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 W. B. Elwood. It consists of a pair of contacts on ferrous 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 reed relay is a type of relay, in which a reed switch is mounted inside a coil. Rudolf F. Graf, "reed relay" Dictionary of Electronics; Radio Shack, 1974-75. Fort Worth, Texas. .

Description

The conventional form of the dry reed switch includes aligned members of magnetizable and electrically conductive metal reeds which have end portions separated by a small gap when the switch is open. The reeds (or two magnetizable and electrically conductive members) are hermetically sealed in opposite ends of a tubular glass envelope.

A magnetic field (such as from an energized coil around the glass tube or a permanent magnet moved towards it) will cause the contacts to pull together, thus completing an electrical circuit Rudolf F. Graf, "reed switch" Dictionary of Electronics; Radio Shack, 1974-75. Fort Worth, Texas. The stiffness of the reeds causes them to separate, and open the circuit, upon removal of the magnetic field. A more complicated configuration contains a nonferrous normally closed contact that opens when the ferrous normally open contact closes. Good electrical contact is assured by plating a thin layer of precious metal over the flat contact portions of the reeds.

Since the contacts of the reed switch are sealed away from the atmosphere, they are protected against atmospheric corrosion. The hermetic sealing of a reed switch make them suitable for use in explosive atmospheres where tiny sparks from conventional switches would constitute a hazard. Reed switches are widely used for electrical circuit control, particularly in the communications field. The leads of the switch must be handled carefully to prevent breaking the glass envelope. There are also versions of reed switches with mercury "wetted" contacts.

In production, a metal reed is inserted in each end of a glass tube and the end of the tube heated so that it seals around a shank portion on the reed. Infrared-absorbing glass is used, so an infrared heat source can concentrate the heat in the small sealing zone of the glass tube. The thermal coefficient of expansion of the glass material and metal parts must be similar to prevent breaking the glass-to-metal seal. The glass used must have a high electrical resistance and must not contain volatile components such as lead oxide and flourides.

Reed switches are commonly used in mechanical systems as proximity switches as well as in door and window sensors in burglar alarm systems and tamperproofing methods; however they can be disabled if they are in a strong, external magnetic field. Reed switches were formerly used in the keyboards for computer terminals, where each key had a magnet and a reed switch actuated by depressing the key. The leads of a reed switch must be handled carefully to prevent breaking the glass envelope.

Reed relays

A reed switch combined with an electromagnet is an electrical relay called a reed relay. In a reed relay, the electromagnet consists of a coil with the reed switch inside the coil. Reed relays are used when high operating speed is required, or where very low-level signals must be switched. The inert atmosphere around the reed contacts ensures that oxidation will not affect the contact resistance. Occasionally mercury-wetted reed relays are used, especially in high-speed counting circuits; such relays must be mounted in a particular vertical orientation otherwise the mercury may bypass the contacts.

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Description

Reed relays

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