REED
RELAYS
APPLICATION DATA
HOW REED RELAYS WORK
MERCURY-WETTED CONTACT RELAYS. ( CONTINUED)
The term reed relay covers dry reed relays and mercury-wetted
contact relays, all of which use hermetically sealed reed switches.
In both types, the reeds (thin, flat blades) serve multiple functions -
as conductor, contacts, springs, and magnetic armatures.
Mercury wetted contacts are fast in operation and have relatively good load
carrying capacity and long life. The mercury films are reestablished at each
contact closure and contact erosion is eliminated.. The mercury films are
stretchable, there is no contact bounce and because it is a mercury contact, the
contact resistance is very low and ideal for low level switching applications.
DRY REED RELAYS
Dry reed relays have become an important factor in the relay field.
They have the advantage of being hermetically sealed and resistant
to atmospheric contamination. They have fast operate and release
times and when operated within their rated contact loads, have very
long life. A typical dry reed switch capsule is shown in Figure 1.
The disadvantages of this type of reed relay are the freezing point of mercury
(-38˚C), poor resistance to shock and vibration and the need to mount the relay in
a near vertical position.
These relays are used for a variety of switching applications such as found in
computers, business machines, machine tool control systems, and laboratory
instruments.
CONTACT COMBINATIONS.
Supporting
Terminal
Supporting
Terminal
Normally
Open
Contacts
Glass
Capsule
The switches used in dry reed relays provide SPST-NO, SPST-NC, SPDT contact
combinations.
The SPST-NO corresponds with the basic switch capsule design (Fig.1).
The SPST-NC results from a combination of the SPST-NO switch and
a permanent magnet strong enough to pull the contacts closed but able to open
when coil voltage is applied to the relay coil.
In typical true SPDT designs, the armature is mechanically tensioned
against the normally closed contact, and is moved to the normally open contact
upon application of a magnetic field. The SPDT contact combination can also be
achieved by joining a SPST-NO switch with an appropriately adjusted SPST-NC
switch, and jumping one side of both switches together to form the movable contact
system.
Latching contacts, defined as contacts which remain in the position to
which they were driven, and staying in that position when coil power is removed
from the relay coil.
Latching switches are manufactured by using a SPST-NO contact, and biasing it
with a permanent magnetic that is strong enough to hold the contacts closed,
but not strong enough to hold the contact closed when coil power is applied to the
coil. The switching process is than reversed by simply reversing the relay coil
polarity to close the switch.
Figure 1. Construction of Switch Capsule of Typical Dry Reed switch (SPST-NO)
In the basic SPST-NO design, two opposing blades are sealed into a
narrow glass capsule and overlapped at their free ends. The contact
area is plated typically with rhodium to produce a low contact resistance
when contacts are drawn together. The capsule is made of glass and
filled with a dry inert gas and then sealed. The capsule is surrounded by
an electromagnetic coil. When the coil is energized, the normally open
contacts are brought together; when the coil voltage is removed, the
blades separate by their own spring tension. Some reeds contain
permanent magnets for magnetic biasing to achieve normally closed
contacts (SPST-NC) or SPDT contact combinations. The current rating,
which is dependent upon the size of the blade and the type and amount
of plating, may range from low level to 1 amp. Effective contact protection
is essential when switching loads other then dry resistive loads.
MERCURY-WETTED CONTACT RELAYS.
Mercury wetted contacts consist of a glass-encapsulated reed with its
base immersed in a pool of mercury and the other end capable of moving
between one or two stationary contacts. The mercury flows up the reed by
capillary action and wets the contact surfaces of the moving end of the
reed as well as the contact surfaces of the stationary contacts. A mercury
to mercury contact is maintained in the closed position. The capsule is
surrounded by an electromagnetic coil and operates in the same manner
MAGNETIC FIELDS
Reed relays in general can be characterized as susceptible to the influences of
external magnetic fields. It is important to keep reed relays at a proper distance
from each other because of the possibility of magnetic-interaction between them.
Proper magnetic shielding must be used to contain stray magnetic fields. When
installing reed relays into equipment, one should be aware of the devices within
that equipment which can produce magnetic fields. The relays being installed into
that equipment should be positioned as far away as possible from any stray
magnetic fields and should be shielded to prevent false operations.
as a dry reed.
Supporting
Leadwire
ELECTRICAL CHARACTERISTICS
UP
SENSITIVITY: The input power required to operate dry reed relays is determined
by the sensitivity of the particular reed switch used, by the number of switches
operated by the coil, by the permanent magnet biasing (if used), and the efficiency
of the coil and the effectiveness of its coupling to the blades. Minimum input
required to effect closure ranges from the very low milliwatt level for a single
sensitive capsule to several watts for multipole relays.
OPERATE TIME: The coil time constant, overdrive on the coil, and the character-
istics of the reed switch determine operate time. With the maximum overdrive
voltage applied to the coil, reed relays will operate in approximately the 200
microsecond range. When driven at rated coil voltage, usually the relays will
operate at about one millisecond.
Movable
contact blade
Glass
Capsule
OPERATING
POSITION
Mercury
pool
RELEASE TIME: With the coil unsuppressed, dry reed switch contacts release
in a fraction of a millisecond. SPST-NO contacts will open in as little as 50
microseconds. Magnetically biased SPST-NC and SPDT switches reclose from
100 microseconds to 1 millisecond respectively.
Figure 2. Miniature Mercury-wetted contact switch (SPDT)
PAGE 68
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