LG MB-390A, Service Manual

Page: 6 / 15

4-4
CIRCUIT DESCRIPTION
• As the door is closed, the contact of MONITOR
SWITCH opens. This switch creates the short circuit to
blow fuse during operation under abnormal condition.
(ie, should the contacts of primary and secondary
switch fail to open the circuit)
• The latches are secured by latch board. The oven light
turns on while the oven is in operation.
WHEN MICROWAVE POWER LEVEL
AND COOKING TIME SET
• The contacts of the timer switch changes to ON
position when the timer knob is rotated.
(While power control knob is set at “Full Power”)
• The contacts of primary and secondary switch close
the circuit.
• Input Volts AC. is applied to the high voltage
transformer through power control switch as shown by
the solid line.
• Turntable rotates.
• The fan motor rotates and cools the magnetron by
blowing the air (coming from the intake holes on the
baseplate) over the magnetron.
• The air is also directed into the oven to exhaust the
vapor in the oven through the upper plate.
• Timer starts rotating.
• 3.2 volts AC is generated from the filament winding of
the high voltage transformer. This 3.2 volts is applied
to the magnetron to heat the magnetron filament
through two noise preventing choke coils.
A high voltage of approximately 2100 volts AC is
generated in the secondary of the high voltage
transformer which is increased by the action of the
diode and charging of the high voltage capacitor.
The negative DC voltage is then applied to the filament
of the magnetron.
WHEN THE DOOR IS OPENED DURING
COOKING
• Both primary switch and secondary switch open to cut
off the primary winding voltage to the high voltage
transformer to stop microwave oscillation.
• Timer, turntable and fan motor stop.
• As the door is opened, if the contact of primary switch
fail to open, the fuse opens due to the large current
surge caused by the monitor switch activation which in
turn stops magnetron, oscillation.
WHEN THE GRILL COOKING FUNCTION
SELECTED
• The contacts of the timer switch change to ON
position.
• The contacts of the grill select switch close the circuit.
• Input volts AC is applied to the grill heater through
thermostat-grill as shown by the solid line.
• The fan motor and turntable rotates.
FUSE
PRIMARY
SWITCH
SECONDARY
SWITCH
MONITOR
SWITCH
L
N
H.V.T
RELAY
TIMER
SWITCH
POWER
CONTROL
SWITCH
FUSE
PRIMARY
SWITCH
SECONDARY
SWITCH
L
N
H.V.T
RELAY
TIMER
SWITCH
POWER
CONTROL
SWITCH
MONITOR
SWITCH
FUSE
PRIMARY
SWITCH
TIMER
SWITCH
GRILL
SELECT
SWITCH
L
N
THERMOSTAT
GRILL
GRILL
HEATER
L
G -Y
N
E