What is a Reed Switch?
Reed switches are made by sealing overlapping strips of magnetic material (frequently Kovar) in a small glass tube. Kovar is used because it has an expansion coefficient that matches the glass. The strip size and overlap is designed to produce specific switch ratings. Just like any other electrical switch, it can perform any desired electrical switching function. A popular application is in security systems where the field of a permanent magnet holds the switch closed when a window closed. When the window is opened the magnet moves away, and when the switch opens an alarm is set off. The “Permanent Magnet Design and Applications Handbook” by Moskowitz presents reed switch information and sketches in a manner that is easy to read and understand.
The first reed switch came into being in 1936. It was an invention of W. B. Elwood at Bell Telephone Laboratories. The reed switch is a specialized type of switch that operates magnetically. A reed switch has the ability to trigger a siren when motion is detected as part of a burglar alarm. It is also commonly used with electrical circuit controls and is an important device in the communications industry. A reed switch consists of a pair of ferrous metal contacts or three springy metal reeds located inside a sealed glass tube. The two-reed switch normally has open contacts that are closed while in operation, referred to as (NO) contacts. The three-reed switch type, on the other hand, has a pair of (NO) and a pair of normally closed contacts, which are referred to as (NC). The reed switch is enclosed in a small tube, which is controlled by a magnet or magnetic field. It works with the help of an additional magnet, which is placed within close range of the switch. When the magnets make c
The basic reed switch consists of two identical flattened ferromagnetic reeds, sealed in a dry inert-gas atmosphere within a glass capsule, thereby protecting the contact from contamination. The reeds are sealed in the capsule in cantilever form so that their free ends overlap and are separated by a small air gap.
An ordinary switch has two electrical contacts in it that join together when you push a button and spring apart when you release it. Rocker switches on wall lights (like the one in the photo up above) push the two contacts together when the switch is in one position and pull them apart when the switch flicks the other way.
A glass enclosed, magnetically operated contact using reeds as the contacting members. Reed switches and sensors come in three basic types: 1. Single Pole Single Throw (SPST) Form A, Normally Open. In the form A the reed contacts CLOSE on the presence of magnetic field. 2. Single Pole Single Throw (SPST) Form B, Normally Closed. In the form B the reed contacts OPEN on the presence of magnetic field. 3. Single Pole Single Throw (SPDT) Form C, also referred to as a changeover reed switch. The form C has two contacts, one that opens and one that closes in the presence of a magnetic field. HOW DOES A REED SWITCH OPERATE? The reed blades act as magnetic flux conductors when exposed to an external magnetic field from either a permanent magnet or an electromagnetic coil. Contacts in a Form A switch, like shown here, close when the magnetic force exceeds the spring force of the reed blades. As the external magnetic force is reduced, the contacts spring back to the open position. WHAT ADVANTAGE
