R-952M - Sharp Home Appliance Service Manual (repair manual). Page 5

R-952M  -  5

Damper position is set automatically by damper motor,
damper switch, damper cam and damper shaft.
These components are operated by a signal that
judges if microwave cooking or other cooking opera-
tion is selected by the CPU unit.

Damper is in the open position, because a portion of
cooling air is channeled through the cavity to remove
steam and vapors given off from the heating foods.
It is then exhausted at the top of the oven cavity into a
condensation compartment.

Damper is in the closed position, so that no hot air will
be allowed to leak out the oven cavity.

1. When power supply cord is plugged in:
1-1. When power supply cord is plugged in, a signal is

sensed in the control unit, and operates shut-off
relay (RY6).

1-2. Contacts of shut-off relay (RY6) close, the damper

motor is energized, opening the damper door.

1-3. When the damper is moved to the open position

by the damper cam, damper switch is closed (ON
position).

1-4. The signal of damper switch is re-sensed in the

control unit and shut-off relay (RY6) is turned off.

1-5. The A.C. line voltage A.C. to the damper motor is

stopped and the motor turns off.

2. When oven is microwave cooking:

Damper is in the open position

3. When oven is convection cooking:
3-1 Damper motor is energized right after the oven is

started.

3-2. When damper is in the closed position (damper

switch is OFF), its signal is sensed by the control
unit, and shut-off relay (RY7) is de-energized.

The door can be opened by pushing the open button on
the control panel.  When the open button is pushed, the
open lever pushes lower latch head on the door
upward.  The upper latch head is linked with the lower
latch head, so now, the door can be opened.

3-3. The damper is held in the closed position during the

cooking operation

3-4. At the end of any cooking, when the fan motor stops,

the relay (RY6) is energized and the damper is
returned to the open position.

NOTE: If the damper door is not in the proper position,

closed durining convection, grill or dual, or open
durning microwave, the control unit will stop oven
operation after 1 minute.

4. Operation of damper is shown below;

Damper operation

Cooking condition

Operation of Damper

Microwave

Convection

Grill; during backed up with con

vection heater

Grill; after convection heater

back- up has stopped.

Dual (Microwave and Convec

tion) when oven tempera
ture above 220˚C is pro
grammed.

When oven  temperature be-
low 200˚C is programmed.

Dual (Microwave and Grill)

Fire sensing condition

Open

Close

Close

Open

Close but open after 20 min-
utes.

Close

Open

Close

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 (ther-
mistor) during it's 32-seconds time base comparing the
obtained voltage measurements. If the most recent  volt-
age measured is 300mV grater than the previous voltage
measured, the LSI judges it as a fire in the oven cavity and
switches off the relays to the high voltage transformer, fan
motor and convection motor. The LSI also stops counting
down and closes the damper door so that no fresh air will
enter the oven cavity. Please refer to the following section
for a more detailed description.

Please refer to the timing diagram on page 4.

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 300mV the LSI makes the
judgment that there is a fire in the oven cavity and will
switch off the relays to the high voltage transformer, fan
motor and convection motor. The LSI also stops count-
ing down and closes the damper door so that no fresh
air will enter the oven cavity.

Upper latch head

Lower latch 
head

Door

SW3 Monitor switch

SW1 Monitored
. latch switch

SW2 Stop switch

Open lever