1. Field of the Invention
The present invention relates generally to a microwave oven of the type including a duty cycle control which periodically energizes and de-energizes the microwave generating system during the cooking process and, more particularly, to a circuit arrangement for providing an improved delay start feature in such an oven without costly changes in the programming or circuitry which controls oven operation.
2. Description of the Prior Art
It is common in commercially available, electronically controlled microwave ovens to provide a delay start feature. Such a feature permits a microwave oven user to program the control circuitry of the oven so that a cooking operation begins at a later time, the delay time being entered by the user into the oven controls. Thus, an operator of an oven desiring to use such a feature places food in the oven; selects a particular power level at which he desires to cook the food and the time duration for the food to cook and enters this information into the oven control system; selects and enters a time delay (usually in the form of a preselected number of minutes) into the control, then usually presses a "start" button on the control signalling the completion of cooking instructions, and leaves the oven unattended.
Microwave ovens heat food by subjecting it to microwave radiation generated by a magnetron. Most microwave ovens vary the cooking power level by using a duty cycle control to operate the magnetron. In the operation of a duty cycle control, the magnetron is switched between a full-on condition and a full-off condition with the percentage of "on" time compared to the total time of each timing period (duty cycle) being varied to change the cooking power delivered to the food. Various specific circuits to provide this duty cycle are well known in the art. These range from simple cam operated mechanical timers having electrical controls to operate the magnetron to more sophisticated systems employing electronic solid state timing and switching elements. Since the duty cycle control does not form a part of this invention, it will not be described in detail. The magnetron therefore represents a first class of load in a microwave oven which is energized intermittently during the cooking process.
As contrasted to the intermittent operation of the magnetron, the microwave oven also includes various other electrical loads which are desirably energized continuously during the cooking process, such as the mode stirrer motor, the magnetron cooling blower and the filament heater for the magnetron.
In microwave ovens having a delay start feature, provision is usually made to control the continuously energized loads separately from the magnetron. More specifically, it is not possible to control these continuously energized loads using the same switching device that controls the magnetron because the latter is operated in an intermittent or duty cycle controlled fashion.
One solution to this problem is to turn on the constantly energized loads when the oven user completes entry of the delay start instructions into the oven controls. In this arrangement, the mode stirrer motor, blower, etc., are energized by activating the start button, or other equivalent input control device and remain energized continuously during the delay time period when the magnetron is idle, waiting for the beginning of the cooking cycle.
The obvious disadvantage of such a scheme is that power used to drive these loads is wasted during the time when they are not functionally needed to support the cooking operation. In addition, the noise caused by operation of these loads is undesirable and distracting.
Another solution to the above mentioned problem is to control the continuously energized loads by means of a separate independent signal developed by the oven control after the time delay period has passed at the beginning of the cooking cycle, i.e. when the magnetron is initially energized. This may be typically done by utilizing an additional input/output port on the microprocessor control with appropriate programming to carry out this function. In addition, an electronic switching element, associated drive electronics and isolators must be employed to implement this approach, resulting in a significantly additional cost. Moreover, it is sometimes difficult to couple additional electronics to an already existing control package without a significant redesign effort.