Specifications

Ver SG 9/200 37 Sharp Electronics Corporation
SENSOR CIRCUITS
Sensors used in microwave ovens are considered peripherals of the control assembly.
They are responsible for sending information back to the microcontroller, which in turn
sets the conditions for the microcontroller to turn power on or off to the select circuits.
The following is a list of sensors used.
Door Sensing and Secondary Interlock Switches:
The secondary interlock switch is mounted in the lower position of the latch hook and the
door sensing switch in the primary interlock system is mounted in the upper position of
the latch hook. They are activated by the latch heads on the door. When the door is
opened, the switches interrupt the power to all high voltage components. A cook cycle
cannot take place until the door is firmly closed thereby activating both interlock
switches. The primary interlock system consists of the door sensing switch and primary
interlock relay located on the control circuit board.
Thermal Cut-Out (Magnetron):
This thermal cut-out protects the magnetron against overheating. If the temperature goes
up higher than 257 degrees F (R-820BK/BW) because the fan motor is interrupted or the
ventilation openings are blocked, the thermal cut-out will open and line voltages to the
high voltage transformer will be cut off and the operation of the magnetron will be
stopped. The thermal cut-out will not resume.
Thermal Cut-Out (Oven):
The thermal cut-out located on the top of the oven cavity is designed to prevent damage
to the oven if the food in the oven catches fire due to over heating produced by improper
setting of the cooking time. Under the normal operation, the oven thermal cut-out
remains closed. However, when abnormally high temperatures are reached within the
oven cavity, the oven thermal cut-out will open at 338 degrees F (R-820BK/BW) causing
the oven to shut down. The thermal cut-out will close in at 311 degrees F (R-
820BK/BW).
Absolute humidity sensor (AH sensor):
This sensor detects humidity or steam generated from the food load. The microcontroller
monitors voltage across two thermistors. One thermistor is in a sealed chamber and the
other is open to air. This sensor is located in the rear exhaust vent and relies on the
venting system of the oven to blow air across it. As humidity builds up on the sensor in
the open air, it will begin to evaporate, which cools the thermistor. When a voltage
difference is developed across the two thermistors, the microcontroller will calculate the
remainder of the cooking time. Refer to pages 39-41 for proper test procedures.