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Electro-Mechanical Defrost Control
A defrost circuit is used on all models having the
automatic defrost feature. This feature is often
called NO FROST or FROST FREE, which simply
means that frost is conveniently and automatically
removed without any action required by the
consumer. (Frost always accumulates where
refrigeration takes place.)
The electrical components which comprise the
defrost circuit are:
• the Heater (or heaters)
• the Control (often called the “defrost timer")
• the Defrost Thermostat (sometimes called the
“Bi-Metal")
The defrost control used on most models is a
simple electro-mechanical timer. It consists of a
clock motor, attached to an insulated housing that
contains a cam, and a single pole, double throw
switch. The terminals are clustered together at one
end of the control housing.
The purpose of the defrost control is to regulate the
frequency of the defrost cycles and the duration
of each cycle. This is accomplished by energizing
the clock motor to drive a set of gears that, in turn,
rotate the cam which operates the switch. The
defrost cycle begins when the cam has rotated to
the position where a mark on the cam aligns with
a mark on the control housing. At this position,
terminal 4 switch blade drops o the edge of the
cam. This opens the rst set of contacts (terminals
3 and 4) and closes the second set of contacts
(terminals 3 and 2). As the cam continues to rotate,
terminal 3 switch blade will drop o the edge of
the cam. This opens the second set of contacts
(terminals 3 and 2) and closes the rst set of
contacts (terminals 3 and 4) thus, ending the defrost
cycle. An audible "SNAP" can be heard each time a
switch blade drops o the edge of the cam (at the
beginning and ending of the defrost cycle).
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