MICROWAVE HEATING OVEN

Abstract: In a microwave heating oven, a safety switch 301 is disposed in an external position outside a heating chamber 100, and switch rod 302, which is made of polypropylene or the like low dielectric constant plastic substance, is disposed through and across the heating chamber 100 and serves as a thermal switch, so that at an abnormally high temperature or at fire of the cooking object, the switch rod 302 melt down thereby to open the safety switch 301 and disengage a microwave generator.

Description: The present invention relates to a microwave heating oven comprising a heating chamber with a front opening therein, adapted for containing and heating an object placed therein; a heating chamber door connected to said heating chamber so as to close the front opening; a microwave generator connected to the heating chamber so as to irradiate and thereby heat said object; a safety switch connected to the microwave generator so that opening the safety switch de-energizes the microwave generator; physical displacement means arranged between the door and the safety switch, the displacement means being arranged so that closing the door means causes the displacement means to close the safety switch and that opening the door causes the displacement means to open the safety switch. Such a microwave heating oven is known from JP-A-60 263 025. In order to make a heating oven compact, a heating oven in which the box of the heating chamber serving as the external case has been tried. For example, a heating oven wherein the control box containing electric parts is disposed at the side of the heating chamber, is shown in JP-A-60 248929, or a heating oven wherein the control box disposed below the heating chamber, is shown in JP-A-60 263 025. FIG.1 shows one of such conventional microwave heating ovens. In FIG.1, the control box 2 containing electric parts is disposed below the heating chamber 1. The front door 3 is disposed at the front opening 1a of such heating chamber 1. The front door 3 has a hook for locking the front door 3. And the safety switch 7 is disposed at the position involving such hook on the side wall of the heating chamber. FIG.2 shows a control circuit of such a conventional microwave heating oven. In FIG. 2, the control circuit comprises, a magnetron 31, a high voltage transformer 32, a high voltage capacitor 33, a diode 34, a cooling fan motor 4, a timer control motor 5, and so on. In the safety standard, for example UL standard, at least two safety switches 7 and 8, responsive to the open-close action of the front door 3 must be included in the design of the microwave. Further, a short switch 10 also must be disposed on such electric circuit, to melt down the fuse 9 in case switches 7 and 8 become stuck closed when front door 3 is open. Furthermore, the conventional microwave heating oven has a thermal switch 11 which is, for example, a fuse switch or a bimetal switch, so as to break off the electric circuit for making microwave 30 and the cooling fan motor 4 in case of abnormal high temperature or fire trouble of food. In practical design of the microwave heating oven, the position of the thermal switch is determined after designing of a passage of the exhaust gas from the cooked food by trial and error of the experiment. As improvement toward simplicity of the configuration of the heating oven proceeds, however, the safety switch responding to the open-close action of the front door must be disposed at the position to be showered by the microwave, which is shown in FIG.1. So that, such a heating oven has the following problems. First, shower of microwave destroys the safety switch, second at receiving such microwave shower, wires of control circuit radiate electric noise which a badly affect the television set or the radio. Further, in a microwave heating oven such as shown in JP-A-60 263025 the walls of the heating chamber serves as the external case, and so the heat is conveyed toward external part easily. Therefore, there is a strong demand for the quick responding thermal switch which can break down the electric circuit in a short time. SUMMARY OF THE INVENTION The purpose of the present invention is to provide a microwave heating oven having safety swiches, at least one of which serves dually as a thermal switch. A microwave heating oven in accordance with the present invention is characterized in that the displacement means being disposed to run across and through the heating chamber so that overheating of the heating chamber causes the displacement means to become overheated; the displacement means being made of a thermally responsive material so as to be rigid at normal operating temperatures of the heating chamber and yet flexible at overheating temperatures of the heating chamber; said displacement means being connected between the door and the safety switch so that when said displacement means becomes flexible, the safety switch is caused to open; and the safety switch being disposed outside a microwave shield. As a result of the above-mentioned configuration, the safety switches are safely shielded from the microwave and hence are not broken by the radiation of the microwave, and on the other hand the switch is quickly opened in the event of abnormally high temperatures or fire trouble. BRIEF DESCRIPTION OF THE DRAWINGS Fig.1 is the cross-sectional side view of the conventional microwave heating oven. Fig.2 is the drawing of the electric circuit for making microwave in accordance with the conventional microwave heating oven. FIG.3 is a cross-sectional side view of a microwave heating oven in accordance with the present invention. FIG.4 is a cross-sectional side view of the microwave heating oven in accordance with FIG.3, showing a case of a fire trouble and action of a safety switch serving as a thermal switch.. FIG.5 is a cross-sectional plan view of the microwave heating oven of FIG.3. FIG.6 is a perspective view of a microwave heating oven in accordance with the present invention. FIG.7 is a circuit diagram of an electric circuit of the microwave heating oven in accordance with the present invention. FIG.8 is a perspective view of a lock mechanism and switches for responding to a front door in accordance with FIG.6. DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of a microwave heating oven in accordance with the present invention is shown in FIG.3, FIG.4, FIG.5, FIG.6, FIG.7 and FIG.8. As shown in FIG.3, a box made of a microwave-shielding substance, such as metal, and constituting a heating chamber 100 of such embodiment is designed to serve also as an external case, so as to make the size of the microwave heating oven compact and reduce cost. Panels 101, 102, 103, 104, 106, 107 and so on, which are for example stainless steel, are made in box shape by known bending and calking and the like. A control box 200 in which a magnetron 201, a high voltage transformer 202, a high voltage capacitor 203, a cooling fan motor 204, and so on are contained, is disposed below the heating chamber 100. At a front opening window of the heating chamber 100, a front door 110 is connected rotatably to the panel 107 by a known mechanism (which is not shown in the drawings). And a front part of the control box 200 is covered by a control panel 120. A timer 207 for controlling the cooking time, (namely radiation time of microwave,) and a knob of the timer 208 are disposed at the front face of the control panel 120. As shown in FIG.6 which is a perspective view of the heating oven in accordance with FIG.3, the front door 110 has a projection part 110b and a hook part 110c on its inner face. The projection part 110b turns on a safety switch 206 disposed on the control box 200 when the front door 110 is closed, and turns off the safety switch 206 when the front door 110 is opened. And the hook part 110c is caught to be locked and released to be unlocked by a known mechanism in the control box 200, responding to the close action and open action of the door knob 110d of the front door 110 (the lock mechanism is not shown in the drawings), respectively. The control panel 120 has openings 120a and 120b, respectively for receiving the projection part 110b and the hook part 110c of the front door 110. And the lock mechanism serves as another safety switch. These mechanism and the safety switches are shown in detail in FIG.8 and explained later. A rear cover 105 is fixed on the rear part of the heating chamber 100, and the space 300, which is enclosed by the panel 102 of the heating chamber 100 and the rear cover 105, constitutes the exhaust passage of the heating oven to exhaust the vapor or the like from the food 130 of heating object by the absorption of the cooling fan 204a. On the inner face of the rear cover 105, a safety switch 301 which turns off or on responding to the open-close action of the front door 110, is disposed. A displacement rod 302, which transmits the open-close action of the front door 110 to the safety switch 301, is disposed through the upper part of the heating chamber 100. And also, it is supported at both end parts in a direction shown by arrow marks A and B in FIG.3 by bushings 304 and at inbetween parts by guide holders 305. The guide holders 305 are held by the panels 102 and 103 of the heating chamber 100. The displacement rod 302 is always charged by a spring of safety switch 301 (which is not shown in figures) so as to be pushed out of the panel 107. When the front door 110 is opened, the displacement rod 302 slides in the direction arrowed A by the force of the spring of the safety switch 301 and is pushed out of the panel 107, so that the right end 302b of the displacement rod 302 is separated from the safety switch 301, and the safety switch 301 is opened. And when the front door 110 is closed, the inner face 110a of the front door 110 contacts with the left end 302a of the displacement rod 302, and so, the displacement rod 302 is withdrawed in the direction arrowed B, and the safety switch 301 is closed. Accordingly, the safety switch 301 is responding to the open-close action of the front door 110. On the bottom of the heating chamber 100, a magnetron 201 which generates the microwave, and a stirrer blade 209 which radiates the microwave equally on the food 130, are disposed. The food 130 is partitioned from the magnetron 201 and the stirrer blade 209 by a partition 140 made of ceramics or the like insulating substance. The safety swich 301 which is mentioned above, also serves as a thermal switch. The thermal switch is for sensing the abnormal high temperature or heat in the heating chamber 100 caused by the fire trouble of the food or the like, and for cutting off the electric circuit of microwave generating circuit 250 and the cooling fan motor 204, so that the opening of the thermal switch 301 prevents propagation of heat to the outside of the heating oven in an early stage. The constitution and operation of the thermal switch is as follows. The displacement rod 302 is made of low dielectric loss materials such as polypropylene, so that it does not ignite at normal heating temperatures of the microwave, but it easily changes its' shape at abnormally high temperatures caused by the fire trouble of the food or the like. Accordingly, in case of the fire trouble of the food which is shown in FIG.4, the center part of the displacement rod 302 changes its shape, and does not withstand the force of the spring of the safety switch 301 (which is not shown in figures). So, the spring returns to normalcy and the right end 302b is pushed in the direction designated by arrow mark A. Then, the safety switch 301 is turned off and cuts off the electric circuit of making microwave 250 and the cooling fan motor 204. FIG.5 is a cross-sectional plan view of the heating oven in accordance with FIG.3. On the center of the heating chamber 100, the food 130 of heating object is disposed. The displacement rod 302, the guide holder 305 and the safety switch 301 are disposed on the right end part of the heating chamber 100. And also these switching mechanisms are disposed on the upper part of the heating chamber 100, so the heating chamber 100 is hardly restricted by these switching mechanisms. When the microwave radiated the food 130 in cooking, the vapor or the like are produced. Such vapor is drafted through the punched holes 102a of the panel 102 in FIG.5 and exhausted out of the heating oven by the cooling fan 204a in FIG.3. The exhaust passage is shown by the arrow marks. FIG.7 shows an example of the electric circuit of this embodiment of the microwave heating oven. In the electric circuit the above-mentioned at least two safety switches 206 and 301 to compl y with the safety standard are provided. The safety switches 206 and 301 are responsive to the open-close action of the front door 110. Further, a short-circuiting switch 280 is also provided in such electric circuit, to turn on in case of failure of the safety switches 206 and 301, so as to melt down a fuse 260. Incidentally, 270 designates a main switch. As shown in FIG.7 and 8, the short-circuiting switch 280 is provided to connect two power lines at the part beyond other safety switch 206, 301 and a fuse 260 in paralel with loads in this apparatus, and serves as lock mechanism of the front door 110. In such electric circuit, a timer motor 207a of a timer 207 which controls the heating time, the cooling fan motor 204 and an electric circuit for making microwave are connected in paralel. Further, the microwave generating circuit 250 comprises the magnetron 201,the high voltage transformer 202, the high voltage capacitor 203, the diode 210 and so on. As mentioned above, the present invention uses a thermal switch 301 as one of safety switches, and the safety switches are disposed at a position thermally isolated from the heating chamber, so that the safety switches of the present invention have high reliability. The electric circuit of the present invention has simplicity, and the microwave heating oven can be made without losing the compactness and the low cost, in comparison with the conventional compact type microwave heating oven.

Claims: A microwave heating oven comprising: (a) a heating chamber (100) with a front opening therein, adapted for containing and heating an object placed therein; (b) a heating chamber door (110) connected to said heating chamber (100) so as to close the front opening; (c) a microwave generator (201) connected to the heating chamber (100) so as to irradiate and thereby heat said object; (d) a safety switch (301) connected to the microwave generator (201) so that opening the safety switch (301) de-energizes the microwave generator (201); (e) physical displacement means (302) arranged between the door (110) and the safety switch (301), the displacement means (302) being arranged so that closing the door means (110) causes the displacement means (302) to close the safety switch (301) and that opening the door (110) causes the displacement means (302) to open the safety switch (301), characterized by (f) the displacement means (302) being disposed to run across and through the heating chamber (100) so that overheating of the heating chamber (100) causes the displacement means (302) to become overheated; (g) the displacement means (302) being made of a thermally responsive material so as to be rigid at normal operating temperatures of the heating chamber (100) and yet flexible at overheating temperatures of the heating chamber (100); (h) said displacement means (302) being connected between the door (110) and the safety switch (301) so that when said displacement means (302) becomes flexible, the safety switch (301) is caused to open; and (i) the safety switch (301) is a spring loaded electrical switch being disposed outside a microwave shield. An oven as in claim 1, characterized in that the safety switch (301) is disposed near one upper corner of the heating chamber (100) opposite to the door (110), so that said displacement means (302) passes along an upper edge of the heating chamber as it passes between the door and the safety switch. An oven as in claim 2, characterized in that said displacement means (302) is made of a low dielectric loss substance. An oven as in claim 3, characterized in that said displacement means (302) is made of polypropylene. An oven as in one of the claims 1 - 4, characterized in that the displacement means comprises a bar (302) of temperature responsive, low dielectric loss material; so that closing of the door pushes the displacement means so as to close the safety switch and opening of the door allows the spring of the safety switch to push the displacement means back to its original position and thereby open the safety switch, and that upon the displacement means becoming flexible, as when the heating chamber overheats, the safety switch opens. An oven as in one of the claims 1 - 5, characterized in that the safety switch comprises a plurality of switches, and the displacement means comprises a plurality of displacement bars activating the plurality of switches.

Inventors: ISHIMURA, YOHZOH, KANAOKA, TOSHIAKI, MATSUKURA, TOYOTSUGU, NAKANO, OKIHIKO
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1985
Main classification: H05B6
IPC codes: H05B6