Basic Switch Characteristics
The Basic Reed Switch
A Reed Switch consists of two ferromagnetic blades (generally composed of iron and nickel) hermetically sealed in a glass capsule. The blades overlap internally in the glass capsule with a gap between them, and make contact with each other when in the presence of a suitable magnetic field. The contact area on both blades is plated or sputtered with a very hard metal, usually Rhodium or Ruthenium. These very hard metals give rise to the potential of very long life times if the contacts are not switched with heavy loads. The gas in the capsule usually consists of Nitrogen or some equivalent inert gas. Some Reed Switches, to increase their ability to switch and standoff high voltages, have an internal vacuum. The reed blades act as magnetic flux conductors when exposed to an external magnetic field from either a permanent magnet or an electromagnetic coil. Poles of opposite polarity are created and the contacts close when the magnetic force exceeds the spring force of the reed blades. As the external magnetic field is reduced so that the force between the reeds is less than the restoring force of the reed blades, the contacts open.
The Reed Switch described above is a 1 Form A (normally open (N.O.) or Single Pole Single Throw (SPST)) Reed Switch. Multiple switch usage in a given configuration is described as 2 Form A ( two normally open switches or Double Pole Single Throw (DPST)), 3 Form A (three normally open switches), etc. A normally closed (N.C.) switch is described as a 1 Form B. A switch with a common blade, a normally open blade and a normally closed blade is described as a 1 Form C (single pole double throw (SPDT)).
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