Circuit Description - Frigidaire MFM-6D Service Manual

CIRCUIT DESCRIPTION

Refer to the "WIRING DIAGRAM" Fig. 1 on page 25.
MICROWAVE COOKING
TIME COOKING
1. When the food is placed inside the oven and door is closed.
1) The low voltage transformer supplies the necessary voltage to the touch control circuit when the power
cord is plugged in.
2) The contacts of the interlock monitor switch open.
This switch creates short circuit to blow 15A fuse and stop magnetron oscillation when door is opened
during operation under abnormal condition (i.e. the contacts of primary interlock switch do not open the
circuit).
3) The contacts of primary interlock switch close the primary circuit.
2. When cooking cycle, power and time are set by touching the function pads and the desired
numerical pads.
1) The function indicating bars are located on the digitron light to indicate that function have been set.
2) The time you set appears in the display window.
3) The touch control circuit memorizes the cooking program you set.
3. When the start pad is touched.
* The RELAY "1" and "2" are controlled by the touch control circuit.
1) 120VAC is applied to the high voltage transformer through the contacts of RELAY "1" (See Fig. 1)
2) Fan motor starts rotating and cools the magnetron by blowing the air coming from the intake on the
rear plate hole.
3) The oven lamp light the inside of the oven.
4) Indicator light turns on to indicate function operation. Cooking time starts count down.
5) 3.3 Volts AC is generated from the filament winding of the high voltage transformer. This filament
voltage is applied to the magnetron to heat the magnetron filament through two noise prventing choke
coils.
6) A high voltage of 2000 Volts AC is generated in the secondary of high voltage transformer and this
secondary voltage is increased by the action of the diode and the charging of the high voltage
capacitor. This resultant DC voltage is then applied to the anode of the magnetron. As shown in Fig. 3
the first half cycle of the high voltage produced in the high voltage transformer secondary charges the
high voltage capacitor. Current flow is in the direction of the dotted-line during the second half cycle, the
voltage produced by the transformer secondary, and the charge of the high voltage capacitor are
combined and applied to the magnetron as shown by the solid line so that oscillations begin. The
disturbance wave generated by the magnetron is prevented by the choke coils of 3.2µH, filter
capacitors of 16pF and the magnetron's shielded case so that TV and radio programs are not impaired
by noise.
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