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#7040748 - Revision A - September, 2016
Electronic Control System
MODEL UW-24
MODEL UW-24
Control Compressor, Condenser Fan, Dual Refrigerant Valve and Evaporator Fan Motors
The control senses evaporator and compartment temperatures via thermistors; one on each evaporator, and one in
each compartment. High Offset (Cut-in) is governed by the evaporator temperatures; Low Offset (Cut-out) is gov-
erned by the compartment/zone temperatures. High Offset and Low Offset are based on the set-point.
If either evaporator temperature is at high offset (calling for cooling), power is supplied to the compressor and con-
denser fan motor, cycling them ON (See Figure 2-16).
Two relays on the control board, which are in series with one another, are used to control the dual refrigerant valve.
The first “pulse” relay controls the length of time the solenoid is energized (500ms / 30 pulses per 1/2 second). The
second “sine” relay controls the negative polarity, or positive polarity of the pulse supplied to the valve’s solenoid
(determined by which evaporator is calling for cooling, ( + ) = Upper Evaporator, ( - ) = Lower Evaporator). When an
evaporator calls for cooling, the bead inside the T-connection of the refrigerant valve is forced to one side or the
other, depending on the polarity of the pulse, supplying refrigerant to the appropriate evaporator (See Figure 2-16).
When the door is closed, the door switch is opened, the control board sense that the door switch is open and turns
off the lighting system (if display lighting is not activated). Depending on which evaporator is calling for cooling, the
corresponding evaporator fan is energized (See Figure 2-16).
NOTE
: When
in Sabbath
Mode, the
evaporator
thermistors still
control cut-in,
except there is
a random 15 to
25 second
delay before
power is sup-
plied to cooling
components.
Figure 2-16. Cooling Signal Trace (Upper Evaporator at High-Offset Temperature, Calling for Cooling)
Ω
2. Temperature
above low
offset
3. Energized
5. Energized
1. High offset tem-
perature detected,
calling for cooling
4. AC pulse,
(+ polarity)
supplied to
solenoid,
valve
opens sup-
plying
refrigerant
to upper
evaporator
