CA1188751A - Microwave oven power conversion arrangement - Google Patents
Microwave oven power conversion arrangementInfo
- Publication number
- CA1188751A CA1188751A CA000383913A CA383913A CA1188751A CA 1188751 A CA1188751 A CA 1188751A CA 000383913 A CA000383913 A CA 000383913A CA 383913 A CA383913 A CA 383913A CA 1188751 A CA1188751 A CA 1188751A
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- CA
- Canada
- Prior art keywords
- tubular member
- center conductor
- probe
- microwave
- current
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Abstract
MICROWAVE OVEN POWER CONVERSION ARRANGEMENT
ABSTRACT OF THE DISCLOSURE
A microwave oven including a heating chamber, a magetron power source, oven control circuity, an indicator and a power conversion arrangement electromagnetically coupled to the heating chamber for tapping a portion of the microwave cooking energy in the heating chamber. The power conversion arrangement includes a probe extending into the heating chamber whereby an AC current is induced in the probe. A rectifier in series with the probe converts the AC into DC current to supply and energize the oven control circuitry and an indicator with DC power.
ABSTRACT OF THE DISCLOSURE
A microwave oven including a heating chamber, a magetron power source, oven control circuity, an indicator and a power conversion arrangement electromagnetically coupled to the heating chamber for tapping a portion of the microwave cooking energy in the heating chamber. The power conversion arrangement includes a probe extending into the heating chamber whereby an AC current is induced in the probe. A rectifier in series with the probe converts the AC into DC current to supply and energize the oven control circuitry and an indicator with DC power.
Description
~ 75~ 9D RG 13l9l BACKGROUND OF THE INVENTION
.
The present invention relates to a new and improved power conversion arrangement and more particulary to a new and improved power conversion arrangement energized by microwave energy in a microwave oven and effective for supplying DC power to control indicator and other circuitry associated with microwave ovens and requiring a DC power source for operation.
Certain electrical sensing devices and other electrically powered elements, such as digital temperature display panels, used in microwave ovens are desirably or necessarily operated with the use of DC power. When such is the case, it is desirable to provide a reliable supply of DC power which is simple in construction and economically feasible in the field in which it is applied.
Accordingly, a primary object of the present invention is to provide a new and improved DC power supply for a microwave oven.
Another object of the present invention is to provide a new and improved power conversion arrangement for supplying DC power to microwave oven control circuitry and peripheral systems operative when the heating chamber of the microwave oven is energized.
Another object of the present invention is to provide a new and improved power conversion arrangement for use in a microwave oven and energized through the use of a portion of the oven operating microwave energy.
Another object of the present invention is to provide a new and improved conversion arrangement for use in a microwave oven and operative in cooperation with the time varying electric and magnetic fields contained within the heating chamber of a microwave oven during operation.
~ S ~ 9D RG 13191 Another object of the present invention is to provide a new and improved microwave oven including a power conversion device which is simple in construction and economically feasible in the microwave oven field.
Another object of the present invention is to provide a new and improved microwave power conversion arrangement for converting alternating current induced in a probe within a microwave oven into direct current and providing a current path to associated DC operated circuitry.
SU~ RY OF THE INVENTION
The present invention, in accordance with one embodiment thereof/ comprises a power conversion arrange-ment coupled to the heating chamber of a microwave oven, which converts microwave power to DC power, and which is energized by a portion of the microwave power supplied to the microwave oven for cooking or heating foodO The invention utilizes a microwave probe extending into the heating chamber and wherein an alternating current is induced. The probe is an extension of a center conductor of a coaxial conductor, including a tubular outer member. Interposed between the ends of the center conductor is a rectifying means which passes only the DC
component of the induced alternating current. The DC
current is smoothed by a capacitor and is supplied to control indicator circuitry of the microwave oven, whereby the circuitry is operative whenever the cooking chamber of the oven is energized. Shielding means is provided to prevent microwave radiation from escaping from the tubular member into the external environmentO
DESCRIPTION OF THE DRAWING
The invention will be better understood from ~ 7~ 9D RG 13191 the following description taken in conjunction with the accompanying drawing, wherein:
FIGURE 1 is a cross-sectional schematic illustration of a microwave oven including a power conversion arrangement constructed according to one embodiment of the present invention;
FIGURE 2 iS a cross-sectional fragmentary schematic illustration of another embodiment of the present invention; and FIGURE 3 iS an illustrative view of a temperature sensing probe usable in a microwave energy containing environment and incorporating a power conversion circuit similar to that shown in FIGURE 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Turning now to a consideration of the drawing, and in particular to FIGURE 1, a microwave oven generally indicated at 10 is shown comprising a heating chamber 1.1 including a top 12 having an aperture 13, sides 14, and a bottom 15. A magnetron 16 is suitably mounted on the top 12 for supplying microwave energy to the heating chamber 11 through an antenna 17. For the sake of illustration, a roast 18 to be cooked is shown in an insulative dish 19 supported on the bottom 15 of the heating chamber 11. Suitably mounted over the aperture 13 on the top 12 of the cooking chamber 11, and shown in enlarged scale to simplify illustration, is a power conversion arrangement 20 constructed in accordance with one embodiment of the present invention~ The arrangement 20 provides DC power to microwave oven control circuitry 21 and an indicator 22. The control circuitry 21 in this embodiment is also provided with a tempera-ture sensor 23 for insertion into the center of the roast 18 for measuring the internal temperature of the mea-t during cooking operation. An electric signal from the temperature sensor 23 to the control circuitry 21 and the indicator 22 is provided along an insulated electric ., ~ 5~ 9D RG 13191 cable 24 or other suitable electric means and through a connector 25 mounted on the top 12 of the heatiny chamber li.
The embodiment of the power conversion arrangement 20 shown in FIGURE 1 comprises a coa~ial conductor including a tubular member 30 and a center conductor 31 supported coaxia:Lly within the tubular member 30. The center conduc-tor 31 extends through and is held in place by a metal cup 32 and a low-loss dielectric member 33 through both of which the center conductor 31 passes. Additionally, the cup 32 is spaced and insulated from the tubular member 30 by an insulative sleeve 34 inserted therebetween. The center conductor 31 is divided into two sections, interconnected by a rectifying means 35. Suitably electrically connected between the center conductor 31 and the tubular member 30 at a location between the insulating member 33 and the rectifying means 35, is a current path including a limiting resistor 36 and a neon bulb 37.
This path conducts current at "flash-over" of the neon bulb, which occurs when the bulb is subjected to a voltage drop in excess of a predetermined threshold.
smoothing capacitor 38 is connected between leads 40 and 41, which in turn are respectively connected to the tubular member 30 and center conductor 31 on the side of the rectifying means 35 opposite the neon bulb 37.
The tubular member 30 of FIGURE 1 is cylindrical and conductive and permits the free flow of current along its sides. Accordingly, the shape of the aperture 13 on the top 12 of the cooking chamber 11 to which the tubular member 30 is mounted is circular. The tubular member 30 includes flanges 42 ~ 75~ 9D RG 13191 by means of which the member 30 is suitably mounted on the cooking chamber top 12.
The center conductor 31 allows the free flow of current -~herethrough and as indicated above is held in place along the longitudinal axis of the tubular member 30 by the cup 32 and the insulating member 33 through which it extends. An extension 44 of the center conductor 31 protrudes into the cooking chamber 11 when the power conversion device 20 is mounted on the top 12 of the chamber. Accordingly, it is subject to the time-varying electric and magnetic fields within the heating chamher 11 when it is energized by the magnetron. This portion of the center conductor is termed a "probe" for purposes of this application.
The low loss dielectric member 33 which assists in positioning the center conductor 31 along the central axis of the tubular member 30 may, for example, comprise a disc of polytetrafluoroethylene sold under the txade mark Teflon. Furthermore, the insulating member 33 is sufficiently rigid for reliably holding the center conductor in place during normal service lifetime. The disc 33 includes a central aperture 50 for allowing the center conductor 31 to pass therethrough. At the upper end, the center conductor 31 is held coaxially positioned in the tubular member 30 by the cup 32. Conslstent with the cylindrical form of the tubular member 30 in FIGURE 1, the cup 32 itself is cylindrical as well and includes a bottom 51 and sides 52. Additionally, the bottom 51 of the cup includes an aperture 53 through which the center conductor 31 extends. The cup 32 is conductive and allows the free flow of electric current along its surface and within the material thereof. The outer ~ 9D RG 13191 diameter of the cup is slightly less than the inner diameter of the tubular member 30, in order to allow placement of the insulating sleeve 34 between -the cup 32 and the tubular member 30. The sides 52 of the cup 32 have a length of approximately a quarter wavelength, as defined by the frequency oE the microwave radiation supplied by the magnetron. This permits the cup 32 to act as a shield preventing the lea]cage of microwave energy from the tubular member 30 into the external environment. However, cup lengths in excess of a quarter wavelength can serve effectively as an end shield for the tubular member 30.
The insulative sleeve 34 is provided between the tubular member 30 and the cup 32 to prevent the flow of current between the cup 32 and the tubular member 30. Additionally, the insula-tive sleeve 34 is slightly longer than the length of the cup 32 to thereby extend beyond the cup at both ends thereof in order to overlap and thereby to eliminate fringe effects between the cup 32 and the tubular member 30 that may result in short circuit.
The rectifying means 35 shown in FIGUR~ 1 is preferably a diode. However, other means for rectifying the current passing through the center conductor may suitably be substituted. These must, however, be able 'co survive the harsh electromagnetic environment within the tubular member 30. For purposes of the instant invention, the diode 35 is a discrete component available on the commercial market. The diode 35 is connected between the two sections of the center conductor 31 a-t a point approximately equal to the product of a positive integer times the quarter wavelength ~ 9D RG 13191 distance measured from either the bottom 51 of the cup 32 or the insulative member 33. The mentioned wave-length is that corresponding to the frequency of the microwave radiation in the cookiny chamber 11. In the embodiment of FIGURE 1, the distance between the bottom 51 oE the cup 32 and the spacer 33 is intended to be approximately a half wavelength. Accordingly, the diode 35 is positioned at approximately the quarter wavelength position or half the distance between the disc 33 and the bottom 51 of the cup 32.
Furthermore, as noted above, positioned between the rectifying means 35 and the disc 33, is the neon-bulb 37 suitably electrically connected between the center conductor 31 and the tubular member 30 at a point of co~tact with the center conductor 31 reasonably close to the quarter wavelength point at which the diode 35 is connected. It may alternatively be connected near other points representing integer multiples of the quarter wavelength distance.
Performance is best at the quarter wavelength position, but for low flash-over point neon bulbs other locations along the center conductor are adequate. It is mandatory that one of the leads to the bulb be connected to the center conductor 31 at a location between the rectifier means 35 and the probe 44 to define a first current path. The resistor 36 is provided between the bulb 37 and the center conductor 31 to limit the current to the bulb.
Capacitor 38, shown in FIGURE 1 between leads 40 and 41 of the tubular member 30 of the center conductor 31, is located on the side of the rectifying means 35 opposite the neon bulb 37. Additionally, for ~ D RG 13191 purposes of the embodiment shown in FIGURE 1, capacitor 38 is shown on the side of the cup 32 opposite the rectifying member 35. However, while it is essential that capacitor 38 be placed on the side of the rectifying means 35 opposite the neon bu:Lb 37, it is not necessary for capacitor 38 to be external to the cup 32.
In operation, the microwave oven shown in FIGURE 1 is connected to a household source of alternating current 55, and the maynetron 56 provides microwave energy to the microwave oven eooking ehamber 11 through antenna 17 in the form of time-varying electric and magnetic fields. The component of the electric field located in the proximity of the probe 44 and parallel thereto induces an alternating current in the probe.
Inasmuch as the probe 44 is physically part of the eenter conductor 31, the alternating current flows through the disc 33 and effeets an alternating current in the section of the center conductor located within the tubular member 30. The standing wave pattern includes a radial electric field which varies in amplitude along the length of the center conductor. The electric field in this embodiment is at a maximum along the eenter conductor 31 at the quarter wavelength point between cup 32 and the disc 33, which is the approximate location of the diode 35 and the current path to bulb 37.
When the eleetric field in -the tubular member 30 creates a potential difference between center conductor 31 and the tubular member 30 on the path including the neon bulb 37, and exceeds a predetermined maximum, the neon bulb flashes over and conducts current. Additionally, a DC eomponent of the alternating current through the center conductor 31 is transmitted through the diode 35, ~ 75~ 9D RG 13191 providing DC power to control circuitry 21 and indicator 22 along a current path including the center conductor 31 and extending through the capacitor 38 by means of leads 40 and 41.
The capacitor 38 substantially smooths out whatever ripple exists in the DC voltage supplied to the capacitor 38. The degree to which the ripple is smoothed is determined by the value of the capacitor 38 In order to reduce the costs of production, the value of the capacitor 38 is as low as possible, consistent with the smoothing requirement of the DC voltage.
The neon bulb 37 assists in regulating the output voltage by helping to maintain it within predetermined limits that eharacterize the voltage drop aeross the neon bulb 32.
The voltage level available at the capacitor 38 is provided through respective leads 40 and 41 to energize the control eircuitry and other peripheral systems such as indieator 32 or other devices ~not shown) including digital displays, audio signal devices, buzzers, or alarm systems. When so powered, the contro] eireuitry 21 can operate to control or shut off the operation of the magnetron 16 or to actuate peripheral systems sueh as a buzzer or alarm system to signal the operator of the microwave oven that cooking has been completed. Additionally, a visual display (not shown) indieatiny the internal temperature of the roast being eooked can be energized by the DC power.
Another embodiment of the instant invention is shown in FIGURE 2 of the drawing, which depicts an alternative power conversion arrangement 60 mounted on the top 12 of the heating cavity of the microwave ~ 9D RG 13191 oven. As in FIGURE 1, the power conversion arrangQment includes a tubular member 61 in the form of a cylinder, having a center conductor 62 coaxially supported therein.
In this embodiment, a feedthrough capacitor 63 is utilized instead of a cup for preventing leakage of microwave radiation into the external environment. The feedthrough capacitor 63 does, however, cooperate with an insulative disc 64 to support the center conductor 62 coaxially in the tubular member 61. An extension 65 of the center conductor 62 extends through the insulating member 64 and functions as a probe receptive to the influences of the electric and magnetic fields within the heating chamber 11. A capacitor 66 is shown electrically connecting the center conductor 62 and the tubular member 61 through leads 67 and 78 on the DC
side of a rectifying means 70 interposed in an interme-diate section of the center conductor 62.
As in the embodiment of FIGURE 1, the power conversion arrangement 60 is suitably mounted on the top 12 of the heating cavity 11. The same conductive and insulative materials indicated in FIGURE 1 for the various components are utilized in FIGURE 2 as well. The tubular member 61 and the inner conductor 62 are separated by distance large enough to prevent the accidental arcing of current through the air separating the two components.
The feed~hrough capacitor 63 of this embod:iment constitutes alternating sheets of conductive material 71 and insulative coatings 72 arranged circumferentially with respec-t to the conductor 62 and having a length along the axis of the conductor 62 of approximately a quarter wavelength as defined by the microwave ~8~ 9D RG 13191 heating frequency to thereby fill the entire radial space between the center conductor 62 and tubular member 61. The feedthrough capacitor 63 of this embodiment acts as a low impedance bypass for the microwave energy between inner conductor 62 and outer conductor 61 but allows the rectified DC component to charge capacitor 66. The capacitor 63 is located a distance of about a half wavelength from the insulative member 64, but distances closest to other integer multlples of a quarter wavelength of the microwave cooking energy can be effectively utilized as well.
The insulative member 64 between center conductor 62 and tubular member 61 can constitute a disc of Teflon, as in FIGURE 1, and has a diameter equal to the inner diameter of the tubular member.
The insulative properties of the disc 6~ prevent current from flowing between the center conductor 62 and the tubular member 61. Although transparent to microwave energy, the diameter of the disc 64 is small compared to the wavelength of the microwave energy in the heating chamberO Accordingly, an insignificant amount of direct radiation takes place from the microwave heating chamber into the region within the tubular member.
Of course, when AC power travels into the power conversion arrangement 60 through center conductor 62, a standing wave pattern including a radial electric field distribution is established within the tubular member 61.
As in the case of the embodiment of FIGURE 1, the rectifying means 70 is a commercially available diode. Again, other rectifying means may be suitably substituted. The position of the diode along the center ~ 7S~ 9D RG 13191 conductor 62 is located at approximately one-half of the distance between the insulative disc 64 and the feedthrough capacitor 42. In view of the fact that this distance is a half wavelength long, the halfway or quarter wavelength point constitutes a position o maximum electric field. For other embodiments in which the distance between the disc 6~ and the feed-through capacitor 63 is other than a half wavelength, it is important that the diode be placed at a location reasonably closed to an intermediate section of the center conductor 62 at whlch the electric field reaches maximum. Accordingly, the diode 70 will be protected from extremes of current flow, thereby extending the useful service life of the diode.
FIGU~E 2 additionally shows a smoothing capacitor 66 provided in suitable electric connection between leads 67 and 63 from the center conductor 62 and the tubular member 61 on the other side o~ the rectifying means 70 opposite the probe 65. As shown in the instant embodiment, the capacitor 66 is a discrete component and is of a value large enough to adequately smooth ripple in the DC current caused by the AC frequency of its source currentl but small enough to minimize production costs of the capacitor.
The embodiment of FIGURE 2 does not show a neon bulb connecting the inner conductor 62 to the tubular member 61. However, one may be provided and would enhance the performance of the embodiment of FIGURE 2. If this is desired, the connection of the neon bulb should be made by suitable electric means, and at a location along the center conductor 62 at which the radial electric field is near a maximum.
~8751 gD RG 13191 As is apparent from FIGURE 2, the probe 65 constituting the section o~ the center conductor 62 extending through the disc 64 is in the shape of a loop of predetermined cross-sectional area. Additionally, the end of the probe 65 makes electrical contact with the inner end of the tubular member 61 and thus allows a current to flow therebetween. In operation, the electric and magnetic fields in the heating chamber act upon the probe 65 to induce an alternating current therein. The magnitude of the alternating current induced depends on the nurnber of magnetic field lines cut by the cross-sectional area of the loop on the probe 65. Furthermore, the magnitude of the alternating current induced in the probe 65 is enhanced by the electirc field components. However, for small loop sizes, the magnetic fields are of primary importance. The rectified current in the center conductor 62 follows a first current path through the loop to the tubular member 61 and thence to the capacitor 66. A second current path is established by the center conductor 62 and lead 68 from the probe 65 through the diode 70, and past the feedthrough capacitor 63 and smoothing capacitor 66 to the above-described control circuitry and indicator to provide DC power to energize sarne. The equipment thereby energized can also include a temperature sensor of the type indicated at 23 in FIGURE 1 which requires a DC current for operation. Upon receiving predetermined signal information from the sensor 23, the control circuitry can shut off or otherwise 39 control the magnetron 56 and its supply of microwave energy to the cooking chamber 11. Alternatively or additionally, a buzzer or other signal device or ~ 5~ 9D RG 13191 indicator can be operated to notify the user of the microwave oven that cookiny has been completed or reached some other predetermined stage.
An alternative embodiment wherein the power supply according to the invention is contained within a temperature sensing probe is shown in FIG. 3.
Referring to FIG. 3, the temperature sensing probe 80 is shown with its sensing tip 81 inserted into a food mass 82 which would be located in the cooking cavity of a conventional microwave oven, generally as shown in FIG. 1. The probe 80 comprises a conductive cylindrical housing 85 having disposed within it a power conversion unit 86 which is substantially similar to unit 60 described hereinbefore in FIG. 2 and which for the sake of bxevity will not be described in great detail. The conversion unit 86 includes a center conductor 87 and an extension 88 thereof, similar in construction and arrangement to like elements disclosed in FIG. 2, which extends outside of the body of the housing 85 through the insulating member 89 to thereby function as a probe receptive to the electric fields within the oven cavity. A capacitor 90, corresponding functionally to the capacitor 66 in FIG. 2, is charged to a DC voltage level, as explained hereinbefore, and this DC voltage is utilized to operate a remo-te indicator system. Specifically, a bulb 92 is coupled in series with a triggerable semiconductor device 93 across the capacitor 90. The device 93, which may be an SCR, has its gate electrode coupled to a comparator circuit 100 via line 105, which responds to the difference between a first reference voltage on line 94 ancl a second sensor voltage on line 95. The 9~ RG ]3191 5~
voltage on line 9~ is established by preselected dropping resistors 96 and 97 coupled across the capacitor 90. The sensor voltage on line 95 is generated by the combined effect of the resistors 101, 102, and 103 taken with the variable temperature sensing resistance 104 or thermistor which is housed in the end of the probe tip 81. The resistor 102 is of the variable type so that an initial set point for the system circuit may be established. When the temperature sensed by the thermistor exceeds this set point, the comparator 100 generates a suitable output on line 105 which is coupled to the gate of the SCR 93 to thereby trigger it on and energize the lamp 92.
Thus, the lamp 92, when lit, signfies that the food has reached the preselected temperature.
Of course, the variable resistor 102 in a commercial embodiment may take the form of a calibrated, operator-adjustable potentiometer to permit selection of set point across normal cooking temperatures.
The length of the sensor probe body indicated as X in FIG. 3 is preferably selected to be an integral multiple of half wavelength of the micro-wave frequency in the cavity in order to achieve minimum feed in the food reference plane.
After reference to the foregoing, modifications of this invention may occur to those skilled in the art.
However, it is to be understood that this invention is not intended to be limited to the particular embodiments shown and described herein, but is intended to cover all modifications coming within the spirit and scope of the invention as claimed.
.
The present invention relates to a new and improved power conversion arrangement and more particulary to a new and improved power conversion arrangement energized by microwave energy in a microwave oven and effective for supplying DC power to control indicator and other circuitry associated with microwave ovens and requiring a DC power source for operation.
Certain electrical sensing devices and other electrically powered elements, such as digital temperature display panels, used in microwave ovens are desirably or necessarily operated with the use of DC power. When such is the case, it is desirable to provide a reliable supply of DC power which is simple in construction and economically feasible in the field in which it is applied.
Accordingly, a primary object of the present invention is to provide a new and improved DC power supply for a microwave oven.
Another object of the present invention is to provide a new and improved power conversion arrangement for supplying DC power to microwave oven control circuitry and peripheral systems operative when the heating chamber of the microwave oven is energized.
Another object of the present invention is to provide a new and improved power conversion arrangement for use in a microwave oven and energized through the use of a portion of the oven operating microwave energy.
Another object of the present invention is to provide a new and improved conversion arrangement for use in a microwave oven and operative in cooperation with the time varying electric and magnetic fields contained within the heating chamber of a microwave oven during operation.
~ S ~ 9D RG 13191 Another object of the present invention is to provide a new and improved microwave oven including a power conversion device which is simple in construction and economically feasible in the microwave oven field.
Another object of the present invention is to provide a new and improved microwave power conversion arrangement for converting alternating current induced in a probe within a microwave oven into direct current and providing a current path to associated DC operated circuitry.
SU~ RY OF THE INVENTION
The present invention, in accordance with one embodiment thereof/ comprises a power conversion arrange-ment coupled to the heating chamber of a microwave oven, which converts microwave power to DC power, and which is energized by a portion of the microwave power supplied to the microwave oven for cooking or heating foodO The invention utilizes a microwave probe extending into the heating chamber and wherein an alternating current is induced. The probe is an extension of a center conductor of a coaxial conductor, including a tubular outer member. Interposed between the ends of the center conductor is a rectifying means which passes only the DC
component of the induced alternating current. The DC
current is smoothed by a capacitor and is supplied to control indicator circuitry of the microwave oven, whereby the circuitry is operative whenever the cooking chamber of the oven is energized. Shielding means is provided to prevent microwave radiation from escaping from the tubular member into the external environmentO
DESCRIPTION OF THE DRAWING
The invention will be better understood from ~ 7~ 9D RG 13191 the following description taken in conjunction with the accompanying drawing, wherein:
FIGURE 1 is a cross-sectional schematic illustration of a microwave oven including a power conversion arrangement constructed according to one embodiment of the present invention;
FIGURE 2 iS a cross-sectional fragmentary schematic illustration of another embodiment of the present invention; and FIGURE 3 iS an illustrative view of a temperature sensing probe usable in a microwave energy containing environment and incorporating a power conversion circuit similar to that shown in FIGURE 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Turning now to a consideration of the drawing, and in particular to FIGURE 1, a microwave oven generally indicated at 10 is shown comprising a heating chamber 1.1 including a top 12 having an aperture 13, sides 14, and a bottom 15. A magnetron 16 is suitably mounted on the top 12 for supplying microwave energy to the heating chamber 11 through an antenna 17. For the sake of illustration, a roast 18 to be cooked is shown in an insulative dish 19 supported on the bottom 15 of the heating chamber 11. Suitably mounted over the aperture 13 on the top 12 of the cooking chamber 11, and shown in enlarged scale to simplify illustration, is a power conversion arrangement 20 constructed in accordance with one embodiment of the present invention~ The arrangement 20 provides DC power to microwave oven control circuitry 21 and an indicator 22. The control circuitry 21 in this embodiment is also provided with a tempera-ture sensor 23 for insertion into the center of the roast 18 for measuring the internal temperature of the mea-t during cooking operation. An electric signal from the temperature sensor 23 to the control circuitry 21 and the indicator 22 is provided along an insulated electric ., ~ 5~ 9D RG 13191 cable 24 or other suitable electric means and through a connector 25 mounted on the top 12 of the heatiny chamber li.
The embodiment of the power conversion arrangement 20 shown in FIGURE 1 comprises a coa~ial conductor including a tubular member 30 and a center conductor 31 supported coaxia:Lly within the tubular member 30. The center conduc-tor 31 extends through and is held in place by a metal cup 32 and a low-loss dielectric member 33 through both of which the center conductor 31 passes. Additionally, the cup 32 is spaced and insulated from the tubular member 30 by an insulative sleeve 34 inserted therebetween. The center conductor 31 is divided into two sections, interconnected by a rectifying means 35. Suitably electrically connected between the center conductor 31 and the tubular member 30 at a location between the insulating member 33 and the rectifying means 35, is a current path including a limiting resistor 36 and a neon bulb 37.
This path conducts current at "flash-over" of the neon bulb, which occurs when the bulb is subjected to a voltage drop in excess of a predetermined threshold.
smoothing capacitor 38 is connected between leads 40 and 41, which in turn are respectively connected to the tubular member 30 and center conductor 31 on the side of the rectifying means 35 opposite the neon bulb 37.
The tubular member 30 of FIGURE 1 is cylindrical and conductive and permits the free flow of current along its sides. Accordingly, the shape of the aperture 13 on the top 12 of the cooking chamber 11 to which the tubular member 30 is mounted is circular. The tubular member 30 includes flanges 42 ~ 75~ 9D RG 13191 by means of which the member 30 is suitably mounted on the cooking chamber top 12.
The center conductor 31 allows the free flow of current -~herethrough and as indicated above is held in place along the longitudinal axis of the tubular member 30 by the cup 32 and the insulating member 33 through which it extends. An extension 44 of the center conductor 31 protrudes into the cooking chamber 11 when the power conversion device 20 is mounted on the top 12 of the chamber. Accordingly, it is subject to the time-varying electric and magnetic fields within the heating chamher 11 when it is energized by the magnetron. This portion of the center conductor is termed a "probe" for purposes of this application.
The low loss dielectric member 33 which assists in positioning the center conductor 31 along the central axis of the tubular member 30 may, for example, comprise a disc of polytetrafluoroethylene sold under the txade mark Teflon. Furthermore, the insulating member 33 is sufficiently rigid for reliably holding the center conductor in place during normal service lifetime. The disc 33 includes a central aperture 50 for allowing the center conductor 31 to pass therethrough. At the upper end, the center conductor 31 is held coaxially positioned in the tubular member 30 by the cup 32. Conslstent with the cylindrical form of the tubular member 30 in FIGURE 1, the cup 32 itself is cylindrical as well and includes a bottom 51 and sides 52. Additionally, the bottom 51 of the cup includes an aperture 53 through which the center conductor 31 extends. The cup 32 is conductive and allows the free flow of electric current along its surface and within the material thereof. The outer ~ 9D RG 13191 diameter of the cup is slightly less than the inner diameter of the tubular member 30, in order to allow placement of the insulating sleeve 34 between -the cup 32 and the tubular member 30. The sides 52 of the cup 32 have a length of approximately a quarter wavelength, as defined by the frequency oE the microwave radiation supplied by the magnetron. This permits the cup 32 to act as a shield preventing the lea]cage of microwave energy from the tubular member 30 into the external environment. However, cup lengths in excess of a quarter wavelength can serve effectively as an end shield for the tubular member 30.
The insulative sleeve 34 is provided between the tubular member 30 and the cup 32 to prevent the flow of current between the cup 32 and the tubular member 30. Additionally, the insula-tive sleeve 34 is slightly longer than the length of the cup 32 to thereby extend beyond the cup at both ends thereof in order to overlap and thereby to eliminate fringe effects between the cup 32 and the tubular member 30 that may result in short circuit.
The rectifying means 35 shown in FIGUR~ 1 is preferably a diode. However, other means for rectifying the current passing through the center conductor may suitably be substituted. These must, however, be able 'co survive the harsh electromagnetic environment within the tubular member 30. For purposes of the instant invention, the diode 35 is a discrete component available on the commercial market. The diode 35 is connected between the two sections of the center conductor 31 a-t a point approximately equal to the product of a positive integer times the quarter wavelength ~ 9D RG 13191 distance measured from either the bottom 51 of the cup 32 or the insulative member 33. The mentioned wave-length is that corresponding to the frequency of the microwave radiation in the cookiny chamber 11. In the embodiment of FIGURE 1, the distance between the bottom 51 oE the cup 32 and the spacer 33 is intended to be approximately a half wavelength. Accordingly, the diode 35 is positioned at approximately the quarter wavelength position or half the distance between the disc 33 and the bottom 51 of the cup 32.
Furthermore, as noted above, positioned between the rectifying means 35 and the disc 33, is the neon-bulb 37 suitably electrically connected between the center conductor 31 and the tubular member 30 at a point of co~tact with the center conductor 31 reasonably close to the quarter wavelength point at which the diode 35 is connected. It may alternatively be connected near other points representing integer multiples of the quarter wavelength distance.
Performance is best at the quarter wavelength position, but for low flash-over point neon bulbs other locations along the center conductor are adequate. It is mandatory that one of the leads to the bulb be connected to the center conductor 31 at a location between the rectifier means 35 and the probe 44 to define a first current path. The resistor 36 is provided between the bulb 37 and the center conductor 31 to limit the current to the bulb.
Capacitor 38, shown in FIGURE 1 between leads 40 and 41 of the tubular member 30 of the center conductor 31, is located on the side of the rectifying means 35 opposite the neon bulb 37. Additionally, for ~ D RG 13191 purposes of the embodiment shown in FIGURE 1, capacitor 38 is shown on the side of the cup 32 opposite the rectifying member 35. However, while it is essential that capacitor 38 be placed on the side of the rectifying means 35 opposite the neon bu:Lb 37, it is not necessary for capacitor 38 to be external to the cup 32.
In operation, the microwave oven shown in FIGURE 1 is connected to a household source of alternating current 55, and the maynetron 56 provides microwave energy to the microwave oven eooking ehamber 11 through antenna 17 in the form of time-varying electric and magnetic fields. The component of the electric field located in the proximity of the probe 44 and parallel thereto induces an alternating current in the probe.
Inasmuch as the probe 44 is physically part of the eenter conductor 31, the alternating current flows through the disc 33 and effeets an alternating current in the section of the center conductor located within the tubular member 30. The standing wave pattern includes a radial electric field which varies in amplitude along the length of the center conductor. The electric field in this embodiment is at a maximum along the eenter conductor 31 at the quarter wavelength point between cup 32 and the disc 33, which is the approximate location of the diode 35 and the current path to bulb 37.
When the eleetric field in -the tubular member 30 creates a potential difference between center conductor 31 and the tubular member 30 on the path including the neon bulb 37, and exceeds a predetermined maximum, the neon bulb flashes over and conducts current. Additionally, a DC eomponent of the alternating current through the center conductor 31 is transmitted through the diode 35, ~ 75~ 9D RG 13191 providing DC power to control circuitry 21 and indicator 22 along a current path including the center conductor 31 and extending through the capacitor 38 by means of leads 40 and 41.
The capacitor 38 substantially smooths out whatever ripple exists in the DC voltage supplied to the capacitor 38. The degree to which the ripple is smoothed is determined by the value of the capacitor 38 In order to reduce the costs of production, the value of the capacitor 38 is as low as possible, consistent with the smoothing requirement of the DC voltage.
The neon bulb 37 assists in regulating the output voltage by helping to maintain it within predetermined limits that eharacterize the voltage drop aeross the neon bulb 32.
The voltage level available at the capacitor 38 is provided through respective leads 40 and 41 to energize the control eircuitry and other peripheral systems such as indieator 32 or other devices ~not shown) including digital displays, audio signal devices, buzzers, or alarm systems. When so powered, the contro] eireuitry 21 can operate to control or shut off the operation of the magnetron 16 or to actuate peripheral systems sueh as a buzzer or alarm system to signal the operator of the microwave oven that cooking has been completed. Additionally, a visual display (not shown) indieatiny the internal temperature of the roast being eooked can be energized by the DC power.
Another embodiment of the instant invention is shown in FIGURE 2 of the drawing, which depicts an alternative power conversion arrangement 60 mounted on the top 12 of the heating cavity of the microwave ~ 9D RG 13191 oven. As in FIGURE 1, the power conversion arrangQment includes a tubular member 61 in the form of a cylinder, having a center conductor 62 coaxially supported therein.
In this embodiment, a feedthrough capacitor 63 is utilized instead of a cup for preventing leakage of microwave radiation into the external environment. The feedthrough capacitor 63 does, however, cooperate with an insulative disc 64 to support the center conductor 62 coaxially in the tubular member 61. An extension 65 of the center conductor 62 extends through the insulating member 64 and functions as a probe receptive to the influences of the electric and magnetic fields within the heating chamber 11. A capacitor 66 is shown electrically connecting the center conductor 62 and the tubular member 61 through leads 67 and 78 on the DC
side of a rectifying means 70 interposed in an interme-diate section of the center conductor 62.
As in the embodiment of FIGURE 1, the power conversion arrangement 60 is suitably mounted on the top 12 of the heating cavity 11. The same conductive and insulative materials indicated in FIGURE 1 for the various components are utilized in FIGURE 2 as well. The tubular member 61 and the inner conductor 62 are separated by distance large enough to prevent the accidental arcing of current through the air separating the two components.
The feed~hrough capacitor 63 of this embod:iment constitutes alternating sheets of conductive material 71 and insulative coatings 72 arranged circumferentially with respec-t to the conductor 62 and having a length along the axis of the conductor 62 of approximately a quarter wavelength as defined by the microwave ~8~ 9D RG 13191 heating frequency to thereby fill the entire radial space between the center conductor 62 and tubular member 61. The feedthrough capacitor 63 of this embodiment acts as a low impedance bypass for the microwave energy between inner conductor 62 and outer conductor 61 but allows the rectified DC component to charge capacitor 66. The capacitor 63 is located a distance of about a half wavelength from the insulative member 64, but distances closest to other integer multlples of a quarter wavelength of the microwave cooking energy can be effectively utilized as well.
The insulative member 64 between center conductor 62 and tubular member 61 can constitute a disc of Teflon, as in FIGURE 1, and has a diameter equal to the inner diameter of the tubular member.
The insulative properties of the disc 6~ prevent current from flowing between the center conductor 62 and the tubular member 61. Although transparent to microwave energy, the diameter of the disc 64 is small compared to the wavelength of the microwave energy in the heating chamberO Accordingly, an insignificant amount of direct radiation takes place from the microwave heating chamber into the region within the tubular member.
Of course, when AC power travels into the power conversion arrangement 60 through center conductor 62, a standing wave pattern including a radial electric field distribution is established within the tubular member 61.
As in the case of the embodiment of FIGURE 1, the rectifying means 70 is a commercially available diode. Again, other rectifying means may be suitably substituted. The position of the diode along the center ~ 7S~ 9D RG 13191 conductor 62 is located at approximately one-half of the distance between the insulative disc 64 and the feedthrough capacitor 42. In view of the fact that this distance is a half wavelength long, the halfway or quarter wavelength point constitutes a position o maximum electric field. For other embodiments in which the distance between the disc 6~ and the feed-through capacitor 63 is other than a half wavelength, it is important that the diode be placed at a location reasonably closed to an intermediate section of the center conductor 62 at whlch the electric field reaches maximum. Accordingly, the diode 70 will be protected from extremes of current flow, thereby extending the useful service life of the diode.
FIGU~E 2 additionally shows a smoothing capacitor 66 provided in suitable electric connection between leads 67 and 63 from the center conductor 62 and the tubular member 61 on the other side o~ the rectifying means 70 opposite the probe 65. As shown in the instant embodiment, the capacitor 66 is a discrete component and is of a value large enough to adequately smooth ripple in the DC current caused by the AC frequency of its source currentl but small enough to minimize production costs of the capacitor.
The embodiment of FIGURE 2 does not show a neon bulb connecting the inner conductor 62 to the tubular member 61. However, one may be provided and would enhance the performance of the embodiment of FIGURE 2. If this is desired, the connection of the neon bulb should be made by suitable electric means, and at a location along the center conductor 62 at which the radial electric field is near a maximum.
~8751 gD RG 13191 As is apparent from FIGURE 2, the probe 65 constituting the section o~ the center conductor 62 extending through the disc 64 is in the shape of a loop of predetermined cross-sectional area. Additionally, the end of the probe 65 makes electrical contact with the inner end of the tubular member 61 and thus allows a current to flow therebetween. In operation, the electric and magnetic fields in the heating chamber act upon the probe 65 to induce an alternating current therein. The magnitude of the alternating current induced depends on the nurnber of magnetic field lines cut by the cross-sectional area of the loop on the probe 65. Furthermore, the magnitude of the alternating current induced in the probe 65 is enhanced by the electirc field components. However, for small loop sizes, the magnetic fields are of primary importance. The rectified current in the center conductor 62 follows a first current path through the loop to the tubular member 61 and thence to the capacitor 66. A second current path is established by the center conductor 62 and lead 68 from the probe 65 through the diode 70, and past the feedthrough capacitor 63 and smoothing capacitor 66 to the above-described control circuitry and indicator to provide DC power to energize sarne. The equipment thereby energized can also include a temperature sensor of the type indicated at 23 in FIGURE 1 which requires a DC current for operation. Upon receiving predetermined signal information from the sensor 23, the control circuitry can shut off or otherwise 39 control the magnetron 56 and its supply of microwave energy to the cooking chamber 11. Alternatively or additionally, a buzzer or other signal device or ~ 5~ 9D RG 13191 indicator can be operated to notify the user of the microwave oven that cookiny has been completed or reached some other predetermined stage.
An alternative embodiment wherein the power supply according to the invention is contained within a temperature sensing probe is shown in FIG. 3.
Referring to FIG. 3, the temperature sensing probe 80 is shown with its sensing tip 81 inserted into a food mass 82 which would be located in the cooking cavity of a conventional microwave oven, generally as shown in FIG. 1. The probe 80 comprises a conductive cylindrical housing 85 having disposed within it a power conversion unit 86 which is substantially similar to unit 60 described hereinbefore in FIG. 2 and which for the sake of bxevity will not be described in great detail. The conversion unit 86 includes a center conductor 87 and an extension 88 thereof, similar in construction and arrangement to like elements disclosed in FIG. 2, which extends outside of the body of the housing 85 through the insulating member 89 to thereby function as a probe receptive to the electric fields within the oven cavity. A capacitor 90, corresponding functionally to the capacitor 66 in FIG. 2, is charged to a DC voltage level, as explained hereinbefore, and this DC voltage is utilized to operate a remo-te indicator system. Specifically, a bulb 92 is coupled in series with a triggerable semiconductor device 93 across the capacitor 90. The device 93, which may be an SCR, has its gate electrode coupled to a comparator circuit 100 via line 105, which responds to the difference between a first reference voltage on line 94 ancl a second sensor voltage on line 95. The 9~ RG ]3191 5~
voltage on line 9~ is established by preselected dropping resistors 96 and 97 coupled across the capacitor 90. The sensor voltage on line 95 is generated by the combined effect of the resistors 101, 102, and 103 taken with the variable temperature sensing resistance 104 or thermistor which is housed in the end of the probe tip 81. The resistor 102 is of the variable type so that an initial set point for the system circuit may be established. When the temperature sensed by the thermistor exceeds this set point, the comparator 100 generates a suitable output on line 105 which is coupled to the gate of the SCR 93 to thereby trigger it on and energize the lamp 92.
Thus, the lamp 92, when lit, signfies that the food has reached the preselected temperature.
Of course, the variable resistor 102 in a commercial embodiment may take the form of a calibrated, operator-adjustable potentiometer to permit selection of set point across normal cooking temperatures.
The length of the sensor probe body indicated as X in FIG. 3 is preferably selected to be an integral multiple of half wavelength of the micro-wave frequency in the cavity in order to achieve minimum feed in the food reference plane.
After reference to the foregoing, modifications of this invention may occur to those skilled in the art.
However, it is to be understood that this invention is not intended to be limited to the particular embodiments shown and described herein, but is intended to cover all modifications coming within the spirit and scope of the invention as claimed.
Claims (11)
1. In a microwave oven including a microwave heating chamber and a source of microwave energy coupled to said chamber, power conversion means for providing a DC power source for energizing electric circuitry associated with said oven comprising:
a pair of spaced coaxial conductors including a tubular member, having an end thereof in communication with said heating chamber, and a center conductor supported coaxially in said tubular member;
said center conductor comprising a probe at one end thereof and extending into said chamber for having an AC current induced therein by microwave energy in said cavity;
rectifying means interposed between said probe and the opposite end of said center conductor for converting said induced AC current to direct current; and means for providing a first current path from the AC side of said center conductor to said tubular member.
a pair of spaced coaxial conductors including a tubular member, having an end thereof in communication with said heating chamber, and a center conductor supported coaxially in said tubular member;
said center conductor comprising a probe at one end thereof and extending into said chamber for having an AC current induced therein by microwave energy in said cavity;
rectifying means interposed between said probe and the opposite end of said center conductor for converting said induced AC current to direct current; and means for providing a first current path from the AC side of said center conductor to said tubular member.
2. The invention of Claim 1, further comprising electric circuitry relating to the operation of the microwave oven and means providing a second current path from the DC side of said center conductor to said electric circuitry, whereby said electric circuitry is energized by DC power.
3. The invention of Claim 1, wherein said means providing said first current path is connected to said center conductor at a point near a maximum in radial electric field strength in said tubular member.
4. The invention of Claim 1, further comprising a shielding means positioned in said tubular member and effective for containing electromagnetic energy therein.
5. The invention of claim 4, wherein said shielding member is located on the side of said rectifying means opposite said probe.
6. The invention of claim 4, wherein said shielding means comprises a quarter wavelength long metal cup electrically insulated from said tubular member.
7. The invention of Claim 4, wherein said shielding means comprises a feedthrough capacitor, constructed of a conductive sheet with at least one insulative layer on one side thereof, wrapped about said center conductor, and substantially filling the radial space between said conductors.
8. The invention of Claim 1, wherein said means providing said first current path comprises a neon bulb having a predetermined flash-over voltage, whereby the voltage level between said center conductor and tubular member is regulated.
9. The invention of Claim 1, wherein said means providing said first current path comprises a looped end of said probe electrically connected to said tubular member.
10. The invention of Claim 1, further comprising a smoothing capacitor connected between the DC side of said center conductor and said tubular member.
11. The invention of Claim 4 t wherein said shielding means and said tubular member cooperatively
11. The invention of Claim 4 t wherein said shielding means and said tubular member cooperatively
Claim 11 continued:
define a half wavelength standing wave cavity, and said rectifying means is located at approximately a quarter wavelength position in said cavity.
define a half wavelength standing wave cavity, and said rectifying means is located at approximately a quarter wavelength position in said cavity.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US18149480A | 1980-08-26 | 1980-08-26 | |
| US181,494 | 1980-08-26 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1188751A true CA1188751A (en) | 1985-06-11 |
Family
ID=22664505
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000383913A Expired CA1188751A (en) | 1980-08-26 | 1981-08-14 | Microwave oven power conversion arrangement |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1188751A (en) |
-
1981
- 1981-08-14 CA CA000383913A patent/CA1188751A/en not_active Expired
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