US 3840810 A
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United States Patent Fritts HIGH FREQUENCY ENERGY GENERATOR LOAD SIMULATOR CIRCUIT  Inventor: Rex E. Fritts, Cedar Rapids, Iowa  Assignee: Amana Refrigeration, Inc., Amana,
 Filed: May 24, 1973  App]. No.: 363,785
 US. Cl 324/158 R, 219/1055, 340/410  Int. Cl G08b 29/00, G0lr 23/00  Field of Search 235/184, 185; 219/1055;
 References Cited FOREIGN PATENTS OR APPLICATIONS 6,404,716 10/1965 Netherlands 323/22 ZD OTHER PUBLICATIONS Basic Simulation Techniques by S. Hori, May 1957,
 Oct. 8, 1974 Automatic Control, pgs. 2, 3, 4, 30, 32, relied on.
Primary Examiner-Gerald Goldberg Attorney, Agent, or FirmEdgar O. Rost; Harold A. Murphy; Joseph D. Pannone  ABSTRACT A circuit is disclosed including semiconductor devices simulates the characteristic load for high frequency energy generators, such as magnetrons, under actual operating conditions in high voltage electrical systems. The circuit facilitates testing switches, timers, rectifiers, transformers, safety interlocks and mechanical or other electrical components for use in, illustrative'ly, microwave oven heating apparatus.
6 Claims, 6 Drawing Figures 15 mime menfieon I LOA D PAIENIEU 31914 3.840. 810
@WJEATEFNTAONF'RON [LOAD INPUT VOLTAGE POINT 1 FULL WAVE RECTIFIER VOLTAGE AT POINT 2 PRIMARY CURRENT AT POINT 3, AND MAGNETRON CURRENT AT POINT 4 PAIENTEBHCI 8:914
SNEEI ear 2 MAGNETRON 70 SIMULATOR CIRCUIT TRANSFORMERS AND AIR CIRCULATORS ON OFF Minn.
HIGH FREQUENCY ENERGY GENERATOR LOAD SIMULATOR CIRCUIT BACKGROUND'OF THE INVENTION 1. Field of the Invention The invention relates to electrical circuits and, more particularly, to means for simulating the electrical load characteristics of high voltage high frequency energy generators.
2. Description of the Prior Art A source of high frequency microwave energy widely used in, illustratively, apparatus for heating and cooking is the magnetron oscillator of the type utilized in radar systems and described in detail in the text Microwave' Magnetrons, Vol. 6, Radiation Laboratory Series, McGraw-Hill Company, Inc., I948 by G. B. Collins. Such generators typically include an anode comprising a plurality of cavity resonators disposed around a central oxide-coated cathode with crossed electric and magnetic fields extending within an interaction region. The electrons emitted are accelerated toward the cavity resonators and rotate in a substantially spoke-like manner to result in the high frequency electrical energy oscillations. The high voltages for the electrical fields between the cathode and anode, as well as the magnetic fields in the case of electromagnets, are provided from low frequency main line AC sources at either 1 or 220 volts Power supplies including high voltage transformers and-rectifiers produce DC voltages at levels of, illustratively, 3,000 6,000 volts with either full or half wave rectifier circuits connected to the secondary windings of the transformer. Separate transformers may be employment to heat the cathode filament.
In the manufacture of apparatus of the type employing subject energy generators numerous electrical and mechanical components are utilized. Switches, timers, rectifiers, transformers and safety interlock switches for controlling and handling the high operating voltages are utilized. Extensive operational, as well as life testing of these components, are conducted to assure compliance with State and Federal regulatory standards by such agencies as the Federal Communications Commission, Department of Health, Education and Welfare and the United States of America Standards Institute in controlling escaping radiation and the satisfactory operation of the end product by the customer. Extensive tests require utilization of energy sources comparable to that utilized in the final end product. Since magnetron energy generators are most commonly employed in the applicable apparatus it is. desirable that such generators or a simulated characteristic operating load be provided for performance testing of all system components. The 600-700 watt microwave energy generator and its power supply represents an investment of approximately one-fourth of the manufacturers cost of the domestic ovens which retail for $300 to $400. For industrial processing apparatus large numbers of such generators are required to supply many kilowatts of energy. Since numerous generators would be required for extensive component testing, a need arises for a suitable means for simulating the characteristic operating load which is low in cost and with substantially no radiation hazards.
The term Microwave used in the description of the electromagnetic energy spectrum having wavelengths in the order of 1 meter to l millimeter and frequencies in excess of 300 MHz.
SUMMARY OF THE INVENTION In accordance with the present invention the characteristic load of a high frequency energy generator is simulated to provide the representative waveforms experienced when the generator and associated power supply are normally operated by a circuit having a plurality of semiconductor devices such as Zener diodes. Such devices are relatively inexpensive and provide a breakdown characteristic closely approximating the generator conduction voltage during operation. Resistors are also provided to limit the current to a value normally experienced in a high voltage generator system. The Zener diodes provide a voltage across the semiconductor junction which is substantially constant and which corresponds to the actual energy generator conduction characteristics. As the voltage from the power supply increases the breakdown voltage is attained and the Zener diodes conduct current through themselves as well as serially connected resistors. As the voltage is further increased more current will flow through the resistors. As the voltage decreases, the current flow will decrease until the Zener breakdown voltage is reached and the current flow ceases.
'The Zener diodes dissipate a portion of the power with the remainder being dissipated by the resistors. The representative waveforms closely approximate those of a characteristic electrical generator load. The present state of the art in the semiconductor devices has advanced to the point that values of approximately volts for each Zener diode can be readily attained. To approximate the average operating voltage level of a conventional magnetron energy generator approximately 30 Zener diodes would be required. It is possible to provide apparatus for testing a large number of components, for example, safety interlock switches by providing for sequentially opening and closing each of such switches and interconnecting the load simulator circuit embodying the teachings of the invention. Numerous other testing apparatus may be realized-utilizing the circuit of the invention, for testing the electrical and mechanical components associated with any apparatus utilizing high voltage high frequency energy generators.
BRIEF DESCRIPTION OF THE DRAWINGS Details of the invention will be understood after consideration of the following description and reference to the accompanying drawings, 'wherein:
' FIG. 1 is a schematic circuit diagram of the illustrative emobodiment;
FIG. 2 is a representative circuit waveform of the input voltage at a predetermined point in the circuit;
FIG. 3 is a representative circuit waveform of a full wave rectified voltage at the input of the simulated load circuit;
FIG. 4 is a representative circuit waveform of the primary current at the input of a high voltage transformer of the power supply and the current at a point in the simulated circuit load;
FIG. 5 is a perspective rear view of an illustrative testing apparatus for safety interlock switches with the door removed; and i FIG. 6 is a perspective view of the front portion of the testing apparatus illustrated in FIG. 5.
DESCRIPTION OF A PREFERRED EMBODIMENT In FIG. 1 a schematic diagram of the circuit of the invention is shown. The details as to the various components of typical microwave heating apparatus have been purposely omitted for the sake of clarity. Such detailed information may be obtained in the copending application of the present inventor, now U.S. Pat. No. 3,766,437, issued Oct. 16, 1973, which describes the safety interlock system for such apparatus. Additionally, further information may be obtained in U.S. Pat. No. 3,514,566 issued May 26, 1972 to R. Ironfield and assigned to the assignee of the present invention. Further, the text Microwave Heating by D. A. Copson, the AV] Publishing Company, Inc., 1962, Westport, Connecticut, particularly pages 261-305 inclusive and pages 334-345 inclusive will be of assistance.
Referring now to FIG. 1 the illustrative circuit comprises a high voltage electrical system having a low frequency low voltage alternating current source 10 comprising conventional line voltages coupled by conductors 12 and 14 with a manually operated switch 16. A conventional high voltage high frequency electromagnetic energy generator power supply includes a step-up transformer 18 which is grounded at 20. The transformer 18 includes primary winding 22 inductively coupled to secondary winding 24. All the electrical components including lights, interlock switches, timers, as well as motors for fans and mode stirrers have been omitted for the sake of clarity in the understanding of the invention. Transformer 18 typically may have a turns ratio of approximately 50:l to supply the desired high voltages for operation of the high frequency electromagnetic energy generator.
Secondary winding 24 is connected to a full wave rectifier network 26 including a plurality of semiconductor diodes 28 poled as indicated for direct current voltage rectification of the high voltages..The direct current output is coupled by means of lead 30 to the means for simulation of the high frequency high voltage energy generator operating characteristics, generally indicated by the dashed line 32 to be hereinafter described. The transformer for electromagnetic energy generators having a solenoid-type magnetic field producing means also provides the required energy for the energizing of the magnets. In many applications the high voltage transformer is provided with a second secondary winding connected to the cathode filament and provides a voltage of, illustratively, approximately 3 to 4 volts. In the instance of high frequency electromagnetic energy generators utilizing permanent magnets the high voltage transformer may be of the saturable core type having a high leakage reactance with the coupling between the primary and secondary windings providing a substantially constant secondary current output over a range of line fluctuations.
The full wave diode rectifier network 26 functions in the manner well known in the art and is connected to the secondary winding at terminals 34and 36. Terminals 38 and 40 will be positive and negative, respectively, as indicated. The positive terminal 38 is con-- nected to ground and terminal 40 which is always negative is connected to lead 30 in such a manner that the pulsed direct current provided at the terminals willsimulate the pulsed direct current provided at the terminals of, illustratively, the anode and cathode of a high frequency energy generator such as a magnetron. The polarities of the terminals 34 and 36 alternate with each half cycle of the alternating current source so that the opposite arms of the network alternately furnish the half-cycle return paths and assure that the voltage in the lead 30 is continuously conducting.
In FIG. 1 the average voltages and currents for the operation of a high frequency electromagnetic energy generator and the characteristic voltage and current in the simulated circuit are indicated by the numerals 1, 2, 3, and 4 shown in the circles at the appropriate circuit points. The embodiment of the invention comprises a plurality of semiconductor junction devices 42 connected to collectively provide atotal conduction voltage value closely approximating the average energy generator characteristics. For the purposes of the description a plurality of Zener diodes have been illustrated which have a characteristic breakdown voltage in a reverse direction at which the insulating properties of the semiconducting material break down causing current made up of electrons which have escaped from the valence band into the conduction band as a result of the influence of the strong electric field, across the junction. The breakdown is caused by the field emission of holes and electrons in the semiconductor material depletion layer. Under the ideal conditions the voltage across the semiconductor junction remains substantially constant and the current flow is limited only by circuit means external to the junction. In accor dance with the invention resistance means 44 is provided having a limiting effect on the current to an average value experienced by the operative high frequency energy generator in a high voltage system.
Referring to FIGS. 2, 3 and 4 the representative circuit waveforms will be illustrated for the circuit points designated within the circle at the appropriate points. In FIG. 2 the primary voltage is the alternating current line voltages at point 1 represented by the modulating curve 46. The full wave rectified voltage at point 2 or, terminal 40 connected to the lead 30 is indicated by the waveform 48. With the illustrative simulation circuit means incorporating Zener diodes 42, the collective breakdown voltage value corresponds with the average conduction voltage characteristics of the energy generator being simulated. The diodes dissipate a portion of the power from the voltage at point 2. As the voltage from the power supply 18 and 26 increases a value is attained substantially equal to the collective Zener breakdown voltages and the devices 42 conduct current through themselves as well as resistance means 44. As the voltage increases further more current will flow limited to the average current normally experienced by an operative energy generator. As the voltage decreases the current flow decreases until the Zener breakdown voltage is reached at which point current flow will stop.
In FIG. 4 the representative current waveforms at the input of the primary winding point 3, is shown as well as the current at point 4 of the simulated circuit or the average conduction current condition through the Zener diodes is indicated by pulsed waveform 50, which would be substantially equal to the average operating characteristics of a high frequency electromagnetic energy generator. The simulator circuit described herein may also be provided with half wave direct current rectification and approximate closely the waveforms experienced when the energy generator is operated from such a rectified source.
Referring to FIGS. 5 and 6 apparatus utilizing the simulation means the invention for the testing of components of an oven apparatus will be described. In FIG. 5 the testing console 52 provides for the testing of a plurality of the safety interlock switches utilized in the door actuators and latching arrangements for microwave ovens. A number of switches 54 are circumferentially mounted in a circular array on plate member 56 supported within cabinet 58. The lever arms 60 of switches 54 which actuate the two pole switch are disposed in the same direction so as to be sequentially contacted and depressed by means of a rotatable arm 62 rotated about pivot 64. The motor for energizing the arm is mounted behind plate 56. The contacting surface of arm 62 for engaging the switch lever arms comprises a cam surface 66. The direction of the travel of the rotatable member is indicated by arrow 68. As each of the switches are contacted the high voltage high frequency simulator circuit is electrically connected to each of the switches to carry the high voltages required in theoperative oven. The simulator circuit may occupy the bottom portion of the cabinet 58 and is indicated generally by the numeral 70. In order to prevent the overheating of the high voltage transformers suitable air circulators may be provided in this portion of the cabinet. The door member for enclosing the rear of the test apparatus has been removed to disclose the internal structure and would be supported by the hinges 72.
Referring to FIG. 6 the front view of the testing apparatus 52 is shown and suitable louvers 74 are provided in the lower portion of the wall member 76. In the upper portion a plurality of electromechanical actuated counters 78 are provided with suitable windows for recording the number of cycles clocked by each of the sequentially disposed safety interlock switches. Indicator lights 80 and 82 may be provided in order to signal any failure of the components. Each of the switches 54 of the type illustrated are provided with terminal leads 84, 86 and 88 and suitable conductors are shown extending through insulators in the plate 56 to interconnect these switches to the simulator circuit as well as counting means 78. Since the disclosed electrical components are desirably cycled through many thousands or tens of thousands of cycles over a lengthy time span, the importance of the simulator circuit of the invention in providing for the simplification of the testing apparatus as well as any radiation hazards may be noted. In simulating of the average voltage and current charac teristics of a high frequency electromagnetic energy generator, such as a magnetron, wherein each of the switches is required to carry high voltages of between 3,000 6.000 volts, the total amount of semiconductor devices of the Zener diode type in accordance with the present state of the art of approximately 100 volts for each diode would require approximately 30 diodes in series for each testing console. Suitable compensation in the number of semiconductor devices will be possible to simulate any desired electrical characteristics for the components being tested.
While a preferred illustrative embodiment of the testing apparatus and simulator circuit has been described, numerous variations and modifications will be readily evident to those skilled in the art. It is intended, therefore, that the preceding description of the invention and the'illustrative embodiments be considered in the broadest aspects and not a limiting sense.
1. A high voltage electrical testing system comprising:
A low frequency alternating current voltage source;
a step up transformer having primary and secondary windings connected to said source;
direct current high voltage rectification means connected to said secondary winding; and solid state means for generating substantially the same electrical conduction characteristics of a high frequency electromagnetic energy generator connected to said rectification means;
said solid state generation means including a plurality of semiconductor devices collectively having a breakdown voltage value substantially equal to the average operating voltage characteristics of said generator; and
current limiting means connected to said semiconductor devices having a value to control the value of current through said devices to be substantially equal to the average conduction current characteristics of an operative generator.
2. A system according to claim 1 wherein said semiconductor devices comprise Zener diodes.
3. Apparatus for testing components for use in high frequency electromagnetic energy ovens comprising:
means for generating the average electrical operating conditions of an-electromagnetic energy generator including: a voltage source; transformer and direct current high voltage rectification means connected to said source; semiconductor means having a breakdown voltage value substantially equal to the average operating voltage characteristics of said generator; and current limiting resistance means connected to said semiconductor means to control the flow of current to a value substantially equal to the average conduction current characteristics of an operative generator; means for mounting a predetermined number of said components; and means for conductively connecting each of said components to said generating means in a predetermined manner. 4. The apparatus according to claim 3 wherein said semiconductor means comprise Zener diodes.
5. The apparatus according to claim 3 wherein said components are mounted in a circular array.
6. The apparatus according to claim 5 and rotatable said components to said generating means.