Microwave oven

Abstract: A microwave oven with a lighting device for illumination of the oven cavity. The lighting device comprises a smaller hole in the cavity wall, the small dimensions of said hole substantially eliminating leak of microwave through the hole, and a light-intensive glowlamp which is smaller than the hole and which is positioned outside the cavity in close connection with the hole.

Description: The invention is directed to a microwave oven comprising an oven cavity enclosed by a cavity wall, a microwave source for feeding microwave energy of a substantially uniform wavelength into said cavity, and a lighting device for illumination of the cavity, said lighting device comprising a light source arranged at the cavity wall and a microwave seal substantially preventing leak of microwaves to the external surroundings of the cavity through said lighting device. Ovens having lighting devices are known of many different shapes, partly ovens in which only microwave energy is used, partly so called combi-ovens in which hot air as well as specific grill elements are used. The expression microwave oven in the following is intended to cover ovens in general in which microwaves are used. In a microwave oven the lighting device is usually provided in the roof of the cavity or in anyone of its side walls. As a light source is used a kind of glowlamp arranged in a fitting in the cavity wall. Lighting devices of this type must comply with a number of requirements. It must provide an even lighting of the complete cavity of desirable light intensity. The lamp fitting in the cavity wall is not allowed to bring with it more than a neglectible leak of microwaves out from the cavity, under all circumstances said leak must be well below the limits set up by authorities in different countries. Leaking microwaves may also disturb the control electronics of the microwave oven and should be minimized also of this reason. The lamp fitting and the glowlamp itself may only influence the microwave field distribution in the cavity to a neglectible degree, because an evne field distribution is of prime importance for the result of cooking. A sufficient life time of the glowlamp must be guaranteed, which demands a specific shape of the filament in order to minimize its antenna function with respect to microwaves and thereby its possibility of being excited by the same, alternatively a so protected position for the lamp that the filament is reached only by a small amount of the microwaves. The heat radiation of the lamp must be limited in order not to disturb surrounding components, which may be a problem if, for example, the lamp is arranged behind a protecting perforated plate, demanding a more powerful light source and in consequence a higher electric power and heat radiation. The lamp and the lamp fitting should occupy a space which is as small as possible both inside and outside the cavity. In general the lighting device should be suitable for production and able to be produced at low cost, among other things by requiring least possible change of the cavity wall and the use of a standard type glowlamp. In so called combi-ovens with a grill element the temperatures in the vicinity of the grill elements may amount to 300°-400°C. The lighting device must then withstand such temperature levels. Prior art lighting devices for microwave oven comply with some of the listed requirements, some of them to a certain degree, others not at all. DE-A-36 41 420 discloses a lighting device of which the flowlamp is positioned in a hole in the cavity wall by means of metal carrier proviced on the outside of the cavity. In order to minimize the microwave leak through said hole and carrier a specific sleeve of metal net is provided around the lamp. In comparison with the requirements set up in the foregoing it may be concluded among other things that the lighting device is of a relatively complicated construction and that the metal carrier thereof demands a considerable space outside the cavity. DE-A-36 43 307 discloses a lighting device of which the glowlamp is surrounded by a sleeve of a paramagnetic material. The lamp is arranged in a so called lamp housing provided on the outside of the cavity wall. Inwardly the cavity the lamp and lamp housing is covered by a protective glass. The protective glass and its fitting into the cavity wall generate microwave field disturbances and some microwave leak through the lamp housing. The purpose of said paramagnetic sleeve is to minimize said disturbances and leak. In correspondance with the construction described above also this one is complicated and space demanding. DE-A-36 43 364 discloses a lighting device of which the glowlamp is carried by a lmap fitting arranged on the inside of the cavity wall, said fitting being in some embodiments sunk into a recess in the cavity wall. The lamp protrudes as a hole into the cavity room and is covered by a protective glass. The protruding position of the glowlamp means that the filament as well as the supporting legs thereof must be dimensioned in relation to the wavelength of the microwaves in order not to start glowing under influence from the microwaves. Accordingly, this construction puts specific demands on the shaping of the lamp. The position of the glow lamp in the cavity in combination with the recessed positioning of its socket in the cavity wall will influence the field distribution in the cavity. EP-B-120 536 discloses a lighting device of which the glowlamp is arranged in a so called wave guide sleeve at a hole in the cavity wall. Said sleeve as well as the position of the filament in the same are so selected that microwave leak from the cavity through the hole in the cavity wall is minimized to an acceptable level. As a whole this construction is relatively complicated and space demanding. A similar construction is disclosed in US-A-2 526 226 including a tube having alt cut-off wavelength which is below the wavelength of the microwaves and showing corresponding drawbacks. DE-A-3 703 937 discloses a further construction including a cage mounted in the cavity wall for accommodating the lamp. This disclosure specifically underlines the problem to safeguard a good metallic contact between the cage and the cavity wall, and thereby also demonstrates the drawbacks involved with the use of sealing means like said cages, tubes and sleeves. The object of invention is to provide a microwave oven of the type mentioned in the introduction having a lighting device without the drawbacks of the prior art constructions and essentially fullfilling the requirements set up above. The object is obtained by a microwave oven which is characterized in that said microwave seal is obtained by means of a preferably circular hole in the cavity wall of a selected smaller sixe in relation to the wavelength of the microwaves which is such that the dimensioning of the hole substantially prevents passage of microwaves throught he hole, and in that said light source comprises a light-intensive glowlamp positioned outside the cavity, having a short filament and small dimensions relative to the hole, and being arranged right in front of in close connection with the hole. The lighting device according to the invention fullfils the requirements which have been set up and provides an even illumination in the cavity due to the position of the glowlamp in close connection with the outside of the cavity wall. By the adaption of the hole diameter to the wavelength of the microwaves a complete control of the microwave leak to the surroundings is obtained. The lighting device is simple and may be produced at low cost and means a neglectible infringement on the cavity wall and thereby also a minimum influence on the microwave field distribution in the cavity. A further improvement of the light distribution in the cavity is obtained by a preferred embodiment of the invention, in which the hole in the cavity wall is covered by a protective glass which is so shaped that it also has the function of a light spreading lens. Still a further embodiment according to the invention is characterized in that the lighting fitting comprises a reflector enclosing the glowlamp so as to direct backwardly and sidewardly radiating light from the same substantially in the direction of the hole in the cavity wall. The hole provided in the cavity wall has a diameter according to the invention which is less than or equal to 25 mm and preferably 14±1 mm. The hole in the cavity wall is preferably circular with a diameter of 14 mm, the lamp and the reflector having then also a circular shape in the plane of the cavity wall, and being combined with a lamp carrier which is fixed by means of a spring holding the reflector pressed against the cavity wall, directly or via said protective glass. The protective glass is adequatly provided with a surrounding part having a track which adjoins to the edge of the reflector and a central part protroduding into the cavity. The invention will be described in more detail in the following with reference to the drawing on which a microwave oven is schematically shown in a vertical section through a lighting device arranged in the roof of the oven cavity. In the figure, 1 refers to the schematically disclosed microwave oven housing. 2 shows the cavity room, and 19 shows the microwave source, also schematically. In the cavity roof 3 is provided a lighting device 4 according to the invention. The lighting device 4 comprises acircular hole 5 in the cavity roof and a light intensive glowlamp 6 arranged outside the cavity right in front of the hole 5. In order to guarantee a good light distribution in the cavity through the hole the glowlamp is firstly of a smaller diameter than the hole, secondly positioned in close connection with the cavity roof. The lamp is inserted into a lamp socket 7 and is supplied with current trough a wire 8 connected to said socket. The lamp socket 7 is combined with a reflector 9 having the task to catch scattered light from the glowlamp and to direct the same towards the hole 5. The hole 5 is covered by a protective glass comprising a surrounding part 10 adjoining the outside of the cavity roof, and a central part with a vaulted top 11, which is inserted into the cavity through the hole. The surrounding part 10 has a track 12 in which the front edge 13 of the reflector 9 rests. The inside of the vaulted top 11 is covered with balls 14, being preferably of the same material as the protective glass and providing a light spreading function in combination with the vaulted shape. The lamp socket 7, the lamp 6, the reflector 9 and the protective glass 10, 11 are fixed by means of a spring 15. The spring 15 is tensioned between two attachments 16, 17, being fixedly connected to the cavity 2 or the housing 1. The spring 15 presses the lamp socket 7 and thereby the edge 13 of the reflector 9 against the track 12 in the protective glass, which is thereby pressed against the outside of the cavity roof 3. For lamp replacement, the lamp socket 7 is released by releasing the spring 15. The lamp is replaced and thereafter the spring 15 is again fastened. From this it may be concluded that also the assembling of the lighting device during manufacture of the oven requires few and simple manoeuveres. The drawing shows the lighting device according to the invention in its natural size. Typically the hole has the diameter 14 mm, the cross section diameter of the lamp being in this case about 8 mm. The distance between the filament 18 of the lamp and the cavity wall is about 7 mm. The lamp, which is preferably a halogen lamp, is operated by an alternating voltage of 6V or 12V. In a lamp of this type the length of the filament is about 3 mm. The short length of the filament contributes to an increased life time of the lamp and an improved mechanical strength and ability to withstand vibrations which are generated at the handling of the microwave oven door. A halogen lamp furthermore provides for the desirable temperature resistancy when the lighting device is used in the so called combi-ovens described above. A hole diameter of 14 mm and other measures in accordance with what has been mentioned above provides for a typical microwave leak of maximum 0,1 mV/cm², which is clearly below the tolerable limit values and which will not disturb the control electronics of the oven. Above and in the patent claims the ligthing device has been described as comprising one hole and one glowlamp. Obviously, the same may be comprised in a set of holes and corresponding glowlamps.

Claims: A microwave oven comprising an oven cavity (2) enclosed by a cavity wall (3), a microwave source (19) for feeding microwave energy of a substantially uniform wavelength into said cavity, and a lighting device (4) for illumination of the cavity (2), said lighting device comprising a light source arranged at the cavity wall and a microwave seal substantially preventing leak of microwaves to the external surroundings of the cavity through said lighting device, characterized in that said microwave seal is obtained by means of a preferably circular hole (5) in the cavity wall of a selected smaller size in relation to the wavelength of the microwaves which is such that the dimensioning of the hole substantially prevents passage of microwaves through the hole, and in that said light source (4) comprises a light-intensive glowlamp (6) positioned outside the cavity (2), having a short filament (18) and small dimensions relative to the hole (5), and being arranged right in front of in close connection with the hole. A microwave oven as claimed in Claim 1, wherein said hole (5) in the cavity is covered by a protective glass (11) which is so shaped that it has also the function of a light spreading lens and the lighting device (4) comprises a reflector (9) enclosing the glowlamp (6) in order to direct backwardly and sidewardly radiating light from the lamp towards the hole (5) in the cavity wall characterized in that the hole (5) in the cavity wall (3) is circular with a diameter of 14 mm, that the lamp (6) and the reflector (9) are circular in the plane of the cavity wall, and being combined by means of a lamp socket (7), which is fixed by means of a spring (15) keeping the front edge of the reflector (9) pressed against the outside of the cavity wall (3) directly or via a part of the protective glass which surrounds the hole. A microwave oven as claimed in Claim 2, characterized in that the surrounding part (10) of the protective glass is circular and provided with a track (12) for the edge (13) of the reflector, and comprising a central part which is fitted into the hole, said part having a light spreading vaulted top (11) which is inserted into the cavity.

Inventors: FREDRIKSSON OVE ROBERT, LEVOSKA ILMARI, FREDRIKSSON, OVE ROBERT, LEVOSKA, ILMARI
Applicants: WHIRLPOOL EUROP, WHIRLPOOL EUROPE B.V.
Application year: 1990
Main classification: H05B6
IPC codes: F24C7, H05B6, A01K61