1. Field of the Invention
The present invention relates to a composite cooking appliance consisting of a gas oven and a microwave oven, or a cooking appliance such as a gas oven, and it particularly relates to a temperature control device therefor utilizing gas combustion.
2. Description of the Prior Art
The remarkable advance of semiconductor technology has resulted in sophistication of control circuits, miniaturization of such circuits by higher integration, and reduction of the costs of such circuits by mass-production, and these electronic control circuits have come to be also widely used in household electric equipment.
The technique using intelligence based on this electronic control is making rapid inroads into various heating apparatuses including electric ovens, microwave ovens, gas ovens, and combinations thereof.
One of the most important factors in cooking appliances based on gas combustion is exact temperature control. Since the quality of cooking depends on temperature, it is important that a preset temperature suitable for a given heating load be maintained accurately. Conventional cooking appliances using gas have temperature control means, most of which have been liquid expansion control methods and bimetals.
A gas oven will be taken up by way of example. There is an arrangement wherein a main burner is fired to produce hot air, which is fed into the heating chamber of the oven to cook a heating load therein. The operation of a gas oven of this arrangement will be described with reference to FIGS. 4 and 5. When the user sets the knob at a certain temperature, two temperatures, upper and lower limit temperatures T.sub.1 and T.sub.2, are set by a control circuit. When cooking is started, the main burners start firing and eventually the upper limit temperature T.sub.1 will be reached at time t.sub.2. Then, the main burners stop firing, so that the temperature in the oven heating chamber gradually lowers until the lower limit temperature T.sub.2 is reached at time t.sub.3. Then, the main burners start firing again, repeating this cycle henceforth. If the preset temperature is changed to a higher one after passage of time t.sub.7 (the upper and lower limit temperatures being T.sub.3 and T.sub.4, respectively), the upper limit temperature T.sub.3 is reached at time t.sub.8, as shown in FIG. 4, and henceforth the same operation as described above is repeated.
Thus, the conventional gas oven is designed to control temperature by the on-off operation of the main burners to maintain the oven heating chamber temperature at a preset value, but this design has the following drawbacks.
The pressure of household gas differs with districts. Even in the same district, the gas pressure available for the gas oven installed in a home will always very owing to the turning on and off of gas in other homes or in another room in the same home. Thus, if the gas pressure drops below the normal value, this decreases the rate of heat generation by the main burners, thus requiring a longer time than usual to reach the preset temperature and hence a longer cooking time.
Further, since the ambient temperature of the air surrounding the gas stove differs greatly between midsummer and midwinter, the cooking time will be different in midsummer, particularly in a drafty room.
Furthermore, because of the design to which turns off all of the main burners upon reaching of the upper limit temperature and turns on all of the main burners upon reaching of the lower limit temperature, the actual difference in temperature between the two extremes is as great as about 10.degree. C., showing that this design fails to attain the purpose of maintaining a temperature at a constant value. Furthermore, the need for frequently turning on and off the main burners entails the drawback of shortening the life of the control circuit system, particularly the relay.