A
reed relay is one or more
reed switchThe reed switch is an electrical switch operated by an applied magnetic field. It was invented at Bell Telephone Laboratories in 1936 by W. B. Ellwood. It consists of a pair of contacts on ferrous metal reeds in a hermetically sealed glass envelope...
es controlled by an
electromagnetAn electromagnet is a type of magnet in which the magnetic field is produced by the flow of electric current. The magnetic field disappears when the current ceases.- Introduction :...
. 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
corrosionCorrosion is the disintegration of a material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means a loss of electrons of metals reacting with water and oxygen. Weakening of iron due to oxidation of the iron atoms is a...
, and are usually plated with
silverSilver is a chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
, which has very low resistivity but is prone to corrosion when exposed, rather than corrosion-resistant but more resistive
goldGold is a chemical element with the symbol Au and an atomic number of 79. It has been a highly sought-after precious metal for coinage, jewelry, and other arts since the beginning of recorded history. The metal occurs as nuggets or grains in rocks, in veins and in alluvial deposits. Gold is...
as used in the exposed contacts of high quality relays.
A
reed relay is one or more
reed switchThe reed switch is an electrical switch operated by an applied magnetic field. It was invented at Bell Telephone Laboratories in 1936 by W. B. Ellwood. It consists of a pair of contacts on ferrous metal reeds in a hermetically sealed glass envelope...
es controlled by an
electromagnetAn electromagnet is a type of magnet in which the magnetic field is produced by the flow of electric current. The magnetic field disappears when the current ceases.- Introduction :...
. 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
corrosionCorrosion is the disintegration of a material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means a loss of electrons of metals reacting with water and oxygen. Weakening of iron due to oxidation of the iron atoms is a...
, and are usually plated with
silverSilver is a chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
, which has very low resistivity but is prone to corrosion when exposed, rather than corrosion-resistant but more resistive
goldGold is a chemical element with the symbol Au and an atomic number of 79. It has been a highly sought-after precious metal for coinage, jewelry, and other arts since the beginning of recorded history. The metal occurs as nuggets or grains in rocks, in veins and in alluvial deposits. Gold is...
as used in the exposed contacts of high quality relays. Multiple reed switches can be inserted into a single
bobbinA bobbin is a spindle or cylinder, with or without flanges, on which wire, yarn, thread or film is wound. Bobbins are typically found in sewing machines, cameras, and within electronic equipment....
and actuate simultaneously. Reed switches have been manufactured since the 1930s.
As the moving parts are small and lightweight, reed relays can switch much faster than relays with armatures
http://www.kpsec.freeuk.com/components/relay.htm. 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 SystemThe Bell System was the AT&T monopoly that provided telephone service in the United States from 1877 to 1984 when it was broken up into separate companies by a Federal mandate....
electromechanical
telephone exchangeIn the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
s. 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 switchIn electronics, a crossbar switch is a switch connecting multiple inputs to multiple outputs in a matrix manner....
ing 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 codeIn 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...
(74210 variant) for easy validity checking by
wire spring relayA wire spring relay is a type of relay, primarily manufactured by the Western Electric Company for use by the Bell System in electromechanical telephone exchanges. It was licenced for use around the world, and was commonplace in Japan...
logic.
Crosspoint switch
In the Bell System
Stored Program Control exchangeStored Program Control exchange is the technical name used for telephone exchanges controlled by a computer program stored in the memory of the system. Early exchanges such as Strowger, panel, rotary, and crossbar switches were electromechanical and had no software control...
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 switchThe Number One Electronic Switching System, the first large scale Stored Program Control telephone exchange or Electronic Switching System in the Bell System, was introduced in Succasunna, New Jersey, in May 1965. The switching fabric was composed of reed matrixes controlled by wire spring relays...
, the cores were made of a magnetically
remanentRemanence is the magnetization left behind in a medium after an external magnetic field is removed. It is denoted in equations as . In engineering applications it is often assumed that the magnetization M is synonymous with the residual flux density B hence the remanence is frequently denoted as...
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
coilsAn electromagnetic coil is formed when a conductor is wound around a core or form to create an inductor or electromagnet. One loop of wire is usually referred to as a turn, and a coil consists of one or more turns. For use in an electronic circuit, electrical connection terminals called taps are...
were wired for coincident current selection similar to a
magnetic core memoryMagnetic core memory, also known as Forrester memory or ferrite-core memory, is an early form of random access computer memory. It uses small magnetic ceramic rings, the cores, through which wires are threaded to store information via the polarity of the magnetic field they contain...
, so operating the contacts for one crosspoint would release the other crosspoints in its row and column.
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.
Because the individual crosspoints were more expensive than those of
crossbar switchIn electronics, a crossbar switch is a switch connecting multiple inputs to multiple outputs in a matrix manner....
es, 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 themselves were of remnant 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 whole, and either repaired at a local workbench or shipped to a repair shop.
Stromberg-CarlsonStromberg-Carlson was a telecommunications equipment and electronics manufacturing company formed in 1894 as a partnership of Alfred Stromberg and Androv Carlson. Along with four other companies, it controlled the United States national supply of telephone equipment until after World War...
made the similar ESC system, whose reeds were called
crossreed.
Other uses
Reed arrays passed out of use in the mid-1990s, being unnecessary in true electronic (digital) telephone systems such as
DMS-100The DMS-100 Switch is the biggest seller of a line of Digital Multiplex System telephone exchange switches manufactured by Nortel Networks....
and
5ESS switchThe 5ESS Switch is the Class 5 telephone electronic switching system sold by Alcatel-Lucent. This digital central office telephone circuit switching system is used by many telecommunications service providers.-History:...
. Reed relays have continued in their uses outside the telephone industry such as for
automatic test equipmentAutomatic/automated test equipment is any automated device that is used to quickly test printed circuit boards, integrated circuits, or any other related electronic components or modules....
and
electronic instrumentationElectronic instrumentation refers to measuring instruments used to measure the properties of electrical devices.-See also:*Ammeter*Meter *Moving coil meter*Multimeter*Ohmmeter*Oscilloscope*Spectrum analyzer*Voltmeter...
due to their hermetic seal, fast operate time, extended life to 10E9 operations and highly consistent contact performance. Reed relays have also found numerous applications in RF and
microwaveMicrowaves are electromagnetic waves with wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300MHz and 300 GHz. This is an extremely broad definition including both UHF and EHF , and various sources use different boundaries...
switching applications.
External links
List of over 50 reed relay manufacturers