Automatic defrost sensing arrangement for microwave oven

Abstract

A system and method for defrost detection particularly applicable to a microwave oven having an excitation system which normally exhibits relatively little change in voltage standing wave ratio and phase for loads of widely varying dielectric constant values. A discontinuity is periodically introduced into the waveguide coupling microwave energy from the source to the cooking cavity. This discontinuity is effective to cause a substantial change in the magnitude and phase of the electromagnetic field in the waveguide for food objects in the frozen state, while causing relatively little change in these parameters for the same food objects in the thawed state. Hence, the presence of the discontinuity in the waveguide provides a readily detectable difference in field strength at the sensor location in the waveguide between an object in its frozen state and the same object in its thawed state. A sensor responsive to the strength of the electromagnetic field at a predetermined location in the waveguide generates an output signal representative of field strength at that location. This output signal is sampled with the discontinuity present in the waveguide to detect a predetermined relationship between the signal and a reference which when detected indicates the food load has converted from its frozen state to its thawed state.