384 Chapter 13
13.2.5.5 Mercury-Wetted Contact Relays
Mercury-wetted contact relays are a form of reed relays
consisting 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 to the reed by capillary
action and wets the contact surface of the moving end of
the reed as well as the contact surfaces of the stationary
contacts. Thus a mercury-to-mercury contact is main-
tained in a closed position. The mercury-wetted relay is
usually actuated by a coil around the capsule.
Aside from being extremely fast in operation and
having relatively good load-carrying capacity,
mercury-wetted contact relays have extremely long life
since the mercury films are reestablished at each contact
closure and contact erosion is eliminated. Since the
films are “stretchable,” there is no contact bounce.
Contact interface resistance is extremely low.
Three disadvantages of this type of reed relays are:
- The freezing point of mercury is (–38.8°C or
–37.8°F). - They have poor resistance to shock and vibration.
- Some type need to mount in a near vertical position.
These relays are available in a compact form for
printed-circuit board mounting. Multipole versions can
be provided by putting additional capsules inside the
coil. They are used for a great variety of switching
applications such as are found in computers, business
machines, machine tool control systems, and laboratory
instruments.
Mercury-wetted switches can also come as a
nonposition sensitive, mercury-wetted, reed relay that
combines the desirable features of both dry reed and
mercury-wetted capsules. This allows the user to place
the reed relay in any position and is capable of with-
standing shock and vibration limits usually associated
with dry reed capsules. On the other hand, they retain
the principal advantages of other mercury-wetted
switches—no contact bounce and low stable contact
resistance.
Operation of the nonposition-sensitive switch is
made possible by the elimination of the pool of mercury
at the bottom of the capsule. Its design captures and
retains the mercury on contact and blade surfaces only.
Due to the limited amount of mercury film, this switch
should be restricted for use at low-level loads.
Mercury-wetted reed relays are a distinct segment of
the reed relay family. They are different from the dry
reed relays in the fact that contact between switch
elements is made via a thin film of mercury. Thus, the
most important special characteristics of
mercury-wetted relays are:- Contact resistance is essentially constant from opera-
tion to operation throughout life. - Contacts do not exhibit bounce. The amount of
mercury at the contacts is great enough to both
cushion the impact of the underlying members and to
electrically bridge any mechanical bounce that
remains. - Life is measured in billions of operations, due to
constant contact surface renewal. - Contacts are versatile. The same contacts, properly
applied, can handle relatively high-power and
low-level signals. - Electrical parameters are constant. With contact wear
eliminated, operating characteristics remain the same
through billions of operations.
To preserve these characteristics, the rate of change
of voltage across the contacts as they open must be
limited to preclude damage to the contact surface under
the mercury. For this reason, suppression should be
specified for all but low-level applications.Mounting Position. To ensure that distribution of
mercury to the relay contacts is proper, position sensi-
tive types should be mounted with switches oriented
vertically. It is generally agreed that deviation from
vertical by as much as 30° will have some effect on
performance. The nonposition-sensitive mercury-
wetted relay, which is the most common type today, is
not affected by these limitations.Bounce. Mercury-wetted relays do not bounce if oper-
ated within appropriate limits. However, if drive rates
are increased, resonant effects in the switch may cause
rebound to exceed the level that can be bridged by the
mercury, and electrical bounce will result. Altered
distribution of mercury to the contacts, caused by the
high rate of operation, may also contribute to this effect.Contact Resistance. Mercury-wetted relays have a
terminal-to-terminal contact resistance that is some-
what lower than dry reed relays. Typical specification
limit for maximum contact resistance is 0.150:.13.2.5.6 RF RelaysRF relays are used in high-frequency applications,
usually in a 50: circuit. The RF coaxial shielded relay
in Fig. 13-17 can switch up to 200 Vdc at 0.5 A.