1. Field of the Invention
The present invention relates generally to a microwave oven of the type including a duty cycle control to vary power level and, more particularly, to a means for minimizing the lamp flicker and blower motor speed variation which occurs in such an oven as the duty cycle control alternately energizes and de-energizes the magnetron power transformer and causes the available AC line voltage to vary.
2. Description of the Prior Art
A frequently employed method for varying the cooking power level in a microwave oven is duty cycle control. In the operation of a duty cycle control, the power transformer and the magnetron are alternately switched between a full-on condition and a full-off condition. The percentage of "on" time compared to the total time of each timing period is known as the duty cycle. The average power level is a direct function of the duty cycle. Various specific circuits have been employed to effect duty cycle power level control. These range from a simple cam-operated mechanical timer having electrical contacts arranged in series with the primary of the power transformer, to more sophisticated systems employing electronic solid state timing and switching elements.
A drawback to duty cycle power level control results from the effect of the drastically varying load on the AC power source as the power level control cycles the power transformer and magnetron "on" and "off". Most consumer countertop microwave ovens are operated from a standard 120 volt, 15 or 20 amp household branch circuit. The load which the power transformer and the magnetron present is approximately 1500 watts, which is relatively large. When this load is effectively connected across the line, the available voltage drops approximately 3 volts. This is of course subject to variation between individual houses and individual branch circuits, depending for example upon such variables as wire size, length of the conductor supplying the branch circuit, and "stiffness" of the voltage supplied by the power company to the house.
In addition to the power transformer and magnetron there are other devices within the microwave oven which are supplied from the AC power source. Some of these devices are constantly energized while the oven is operating. These include such devices such as an oven lamp used to illuminate the interior of the cooking cavity and a blower motor used to provide cooling air for the magnetron. Such devices are adversely affected by the cyclical variation in available voltage to the extent that they become a subtle annoyance to a user of the oven. The lamp flickers in a cyclical manner and the blower motor rotational speed varies to produce a rhythmical variation in audible sound. These effects are particularly noticeable when a solid state duty cycle power level control is employed which typically has a short timing period, in the order of one or two seconds. The effects are present but less noticeable when a mechanical duty cycle timer is employed, as these devices typically have a timing period in the order of thirty seconds.
By the present invention there is provided a microwave oven circuit which minimizes the above-described effects on constantly energized devices within the microwave oven to the extent that such effects are a result of voltage variations caused by cyclical loading and unloading of the power source as the power level control energizes and de-energizes the power transformer and magnetron.