Specifications

Ver SG 9/200 43 Sharp Electronics Corporation
CONVECTION CIRCUITS
The R-820BK/BW combines the microwave and convection cooking modes together.
The convection process requires air to be circulated around a heater within an enclosed
oven cavity. When the food load is placed within the confines of the cavity, it will also
be subjected to the circulating hot air. This will cause the food load to be cooked more
evenly, quicker, and retain the foods natural juices better than the conventional radiant
method.
The cavity for a micro/convection oven differs from the straight microwave oven. The
cavity must be insulated to retain the hot air within. A heat chamber is added which
includes the heating element, a thermistor to detect oven temperature, and a convection
fan assembly. The cavity must also have a means to block the vent system, which is used
to vent the oven cavity while in the microwave mode. The means to do this is a damper
door and switch assembly. The damper assembly is not used in the R-820BK/BW.
However, the damper assembly is explained so the technician will become familiar with
it.
When the convection mode is selected, the microcontroller will close relays to the heating
element, convection fan, and oven light. The microcontroller will also monitor the
voltage and/or logic levels on the pins leading to the damper switch and oven thermistor.
If the damper switch does not go to the proper logic level for the mode selected (high for
microwave cooking and low for convection cooking) within a specified time the oven
will shut down. If the oven does not detect a voltage change across the oven thermistor
within a specified time the oven will again shut down. These two features are designed
safety features within the microcontroller. Refer to page 45 for a detailed explanation of
the damper assembly.
As the oven continues to operate in the convection mode, the air temperature increases.
The oven thermistor, which has a negative temperature coefficient, will decrease in
resistance. This will cause the voltage being monitored by the microcontroller to
decrease, cycling the relay to the heating element. As the oven cavity cools the
thermistor will also cool. This will increase the temperature going to the microcontroller
and cycle the relay back on. This cycling will continue throughout the programmed
cooking time.
To check the temperature circuit several factors must be considered. Due to the fact that
the air flow pattern is a recirculating system, the heater assembly must remain fairly
airtight. If cool room air is allowed to enter the air flow system, the oven temperature
will always be low and take a long time to reach preheat conditions. Check for air gaps
in the area around the heater box and also by the damper door assembly.