The Tappan Company

John T Lamb

Carl L Anderson

An energization circuit for a microwave oven employs a PTC thermistor to eliminate high transient current surges resulting from energization and deenergization of induction elements therein to provide circuit protection both during oven start and operation. The energization circuit comprises first and second parallel circuit portions for energizing a transformer used to energize a magnetron that produces microwave energy, the first circuit portion including a normally open start switch and a current limiting PTC thermistor coupled in series relation and the second circuit portion including a relay operated switch, a relay for operating the relay operated switch, and a relay circuit portion connected to the first circuit portion between the start switch and PTC thermistor for energizing the relay upon closure of the start switch and then through the PTC thermistor upon opening of the start switch. During start-up, power is briefly supplied to the magnetron through the PTC thermistor to reduce high current surge flow to the transformer until the relay operated switch closes and supplies full power to the transformer and thus to the magnetron. The relay first is energized directly through the start switch upon closure of the latter and then by reverse current flow through the PTC thermistor upon closing of the relay operated switch and opening of the start switch.

Energization circuit for a microwave oven

A protector switch is added in the power supply to open on failure of the door interlock switch to open when the oven door is opened. The monitor switch that closes as the door opens, after the door switch should open, will if the latter does not occur create a power short-circuit ahead of the oven; in this circuit there is a heat responsive electrical resistance element that mechanically holds the protector switch closed against a switch opening bias. Such element will fail under the short circuit current and release the protector switch to shut off the power to the oven. The resistance element can be a wire of predetermined resistance, with current limiting capability, or an off-the-shelf control resistor.

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Mansfield, OH, United States of America

John T Lamb