Tips on Applying Relays
15.4
While most causes of problems in relay applications occur in the coil or
contact systems, a few other more general precautions are in order.
15.4.1
Almost all AC operated relays will exhibit some audible noise or hum. The
acceptable level of hum in a given application should be discussed by the user
and manufacturer.
15.4.2
If dust-covered relays are used in varying temperatures, (day and night) and
high relative humidity, condensation may occur. This can result in a decrease
in insulation resistance and possible a short circuit. Holes in the case to
allow the relay to breathe or a hermetically-sealed relay may be needed.
15.4.3
In cleaning relays on a printed circuit board, some fluids are very corrosive
to copper and other types can attack the adhesive used on the coil tape. The
user should give the manufacturer complete details on the cleaning operation,
so a proper relay can be furnished.
15.4.4
Avoid mounting relays (particularly commercial-type relays) on a large chassis
printed circuit boards. The operation of a solenoid or contactor on the same
mounting surface may cause false operation of the relay contacts due to
transmitted shock or vibration.
15.4.5
Be aware that if the DC coils of a large and a small relay are paralleled, the
small relay may make a false operation when the coil circuit is opened. What
occurs, is that the field in the large coil collapses more slowly than that of
the small one and the inductive discharge of the large coil will drive current
through the small relay in the reverse direction, causing the small relay to
pick up momentarily. Isolation diodes can prevent this.
15.4.6
Dust-covered relays should always be mounted with the closed end upward. If
they are mounted with the closed end downward, they may fill with water from
condensation or accidental oversprays. This can result in rusting of internal
parts. Hinge binding, a decrease in insulation resistance, and possibly a
short circuit between the contacts or even contact to coil.