Reed Switch Review

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Reed Switch Review

A Reed switch is an electrical or mechanical switch that is actuated by an applied magnetic field. Origianlly at Bell Telephone Laboratories in 1936 by W. B. Elwood (see Figure A). These switcehs consists of contacts on conductive metal reeds in a hermetically sealed package. The contacts may be configured normally open or closed, and will either close or open when a magnetic field is present.

Figure A
Reed switch diagrams from Ellwood's patent, U.S. Patent 2,264,746

The Reed switches, illustrated below, tend to be more reliable than limit switches, due to their simplified construction. The switches are constructed of flexible ferrous strips (reeds) and are placed near the intended travel of the valve stem or control rod extension.

When using reed switches, the extension used is a permanent magnet. As the magnet approaches the reed switch, the switch shuts. When the magnet moves away, the reed switch opens. This ON/OFF indicator is similar to mechanical limit switches. By using a large number of magnetic reed switches, incremental position can be measured. See Figure B

Failures are normally limited to a reed switch which is stuck open or stuck shut. If a reed switch is stuck shut, the open (closed) indication will be continuously illuminated. If a reed switch is stuck open, the position indication for that switch remains extinguished regardless of valve position.

Figure B

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