB - change-over contact There are also switch having two contact points, wherein said movable switch member establishes the connection between two conductors either or separated ( in protecting normal ): DM - Double Make (NO) DB - Double Break ( NC) DB -DM - Change-over contact
P Pole - number of switch contacts ( Single, Double, ... )
Order of abbreviations: A contact assembly is marked as follows: 1 Number of poles ( pole ) 2 switch positions ( Throws ) 3 idle state ( normal position ) Occasionally still follows a break- make- specification (usually omitted)
Rest, work and exchange contacts: NC - Normally Closed = normally closed contact; also: Break NO - Normally Open Normally Open; also: make CO - Changeover = changeover contact; also: Break - Make (B- M)
Throw T - Number of switching positions ( Single, Double)

A few examples:

Switching of relays with transistors

In the control of the relay coil connected to a transistor is to be noted that results in a high voltage of opposite polarity through the coil of the relay by means of self-induction of the switch-off of the current. This voltage exceeds the rated voltage of the relay and can significantly if the maximum blocking voltage of the transistor destroy it.

In order to prevent the destruction of the switching transistor (T1 in the figure), one closes this reverse voltage by a freewheeling diode ( D1 in the figure) short or limited it to the forward voltage of the diode. However, the result is that the magnetic field in the coil collapses slowly and the switching times of the relay significantly increases - which in turn reduced by a stronger switching arc, the life of the switching contacts.

The disadvantages with respect to the switching time of variant A was dissolved by adding a zener diode ( ZD 1 in the figure, variant B ), the zener voltage should correspond to a benchmark of approximately the rated voltage of the relay - the magnetic field in the coil can break down much faster. However, it should be noted that the blocking voltage of the switching transistor needs to be still larger than the operating voltage plus the Zener voltage in order to prevent its destruction.

There are other protection circuits, for example with parallel or protective resistance matched to the inductance of the coil RC network ( snubber ). These measures operate regardless of polarity and are also suitable for relays with AC operation.

Some types of relays have already built a freewheeling diode or a protection resistor.

Furthermore, there are specific, suitable for switching inductive loads switching transistors, which in turn have built a limiter (eg, the Darlington transistor 2SD1843 ).

Historical to the relay

Telegraphy

Around the year 1820 is focused around the research for electronic data transmission to electromagnetism. Early models were designed by André- Marie Ampère, Pierre- Simon Laplace, and many others. Became well-known example Joseph Henry in 1835, who developed the messaging of his lab to his house himself, or Charles Wheatstone, who together with William Fothergill Cooke introduced in 1837 Eisenbahntelegraphie in England. Samuel Morse improved by correspondence with J. Henry the relay so that it also responded to weaker stimuli and put it as a signal amplifier. Although the idea of a telegraph existed since the mid 18th century, but the relay was ultimately the key to success. It had all 30 km to be inserted into the signal path of the telegraph wires, to regenerate the incoming weak signals again. Thus the basis was created to transmit impulses over long distances. The first demonstration of the telegraph took place in 1844 between Washington, DC and Baltimore instead. Based on the relay stations of the mail, where the mail rider could exchange their horses for fresh, was christened the new device relays.

Telephone switching

A major impetus to the further spread of the relay was the introduction of the participants dial into the telephone switching equipment end of the 19th century. The first autodialer in Germany was taken on July 10, 1908 in Hildesheim in local transport with 900 participants in operation. The national telephone traffic was automated gradually from 1923 and would not have been possible without the massive use of relay technology.

Relay at the beginning of computer development

The relay also enabled the development of the computer that was first built in 1941 by Konrad Zuse under the name "Z3 " with 2,000 relay for the display unit and the memory.

In the mid- 40s were relay in computer technology largely replaced by electron tubes, however. Later, the function of the transistors and integrated circuits, has been adopted (IC).

Electric power Hermetic seal Railway signal Flyback diode Power-system protection Electrical impedance Clock rate Z3 (computer)

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