LG MB-387W Service Manual - Page 10

CIRCUIT DESCRIPTION GENERAL DETAILS • The low voltage transformer supplies the necessary voltage to the micom controller when power cord is plugged in. • When the door is closed, the primary switch is ON, the secondary switch is ON, and the monitor switch opens (contact COM and NO). WHEN SELECTING COOKING POWER LEVEL AND TIME • The micom controller memorizes the function you set. • The time you set appears in the display window. • Each indicator light turns on to indicate that the stage has been set. WHEN TOUCHING THE START PAD • The coil of the relay is energized by the micom controller. • Power input is supplied to the high voltage transformer through the fuse to the primary switch and relay 2. • Turntable rotates. power of microwave oven as POWER LEVEL. (refer to page 1-1) WHEN THE DOOR IS OPENED DURING COOKING • Both the primary switch and relay 2 are cut off primary winding voltage of the high voltage transformer. • ON-OFF of relay 2 is coupled electrically with opening and closing of the secondary switch. • When the door is opened, the secondary switch is opened and when the door is closed, the secondary switch is closed. • The cooking time stops counting down. • Relay stops functioning. • As the door is opened, if the contact of primary switch and relay 2 and/or secondary switch fails to open, the fuse opens due to the large current surge caused by the monitor switch activation, which in turn stops magnetron oscillation. FUSE PRIMARY SWITCH FUSE PRIMARY SWITCH L MONITOR SWITCH N H.V. TRANSFORMER RELAY 2 MICOM CONTROLLER SECONDARY SWITCH • The fan motor rotates and cools the magnetron by blowing the air (coming from the intake on the baseplate). • The air is also directed into the oven to exhaust the vapor in the oven through the upper plate. • Cooking time starts counting down. • 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 high voltage diode and charging of the high voltage capacitor. • The negative 4,000 Volts DC is applied to the filament of the magnetron. WHEN THE OVEN IS SET AT ANY LEVEL EXCEPT MAXIMUM. • The micom controller controls the ON-OFF time of relay 2 by the applied signal to vary the average output L MONITOR SWITCH H.V. TRANSFORMER N RELAY 2 MICOM CONTROLLER SECONDARY SWITCH WHEN TOUCHING THE START KEY WITH THE GRILL COOKING FUNCTION SELECTED • The contacts of the primary switch and the secondary switch close the circuit. • A.C. voltage is applied to the grill heater through grill thermostat as shown by the solid line. GRILL THERMOSTAT L L G -Y E N N GRILL HEATER RELAY 3 • Turntable rotates. • The fan motor rotates. • The air is also directed into the oven to exhaust the vapor in the oven through the base plate and upper plate. 4-4