US3679908A

US3679908A - Safety device for sensing preventing radiation leakage from microwave-producing apparatus

Info

Publication number: US3679908A
Application number: US98378A
Authority: US
Inventors: Lamberto Mazza
Original assignee: Industrie Zanussi SpA
Current assignee: Industrie Zanussi SpA
Priority date: 1970-01-17
Filing date: 1970-12-15
Publication date: 1972-07-25
Anticipated expiration: 1989-07-25
Also published as: FR2080555B3; JPS541939B1; DE2029559B2; DE2029559C3; DE2029559A1; GB1298163A; FR2080555A3

Abstract

A microwave-leakage preventing safety device, especially adapted for microwave cooking ovens and like appliances, which is characterized by a radiation sensitive gas lamp (or gas diode) placed in the vicinity of the point at which radiation leakages are expected, for example the oven door. The gas lamp is a part of an electric safety circuit which cuts off the current feed to the microwave generator proper whenever the detecting lamp has radiations impinging thereon.

Description

United States Patent Mazza 1 July 25, 1972 [54] SAFETY DEVICE FOR SENSING 250/83.6

PREVENTING RADIATION LEAKAGE FROM MICROWAVE-PRODUCING [561 CM APPARATUS UNlTED STATES PATENTS [72] Inventor: Lumber-to Mazza, Pordenone, Italy 3,350,702 10/ I967 Herman ..340/228 n 1 Assignees lndustrie A. Zanussi P Pordenone. 3,063,0l0 11/ I962 Richardson .340/-..8

Italy Primary livaminer-Herman J. Hohauser 2 Filed; 15 1970 AlmrneyHolman& Stern [21] Appl. No.: 98,378 [57] ABSTRACT A microwave-leakage preventing safety device, especially [30] Foreign Application Priority Da adapted for microwave cooking ovens and like appliances, which is characterized by a radiation sensitive gas lamp (or gas Jan. 17, 1970 Italy ..l95l0 A/70 diode) placed in the vicinity of the point at which radiation leakages are expected, for example the oven door, The gas [52] U.S. Cl ..307/ll7, 2l9/l0.55, 325/l49, lamp is a pan of an electric safety circuit which cuts off the 325/364 340/228 current feed to the microwave generator proper whenever the [51 Int. Cl. ..n0lh 35/00 detecting lamp has radiations impinging thereon. [58] Field of Search ..307/92, 94, I17, 116;

2l9/10.55 X; 325/149 X, 363, 364 X; 340/228;

VIVIV 3 Claims, 3 Drawing Figures AA AAL W" H SAFETY DEVICE FOR SENSING PREVENTING RADIATION LEAKAGE FROM MICROWAVE- PRODUCING APPARATUS This invention relates to a safety device against leakage of radiations which could take place in microwave-producing apparatus, more particularly ovens, especially at the doors which give access to the treatment environment.

As is known, microwave-producing apparatus should be equipped with appropriate protection adapted to prevent the emergence of radiations from the treatment environment. Special difficulties have been encountered in producing devices adapted to prevent the emergence of radiations at the sealing regions between the door and the enclosing walls of the apparatus, since the resilient seals as usually provided rapidly deteriorate with the lapse of time and, finally, do not function adequately. A doubtlessly improved approach in this direction has been to place, about the door outline, a waveguide assembly which is capable of sensing the radiation leakages through the above mentioned regions, and which is connected to a crystal detector intended to block the feeding to the microwave generator immediately when the radiation exceeds a predetermined safety level. This approach, however, is inadequate in the case where the door is made of a shielding glass rather than a metal, in order to permit the visual inspection of the interior of the apparatus. As a matter of fact, the glass may be shattered, thus causing a considerable radiation leakage through the door body, a leakage which is not sensed by the waveguide assembly, or the detector, either. Similar difficulties are experienced with a shielding glass or any other wall of the apparatus, in which fears exist of radiation leakages at welding seam locations or other junction areas.

These shortcomings are overcome by a device according to this invention, which is characterized in that it comprises at least a microwave-responsive gas tube which is located in the vicinity of possible radiation leakage regions, for example in the neighborhood of the oven door, said tube being electrically, inserted in the control circuit of a controlled diode, the latter being electrically inserted, in turn, in the feeding circuit of a relay whose energization controls the power supply to the electric circuit which feeds the microwave generator.

By so doing, any radiation leakage which is sensed by the gas tube(s) causes a decrease of the resistance thereof, a fact which, in turn, causes the controlled diode to become conductive, this inturn causing the energization of the relay and finally the discontinuance of the power feed to the microwave generator.

Features and advantages of the present invention will be better understood from the ensuing detailed description of a microwave oven equipped with a device according to the invention, the latter being arranged in the neighborhood of the oven door. The oven, which will be described by way of example only, is diagrammatically shown in the accompanying drawings, wherein:

FIG. 1 shows the oven in front elevational view.

HO. 2 shows the oven in side elevational view, partly in section, the door being left ajar. I

FIG. 3 shows the wiring diagram of the radiation detecting safety device with which the oven is equipped.

The microwave oven shown in FIGS. 1 and 2 comprises a substantially prismatic body 1, in whose interior a cooking chamber 2 is confined, an emitting antenna 3 of a microwave generator 4 protruding thereinto. The access to the cooking chamber 2 is through an opening 5 which is encircled by four curled resilient blades 6 and the chamber has a door 7. The door in hinged at 8 to the front wall of the oven and can be operated between two end positions (wide open and fully closed) as defined by the engagement of either of two rollers 9 (a single roller has been shown) with either of two notches l0 and 11 formed in each of two levers 12 (a single lever being shown) hinged to the door sidewalls and held against the rollers 9 by respective springs 13 (only one has been shown). In addition, the door 7 has a handle 14 and a transparent central area 15 consisting of a glass plate intended to act as a shield against the microwaves as emitted by the generator 4 while food is being cooked. The seal which is required between the door 7 and the oven front wall is provided by four resilient blades 6 which are pressed by the door against the oven wall as the door is pushed towards the closed position, shown in dotted lines in FIG. 2.

The oven described herein, has a device intended to block the operation of the microwave generator both if the door is opened and in the case of an exceedingly high radiation leakage caused by a poor seal provided by the resilient blades 6, or by breakage of the door shielding glass 15. The device comprises a gas lamp 16, located above the door in a chamber 17 enclosed by a microwave-transparent wall 18, and is serially connected with a first relay coil 19 having a low energization current, with a resistor 20 and a potentiometer 21, in the control circuit of a controlled diode (SCR) 22 whose feeding circuitry comprises an AC source (not shown), a normally closed pushbutton switch 23, a full wave rectifier 24 and a second relay coil 25. The two relay coils l9 and 25 control respective

movable contacts

26 and 27, the first of which is normally open and the second normally closed, said contacts being serially connected in the electric feeding circuit of the microwave generator 4.

In operation, until such time as the oven door 7 is closed, the seal provided by the blades 6 is largely sufficient and the glass portion 15 of the door is intact, no radiation leakages can occur, so that the resistance of the lamp remains high, the controlled diode 22 is cut off, the relay 25 is de-energized and the contact 27 is closed. The small current flowing through the lamp is however sufficient to keep the relay l9 energized, thereby permitting the generator 4 to be fed with (contact 26 shown in H6. 3 being in its closed position).

If, conversely, as a consequence of the opening of the door or unsatisfactory operation of the blades 6, or shattering of the shielding glass 15, a radiation flow emerges from the cooking environment and impinges on the gas lamp, the resistance of the lamp decreases and the voltage taken by the potentiometer 21 increases until the controlled diode 22 becomes conductive. As a result, the relay 25 is energized and the movable contact 27 is opened and cuts power supply to the microwave generator.

In order that feeding may be reinstated, the pushbutton 27 should be depressed so as to deactivate the controlled diode 22, but not without removing the condition which caused the radiation leakage. This pushbutton can also be coupled for operations with the starting pushbutton of the microwave generator (not shown). 7

The relay 19 has the function of cutting off the feed to the microwave generator, should the gas lamp break.

That which has been shown and described is but an example of how the device according to the invention can be embodied. It is apparent that a number of changes can be introduced therein without departing from the scope of the invention. For example, the single lamp 16 of H6. 1 could be replaced by a number of lamps connected in parallel and in alignment, in a circuit such as that of FIG. 3, after having adjusted the ohmic values. Likewise, the gas lamp 16 could be replaced by any other gas tube, such as a gas diode. ln addition, the device according to the invention could be employed, not only for an oven door, but also for any other component part of the oven in which radiation leakages are likely to occur, for example where there are welding seams, other shielding plate glass walls, and so forth. Obviously, in such a case, the gas tubes should be provided at all the parts of the oven which could be regarded as being critical with respect to radiation leakage hazard. Lastly, the device according to the invention can be applied, in addition to microwave ovens, also to other kind of microwave-producing devices, such as driers of the kind disclosed and claimed in the Italian Patent specification No. 834,762, filed May 28, 1968 in the name of the same applicants.

What is claimed is:

l. A safety device for sensing and preventing radiation leakage from a microwave-producing apparatus comprising in combination: a first series branch including at least one gastube which is sensitive to microwave radiation and is located in a region of possible microwave radiation leakage, a first relay coil electrically connected in series with said gas-tube and arranged to operate a first set of contacts which are inserted in a power supply circuit of the micro-wave producing apparatus so as to break the power supply to the micro-wave producing apparatus upon de-energization of said first relay coil, and means sensitive to current passing through said first series branch; a second series branch connected in parallel with said first series branch and including a controlled diode and a second relay coil connected in series therewith, the

second relay coil connected to operate a second set of contacts which are inserted in said power supply circuit so as to break the power supply therein upon energization of said second relay coil; and a dc source supplying direct current to said first and second series branches, whereby, upon the gastube sensing microwave leakage, the power supply to the microwave-producing apparatus is interupted.

2. A safety device as in claim 1, wherein the microwaveproducing apparatus is comprised in a microwave cooking chamber having a door, and where said gas tube is disposed in the vicinity of the door opening region.

3. A device according to claim 1, characterized in that said gas tube is a gas lamp.

Claims

1. A safety device for sensing and preventing radiation leakage from a microwave-producing apparatus comprising in combination: a first series branch including at least one gas-tube which is sensitive to microwave radiation and is located in a region of possible microwave radiation leakage, a first relay coil electrically connected in series with said gas-tube and arranged to operate a first set of contacts which are inserted in a power supply circuit of the micro-wave producing apparatus so as to break the power supply to the micro-wave producing apparatus upon de-energization of said first relay coil, and means sensitive to current passing through said first series branch; a second series branch connected in parallel with said first series branch and including a controlled diode and a second relay coil connected in series therewith, the second relay coil connected to operate a second set of contacts which are inserted in said power supply circuit so as to break the power supply therein upon energization of said second relay coil; and a d.c. source supplying direct current to said first and second series branches, whereby, upon the gas-tube sensing microwave leakage, the power supply to the microwave-producing apparatus is interupted.

2. A safety device as in claim 1, wherein the microwave-producing apparatus is comprised in a microwave cooking chamber having a door, and where said gas tube is disposed in the vicinity of the door opening region.