R-212M (serv.man2) - Sharp Home Appliance Service Manual (repair manual). Page 5

R-212M 5

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 (SW2) is mechanically controlled
by oven door, and monitors the operation of the moni-
tored latch switch (SW1).

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

cycle of a cooking program, the monitored latch
switch (SW1) and stop switch (SW3) must open their
contacts first. After that the contacts (COM-NC) of the
monitor switch (SW2) 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 F1 will blow, because the
contacts (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 27-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.

27 sec. ON

Approx. 70%

100%

HIGH

MEDIUM HIGH

MEDIUM

MEDIUM LOW

LOW

21 sec. ON

6 sec. OFF

12 sec. OFF

18 sec. OFF

22 sec. OFF

Approx. 50%

15 sec. ON

Approx. 30%

9 sec. ON

Approx. 10%

5 sec. ON

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

IMPORTANT
When the timer knob is at "0" the oven is in the OFF
condition.
When the oven door is closed, the monitor switch contacts
(COM-NC) are open.
When the microwave oven is plugged in a wall outlet (230
- 240V 50Hz), the noise filter and the control panel are
energized.

HIGH COOKING
When the door is closed, the contacts (COM-NO) of the
monitored latch switch (SW1) and the monitor switch
(SW2) are closed. When the microwave power level is set
to 800W by pressing the microwave power level button
and the cooking time is selected by turning the timer knob,
the following components are energized: H.V. transformer,
Magnetron, Oven lamp, Turntable motor and Fan motor,
H.V. capacitor, H.V. rectifier.

CONNECTED COMPONENTS

RELAY

Oven lamp, Fan motor, Turntable motor

RY1

High voltage transformer

RY2

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

the 

high voltage

 transformer. This voltage is converted

to approx 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
A.C). 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+RY2 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