R-209 - Sharp Home Appliance Service Manual (repair manual). Page 6

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1. When the timer knob is at "0", the oven is OFF condi-

tion.

2. Closing the oven door activates the monitored latch

switch and the latch switch.

IMPORTANT
When the oven door is closed, the contacts (COM-NC) of
the monitor switch  must be open.
When the microwave oven is plugged in a wall outlet, rated
voltage is supplied to the noise filter and the control unit.

HIGH COOKING
Set the cooking control to HIGH and then set the timer.

1. Following components are energized.

High voltage rectifier

High voltage transformer

High voltage capacitor

High voltage fuse

Turntable motor

Timer motor

Fan motor

Magnetron

Oven lamp

Control unit

CONNECTED COMPONENTS

RELAY

High voltage transformer

RY1

2. Rated 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.

3. 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..

4. 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.

5. When the cooking time is up, the timer returns to "0",

the bell rings and the contacts of the timer switch are
opened. The following components are turned off.

High voltage rectifier

High voltage transformer

High voltage capacitor

High voltage fuse

Turntable motor

Timer motor

Fan motor

Magnetron

Oven lamp

Control unit

6. When the oven door is opened during a cook cycle, the

switches come to the following position and they are
common to the other cooking conditions too.

          CONDITION
DURING

DOOR OPEN

SWITCH

CONTACT

COOKING

(NO COOKING)

Monitored latch switch COM-NO

Closed

  Open

Latch switch

COM-NO

Closed

  Open

Monitor switch

COM-NC

Open

  Closed

The circuits to the high voltage transformer , turntable
motor , timer motor , fan motor , oven lamp  and control

unit are cut off when the contacts (COM-NO) of the
monitored latch switch  and latch switch  are made
open.
The timer stops to indicate how much cooking time
remains.

7. MONITOR SWITCH CIRCUIT

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

7-1 When the oven door is opened during or after the

cycle of a cooking program, the monitored latch
switch  and latch switch  must open their contacts
(COM-NO) first. After that the contacts (COM-NC) of
the monitor switch  can be closed.

7-2. When the oven door is closed, the contacts (COM-

NC) of the monitor switch  must be opened first. And
then the contacts (COM-NO) of the monitored latch
switch  and the latch switch  are closed.

7-3. When the oven door is opened and the contacts

(COM-NO) of the monitored latch switch  and con-
tacts of the timer switch remain closed. The fuse
T6.3A will blow, because the monitor switch is closed
and a short circuit is caused.

When the microwave oven is preset for variable cooking
power, rated voltage is supplied to the high voltage trans-
former intermittently within a 32-second time base through
the relay contact which is coupled with the current-limiting
relay. 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.

1. If the oven is set for more than 20 minutes at HIGH,

after the first 20 minutes the power level will automati-
cally adjust itself to 70% power to avoid overcooking.

2. If the oven is operated for more than 90 minutes

continuously (at any power levels), the relay RY1
opens. And the circuit to the high voltage transformer
will be cut off.

32 sec. ON

Approx. 70%

100%

HIGH

MED HIGH

MED

MED LOW

LOW

24 sec. ON

8 sec. OFF

14 sec. OFF

10 sec. OFF

26 sec. OFF

Approx. 50%

18 sec. ON

Approx. 30%

12 sec. ON

Approx. 10%

6 sec. ON