Sharp R-870B Service Manual Download Page 9

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R-870B

Specified cooking

Limited power

Time base

Cooking mode

time (minutes)

output (%)

(seconds)

Microwave
100% Power

Top heaters

Bottom heater

Top browner and

10 (Top)

Bottom browner

10 (Bottom)

or Oven cooking

NOTE:
1. In case of Automatic operations, the limitations of

power output are not carried out.

2. In case that the STOP/CLEAR pad is pressed or the

oven door is opened during cooking, the limitations of
power output are carried out after the total cooking time
beyond the specified cooking time.

3. In case of the two or more same cooking modes are

carried out, the limitations of power output are carried
out after the total cooking time beyond the specified
cooking time.

4. In case of the two or more different cooking modes are

carried out, the specified cooking time is started to
count from the point when the cooking mode is changed.

5. If the cooking mode has the power level display, the

power level is also displayed when the limitations of
power output are carried out.

ABSOLUTE HUMIDITY SENSOR (AH SENSOR)
COOKING CONDITION

In case where the AH sensor is used (SENSOR RICE
MENU, SENSOR REHEAT DISH or SENSOR STEAM
MENU), the foods are cooked in microwave cooking mode
without figuring time, power level or quantity. When the
oven senses enough steam from the food, it relays the
information 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 devel-
oped. 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 experiment 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. ;

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.

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
judgment that there is a fire in the oven cavity and will
switch off the relays to the power transformer, fan

MICROWAVE

AH SENSOR