Thermal Fuses; Low Ambient Heater - Fisher & Paykel 790 Active Smart Service Manual

6.1.7

Thermal Fuses

There are two thermal fuses mounted in the wiring harness of the defrost element, having a tripping
0
temperature of 72 C (161
heater assembly.
These fuses in both leads of the element protect the refrigerator from any over heating through failure of the
element itself or a triac failure in the power/control module. Both sides are protected in case phase and
neutral are reversed.
NOTE: Care should be taken if manually defrosting the evaporator (i.e.. using heat guns), to ensure that the
thermal fuses are not over heated.
6.1.8

Low Ambient Heater

In low ambient temperatures a 12 Volt, 7 Watt low power heater is used to keep the temperature in the
provision compartment above freezing. The ambient heater is controlled by the power/control module which
runs the heater at 58% duty cycle to give 4.1 watts of heat. This is achieved by the use of pulse width
modulation (PWM). The heater is situated in the air duct of the "T" models and in the divider partition on "B"
models. The purpose of the element is to warm the area if the ambient becomes too low. The element is on
when the cabinet cycles off. The low ambient heater operates during both the compressor on and off cycles
when the percentage of compressor run time averaged over the previous four cycles drops below 65%. It
switches off when the percentage run time increases to above 70%. The heater will always be switched off
during the defrost cycle and whenever the PC door is open. There may be less than 4 cycles in the
calculation if a defrost has occurred or there were long cycle times.
0
F). Once open circuit they can not be reset. Replacement is part of the element
"T" MODEL
Diagram 6.1.8
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