Reed relay

Last updated
(from top) Single-pole reed switch, four-pole reed switch and single-pole reed relay. Scale in centimeters. Reedrelay.jpg
(from top) Single-pole reed switch, four-pole reed switch and single-pole reed relay. Scale in centimeters.

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.

Relay electrical switch

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.

Electromagnet Type of magnet in which the magnetic field is produced by the flow of electric current

An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. The magnetic field disappears when the current is turned off. 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 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.

Reed switch

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. It consists of a pair of contacts on ferromagnetic metal reeds in a hermetically sealed glass envelope. The contacts may be normally open, closing when a magnetic field is present, or normally closed and opening 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 reed returns to its original position.


Compared with armature-based relays, reed relays can switch much faster, as the moving parts are small and lightweight, although switch bounce is still present. [1] Also they require less operating power and have lower contact capacitance. Their current handling capacity is limited but, with appropriate contact materials, they are suitable for "dry" switching applications. They are mechanically simple, making for reliability and long life.

Memory device

A few million reed relays were used from the 1930s to the 1960s for memory functions in Bell System electromechanical telephone exchanges [2] . Often a multiple-reed relay was used, with one of the reeds latching the relay, and the other or others performing logic or memory functions. Most reed relays in the crossbar switching systems of the 1940s through the 1970s were packaged in groups of five. Such a "reed pack" was able to store one decimal digit, encoded in a two-out-of-five code (74210 variant) for easy validity checking by wire spring relay logic.

Bell System telephone service provider

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.

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.

Two-out-of-five code

In telecommunication, a two-out-of-five code is an m of n code that provides exactly ten possible combinations, and thus is popular for representing decimal digits using five bits. There are ways to assign weights to each bit such that the set bits sum to the desired value, with an exception for zero.

Such an electrically latching reed relay requires continuous power to maintain state, unlike magnetically latching relays, such as ferreed (ferrite and reed relay) or the later remreed (remanent reed relay).

Crosspoint switch

In the Bell System Stored Program Control exchange systems of the 1970s, reed relays were no longer needed for data storage, but tens of millions of them were packaged in arrays for voice path switching. In the 1ESS switch, the cores were made of a magnetically remanent alloy, so the relay could latch magnetically instead of latching electrically. This "Ferreed" method reduced power consumption and allowed both contacts to be used for voice path. The coils were wired for coincident current selection similar to a magnetic core memory, so operating the contacts for one crosspoint would release the other crosspoints in its row and column.

Remanence or remanent magnetization or residual magnetism is the magnetization left behind in a ferromagnetic material after an external magnetic field is removed. Colloquially, when a magnet is "magnetized" it has remanence. The remanence of magnetic materials provides the magnetic memory in magnetic storage devices, and is used as a source of information on the past Earth's magnetic field in paleomagnetism.

Electromagnetic coil electrical component

An electromagnetic coil is an electrical conductor such as a wire in the shape of a coil, spiral or helix. Electromagnetic coils are used in electrical engineering, in applications where electric currents interact with magnetic fields, in devices such as electric motors, generators, inductors, electromagnets, transformers, and sensor coils. Either an electric current is passed through the wire of the coil to generate a magnetic field, or conversely an external time-varying magnetic field through the interior of the coil generates an EMF (voltage) in the conductor.

Each input of the array had, besides the two talk wires, a P lead for controlling the crosspoints on that level. Two coils on each crosspoint were wired in series with all the others on that level, to the P lead. Each output of the array also had a P lead with two coils on each crosspoint of that output level. The two windings controlled by the same level were unequal, and were wound around opposite ends of the reed, in opposing polarity. When a pulse passed through the crosspoints of a level, the two ends of each reed were magnetized north to north or south to south, thus repelled each other and opened the crosspoint in all except the selected crosspoint.

The selected crosspoint had current passing through both its input P lead and its output P lead, thus through all four windings. On each end of the ferreed, the windings provided by the two different P leads were opposed to each other, and the greater one predominated when both were energized. This being the input P lead at one end of the ferreed, and the output P lead at the other end, the two ends of that particular ferreed were magnetized north to south, hence attracted each other and closed the contact. Current was applied by the pulser only to set up the connection. The P leads remained dry and the crosspoint remained closed until such time as another connection was made involving one of the levels. [3]

Because the individual crosspoints were more expensive than those of crossbar switches, while the control circuitry was cheaper, reed arrays usually had fewer crosspoints and were more numerous. This required them to be arranged in more stages. Thus, while a telephone call in a typical crossbar exchange like 5XB passed through four switches, a call in a reed system such as 1ESS typically passed through eight.

In the later 1AESS, the reeds were of remanent magnetic material. This "Remreed" design allowed further reduction in size and power consumption. A "grid" of 1024 2-wire crosspoints, arranged as two stages of eight 8x8 switches, was permanently packaged in a box. Despite the sealed contacts, plating with silver rather than with precious metals resulted in reed arrays being less reliable than crossbar switches. When one crosspoint failed, the grid box was quickly replaced as a unit, and either repaired at a local workbench or shipped to a repair shop.

Stromberg-Carlson made the similar ESC system, whose reeds were called crossreed.

Reed relays were extensively used in the British TXE family of telephone exchanges.

Other uses

Reed arrays passed out of use in the mid-1990s, being unnecessary in digital telephone systems such as DMS-100 and 5ESS switch. Reed relays have continued in their uses outside the telephone industry, such as for automatic test equipment and electronic instrumentation due to their hermetic seal, fast operate time, extended life to 109 operations and highly consistent contact performance. Reed relays have also found numerous applications in RF and microwave switching applications. [4] They are also used in applications which make use of their extremely low leakage current (in the order of femtoamperes) such as photomultiplier detectors and other extremely low current handling circuits. Reed switches can also be manufactured to withstand several kilovolts and are still used as high-voltage relays in place of more costly sulfur hexafluoride or vacuum relays.

See also

Related Research Articles

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.

Ladder logic was originally a written method to document the design and construction of relay racks as used in manufacturing and process control. Each device in the relay rack would be represented by a symbol on the ladder diagram with connections between those devices shown. In addition, other items external to the relay rack such as pumps, heaters, and so forth would also be shown on the ladder diagram.

Nonblocking minimal spanning 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.

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.

The Crossbar Latch is a technology invented by Hewlett-Packard in October 2001, that HP claims could replace transistors in some applications. This would allow the construction of integrated circuits made entirely from memristors, which HP states could be simpler and less expensive to build. HP says that it is possible for memristors to eventually replace transistors in the way that transistors replaced the vacuum tube.

SP-1 was the name of a computerized telephone exchange manufactured by Northern Electric in Canada. It was introduced in 1971

Marker (telecommunications) type of special purpose control system used in electromechanical telephone central office switches

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.

Wire spring relay

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.

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.

A class-4, or tandem, telephone switch is a U.S. telephone company central office telephone exchange used to interconnect local exchange carrier offices for long distance communications in the public switched telephone network.

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.

Number One Electronic Switching System

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.

Telephone exchange telecommunications system used in public switched telephone networks or in large enterprises

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.

A reed receiver or tuned reed receiver (US) was a form of multi-channel signal decoder used for early radio control systems. It uses a simple electromechanical device or 'resonant reed' to demodulate the signal, in effect a receive-only modem. The encoding used is a simple form of frequency shift keying.


  1. The electronics clubs."Relays".
  2. "How do reed relays compare with other switching technologies?". Retrieved 23 October 2017.
  3. Ahamed, Syed V.; Lawrence, Victor B. (2012-12-06). Design and Engineering of Intelligent Communication Systems. Springer Science & Business Media. ISBN   9781461562917.

{Authority control}}