EP0089838B1

EP0089838B1 - Microwave oven with monitor circuit

Info

Publication number: EP0089838B1
Application number: EP83301543A
Authority: EP
Inventors: Isao Matsumoto
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 1982-03-19
Filing date: 1983-03-18
Publication date: 1989-02-01
Also published as: AU1255083A; DE3379147D1; JPS58142892U; EP0089838A1; AU557262B2

Description

The present invention relates to a microwave oven having a monitor circuit.
Such a monitor circuit is needed in a microwave oven to prevent microwaves from accidentally being scattered from the cavity of the microwave oven. This could otherwise occur when an interlock door switch becomes faulty. Different countries all around the world impose safety standards which require microwave ovens to be provided with such monitor circuits.
Fig. 1 shows a known 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 oven main fuse 4 rated at about 6-16 A. A first interlock switch 5, which is closed when a latch door is closed, is connected in series with the main fuse 4 and with 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 in series with the primary winding of the transformer 9.
A monitor switch 11 is connected in series with a monitor resistor 12. This switch 11 is mechanically opened when the latch door is closed, and closed when the door is opened. The terminal of the monitor switch 11 remote from 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, and the terminal 14 is directly connected to the AC plug 3.
Power is supplied to the touch control circuit 6 by way of a power transformer 15. A rectifier 16 is connected for converting AC power to DC power. The touch control circuit 6 uses DC power from the rectifier 16 for controlling the microwave oven.
A conventional microwave generating circuit, including a high voltage condenser 17, a high voltage diode 18, a magnetron 19 and the like are connected to the secondary winding of the transformer 9.
In a microwave oven equipped with the known circuit of Fig. 1, consider the case where the second interlock switch 10 is faulty, through being damaged, when foodstuff is being cooked within the microwave oven. When the latch door is opened, the monitor switch 11 is closed, so that a closed loop including the oven main fuse 4, the monitor switch 11, the monitor resistor 12 and the second interlock switch 10 is formed as indicated by the arrow A. A large monitor current flows through the loop, and the main fuse 4 blows thereby to cut the power supply to the microwave oven.
It is preferable to avoid such a large rush current from flowing through the house main fuse 2 as might cause it to blow. For this purpose, the monitor resistor 12 is needed.
Unfortunately, the value of the microwave oven main fuse 4 is not fixed, but rather is variable from one country to another and may be rated at anything from 6-16 A. Depending upon this fuse rating, it may happen that the monitor current can reach such a magnitude that the house main fuse 2 is not adequately protected by the fuse 4 so that it may blow despite the provision of the monitor resistor 12. This is contrary to normal safety standards, which demand that the house main fuse 2 should not be blown when the monitor circuit is operated.
Published European patent application EP-A-0074408, which is relevant only under Article 54(3), discloses a microwave oven which has a monitor circuit including a door switch which is closed upon opening of the oven door, and a resistor and fuse in series with this switch. When a short-circuit fault occurs in the main door switch, a short-circuit current flows through the monitor resistor to blow the monitor fuse, so that monitoring is possible with a relatively small current value. This avoids adverse effects upon the power source. During a short starting time a push button cooking starting switch energizes a self-holding relay via a circuit which includes the monitor resistor and fuse so as to avoid rush current on starting.
After said short starting time, said self-holding relay is maintained energized by a current which no longer flows through said monitor circuit.
United States patent specification US-A-4054769 discloses a short-circuiting monitor circuit for a microwave oven. A heat-responsive electrical resistance element is connected in the monitor circuit and normally holds closed mechanically an additional protector switch which is in series with the main door switch. In the event of failure of the main door switch, the short circuiting current which flows in the monitor circuit causes the resistance element to fail, and the protector switch opens.
According to the present invention, there is provided a microwave oven having a door interlock switch connected in series with a power utilization circuit of the oven, said interlock switch being arranged to be opened upon opening of a door of the oven; a monitor circuit connected in parallel with said power utilization circuit and including a monitor switch which is arranged to be closed upon opening of a door of the oven; and a power control circuit which is connected to a point on the monitor circuit so that said power control circuit is powered continuously by current which flows in a part of the monitor circuit, said power control circuit being responsive to the cutting of the power supplied thereto by way of said part of the monitor circuit to disable the power utilization circuit; said monitor circuit being connected across power supply terminals coupled to said power utilisation circuit so that if the interlock switch fails to open when the oven door opens, causing power to be applied to said terminals while said-monitor switch is closed, the current flowing through the monitor circuit causes a monitor fuse which is connected in said part of the monitor circuit, to blow, thereby to cut the power supply to said power control circuit and thereby disable the power utilization circuit, said monitor fuse being of lower rating than a further fuse connected in series with the power utilization circuit.
According to the present invention, there is also provided a microwave oven having a door interlock switch connected in series with a power utilization circuit of the oven, said interlock switch being arranged to be opened upon opening of a door of the oven; a monitor circuit connected in parallel with said power utilization circuit, and including a monitor switch which is arranged to be closed upon opening of the door of the oven and a current-limiting resistor in series with said monitor switch; and a power control circuit which includes a touch key panel and a power transformer the primary winding of which is connected to a point on the monitor circuit so that the power control circuit is continuously powered by current which flows through said primary winding and through a part of said monitor circuit, which part excludes the monitor switch and the current-limiting resistor, said control circuit being responsive to the cutting of power supply thereto to open the contact of a power switch connected in series with the power utilization circuit; said monotor circuit being connected across power supply terminals coupled to said power utilization circuit so that if the interlock switch fails to open when the oven door opens, causing power to be applied to said terminals while the monitor switch is closed, the short-circuit current flowing through the monitor circuit causes a monitor fuse which is connected in said part of the monitor circuit, to blow, thereby to cut said current to said primary winding and thus the power supply to said power control circuit, said monitor fuse being of lower rating than a further fuse connected in series with the power utilization circuit, said monitor fuse also conducting current which flows through said primary winding so as to protect against short circuiting in the power control circuit.
The present invention will become more fully understood from the detail 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 known 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.
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 monitor fuse 20 is connected to a power side terminal of the microwave oven main fuse 4 and the monitor switch 11. The current capacity of thus fuse 20 is about 0.5-1 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 control 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.
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 and the latch door is closed. Responsive to this operation, the monitor switch 11 is opened, and the first and the second interlock switches 5 and 10 are closed, 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 transformer 15 in the touch control circuit 6 to operate the CPU 22.
Let us assume that during such an operation, the second interlock switch 10 fails so that it cannot open. In such a case, when the latch door is opened, the monitor switch 11 is closed, so that the power energy is momentarily 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 as indicated with an arrow B. Hence, the monitor fuse 20 is blown.
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 no longer receives power. The cooking start relay 25 also becomes deenergized, so that the cooking start relay terminals 26 are opened. The primary winding of the high voltage transformer 9 is thereby isolated from the power source, and the magnetron can no longer generate microwaves. Therefore, the microwave leakage from the oven cavity can be prevented.
Thus, the monitor fuse 20 is permitted to blow when the second interlock switch 10 cannot be opened due to damage. The power supply to the touch control circuit 6 can be cut to stop energization of the magnetron 19. The oven 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 to the power source side of the main fuse 4. In place of this circuit connection of Fig. 2, it is 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, to the side including the high voltage transformer 9. Again, when the second interlock switch 10 fails and the latch door is opened, the monitor fuse 20 can blow to permit the cooking start relay terminals 26 to open.
- The monitor fuse 20 also serves as a fuse for protecting against short- circuiting 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 operates successfully without any problem of failure. If necessary, an additional monitor circuit can be provided for protecting the first interlock switch 5.
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 scope of the present invention as claimed.

Claims

1. A microwave oven having a door interlock switch (10) connected in series with a power utilization circuit (9) of the oven, said interlock switch (10) being arranged to be opened upon opening of a door of the oven; a monitor circuit (11, 12, 20) connected in parallel with said power utilization circuit (9) and including a -monitor switch (11) which is arranged to be closed upon opening of a door of the oven; and a power control circuit (6) which is connected to a point on the monitor circuit (11, 12, 20) so that said power control circuit (6) is powered continuously by current which flows in a part of the monitor circuit, said power control circuit (6) being responsive to the cutting of the power supplied thereto by way of said part of the monitor circuit to disable the power utilization circuit (9); said monitor circuit being connected across power supply terminals coupled to said power utilization circuit (9) so that if the interlock switch (10) fails to open when the oven door opens, causing power to be applied to said terminals while said monitor switch (11) is closed, the current flowing through the monitor-circuit causes a monitor fuse (20) which is connected in said part of the monitor circuit, to blow, thereby to cut the power supply to said power control circuit (6) and thereby disable the power utilization circuit (9), said monitor fuse (20) being of lower rating than a further fuse (4) connected in series with the power utilization circuit (9).

2. A microwave oven according to claim 1, wherein said power control circuit (6) comprises a power transformer (15) the primary winding of which is connected to said point in the monitor circuit.

3. A microwave oven according to claim 1 or claim 2 wherein one of said power supply terminals is located between a commercial power source and said further fuse (4).

4. A microwave oven according to claim 1 or claim 2 wherein one of said power supply terminals is located between said further fuse (4) and a further door interlock switch (5) which is operable to be closed when the door is closed and is connected in series with said power utilization circuit (9).

5. A microwave oven according to any preceding claim wherein said monitor fuse (20) is rated at 0.5-1 A.

6. A microwave oven having a door interlock switch (10) connected in series with a power utilization circuit (9) of the oven, said interlock switch (10) being arranged to be opened upon opening of a door of the oven; a monitor circuit (11, 12, 20) connected in parallel with said power utilization circuit (9), and including a monitor switch (11) which is arranged to be closed upon opening of the door of the oven and a current-limiting resistor (12) in series with said monitor switch; and a power control circuit (6) which includes a touch key panel (21) and a power transformer (15) the primary winding of which is connected to a point on the monitor circuit (11, 12, 20) so that the power control circuit (6) is continuously powered by current which flows through said primary winding and through a part of said monitor circuit, which part excludes the monitor switch (11) and the current-limiting resistor (12), said power control circuit (6) being responsive to the cutting of power supply thereto to open the contact of a power switch (26) connected in series with the power utilization circuit (9); said monitor circuit (11, 12, 20) being connected across power supply terminals coupled to said power utilization circuit (9) so that if the interlock switch (10) fails to open when the oven door opens, causing power to be applied to said terminals while the monitor switch (11) is closed, the short-circuit current flowing through the monitor circuit causes a monitor fuse (20) which is connected 'in said part of the monitor circuit, to blow, thereby to cut said current to said primary winding and thus the power supply to said power control circuit (6), said monitor fuse (20) being of lower rating than a further fuse (4) connected in series with the power utilization circuit (9), said monitor fuse (20) also conducting current which flows through said primary winding so as to protect against short circuiting in the power control circuit (6).