R-291KM - Sharp Home Appliance Service Manual (repair manual). Page 7

R-291(K)M

5 – 1

R218L(W)

1. When the timer knob is at “0”, the oven is OFF condition.

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 fuse holder assembly.

                                            Figure O-1 on page 12-1 

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

Function sequence 

                           

1. Following components are energized.

High voltage capacitor   High voltage fuse

Turntable motor         Timer motor         Fan motor

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

volts A.C. on the secondary winding.

ment and the high voltage (2000 volts) is sent to the voltage dou-
bling circuit, where it is doubled to negative voltage of
approximately 4000 volts D.C..

4. The 2450 MHz microwave energy produced in the magnetron gen-

erates 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

High voltage capacitor     High voltage fuse

Turntable motor         Timer motor          Fan motor

Magnetron               Oven lamp

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.

The circuits to the high voltage transformer, turntable motor, timer

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 and the relay
RY1.

1) When the oven door is opened during or after the cycle of a

cooking program, the monitored latch switch and latch switch

(COM-NC) of the monitor switch can be closed.

2) When the oven door is closed, the contacts (COM-NC) of the

(COM-NO) of the monitored latch switch and the latch switch
are closed.

3) When the oven door is opened and the contacts (COM-NO) of

the monitored latch switch and contacts of the timer switch and
the contact of the relay RY1 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  transformer 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 correspond to the

percentage of microwave power, because approx. 3 seconds

CONNECTED COMPONENTS

RELAY

High voltage transforme                                     RY1

          CONDITION

SWITCH CONTACT 

DURING 

COOKING

DOOR OPEN 

(NO COOKING)

Monitored latch 
switch 

COM-NO          Closed 

Open

Latch switch               COM-NO          Closed 

Open

Monitor switch             COM-NC           Open               Closed

HIGH

32 sec. ON

MEDIUM HIGH

700 W

800 W

29 sec. ON

3 sec. OFF

4 sec. OFF

500 W

M

U

I

D

E

M

21 sec. ON

200 W

W

O

L

6 sec. ON

10 sec. OFF

6 sec. ON

10 sec. OFF