Priniciples of Electromechanical Relay Operation
3.32 Reed and Mercury Wetted Relays
The magnetic reed switch was originally developed in the early 1940s, however,
there was no commercial demand at the time since high reliability, extremely
fast operation, and long life were not considered as important as they are
today. The combination of lack of knowledge and lack of necessity caused this
new switching concept to remain relatively unnoticed by the engineering world
until the mid-1050s.
As manufacturing techniques improved the advantages of reed switches gained
wide recognition, the reed relay industry grew by leaps and bounds.
Hermetically sealed contaces in a clean atmosphere are unaffected by dust,
corrosion, or oxidation, and design allows no opportunity for sticking,
binding, or wearing of hinged joints. With proper circuitry, magnetic reed
switches can offer a life span in excess of 100 million operations. Although
slower than transistorized logic, reed relays are sufficiently faster than
conventional relays, which allows their use in high speed switching
applications. The reed relay provides package density which approaches that
of its solid state counterpart, but is far greater than is available with
convention relays. When compared to transistorized logic, the necessary
coupling circuitry between the logic and input and output devices is less
complicated and less expensive.
The principle of operation is fairly simple. A basic magnetic reed switch
consists of a pair of low reluctance, ferromagnetic, slender flattened reeds,
hermetically sealed into a glass tube with a controlled atmosphere in
cantilever fashion so that the ends align and overlap-but with a smaller air
gap.
Since the reeds are ferromagnetic, the extreme ends will assume opposite
magnetic polarity when brought into the influence of a magnetic field. When
the magnetic flux density is sufficient, the attraction forces of the opposing
magnetic poles overcomes the reed stiffness causing them to flex toward each
other and make contact. This operation can be repeated millions of times at
extremely high speeds.
Energizing the coil sets up a magnetic field which acts in the same manner as
the permanent magnet in the case above. A form "B" (normally closed) contact
arrangement is made by placing a permanent magnet adjacent to form "A"
(normally open), reed switch, thus closing it. Energizing the coil
surrounding the switch and magnet cancels out the permanent magnet to open the
switch, when the proper coil voltage polarity is applied.
Mercury wetted capsules are similar to dry reed switches in that the
mechanical elements are ferromagnetic reed enclosed in a hermetically sealed
glass envelope. A small amount of mercury is placed in the glass capsule
prior to sealing it. The mercury wets the contact surfaces and provides a new
and clean contact surface at every operation due to the capillary action of
the liquid mercury.
Further information relative to reed relays may be found in Chapters 12 and
13.