Wire spring relay

Last updated May 05, 2019

A wire spring relay is a type of relay, that has springs made from drawn wires of nickel silver, rather than cut from flat sheet metal as in the flat-spring relay. This class of relays provided manufacturing and operating advantages over previous designs. Wire spring relays entered mass production in the early 1950s. ^[1]^[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 separate 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 marker is a type of special purpose control system that was used in electromechanical telephone central office switches. Switches employing markers belong to a class of switches known as "common control", as the purpose of a marker is to control the closure of contacts in the switching fabric that connect a circuit between the calling party and the called party. This is in contrast to "direct control" switches, where the switching elements were controlled directly by the customer's dial, such as the Step by Step switch. The term marker came from its use to mark a path of links through the switching fabric. A marker's comprehensive view of the switching fabric allowed it to find and assemble a path from one terminal to another, if the links were available, unlike the earlier graded progressive systems in which a path might not be found.

In electronics, a crossbar switch is a collection of switches arranged in a matrix configuration. A crossbar switch has multiple input and output lines that form a crossed pattern of interconnecting lines between which a connection may be established by closing a switch located at each intersection, the elements of the matrix. Originally, a crossbar switch consisted literally of crossing metal bars that provided the input and output paths. Later implementations achieved the same switching topology in solid state semiconductor chips. The cross-point switch is one of the principal switch architectures, together with a rotary switch, memory switch, and a crossover switch.

Wire spring relays were primarily manufactured by the Western Electric Company for use in electromechanical telephone exchanges in the Bell System. The design was licensed for use around the world, and was commonplace in Japan.

A telephone exchange is a telecommunications system used in the public switched telephone network or in large enterprises. An exchange consists of electronic components and in older systems also human operators that interconnect (switch) telephone subscriber lines or virtual circuits of digital systems to establish telephone calls between subscribers.

The Bell System was the system of companies, led by the Bell Telephone Company and later by AT&T, which provided telephone services to much of the United States and Canada from 1877 to 1984, at various times as a monopoly. On December 31, 1983, the system was divided into independent companies by a U.S. Justice Department mandate.

Manufacturing of wire spring relays greatly declined in the late 20th century due to the introduction of digital electronic switching systems that used them in very small numbers.

Description

A relay has two major parts, the electromagnet and the contacts. The electromagnet may have a resistance between 15 and 200 ohms, and is often designed to operate satisfactorily at a common telephony voltage, such as 24 or 48 volts.

An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. Electromagnets usually consist of wire wound into a coil. A current through the wire creates a magnetic field which is concentrated in the hole in the center of the coil. The magnetic field disappears when the current is turned off. The wire turns are often wound around a magnetic core made from a ferromagnetic or ferrimagnetic material such as iron; the magnetic core concentrates the magnetic flux and makes a more powerful magnet.

The ohm is the SI derived unit of electrical resistance, named after German physicist Georg Simon Ohm. Although several empirically derived standard units for expressing electrical resistance were developed in connection with early telegraphy practice, the British Association for the Advancement of Science proposed a unit derived from existing units of mass, length and time and of a convenient size for practical work as early as 1861. The definition of the ohm was revised several times. Today, the definition of the ohm is expressed from the quantum Hall effect.

Telephony is the field of technology involving the development, application, and deployment of telecommunication services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is intimately linked to the invention and development of the telephone.

The electromagnet can also be modified by the insertion of metallic slugs (lumps) to create a brief delay before pulling in the contacts (slow operate), or hold the contacts in place briefly after power is removed (slow release).

A wire spring relay typically has many individual contacts. Each contact is either a fixed contact, which does not move, or is a moving contact, and is made from a short piece of wire. The contact points are made from small blocks of precious metals, such as palladium, which are spot-welded to the wire springs. The majority of the wire spring relays manufactured in the 1960s had twelve fixed contacts. A normally open (make) contact, a normally closed (break) contact, or both can be provided for each fixed contact. A moving contact consists of two wires projecting out of the base of the relay, bent slightly inwards in order to exert pressure against the armature.

Palladium is a chemical element with symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by William Hyde Wollaston. He named it after the asteroid Pallas, which was itself named after the epithet of the Greek goddess Athena, acquired by her when she slew Pallas. Palladium, platinum, rhodium, ruthenium, iridium and osmium form a group of elements referred to as the platinum group metals (PGMs). These have similar chemical properties, but palladium has the lowest melting point and is the least dense of them.

The moving contacts are held away from the fixed contacts by a phenolic paper pattern called a "card". By changing the depth of the cuts on this form, the contacts can be made to make or break earlier or later than others. This can be used to transfer electrical control or power from one source to another by having a "make" contact operate before the corresponding "break" contact does.

Phenolic paper is a material often used to make printed circuit board substrates. It is a very tough board made of wood fibre and phenolic polymers. It is most commonly brown in colour, and is a fibre reinforced plastic. These PCB materials are known as FR-1 and FR-2 – FR-2 is rated to 130 °C, FR-1 is rated to 105 °C.

For the stored program control exchanges of the early 1970s, many relays were made with steel cores that remained magnetized after current ceased to flow in the winding. ^[3] This magnetic latching feature, different from the use of slugs to delay relay operation, was used in the arrays of reed relays that switched connection paths in the early models of electronic switching systems. A miniature wire spring relay was also produced, starting in approximately 1974 as part of the 1A redesign of the 1ESS switch.

Use in logic

Wire spring relays could be interconnected to create the typical combinational circuits that were later used in silicon design.

The contacts of one or more relays can be used to drive the coil of another relay. To make an OR gate for example, the contacts of several input relays may be placed in parallel circuits and used to drive the electromagnet of a third relay. This, along with series circuits and more complicated schemes such as multiply wound electromagnets, allows the creation of AND gates, OR gates and Inverters (using the normally closed contact on a relay). Using these simple circuits in combination with De Morgan's Laws, any combinational function can be created using relays.

When output wires are fed back as inputs, the result is a feedback loop or sequential circuit that has the potential to consider its own history. Such circuits are often suitable as memories.

Use as memory

Memory circuits in the form of latches can also be created by having a relay contact complete the circuit of its own coil when operated. The relay will then latch and store the state to which it was driven. With this capability, relays were used to create special purpose computers for telephone switches in the 1930s. Starting in the 1950s, these designs were converted to wire spring relays, making them faster and more reliable. The majority of wire spring relays were used in 5XB switches.

Most wire spring relays have a permalloy core, and require continuous power to maintain state. Some have a steel core, making them magnetically latching relays, similar to the ferreed and remreed types of reed relay.

Reed relays are smaller and cheaper, thus better suited to data storage. They were used in conjunction with wire spring relays, for example to store digits for sending to other crossbar switching offices. In a multi-frequency sender (the part of a switch which sends routing information about outgoing calls over trunk lines), for example, wire spring relays direct the dialed digits one at a time from reed relay packs to frequency generators, under sequential control of logic implemented with wire spring relays. At the other end, similar relays steered the incoming digits from the tone decoder to a reed relay memory. In such uses, two-out-of-five codes and similar schemes checked for errors at both ends.

Use in carrier systems

Wire spring relays were also incorporated into the signaling channel units of carrier systems.

Related Research Articles

In electrical engineering, a switch is an electrical component that can "make" or "break" an electrical circuit, interrupting the current or diverting it from one conductor to another. The mechanism of a switch removes or restores the conducting path in a circuit when it is operated. It may be operated manually, for example, a light switch or a keyboard button, may be operated by a moving object such as a door, or may be operated by some sensing element for pressure, temperature or flow. A switch will have one or more sets of contacts, which may operate simultaneously, sequentially, or alternately. Switches in high-powered circuits must operate rapidly to prevent destructive arcing, and may include special features to assist in rapidly interrupting a heavy current. Multiple forms of actuators are used for operation by hand or to sense position, level, temperature or flow. Special types are used, for example, for control of machinery, to reverse electric motors, or to sense liquid level. Many specialized forms exist. A common use is control of lighting, where multiple switches may be wired into one circuit to allow convenient control of light fixtures.

In telecommunications, an electronic switching system (ESS) is a telephone switch that uses digital electronics and computerized control to interconnect telephone circuits for the purpose of establishing telephone calls.

The Strowger switch is the first commercially successful electromechanical stepping switch telephone exchange system. It was developed by the Strowger Automatic Telephone Exchange Company founded in 1891 by Almon Brown Strowger. Because of its operational characteristics it is also known as a step-by-step (SXS) switch.

A nonblocking minimal spanning switch is a device that can connect N inputs to N outputs in any combination. The most familiar use of switches of this type is in a telephone exchange. The term "non-blocking" means that if it is not defective, it can always make the connection. The term "minimal" means that it has the fewest possible components, and therefore the minimal expense.

The reed switch is an electrical switch operated by an applied magnetic field. It was invented at Bell Telephone Laboratories in 1936 by Walter B. Ellwood. In its simplest and most common form, it consists of a pair of ferromagnetic flexible metal reeds contacts in a hermetically sealed glass envelope. The contacts are usually normally open, closing when a magnetic field is present, or they may be normally closed and open when a magnetic field is applied. The switch may be actuated by an electromagnetic coil, making a reed relay, or by bringing a permanent magnet near the switch. When the magnetic field is removed, the reeds in the reed switch return to their original position.

In electrical controls, a stepping switch or stepping relay, also known as a uniselector, is an electromechanical device that switches an input signal path to one of several possible output paths, directed by a train of electrical pulses.

An electric bell is a mechanical bell that functions by means of an electromagnet. When an electric current is applied, it produces a repetitive buzzing or clanging sound. Electric bells have been widely used at railroad crossings, in telephones, fire and burglar alarms, as school bells, doorbells, and alarms in industrial plants, since the late 1800s, but they are now being widely replaced with electronic sounders. An electric bell consists of one or more electromagnets, made of a coil of insulated wire around an iron core, which attract a springy iron armature with a clapper. When an electric current flows through the coils, the electromagnet creates a magnetic field which pulls the armature towards it, causing the clapper to strike the bell.

Contactor Electronic circuit element serving as a switch

A contactor is an electrically-controlled switch used for switching an electrical power circuit. A contactor is typically controlled by a circuit which has a much lower power level than the switched circuit, such as a 24-volt coil electromagnet controlling a 230-volt motor switch.

A reed relay is a type of relay that uses an electromagnet to control one or more reed switches. The contacts are of magnetic material and the electromagnet acts directly on them without requiring an armature to move them. Sealed in a long, narrow glass tube, the contacts are protected from corrosion. The glass envelope may contain multiple reed switches or multiple reed switches can be inserted into a single bobbin and actuate simultaneously. Reed switches have been manufactured since the 1930s.

The Number Five Crossbar Switching System is a telephone switch for telephone exchanges designed by Bell Labs and manufactured by Western Electric starting in 1947. It was used in the Bell System principally as a Class 5 telephone switch in the public switched telephone network (PSTN) until the early 1990s, when it was replaced with electronic switching systems. Variants were used as combined Class 4 and Class 5 systems in rural areas, and as a TWX switch.

The Number One Crossbar Switching System, was the primary technology for designing urban telephone exchanges in the Bell System in the mid-20th century. Its switch fabric used the new electromechanical crossbar switch to implement the topology of the panel switching system of the 1920s. The first 1XB system was installed in the PResident-2 office at Troy Avenue in Brooklyn, New York which became operational in February 1938.

The Number One Electronic Switching System (1ESS) was the first large-scale stored program control (SPC) telephone exchange or electronic switching system in the Bell System. It was manufactured by Western Electric and first placed into service in Succasunna, New Jersey, in May 1965. The switching fabric was composed of a reed relay matrix controlled by wire spring relays which in turn were controlled by a central processing unit (CPU).

TXE, was a family of telephone exchanges developed by the British General Post Office (GPO), designed to replace the ageing Strowger systems.

Relay logic is a method of implementing combinational logic in electrical control circuits by using several electrical relays wired in a particular configuration.

A magnetic starter is an electromagnetically operated switch which provides a safe method for starting an electric motor with a large load. Magnetic starters also provide under-voltage and overload protection and an automatic cutoff in the event of a power failure.

The Panel Machine Switching System is an early type of automatic telephone exchange for urban service, introduced in the Bell System in the 1920s. It was developed by Western Electric Laboratories, the forerunner of Bell Labs, in the U.S., in parallel with the Rotary system at International Western Electric in Belgium before World War I. Both systems had many features in common.

References

↑ Arthur C. Keller, A New General Purpose Relay for Telephone Switching Systems , Bell System Technical Journal, v.31(6), 1023 (November 1952)
↑ A.L. Quinlan, Automatic Contact Welding in Wire Spring Relay Manufacture, Bell System Technical Journal, v.33(4), 897 (July 1954)
↑ A. Feiner, The Ferreed , Bell System Technical Journal, v.43(1), 1 (January 1964)

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[1] Arthur C. Keller, A New General Purpose Relay for Telephone Switching Systems , Bell System Technical Journal, v.31(6), 1023 (November 1952)

[2] A.L. Quinlan, Automatic Contact Welding in Wire Spring Relay Manufacture, Bell System Technical Journal, v.33(4), 897 (July 1954)

[3] A. Feiner, The Ferreed , Bell System Technical Journal, v.43(1), 1 (January 1964)