Countertop microwave (transformer type): field service manual

Line power (120 V) enters through the cord, a noise filter and the line (main) fuse. The control board and low-voltage parts (display, turntable motor, light, cooling fan) are fed through the door interlock switches and relays or a triac on the control.

When cooking starts and the door is proven closed, a relay (or triac) on the control closes and energizes the primary of the HV transformer. The transformer has two secondaries: a low-voltage winding that heats the magnetron filament, and a high-voltage winding that feeds the HV capacitor and HV diode.

The capacitor and diode form a doubler: on one half of the AC cycle the diode charges the capacitor, and on the next half the capacitor voltage adds to the winding voltage. The result is a pulsating DC of several thousand volts, negative with respect to the chassis, applied to the magnetron.

The magnetron converts that energy into microwaves at about 2.45 GHz, which travel down the metal waveguide and enter the cavity behind the waveguide cover. A turntable or stirrer spreads the energy around the food.

Power levels below High are made by duty cycling: the control switches the whole HV circuit on and off over a cycle that can be up to roughly 30 seconds long. On a low setting the magnetron may run only a small fraction of each cycle.

A cooling fan blows air across the magnetron and transformer. Thermal cutouts on the magnetron and the cavity open the circuit if those parts overheat, typically because the fan stopped or the vents are blocked.

Three door switches (primary, secondary and monitor) make sure microwaves are only produced with the door shut; the monitor switch deliberately blows the line fuse if the others fail to open. See the door interlock component entry.