US3753003A

US3753003A - Safety device against radiation leakage from microwave apparatus

Info

Publication number: US3753003A
Authority: US
Inventors: L Mazza
Original assignee: Industrie Zanussi SpA
Current assignee: Industrie Zanussi SpA
Priority date: 1970-10-07
Filing date: 1971-10-05
Publication date: 1973-08-14
Anticipated expiration: 1990-08-14
Also published as: ES208792U; JPS5734634B1; ES208792Y

Abstract

A safety device for microwave ovens is disclosed, which prevents radiation leakage from such an oven, the device being of the kind where a gas lamp is located in the area where radiation leakages may occur, said lamp being connected serially with a potentiometer which cooperates with a controlled diode which drives a relay which cuts the feed to the microwave generating circuit, the safety device being characterized by further comprising a bistable circuit and an amplifier which are connected to one another in cascade arrangement, the bistable circuit and the amplifier being inserted between the potentiometer voltage tap and the control electrode of the controlled diode. By such an arrangement, the sensitivity, the promptness and the reliability of the safety device of the kind described are considerably improved.

Description

United States Patent [191 Mazza 1 SAFETY DEVICE AGAINSTRADIATION LEAKAGE FROM MICROWAVE APPARATUS [301 si nmil sa iqnlir eritriqala Oct. 7. i970 .ltaly 30700 A/70 [52] US. Cl 307/117, 325/149, 325/364, 340/228 [51] Int. Cl. H0lh 35/00 [58] Field of Search 307/92, 94, 116, 307/117; 219/10, 55; 325/149, 363, 364; 340/228 [56] References Cited UNITED STATES PATENTS 10/1967 Herman ..340/228 11/1962 Richardson 340/228 [45.1 Aug. 14, 1973 Primary Examiner-Herman J. Hohauser Attorney-Holman 8L Stern ABSTRACT A safety device for microwave ovens is disclosed, which prevents radiation leakage from such an oven, the device being of the kind where a gas lamp is located in the area where radiation leakages may occur, said lamp being connected serially with a potentiometer which cooperates with a controlled diode which drives a relay which cuts the feed to the microwave generating circuit, the safety device being characterized by further comprising a bistable circuit and an amplifier which are connected to one another in cascade arrangement, the bistable circuit and the amplifier being inserted between the potentiometer voltage tap and the control electrode of the controlled diode. By such an arrangement, the sensitivity, the promptness and the reliability of the safety device of the kind described are considerably improved. I

7 Claims, 2 Drawing Figures MICROWAVE 0* GENERATOR 7 minnow: 14 ms SHEET 1 0F 2 Aadmber-fo M41 1 INVENTOR. WW

PATENTEU MIR 1 4 I975 SHEH 2 BF 2 EOCEMZMU In U.S. Patent application Ser. No. 98,378 filed on Dec. 15, 1970 in the name of the same applicant, now Pat. No. 3,679,908, a safety device against radiation leakage from microwave apparatus has been disclosed and claimed, in which the operation is based on the ionization by the agency of microwaves of at least a gas lamp which is situated in the neighborhood of the area where possible radiation leakages from the internal chamber of the apparatus may be experienced. As a matter of fact, this gas lamp is electrically and serially connected with a potentiometer from which the control voltage is drawn for a controlled diode which, in turn, is electrically and serially connected with a relay whose energization controls the opening ot the microwave generator feeding circuit. Thus, the variation of the resistance of the lamps, which is due to ionization by the agency of the microwaves causes the controlled diode to be primed and thus the relay to be energized and the microwave generator to be deactivated.

An object of the present invention is to provide a device according to the main patent but having an enhanced sensitivity and improved promptness and reliability of actuation, and also to make the sensitivity adjustment of the device less critical, all this having in mind the final objective of improving the safety factor of the device.

Such a desirable improvement of sensitivity, promptness and reliability in entering action is obtained, according to this invention, by inserting between the voltage tap of the potentiometer and the control electrode of the controlled diode, a bistable circuit and an amplifier, mutually connected in cascade fashion, whereas the sensitivity adjustment is made less critical by providing an impedance match between the circuit connected to the potentiometer and the unblocking impedance of the controlled diode.

The features of the present invention will be better understood with the aid of the ensuing detailed description of a microwave oven equipped with a safety device made according to the present invention. Such an oven, which is disclosed and will be described herein by way of example only, is diagrammatically depicted in the accompanying drawings, wherein:

FIG. 1 shows the device in side elevational view, partly in section with the door set ajar, and

FIG. 2 shows the circuit diagram of the safety antiradiation device with which the oven is equipped.

The microwave oven shown in FIG. 1 comprises a substantially parallelepipedal body 1, having a cooking chamber 3 in its interior, into which projects the emitting antenna 3 ofa microwave generator 4. An opening 5, surrounded by four resilient curled laminae 6 and having a closing door 7, gives access to the cooking chamber. The door 7, hinged and displaceable between a fully open and a fully closed position, has a handle 14 and a central transparent portion 15 consisting of a glass plate adapted to act as a shield against the microwaves as emitted by the generator 4 during the food cooking stage. A gas lamp 16, located in a cavity 17 protected by a microwave-pervious wall 18, is an integral part of the device whose task is to block the operation of the microwave generator if radiation leakage occurs. For a more comprehensive disclosure of the device, reference should be had to the U.S. Patent application Ser. No. 98,378, now Pat. No. 3,679,908.

The gas lamp 16 is a part of an anti-radiation safety device whose circuitry diagram is shown in FIG. 2 and comprises a source of alternating current (not shown) which, via a pushbutton switch 20, feeds two circuit branches in parallel, the first of which comprises a diode 21 and the parallel connection of a capacitor 22 and a series connection composed by a relay 23 for controlling a normally open contact 24 inserted in the microwave generator (4) feeding circuit, a resistor 25, the gas lamp [6, a resistor'26 and a potentiometer 27, whereas the second branch comprises an SCR diode 30 and a relay 28 which respectively control a normally closed contact 29 inserted in the feeding circuit of the microwave generator and a normally open contact 46 connected in parallel with respect to the SCR diode 30.

The slider of the potentiometer 27 and the control electrode of the SCR, diode 30 are mutually connected by means of a circuit assembly comprising a snapping circuit of the bistable type and an amplifier. The bistable circuit comprises two

npn transistors

31 and 32 having their emitters connected to the ground through a common resistors 33 and their collectors connected to a positive feeding terminal 34 via the

respective resistors

35 and 36. The base of the transistor 31 is connected to the slider of the potentiometer 30 via a variable resistor 38, whereas the base of the transistor 32 connected to the ground via a resistor 39 via the parallel connection of a resistor 40 with a capacitor 41. The amplifier comprises, in turn, an npn transistor 42 having its base directly connected to the collector of the transistor 31, its collector directly connected to the positive terminal 34 and its emitter connected to ground via a resistor 43 and to the control electrode of an SCR diode via a resistor 44. Between the control electrode of the SCR diode 30 and the ground there is arranged, in addition, a reverse bias diode 45.

in operation, until such time as no microwave leakage is experienced, the resistance of the lamp 16 remains high, so that the current flowing therethrough is low even if it is enough as to keep the relay 23 energized (thus the contact 24 is closed) and the transistor 31 conducting. By virtue of the conduction of the latter, the transistor 42 is cut off and consequently so is the SCR diode 30. The relay 28 is thus de-energized so that the contact 29 is closed and permits that the microwave generator 4 be fed.

If, however, due to the opening of the door 7 or the malfunction of the blades 6 or breakage of the shielding glass 15 a radiation leakage occurs, the radiations impinge on the lamp 16 causing the ionization thereof. The result is a strong resistance drop which causes the flow of a heavy current composed by a substantially direct current (rectified by the diode 21 and partially clipped by the capacitor 22) and by an impulsive portion consisting of irregular pulses which are indirectly originated by the radiations. The capacitor 37 blocks the direct current of the signal and sends to the base of the transistor 31 the alternating sequence of positive and negative pulses only, as originated by the pulses due to the radiations as aforesaid. Inasmuch as the transistor 31 is of the npn type, the first incoming negative pulse switches it to a cutoff state (the reasoning should be reserved if the transistor 31 would be of the pnp type). The cutoff of the transistor 31, in addition to rendering the transistor 32 conducting, that which has the task of shortening the switching times of the snap circuit while simultaneously rendering it stabler due to the presence of a delay circuity formed by the resistor 40 and the capacitor 41, causes the conduction of the transistor 42 and thus the application to the control electrode of the SCR diode 30 of a voltage which is sufficient to prime it. Then the relay 28 is energized and opens the contact 29, the result being the deactuation of the microwave generator and the closing of the contact 46 which keeps the relay 28 energized. In order to reinstate the feed, and that obviously after having removed the cause of the radiation leakage, it is necessary to press the-button 20 so as to deenergize the relay 28, which, in turn, reopens the movable contact 46 and closes the movable contact 29 again. By releasing the pushbutton 20 the device is fed once more and is brought back to its initial condition.

As desired, the device according to the present invention affords, in addition to a high sensitivity, an easy adjustment of sensitivity (through the potentiometer) in that, since the input impedance of the snapping circuit is high, an impedance match is obtained with the source of control pulses (drawn between the slider and the lower end of the potentiometer 27). Likewise, the impedance match is obtained also between the output of the snap circuit and the input of the current amplifier and also between the output of the latter and the impedance of the control circuit of the SCR diode.

What is claimed is:

1. A safety device for sensing and preventing radiation leakage from a microwave-producing apparatus having a microwave feeding circuit, said device comprising, in combination, a first circuit branch including a gas lamp which is sensitive to microwave radiation and which is disposed in a region of possible microwave radiation leakage from the microwave-producing apparatus, and a potentiometer electrically coupled with said gas lamp, said potentiometer having a voltage path providing a control voltage therefrom; a second circuit branch including a controllable diode having a control electrode, and a relay means serially connected to said controllable diode for energization thereby, said relay means upon energization opening the microwave feeding circuit; bistable circuit means receiving said control voltage from said potentiometer voltage tap for changing output states in response thereto; and amplifier means coupled between said bistable circuit means and said control electrode of said controllable diode for triggering saidcontrollable diode.

2. A device according to claim 1, further including means effecting impedance matches between the output circuit of the control voltage and the input of the bistable circuit means, the output of said bistable circuit means and the input of said amplifier means and the amplifier output and said controlled diode input.

3. A device according to claim 1, characterized in that in parallel with the controlled diode there is arranged a normally open movable contact which is driven to become closed by the energization of said relay.

4. A device according to claim 1, characterized in that between the potentiometer voltage tap and the bistable circuit, there is inserted a capacitor for blocking the DC component of the control voltage taken from the potentiometer.

5. A device according to claim 1, characterized in that said bistable circuit means comprises a transistor the conductivity of which is dependent on the voltage drawn from the potentiometer.

6. A device according to claim 5, characterized in that said bistable circuit means further comprises a second transistor which is common-emitter connected with respect to the first transistor, the base of the second transistor being connected to the collector of the first transistor through a delay circuit.

7. A device according to claim 6, characterized in that said amplifier means comprises a transistor, whose base is connected to the collector of the first transistor of said bistable circuit means and the collector-emitter circuit comprises a load resistor across which the control voltage for the controlled diode is drawn.

Claims

1. A safety device for sensing and preventing radiation leakage from a microwave-producing apparatus having a microwave feeding circuit, said device comprising, in combination, a first circuit branch including a gas lamp which is sensitive to microwave radiation and which is disposed in a region of possible microwave radiation leakage from the microwave-producing apparatus, and a potentiometer electrically coupled with said gas lamp, said potentiometer having a voltage path providing a control voltage therefrom; a second circuit branch including a controllable diode having a control electrode, and a relay means serially connected to said controllable diode for energization thereby, said relay means upon energization opening the microwave feeding circuit; bistable circuit means receiving said control voltage from said potentiometer voltage tap for changing output states in response thereto; and amplifier means coupled between said bistable circuit means and said control electrode of said controllable diode for triggering said controllable diode.