CII Mid-Range Relays
AEROSPACE, DEFENSE & MARINE /// HIGH PERFORMANCE RELAYS
Selection and Application Guide
A. Contacts must be rated for the load. Current rating, type of load
(resistive, lamp, motor, inductive, and so forth), impedance range,
voltage rating, DC or AC, frequency, single phase or polyphase,
polyphase load balance, and type of switching or transfer should all
be considered. Each of the following switching and transfer functions
places a different requirement on each of the relay contacts and must
be considered when selecting a relay with the proper contact rating:
(1) On-Off Switching - DC, single phase or polyphase
(2) Motor Reversing (AC or DC)
(3) Transferring load between phases of same source
(4) Transferring load between unsynchronized AC sources
B. Power supply characteristics must be taken into account. Voltage
regulation, variations in frequency, ripples and spikes, as well as
steady state conditions, should be included. If more than one power
supply is involved, not only must each be suitable but interaction
between them also should be investigated.
C. Coil (or coils) should be rated so as to have proper operation under
all anticipated conditions.
D. Consideration of environmental conditions anticipated through-
out the service of life, as well as those expected during storage and
transportation before installing the relays in equipment, is mandatory.
Electrical parameters, environmental factors, mechanical stresses, and
compatibility are among the categories for which the relay must be
reviewed.
E. The circuit in which the relay is used, the interlocking feature
employed, the wiring harness, and the associated components should
all be reviewed for assuring mutual suitability.
F. Relays should be hard wired whenever possible, to avoid the
need for additional contact points associated with the relay plug-in
socket arrangement. (Plug-in types should be considered for quick
turnaround times).
G. To permit “safe” isolation of relay circuit in the OFF condition,
and better eliminate an electrical shock hazard, an electromechanical
switching device should be placed between the positive terminal of the
power source and relay coil.
H. Proper transistor control of the relay coil requires a stable
|reference voltage. This can be done by connecting the plus side of the
coil to the positive side of the power source, the minus side of the relay
coil to the collector of an NPN transistor, the emitter of the transistor to
the grounded side of the power source, and the transistor base to the
control voltage. For example, see MIL-R-28776/1.
I. Any switching device controlling the relay coil circuit must be
capable of withstanding, without damage, the sum of the maximum
coil circuitry voltage and the peak value of transient voltage that results
when the coil circuit is opened; for example, a switch controlling a
relay coil that is supplied with a 28V DC line and subjected to a
transient voltage suppressed to 42V must be capable of withstanding
28V + 42V or a 70V surge without damage.
J. In selecting solid state electronic switching devices to
control relay coil circuits, care must be used in selecting a solid state
device with a leakage current (in the “off state”) that is sufficiently low
to permit the relay to drop out.
K. Control of the relay coil circuit by other than step-function
switching may invalidate published relay performance properties such
as pickup and dropout voltages, pickup, dropout, and bounce times.
This selection and application guide is suggested practices from ARP (Aerospace Recommended
Practice) 4005 Concerning proper performance of relays.
Caution:
The use of any coil voltage less than the rated coil voltage may compromise the operation
of the relay. Choosing the proper relay depends primarily on matching the relay to the load,
power supply, and environment. Selection should be limited to items that meet the following
requirements: