MICROWAVE OVEN WITH TIMER DEVICE

Abstract: In a microwave oven a timer device comprised therein is driven by a ripple voltage related to the mains frequency, said ripple voltage appearing in the microwave oven power supply during feeding of microwave energy into the oven cavity.

Description: The invention is directed to a microwave oven with a timer device for selecting and/or indicating a cooking time, said oven comprising a microwave source fed by a power supply in which the mains voltage is transformed to a rectified high voltage which is supplied to the microwave source, a ripple voltage related to the mains frequency appearing in said power supply during generation of microwave energy by the microwave source. According to prior art said timer device is usually controlled via a control unit comprised in said microwave oven, the main purpose of said control unit being to control and supervise the cooking procedure in the oven by controlling the microwave energy which is fed to the oven cavity depending on selected control parameters for the actual food. This kind of control of the timer device is used among others in Philips microwave oven type AVM 730. A control in this manner means that the number of active switching components, e.g. transistors, are used in the control circuit of the timer device. In said Philips microwave oven a TRIAC performs the connection of a motor comprised in the timer device to the mains-AC-voltage when the oven is started. The use of active components means costs and moreover an increased complexity of said control unit. The object of invention is a solution for controlling the timer device in a microwave oven which is less complicated and therefore less costly than prior art solutions. The object of invention is obtained by a microwave oven as described in the introduction, which is characterized in that the said timer device comprises a frequency sensitive control signal input which is connected to a circuit point of said power supply in which said ripple voltage appears, whereby the timer device is activated and advanced dependent on the mains frequency during generation of microwave energy in the oven. By the invention is obtained the advantage of a very much simplified control of the timer device by means of a signal appearing exactly during the progress of the cooking procedure. According to a preferred embodiment the timer device comprises a synchronous motor which is fed via a capacitor being connected to the output of a rectifier bridge comprised in the power supply, whereby the timer device is activated and controlled completely without use of any active components. Further embodiments are evident from the following patent claims. The invention is described more closely in the following with reference to the drawings, in which figure 1 discloses a timer device which is connected according to the invention to a power supply of switch mode type, figure 2a and figure 2b disclose time diagrams of the voltages in points A respectively B in figure 1 when the power supply is not activatged, figure 3a and figure 3b disclose time diagrams of voltages in points A respectively B in figure 1 for an activated power supply. The power supply disclosed in figure 1 is fed from the mains via a fullwave rectifier bridge 1, having its input terminals T1, T2 supplied with the mains AC-voltage 220V. The output terminals T3, T4 of the rectifier bridge are connected to a resonance circuit comprised in said switch mode power supply and having a controllable switch and a transformer feeding a driving voltage via a rectifier to a magnetron of the microwave source 3. A capacitor C1 is provided at the input of said resonance circuit. A capacitor C2 in series with a synchronous motor TM are connected in series between output terminals T3, T4. The circuit disclosed in figure 1 operates in the following way. When the power supply is inactive, that is when microwave energy is not generated by the microwave source 3, the capacitor C1 is charged to a DC-voltage which corresponds to the peak value of the mains voltage. This condition is illustrated in figure 2a, showing the voltage in point A as a function of time. A ripple of double mains frequency emanating from the rectification is suppressed by the capacitor C1. The DC-voltage level in point A is blocked by the capacitor C2 which means that the voltage in point B will have a progress with time as illustrated in figure 2b, that is the voltage zero appears across the motor TM. Accordingly the timer device is not advanced in this situation. When the microwave source starts the generation of microwave energy, that is when the power supply is active, the voltage in point A will have the shape as illustrated in figure 3a in consequence of the fact that the capacitor C1 is charged and decharged at double mains frequency. The DC-component of the voltage according to figure 3a is blocked by the capacitor C2 and a ripple voltage of double mains frequency appears in point B. Said ripple voltage is shown in figure 3b. The ripple voltage is supplied to the synchronous motor TM, then advancing the timer device. Said advance is continued until the power supply is again inactivated and the generation of microwaves is stopped. This may be obtained by having the timer device to act upon a mains switch when a preset cooking time is reached. The function of the timer device in this respect corresponds exactly with prior art and is therefore only schematically illustrated in figure 1. A so called cooking lamp may be connected in parallel with the motor TM in order to be fed as well by the ripple voltage and lighted up during activation of the oven. This is indicated by dashed lines in figure 1 in which GL represents a dimglowing lamp. By the described circuit is obtained very much simplified solution for driving the timer device by means of a signal which automatically appears during feeding of microwave energy to the microwave oven cavity. In the embodiment described above the timer device comprises a synchronous motor driven by the appearing ripple voltage. Obviously the ripple voltage may be used also for driving a time measurement device of a different kind, e.g. a pulse counter, to which the ripple voltage is supplied after being adequately pulse shaped. In a power supply of a kind in which the mains voltage is transformed to a high voltage without any prior rectification, a corresponding ripple voltage may be obtained at an adequate circuit point after rectification of the high voltage of mains frequency.

Claims: Microwave oven with a timer device for selecting and/or indicating a cooking time, said oven comprising a microwave source fed by a power supply in which the mains voltage is transformed to a rectified high voltage which is supplied to the microwave source, a ripple voltage related to the mains frequency appearing in the power supply during generation of microwave energy by the microwave source, characterized in that said timer device comprises a frequency sensitive control input which is connected to a circuit point of said power supply in which said ripple voltage appears, whereby the timer device is activated and advanced dependent on the mains frequency during generation of microwave energy in the oven. A microwave oven as claimed in Claim 1, in which said power supply is of a switch-mode type comprising a mains connected rectifier device feeding a resonance circuit, in which a capacitor provided at the input of said resonance circuit suppresses the ripple voltage when the power supply is inactive, characterized in that the control signal input of the timer device is connected to the output of the rectifier device. A microwave oven as claimed in Claim 2, in which said rectifier device comprises a full wave rectifier bridge, characterized in that said timer device is driven by a synchronous motor, said motor being connected in series with a capacitor between the rectifier bridge output terminals, whereby the synchronous motor is supplied with a ripple voltage of double mains frequency when the power supply is active.

Inventors: BRAUNISCH, ECKART, OENNEGREN, JAN, Önnegren, Jan
Applicants: WHIRLPOOL EUROPE B.V.
Application year: 1989
Main classification: H05B6
IPC codes: F24C7, H05B6