MICROWAVE OVEN WITH MONITOR CIRCUIT

Abstract: A monitor circuit for a microwave oven comprises a series circuit including a monitor switch (11), a monitor resistor (12), and a fuse (20). The series circuit is connected in parallel to a commercial power source. The monitor switch (11) is closed when a latch door is opened. A power input circuit such as a power transformer (15) is connected to the fuse (20) positioned in front of a commercial power source.

Description: IMPROVED MONITOR CIRCUIT FOR MICROWAVE OVEN BACKGROUND OF THE INVENTION The present invention relates to a monitor circuit for a microwave oven to monitor power energy supply to the microwave oven. A monitor circuit for a microwave oven is needed to prevent microwave from erroneously being scattered from a cavity of the microwave oven. This may occur when an interlock switch as a door switch is disordered. All the microwave ovens must be provided with the monitor circuit according to a safety standard in all counteries around the world. FIG. l shows a conventional monitor circuit for a microwave oven. The circuit comprises a commercial power source 1, a house main power fuse 2, an AC plug 3, and a microwave main fuse 4 for a current of about 6-16 A. A first interlock switch 5 is connected which is closed when a latch door is closed. This switch 5 is connected in series to the main fuse 4 and to a primary winding of a high voltage transformer 9 via relay terminals 8 of a relay 7 included within a touch control circuit 6. A second interlock switch 10 is connected between the primary winding of the transformer 9 and the AC plug 3. Thus, the first and the second interlock switches 5 and 10 are connected to the primary winding of the transformer 9. A monitor switch 11 is connected in series to a monitor resistor 12. This switch 11 is mechanically opened when the latch door is closed, and closed when the door is opened. One terminal of the monitor switch 11 opposed to the monitor resistor 12 is connected to a point between the main fuse 4 and the first interlock switch 5. The monitor resistor 12 is connected to the primary winding of the transformer 9. Two terminals 13 and 14 are provided as power input terminals for the touch control circuit 6 which is electronically operated. The terminal 13 is connected to the AC plug 3 via the main fuse 4. The terminal 14 is directly connected to the AC plug 3. A power transformer 15 is provided for supplying power to the touch control circuit 6. A rectifier 16 is connected for converting AC power to DC power. The touch control circuit 6 is responsive to the DC power generated with the rectifier 16 for controlling the microwave oven. To the secondary winding of the transformer 9, a high voltage condenser 17, a high voltage diode 18, a magnetron 19 and the like are connected to generate wicrowave according to the conventional technique. In the conventional microwave oven equipped with the circuit of FIG. 1, it is assumed that the second interlock switch 10 is physically disordered on acconunt of its damage when a foodstuff is cooked within the microwave oven. When the latch door is opened, the monitor switch li is closed, so that a closed loop including the main fuse 4, the monitor switch 11, the monitor resistor 12 and the second interlock switch 10 is formed as indicated with an arrow A. A large monitor current flows through the loop. Therefore, the main fuse 4 is cut to thereby shut out power supply to the microwave oven. It is preferable to avoid that a large rush current flows through the house main fuse 2 enough to cut it. For this purpose, the monitor resistro 12 is needed. Unfortunately, the value of the microwave oven main fuse 4 is dependent on the country not to thereby fix to be for the current of about 6-16 A. It may be occur that a large monitor current flows'in such a degree that the house main fuse 2 is cut even with the connection of the monitor resistor 12. According to the safety standard, the house main fuse 2 should not be cut even when the minotor circuit is operated. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an improved monitor circuit for a microwave oven for preventing a house main fuse from being cut even when a monitor circuit is operated. It is another object of the present invention to provide an improved monitor circuit for a microwave oven for permitting a monitor fuse to serve as a shortcurcuiting protecting fuse in a secondary circuit of a control circuit. Briefly described, in accordance with the present invention, a monitor circuit comprises a monitor fuse whose current capacity is much less than that of a microwave oven main fuse. The monitor fuse is connected to a monitor switch. When the monitor switch is closed, the monitor fuse is cut to thereby shut out power supply to a touch control circuit, in which a primary winding of a high voltage transformer of the microwave oven is not energized. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention and wherein: FIG. 1 shows a configuration of a conventional monitor circuit for a microwave oven; and FIG. 2 shows a configuration of a monitor circuit for a microwave oven according to the present invention. DESCRIPTION OF THE INVENTION FIG. 2 shows a configuration of a monitor circuit for a microwave oven according to the present invention. Like elements of the circuit of FIG. 1 are indicated by like numerals. A minotor fuse 20 is connected to a power side terminal of the microwave oven main fuse 4 and the minotor switch 11. The current capacity of this fuse 20 is about 0.51 A which is much less than that of the main fuse 4 of about 6-16 A. A connection point between the monitor fuse 20 and the monitor switch 11 is connected to the input terminal 13 of a power input circuit such as the power transformer 15 in the touch countrol circuit 6. The monitor switch 11 is connected to the AC plug 3 having three terminals via the monitor resistor 12 and the second interlock switch 10. A touch key panel 21 is provided for key operations for the touch control circuit 6. A CPU 22 is provided within the touch control circuit 6 for controlling the operations of the microwave oven. t An NPN transistor 23 is connected to the CPU 22 via a drive circuit 24. A cook start relay 25 is connected at the collector side of this transistor 23. Relay terminals 26 are related to the cook start relay 25. The relay terminals 26 are connected to the primary winding of the high voltage transformer 9 and to the first interlock switch 5 coupled to the microwave oven main fuse 4. The monitor switch 11 is mechanically opened when the latch door is closed, and mechanically closed when this door is opened. The first and the second interlock switches 5 and 10 are mechanically closed when the latch door is closed, and mechanically opened when this door is opened. Thus, the interlock switches 5 and 10 function to allow the power supply to the remaining portions of this circuit only when the latch door is tightly closed. In operation, a foodstuff is disposed within the microwave oven to thereby dose the latch door. Responsive to this operation, the monitor switch 11 is opened, and the first and the second interlock switches 5 and 10 are dosed, respectively. After the touch key panel 21 is actuated to set a cooking time and a cooking temperature, a cooking start key 21 in the touch key panel 21 is operated to input a cooking start signal into the CPU 22. The cooking start signal is inputted into the CPU 22 and outputted into the NPN transistor 23 via the drive circuit 24. The NPN transistor 23 is made conductive to supply power energy to the cooking start relay 25, so that the cooking start relay terminals 26 are closed. Thus, the commercial power is supplied from the AC plug 3 into the high voltage transformer 9 through the main fuse 4, the first interlock switch 5, the cooking start relay terminals 26, and the second interlock switch 10. The magnetron 19 is energized with a high voltage to generate microwave. Through the monitor fuse 20, the commercial power is supplied to the power stransformer 15 in the touch control circuit 6 to operate the CPU 22. During such a operation, it is assumed that the second interlock switch 10 cannot be physically disconnected. In such a case, when the latch door is opened, the monitor switch 11 is dosed, so that the power energy is supplied from the AC plug 3 into the monitor fuse 20, the monitor switch 11, the monitor resistor 12, and the second interlock switch 10, in a moment, as indicated with an arrow B. Hence, the monitor fuse 20 is made cut. The input terminal 13 of the power transformer 15 in the touch control circuit 6 is made open so that the touch control circuit 6 is not energized with power energy. The cooking start relay 25 is not energized, either, so that the cooking start relay terminals 26 are opened. The primary winding of the high voltage transformer 9 is isolated from the power energy in which the magnetron cannot generate microwave, anymore. Therefore, the microwave leakage from the oven cavity can be prevented. Thus, according to the present invention, the monitor fuse 20 is permitted to be cut when the second interlock switch 10 cannot be disconnected on acconunt of damage. The power supply to the touch control circuit 6 can be stopped to stop energizing the magnetron 19. The main fuse 4 is not cut. In the circuit of FIG. 2, a line leading to the monitor fuse 20, the monitor switch 11, and the monitor resistor 12 is connected at the power source side of the main fuse 4. In place of this circuit connection of FIG. 2, it may be possible that this line leading to the monitor fuse 20 is connected to the point between the main fuse 4 and the first interlock switch 5, namely, at the side including the high voltage transformer 9. In such a case, when the second interlock switch 10 is physically disconnected and the latch door is opened, the monitor fuse 20 can be cut to permit the cooking start relay terminals 26 to be unconductive. The monitor fuse 20 serves as a fuse for protecting a shortcircuiting in the secondary circuit of the cooking control circuit 6. Since the touch control circuit 6 consumes an approximately-constant current, the resistance of the monitor resistor 12 for limiting a current can be freely selected. In the above description, it is assumed that the first interlock switch 5 is not damaged, so that it is exactly operated without any disconnect problem. It may be needed that an additional monitor circuit is provided for protecting the first interlock switch 5, when required. While only certain embodiments of the present invention have been described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the sprit and scope of the present invention as claimed.

Claims: CLAIMS: 1. A monitor circuit for a microwave oven comprising: monitor switch means operable to be closed when a latch door is opened; monitor resistor means; fuse means to form a series circuit including the monitor switch means, the monitor resistor means and the fuse means; the series circuit being in parallel to a commercial power source; and power input means for microwave oven control curcuit for driving the microwave oven, the fuse means being connected at the input side of the power input means. 2. The circuit of daim 1, wherein the power input means comprises a power transformer. 3. The circuit of daim 1, wherein a terminal of the series circuit is connected between the commercial power source and a microwave oven main fuse. 4. The circuit of daim 1, wherein a terminal of the series circuit is connected between a microwave oven main fuse and an interlock switch means operable to be closed when the latch door is opened. 5. The circuit of daim 1, wherein the current capacity of the fuse means is much less than that of a microwave oven main fuse. 6. The circuit of claim 5, wherein the current capacity of the fuse means is about 0.5-1 A. 7. A microwave oven having a monitor circuit (11,12,20) including a switch (11) which is arranged to be closed upon opening of a door of the oven, the monitor circuit (11,12,20) being connected in parallel with a power utilization circuit (9) of the microwave oven and across power supply terminals so that if power is applied to said terminals while said switch (11) is closed, the current flowing through the monitor circuit (11,12,20) causes a fuse (20) of the oven to blow, characterised in that the fuse (20) forms part of said monitor circuit (11,12,20) and is thus connected in parallel with said utilization circuit (9). 8. An oven as claimed in claim 7, including a further fuse (4) connected in series with said utilization circuit (9). 9. An oven as claimed in claim 8, wherein said further fuse (4) is connected in parallel with said monitor circuit (11, 12,20).

Inventors: MATSUMOTO, ISAO
Applicants: SHARP KABUSHIKI KAISHA
Application year: 1983
Main classification: H05B3
IPC codes: H05B6, F24C7