Heatsinks and Relays 383meet. The capsule, surrounded by an electromagnetic
coil, is made of glass and filled with a dry inert gas.
When the coil is energized in the basic Form A contact
combination, the normally open contacts are brought
together; when the field is removed the reeds separate
by their own spring tension.
Some may contain permanent magnets for magnetic
biasing to achieve normally closed contacts (Form B).
Single-pole, double-throw contact combinations (Form
C) are also available. Current rating, which is dependent
on the size of the reed and the type and amount of
plating, may range from low level to 1 A. Effective
contact protection is essential in most applications
unless switching is done dry.
Relay packages using up to four Form C and six
Form A dry reed switches are common, providing
multiple switching arrangements. The reed relay may be
built for a large variety of operational modes such as
pulse relay, latch relay, crosspoint relay, and logic relay.
These relays may also be supplied with electrostatic or
magnetic shields. The relay in Fig. 13-16 has two Form
C contacts.Reed switches have the following characteristics:- A high degree of reliability stemming from their con-
trolled contact environment. - Consistency of performance resulting from a min-
imum number of parts. - Long operational life.
- Ease of packaging as a relay.
- High-speed operation.
- Small size.
•Low cost.
Number of Switches. There appears to be no limit to
the number of switches that can be actuated by a
common coil. However, as the number increases, coilefficiency decreases and power input increases. This
can lead to a practical limitation. On the other hand, the
increase in power required to operate one more switch
capsule is usually less than the total required if the
assembly were split in two. The single contact relay is
the most frequently used but relays with four or more
switches in a single coil are quite common.Sensitivity. The power input required to operate dry
reed relays is determined by the sensitivity of the partic-
ular reed switch used, by the number of switches oper-
ated by the coil, by the permanent magnet biasing (if
used), and by the efficiency of the coil and the effective-
ness of its coupling to the reeds. The minimum input
required to effect closure ranges from milliwatts for a
single capsule sensitive unit to several watts for a multi-
pole relay.Operate Time. Coil time constant, overdrive, and the
characteristics of the reed switch determine operate
time. With maximum overdrive, reed relays will operate
in approximately 200μs or less. Drive at rated voltage
usually results in a 1 ms operate time.Release Time. With the relay coil unsuppressed, dry
reed switch contacts release in a fraction of a milli-
second. Form A contacts open in as little as 50μs.
Magnetically biased Form B contacts and normally
closed contacts of Form C switches reclose from 100μs
to 1 ms, respectively.
If the relay coil is suppressed, release times are
increased. Diode suppression can delay release for
several milliseconds, depending on coil characteristics,
drive level, and reed release characteristics.Bounce. As with the other hard contact switches, dry
reed contacts bounce on closure. The duration of
bounce is typically quite short and is in part dependent
on drive level. In some of the faster devices, the sum of
operate time and bounce is relatively constant so as
drive is increased, the operate time decreases and
bounce increases.
While normally closed contacts of a Form C switch
bounce more than normally open contacts, magneti-
cally biased Form B contacts exhibit essentially the
same bounce as Form A.Contact Resistance. Because the reeds in a dry reed
switch are made of a magnetic material that has a high
volume resistivity, terminal-to-terminal resistance is
somewhat higher than in some other types of relays.
Typical specification limit for initial maximum resis-
tance of a Form A reed relay is 0.200:.Figure 13-16. Coto Technology 2342 multipole relay,
Courtesy Coto Technology.