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Instrumentation Table of Contents
Sensors and Transducers Suppiers
Reed Switch Companies
Hermetic Switch, Inc. manufactures reed switches, proximity sensors and provides custom switching and sensing solutions. Our manufacturing organization is known to be very precise and capable of fabricating the most difficult types of switches. www.hermeticswitch.com |
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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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