R-252M (serv.man2) - Sharp Home Appliance Service Manual (repair manual). Page 5

R-252M   5

Closing the door activates all door interlock switches
(monitored latch switch and stop switch).

IMPORTANT
When the oven door is closed, the monitor switch contacts
(COM-NC) must be open.
When the microwave oven is plugged in a wall outlet (230
- 240V 50Hz), the noise filter and the control panel are
energized.

100P (HIGH) COOKING
When the door is closed, the contacts (COM-NO) of the
monitored latch switch and the monitor switch are closed.
When the microwave power is set to 100P position by
pressing the power level pad and cooking time is entered
by touching the time pads, the following components are
energized: H.V. transformer, Magnetron, Oven lamp, Turn-
table motor and Fan motor.

CONNECTED COMPONENTS

RELAY

Oven lamp, Fan motor, Turntable motor

RY1

High voltage transformer

RY3

1. The line voltage is supplied to the primary winding of

the 

high voltage

 transformer. The voltage is converted

to about 3.3 volts A.C. output on the filament winding
and high voltage of approximately 2000 volts A.C. on
the secondary winding.

2. The filament winding voltage (3.3 volts) heats the

magnetron filament and the high voltage (2000 volts) is
sent to the voltage doubling circuit, where it is doubled
to negative voltage of approximately 4000 volts D.C..

3. The 2450 MHz microwave energy produced in the

magnetron generates a wave length of 12.24 cm. This
energy is channelled through the waveguide (transport
channel) into the oven cavity, where the food is placed
to be cooked.

4. When the cooking time is up, a signal tone is heard and

the relays RY1+RY3 open. The circuits to the oven
lamp, 

high voltage

 transformer, fan motor and turntable

motor are cut off.

5. When the door is opened during a cook cycle, the

switches come to the following condition.

          CONDITION

DURING

DOOR OPEN

SWITCH

CONTACT

COOKING

(NO COOKING)

Monitored switch

COM-NO

Closed

  Open

COM-NO

Closed

  Open

Monitor switch

COM-NC

Open

  Closed

Stop switch

COM-NO

Closed

  Open

The circuits to the 

high voltage

 transformer, fan motor

turntable motor, and the oven lamp are cut off when the
monitored latch switch and stop switch are made open.

6. MONITOR SWITCH CIRCUIT

The monitor switch is mechanically controlled by oven
door, and monitors the operation of the monitored latch
switch.

6-1 When the oven door is opened during or after the

cycle of a cooking program, the monitored latch
switch and stop switch  must open their contacts first.
After that the contacts (COM-NC) of the monitor
switch can be closed.

6-2. When the oven door is closed, the contacts (COM-

NC) of the monitor switch (SW2) must be opened first.
After that the contacts (COM-NO) of the monitored
latch switch (SW1) and stop switch (SW3) are closed.

6-3. When the oven door is opened and the contacts

(COM-NO) of the monitored latch switches (SW1)
remain closed, the fuse will blow, because the con-
tacts (COM-NC) of the monitor switch are closed and
a short circuit is caused.

When the microwave oven is preset for variable cooking
power, the line voltage is supplied to the high voltage
transformer intermittently within a 32-seconds time base
through the relay contact which is coupled with the cur-
rent-limiting relay RY2. The following levels of microwave
power are given.

SETTING;

NOTE: The ON/OFF time ratio does not exactly corre-

spond to the percentage of microwave power,
because approx. 3 seconds are needed for heat-
ing up the magnetron filament.

32 sec. ON

24 sec. ON

18 sec. ON

12 sec. ON

6 sec. ON

8 sec. OFF

14 sec. OFF

20 sec. OFF

26 sec. OFF

HIGH

MEDIUM HIGH

MEDIUM

MEDIUM LOW

LOW

Approx. 70%

100%

Approx. 50%

Approx. 30%

Approx. 10%

  OPERATION SEQUENCE