A reed relay is a type of electrical switch that uses a magnetic field to activate a set of contacts. These contacts, which are made of thin metal reeds, are sealed inside a glass or plastic enclosure. When an electrical current is applied to a coil surrounding the reeds, a magnetic field is created which attracts the reeds together, closing the contacts and allowing electrical current to flow. This article will explore the reed relay working principle, including its components.
How Does a Reed Relay Work?
The reed relay, like the armature relay, uses an electromagnetic coil. The contacts are thin reeds of magnetic material and are usually located inside the coil. When there is no coil current, the reeds are in the open position. When there is current through the coil, the reeds make contact because they are magnetized and attract each other.
Reed relays are simple, fast, and highly reliable relays that are one or more reed switches controlled by an external magnet.
The switch is made from two thin, flat strips of ferromagnetic material called reeds, with contacts on the overlapping ends. Leads are connected to the outside ends of the reeds and the entire assembly is sealed in a hermetic glass tube. The tube holds the leads in place (with a small gap between the contacts for normally open switches).
Normally closed switches, less common than normally open, are made in one of two ways. The first method is to make the switch so that the contacts are touching each other. The second method uses a small permanent magnet to hold normally open contacts together. The field from the coil opposes the field of the magnet, allowing the contacts to open.
To reduce the oxidation of contacts greatly (or eliminate it), the environment inside the glass tube is often pressurized with a gas that contains no oxygen. A good general-purpose gas has about 97% nitrogen and 3% hydrogen. For high-voltage switching (above 500V), the glass tube is evacuated. Although more costly, this provides the highest standoff voltage switch.
To lower the characteristic capacitance and leakage currents of reed relays (and, less commonly, electromechanical relays), an electrostatic shield is sometimes added between the switches and the coil. The shield is typically metal foil that is terminated to a pin connected to some low impedance point in the circuit. Common low impedance connection points are signal low and guard.
An electromagnetic shield can also be added around the outside of the coil. This type of shield keeps the field generated by the energized coil from interfering with other components. It also protects the switch so that random state changes do not occur due to external EMI. (Electromagnetic shields are not placed between the coil and switch since they “protect” the switch and prevent operation.) This type of shield is connected in the same way as an electrostatic shield.
The high permeability of the ferromagnetic reeds in a reed relay causes a pronounced skin effect with increasing signal frequency. Skin effect is current conduction along only the outer surface of a conductor at high frequencies. This causes a dramatic increase in resistance and a decrease in inductance. Signal frequencies carried by reed relays are usually limited to 10MHz or less due to this effect.
Reed relays are rated for lower currents than armature relays and are susceptible to shock and vibration and can “bounce” when contacts are closed. Applications include pulse counting, position sensors, alarm systems, and overload protection.
History of reed relay
The original idea of such a function mix, which was the invention of the reed switch, was proposed in 1922 by a professor from Leningrad Electrotechnical University, V. Kovalenkov, who lectured on ‘‘magnetic circuits’’ from 1920 until 1930. Kovalenkov received a U.S.S.R. inventor’s certificate registered under No. 466.
In 1936, the American company ‘‘Bell Telephone Laboratories’’ launched research work on reed switches. Already in 1938, an experimental model of a reed switch was used to switch the central coaxial cable conductor in a high-frequency telecommunication system, and in 1940 the first production lot of these devices called ‘‘Reed Switches,’’ was released.
At the end of the 1950s, some Western countries launched the construction of quasi-electronic exchanges with a speech channel (which occupied over 50% of the entire equipment of an exchange) based on reed switches and control circuits on semiconductors. In 1963, the Bell Company created the first quasi-electronic exchange of ESS-1 type designed for an intercity exchange. In a speech channel of such an exchange, more than 690,000 reed switches were used. In the ensuing years, the Western Electric Company arranged a lot of production of telephone exchanges based on reed switches with a capacity from 10 up to 65,000 numbers. By 1977, about 1,000 electronic exchanges of this type had been put into operation in the U.S.A.
In, Japan the first exchange of ESS-type was put into service in 1971. By 1977, the number of such exchanges in Japan was estimated in the hundreds. In 1956, Hamlin Co. launched a lot of production of reed switches and soon became the major producer and provider of reed switches for many relay firms. Within a few years, this company-built plants producing reed switches and relays based on them in France, Hong Kong, Taiwan, and South Korea. Under its licensed plants in Great Britain and Germany, it also started to produce reed switches in those countries. By 1977, Hamlin produced about 25 million reed switches, which was more than half of all its production in the U.S.A. Reed switches produced by this firm were widely used in space-qualified hardware, including man’s first flight to the Moon (the Apollo program). The cost of each reed switch thoroughly selected and checked for this purpose reached $200 apiece.
In the former Soviet Union lot production of reed switches was launched in 1966 by the Ryazan Ceramic–Metal Plant (RCMP). Plants of the former Ministry of Telecommunication Industry (its 9th Central Directorate in particular) were also involved in the production of weak-current relays based on reed switches. At the end of the 1980s, there were 60 types of reed relays produced in the U.S.S.R. The total amount of such relays reached 60 to 70 million a year. Economic crises in Russia led to a steep decline in the production of both reed switches and reed relays. In 2001, plants producing relays (those which were still working in Russia) ordered only about 0.4 million reed switches for relay production.
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