Magnetron Moding Interrupter Control Circuit

Abstract

A magnetron operating control circuit is disclosed including means for sensing the commencement of electromagnetic energy radiation and means derived from the sensing means to interrupt the application of line voltages to the magnetron power supply and allow the voltages to decrease for a sufficient time to shift to desired normal operating mode conditions when the supply is reenergized. The circuit incorporates a flexible sensing feature which accommodates the time of the interruption cycle to variations in tubes, line voltages and other circuit parameters, particularly in microwave oven apparatus to substantially reduce the magnetron moding problems.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to electrical circuits for operating magnetron energy generators in microwave oven apparatus.

2. Description of the Prior Art

In microwave cooking an energy source commonly employed in the generation of high frequency electromagnetic oscillations is the magnetron energy generator. Such devices employ crossed electric and magnetic fields tranversing an intraction region between a central cathode and cavity resonators defined by an anode member. The emitted electrons interact in energy-exchanging relationship with the electrical energy stored in the cavity resonators and extremely high oscillations are generated, typically, at microwave frequencies of 2450 MHz in the electromagnetic energy spectrum. For the purposes of the present invention the term "microwave" refers to radiation in that portion of the electromagnetic energy spectrum having wavelengths of from 1 meter to 1 millimeter.

Magnetron tubes are typically operated by a power supply utilizing main line AC voltages which are rectified to provide DC voltages in the region of 4,000 to 6,000 volts. Examples of prior art magnetron power supply circuits are disclosed in U.S. Letters Pat. No. 3,396,342 issued on Aug. 6, 1968 to A.E. Feinberg. Such circuits are especially useful in energizing magnetrons having permanent magnets rather than electromagnets to provide the desired magnetic fields which usually extend parallel to the axis of the cathode member. The circuit disclosed in the referenced patent is of the high leakage reactance transformer type which seeks to substantially minimize any fluctuations in the DC output voltages due to variations in the AC line voltages. In such circuits the secondary winding is inductively coupled to the primary winding with full wave rectifying circuit means connected in series with the secondary winding and cathode. In some applications magnetron power supplies provide the cathode filament and anode voltages simultaneously and no anode current is drawn until the cathode filament has reached operating temperature. As a result of the initial lack of a load, open circuit transient voltages high as 12 to 15 thousand volts can result when the power supply is energized. The high transient surge currents present a problem in that once the anode voltage rises to the level which allows the magnetron to oscillate the tube may operate at undesirable higher order modes characterized by lower efficiency and high temperatures which shorten tube life. The oscillations, therefore, are desired in a particular operating mode, typically the "pi" mode which is at a lower anode voltage level and provides for stable operation. A "mode shift" is desirably instituted before the higher order mode oscillations become self-sustaining.

One method of coping with the transient voltages surge problem discussed in the prior art includes a separate low voltage filament transformer for preheating the cathode before the main anode voltages are applied. Additional circuitry and components are required for such a separate preheat cycle. In microwave ovens the operator is required to operate the device in separate steps with a preheat period before the main high electrical voltages are applied to the magnetron.

Another suggested solution to the problem involves the momentary interruption of the primary circuit for both the filament and anode voltages at a predetermined time after the magnetron is energized. Magnetron moding is effectively controlled by allowing the anode voltages to decay below the operating point but not low enough to allow excessive cathode filament cooling to occur before the primary circuit is reenergized. The anode voltage will then rise to the normal operating frequency mode after the predetermined interruption and the magnetron will oscillate in this mode. An example of circuits utilizing this principle is found in the copending patent application entitled "Magnetron Starting Circuit" Ser. No. 185,624 filed Oct. 1, 1971 by Donald E. Peterson and assigned to the assignee of the present invention. The interrupt time period is selected as a compromise considering the variations in magnetron tubes, domestic line voltages and other circuit parameters to select the optimum average time period. since it is desirable to provide fast, low cost equipment, improvements in power supply circuits for simultaneously applying the anode and cathode filament voltages and operating of the magnetron in the proper operating mode are continually being explored.

SUMMARY OF THE INVENTION

In accordance with the present invention a magnetron operating control circuit for utilization in microwave oven apparatus embodiment is disclosed which is based on the sensing of the commencement of electromagnetic energy radiation to derive control signals to interrupt the application of the full anode line voltages for a sufficient time to assure that continued operation of the magnetron generator will be in the normal operating mode frequency. The sensing means control the interruption of the application of the simultaneous voltages rather than rigid delay timer means which represent an approximate compromise of the individual tube performance variables as well as circuit variations. The disclosed magnetron control circuit provides for a customized operation to assure that the proper magnetron operating mode frequency is achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

Details of an illustrative embodiment of the invention will be readily understood after consideration of the following description and reference to the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of the electrical circuit of the invention;

FIG. 2 is an isometric view of a microwave oven apparatus embodying the invention with a portion of the outer casing and waveguide launching means broken away to reveal internal structure; and

FIG. 3 is a vertical cross-sectional view of the microwave oven apparatus illustrated in FIG. 2.

DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to the drawings, particularly FIGS. 2 and 3, the microwave oven apparatus 10 utilizing the radiation sensing interrupter magnetron operating control circuit 12 will now be described. Top and bottom conductive walls 14 and 16 define with sidewalls 18 an enclosure 20. An access opening is provided for front loading and means for closing the access opening comprise door assembly 22 which is actuated by handle 24. A control panel member 26 adjacent to the door assembly provides for the mounting of timers 28 and 30 as well as the start, stop and light buttons 32, 34 and 36.

The magnetron energy generator, indicated generally by block 38, provides the electromagnetic energy for radiation within the enclosure 20. Magnetrons are considered to be well-known in the art and additional information may be obtained from the "Microwave Magnetrons," Radiation Laboratory Series, Vol. 6, by G. B. Collins, McGraw-Hill Book Company, Inc., 1948, as well as the referenced copending application. The high voltage power supply and all electrical circuits are indicated by box 40. The electromagnetic energy is launched form the magnetron anode resonators by means of an antenna 42 within dielectric dome 44, extending within a rectangular waveguide launching section 46 supported by top wall 14 of the oven enclosure. The waveguide is closed at one end by a terminating wall 48 having perforations 50 which are utilized for the detection of any electromagnetic energy radiation once the magnetron generator commences to oscillate. The antenna member 42 is spaced from the terminating wall 48 approximately 1/4 of a wavelength at the operating frequency of the apparatus for optimum directivity. Distribution of the energy in a plurality of modes is accomplished by any of the well-known means including, for example, a mode stirrer 52 comprising a plurality of paddles 54 supported by a shaft 56 which is actuated by motor 58 supported on top wall 14. A dielectric plate 60 spans the indentation in the bottom wall 16 and supports the articles to be cooked or heated within the enclosure 20. The microwave energy utilized in such apparatus operates at the F.C.C. assigned frequency of 2450 MHz.

In accordance with the invention the magnetron moding interrupter control circuit commences with electromagnetic energy radiation sensing means such as, for example, a semiconductor diode 62. Such devices typically are unidirectional and comprise a chip of silicon or germanium and an electrode. To sense when the magnetron commences oscillation, albeit it at the higher order modes, the novel circuit incorporates the diode sensing means 62 in close proximity to the perforations 50 in the back terminating wall 48 of the waveguide launching section 46 near antenna 42.

Referring now to FIG. 1 a schematic of the radiation sensing and interrupter magnetron operating control circuit will now be described. For the sake of simplicity the details of the high voltage magnetron power supply circuit including the DC rectification means and transformer have been omitted in order that attention may be focused on the interrupter circuit. An example of a magnetron power supply circuit under consideration may be had by referring to the aforereferenced copending patent application of Donald A. Peterson as well as the power supply circuit disclosed in U.S. Pat. No. 3,396,342 issued to A. E. Feinberg. In the present description both the filament power and anode voltages are applied simultaneously to the magnetron 38 through the power supply 40 from a conventional domestic AC line voltage source 64, typically 115-120 volts by means of electrical leads 66 and 68.

The novel circuit comprises, first, primary AC line voltage source 64, power relay 70 and semiconductor diode 72. Diode 72 rectifies the line voltage which is filtered through resistor R1 and capacitor C1 to the gate of the silican controlled rectifier SCR1 and a diode bridge 76. The current flows through SCR1 closing relay 70 and results in the application of voltages to magnetron 38 through power supply 40. As the magnetron filament heats emission starts and the magnetron anode voltage simultaneously rises until the magnetron operates at either the adjacent or the normal operating mode.

The commencement of the the oscillations results in generation of electromagnetic energy which is detected by the sensing diode 62 in close proximity to the antenna 42. The energy detected derives a signal which is filtered and amplified by voltage amplifier 78 with the resultant signal further filtered and applied to paired transistors 80 and 82. The amplifier 78 is biased by a 12 volt DC supply 84 and the diode filtering circuit includes resistors R2 and R3 and R4. In addition capacitors C2 and C3 are utilized for the detected electromagnetic energy radiation signal through diode 62. The generated signal is fed into the amplifier 78 through resistors R5 and R6 and resistors R7, R8 and R9 are utilized in the amplifier branch to feed transistors 80 and 82.

The amplified signal is conducted by means of resistors R10, R11 and R12 biasing the base of the transistor 82 and capacitor C4 is connected to ground.

Transistors 80 and 82 are biased by the 12 volt source 84 through resistor R13. The output of transistor 80 is applied by resistors R14, R15, R16, R17 and R18 and charging capacitor C5 to a unijunction circuit 86. The unijunction circuit 86 controls the firing of the silicon controlled rectifier SCR2 through a gate including resistor R19 circuit with resistor R20 to ground. The firing of the SCR 2 renders transistor 88 conductive with its base electrode biased by resistor R21 with the collector grounded through resistor R22 after the gate of the unijunction reaches a predetermined level.

OPERATION OF THE SENSING-INTERRUPTER CIRCUIT

The circuit operates initially when power source 64 activates diode 72. The line voltage is rectified, filtered and applied to gate SCR1 on. Current flows through relay 70, and diode bridge 76 to thereby close relay 70 and apply simultaneous voltages to the cathode filament and anode of magnetron 38 through power supply 40. Upon commencement of the electromagnetic energy oscillations energy is detected by diode 72, filtered and the resultant signal is applied to amplifier 78 and the subsequent transistors 80 and 82 with the amplified signal being applied to unijunction circuit 86.

Unijunction circuit 86 controls SCR 2 and at a predetermined level transistor 88 becomes conductive. The conductive condition of transistor 88 results in grounding the gate of SCR1 and opens to interrupt primary voltage source 64. The relay remains open a period of time determined by charging capacitor C5 whereupon transistor 88 stops conducting thereby allowing the SCR1 to becomes conductive and reclose power relay 70 to reapply the voltages to the magnetron through power supply 40 and permit operation of the energy generator in the normal operating mode. As SCR2 becomes conductive, the supply voltage to the unijunction circuit 86 is effectively shorted out to thereby prevent subsequent firings except when the main source 64 or electromagnetic energy radiation from the magnetron 38 are turned off. Recycling occurs when the source 64 or radiation are turned on and/or sensed by the disclosed circuit.

CIRCUIT SPECIFICATIONS

To assist in the practice of the invention some suggested components and electrical parameters utilized in the novel sensing and interrupter circuit are listed in the table below.

Diode 62 - IN830

Diode 72 - IN206

Amplifier 78 - No. 709 Voltage Amplifier

Transistor 80 - 2N697

Transistor 82 - 2N697

R1 -- 33k ohms

R2 -- 1k ohms

R3 -- 1k ohms

R4 -- 500 ohms

R5 -- 100 ohms

R6 -- 100 ohms

R7 -- 470k ohms

R8 -- 4.7k ohms

R9 -- 100k ohms

R10 -- 2.2k ohms

R11 -- 4.7k ohms

R12 -- 1k ohms

R13 -- 2.2k ohms

R14 -- 2.2k ohms

R15 -- 2.2k ohms

R16 -- 15k ohms

R17 -- 2.2k ohms

R18 -- 4.7k ohms

R19 -- 220 ohms

R20 -- 2.7k ohms

R21 -- 220 ohms

R22 -- 1.5k ohms

C1 -- 10/200 volts

C2 -- .01 microfarad

C3 -- 500 picofarads

C4 -- 15/20 volts

C5 -- 68 microfarads/50 Volts

Unijunction 86 -- Commercially Available

Scr1 and 2 -- Commercially Available

There is thus disclosed a novel circuit for operating magnetron tubes which readily adapts to individual tube and circuit variations to assure operation in the "pi" operating mode. The sensing of the electromagnetic radiation to derive the control signal for interruption of the application of filament and anode voltages can result in a time cycle which will be neither to short nor too long. Typically, such interrupt cycles will be a varying duration and occur at times ranging from 4 to 10 seconds after the circuit is initially energized. Numerous variations and substitutions will be evident to those well versed in the art and the preceding description of an illustrative embodiment is to be considered in its broadest aspects.

Claims

We claim:

1. In combination:

an electromagnetic energy generator;

means for energizing said generator including an Ac voltage source and power supply comprising a transformer power relay and high voltage DC rectification means;

means for energizing said voltage source to operate said generator including a power relay and semiconductor means;

means for sensing commencement of operation of said generator comprising an energy radiation detector to derive an electrical signal;

means for filtering, amplifying and coupling said derived signal to a interrupt circuit means including semiconductor means and a charging capacitor to fire said last named means to interrupt operation of said generator for a predetermined period of time determined by storage of electrical energy in said capacitor by opening said power relay.

2. The combination according to claim 1 wherein said energizing means comprise a diode rectifier, a first silicon controlled rectifier, and electrical filter means.

3. The combination according to claim 2 wherein said interrupt circuit means includes a unijunction device and a second silicon controlled rectifier interconnected to said first silicon controlled rectifier by a transistor device having a grounded electrode when in the conductive state to ground said first rectifier and allow said power relay to open deenergizing said voltage source.

4. A microwave oven apparatus comprising:

an enclosure;

a magnetron energy generator,

means for coupling from said generator to be radiated within said enclosure;

means for energizing said generator including an AC voltage source and power supply comprising a transformer power relay, and high voltage DC rectification means;

means for energizing said voltage source to operate said generator;

means for sensing radiation of energy in said coupling means to derive and electrical signal;

electrical circuit means for filtering, amplifying and utilizing said derived signal to interrupt operation of said magnetron for a predetermined period of time by opening said power relay.

5. The apparatus according to claim 4 wherein said magnetron includes an output antenna.

6. The apparatus according to claim 4 wherein said energy coupling means comprise a hollow waveguide transmission section.

7. The apparatus according to claim 4 wherein said magnetron includes an output antenna and said energy coupling means comprise a hollow waveguide transmission section with said antenna disposed therein.

8. The apparatus according to claim 7 wherein means for sensing energy radiation comprise a diode rectifier adapted to detect energy radiated from said antenna.