Sharp R-880B Service Manual page 9

Selected Temp- Top Heaters
erature (˚C) Power (%)
250
230
220
200
180
160
150
130
70
40
4. Upon completion of the cooking time, the audible
signal will sound, and oven lamp, turntable motor,
cooling fan motor and convection motor are de-ener-
gized. At the end of the convection cycle, if the cavity
air temperature is above 120˚C, the circuit to (RY5) will
be maintained (by the thermistor circuit) to continue
operation of the cooling fan motor until the temperature
drops below 104˚C, at which time the relay will be de-
energized, turning off the fan motor. Relay (RY1) will
however, open as soon as the convection cycle has
ended, turning off the convection fan motor.
ABSOLUTE HUMIDITY SENSOR (AH SENSOR)
COOKING CONDITION
In case where the AH sensor is used (REHEAT SENSOR
or SENSOR INSTANT ACTION), the foods are cooked in
microwave cooking mode or grill cooking mode without
figuring time, power level or quantity. When the oven
senses enough steam from the food, it relays the informa-
tion to its microprocessor which will calculate the remaining
cooking time and power level needed for best results.
When the food is cooked, water vapour is developed. The
sensor "senses" the vapour and its resistance increases
gradually. When the resistance reaches the value set
according to the menu, supplementary cooking is started.
The time of supplementary cooking is determined by ex-
periment with each food category and inputted into the LSI.
An example of how sensor works:
1. Potatoes at room temperature. Vapour is emitted very
slowly.
2. Heat potatoes. Moisture and humidity is emitted rap-
idly. You can smell the aroma as it cooks. ;
AH SENSOR
Bottom Heater
Power (%)
40 40
40 40
40 40
30 70
20 70
10 70
10 70
10 60
10 40
10 30
3. Sensor detects moisture and humidity and calculates
cooking time and variable power.
AH SENSOR COOKING SEQUENCE
1. In case the AH sensor cooking condition is started, the
coil of shut-off relays (RY5+RY6) are energized, the
oven lamp and cooling fan motor are turned on, but the
power transformer is not turned on.
NOTE: The oven should not be operated on AH SENSOR
COOKING immediately after plugging in the unit.
Wait t wo minutes before cooking on AH SENSOR
COOKING CONDITION.
2. After about 16 seconds, the cook relay (RY2) is ener-
gized. The power transformer is turned on, microwave
energy is produced and first stage is started. The 16
seconds is the cooling time required to remove any
vapour from the oven cavity and sensor.
(Figure O-2)
NOTE: During this first stage, do not open the door or
touch STOP/CLEAR pad.
There is a menu which the oven operates in grill
cooking mode.
3. When the sensor detects the vapour emitted from the
food, the display switches over to the remaining cooking
time and the timer counts down to zero. At this time, the
door may be opened to stir food, turn it or season, etc.
NOTE: In case where a small quantity of food is cooked,
the oven will stop without displaying the remaining
cooking time. In case of "Reheat Pie" of Sensor
Instant Action, the relay (RY2) is turned off and
power transformer is turned off. And then the
relays (RY3 + RY4) are energized. The top and
bottom heaters are turned on.
4. When the timer reaches zero, an audible signal sounds.
The shut-off relay (RY5+RY6) and cook relay (RY2)
are de-energized and the power transformer, oven
lamp, etc. are turned off.
FIRE SENSING FEATURE (MICROWAVE MODE)
This model incorporates a sensing feature which will stop
the oven's operation if there is a fire in the oven cavity
during microwave cooking. This accomplished by the LSI
repeatedly measures the voltage across the temperature
measurement circuit (thermistor) during it's 32-seconds
time base comparing the obtained voltage measurements.
If the most recent voltage measured is 700mV grater than
the previous voltage measured, the LSI judges it as a fire
in the oven cavity and switches off the relays to the power
transformer, fan motor turntable motor and the oven lamp.
The LSI also stops counting down. Please refer to the
following section for a more detailed description.
Operation
Please refer to the timing diagrams below.
1. The thermistor operates within a 32-seconds time base
and it is energized for three (3) seconds and off for 29
seconds. Two (2) seconds after the thermistor is
energized, the voltage across the temperature meas-
urement circuit is sampled by the LSI and twenty one
(21) seconds after the thermistor is cut off the LSI turns
on the cooling fan for six (6) seconds.
2. The above procedure is repeated. If the difference
between the first voltage measured (in step 1) and
the voltage measured when the procedure is repeated
(step 2) is greater than 700mV the LSI makes the
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R-880B