Microwave oven power control system
This is a continuation of application Ser. No. 090,684, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Title of the Invention 
     The present invention relates generally to a microwave oven of the type including a duty cycle control to vary power level, and more particularly to a means for decreasing the magnetron power by way of a plurality of intermediate power levels during the thawing of different food products. 
     2. Description of the Prior Art 
     An often used method for thawing processed and prepared foods in a microwave oven involves setting a program timer for a desired span of time while keeping the magnetron power level relatively low. The thawing period and the power level may depend upon the quality of nature of the food, however, and thus deciding the time-power level conditions is known to be intricate. 
     In the meantime there has been developed a control system called a duty cycle control in which the power transformer and the magnetron are alternately switched between a full-on operation and a full-off operation. The ratio of &#34;on&#34; time compared to the total operation time is known as the duty cycle. The average magnetron power level is the actual power output of the microwave oven. 
     Various particular circuits have been proposed to effect duty cycle power control. These control systems range from a simple cam-functioned mechanical timer, to more advanced systems adopting electronic timing and switching elements. 
     Although the above-described methods and systems are in practical use, no appropriate power control systems have been made available that function in relation to the temperature of the food undergoing thawing. 
     SUMMARY OF THE INVENTION 
     Accordingly, one object of this invention is to provide a novel circuit employing duty cycle control techniques for thawing foods by means of a plurality of decreased magnetron power levels depending upon the food temperature. 
     It is another object of the invention to provide a novel circuit for duty cycle control with which frozen foods can be effectively thawed back to their original conditions without being impaired. 
     In accordance with this invention, a temperature sensing probe is to be provided, being connected to a comparating block which comprises an amplifier, a plurality of variable resistors and a plurality of comparators. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
     FIG. 1 is a simplified schematic diagram of a magnetron power control system employed in a microwave oven illustrating the general principles of the invention; 
     FIG. 2 is a schematic diagram showing the connections of the elements constituting the comparator shown in FIG. 1; 
     FIG. 3 is a graph illustrating the volt-temperature characteristic of the thermal sensor shown in FIG. 2; 
     FIG. 4 is an illustration of the output level characteristics of the comparator elements shown in FIG. 2; 
     FIG. 5 is a circuit diagram of the magnetron power control circuit to be connected to the comparator illustrated in FIG. 2; 
     FIG. 6 is an illustration of the timing intervals of the shift register on which the firing of the thyristor shown in FIG. 5 depends; and 
     FIG. 7 is a graph illustrating the power level-temperature characteristic of the magnetron employed in a microwave oven of the present invention shown in FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, and more particularly to FIG. 1 thereof, a simplified microwave oven control system includes a cooking cavity 1 in which food 3 to be defrosted is placed on a suitable disk on a dielectric member 2, being exposed to the microwave energy generated by the magnetron 4, which is secured in the ceiling of the heating cavity 1. 
     The power supply circuit of the magnetron 4 includes a high power transformer having a high voltage secondary winding connected to energize the magnetron through a conventional half-wave voltage doubler 5 comprising a series capacitor and a rectifying diode connected across the magnetron anode and cathode terminals. 
     The primary winding of the power transformer 6 is connected to a thyristor 7 which alternately energizes and deenergizes the power transformer 6 in response to the gate signals applied to the gate thereof. The thyristor 7 is also connected to an AC power source 8, such as a 100 V, 50/60 Hz household branch circuit, through a normally open switch 32a which is energized by the switching relay 32. 
     Additionally, a thermal sensing probe 9 is provided inside the heating cavity 1 having a stick-like thermal sensing element 9 1 , such as a thermistor, which is to be inserted in the food. Preferably, that part of the probe 9 which is inserted into the food is detachable. 
     In operation the thermal sensing element 9 1  senses the temperature of the food and converts the information into electric signals. 
     In accordance with the invention, a particular type temperature comparator 11 is connected in series with the probe 9. 
     The results of the comparison are transferred to a gate terminal control block 18. One of the comparator signals is also applied to an inverter 27, and then to a flip-flop reset block 29 through an integration circuit 28. 
     The flip-flop reset block 29 is provided both with set signals from a cook-start switch 30 and with a reset signal from the integration circuit 28. 
     In operation a user begins cooking with a push on the cook-start switch 30, giving the flip-flop reset block 29 a set signal S. 
     Then the flip-flop reset block 29 will be turned on, energizing a driving block 31 which consequently will turn the relay 32 on to close the normally open switch 32a, thus forming the power supply circuit. 
     Since the inverter 27 is connected with a signal line communicating with the comparator 11 and the gate terminal control block 18, and this signal line corresponds to the lowest power level, the flip-flop reset block 29 will be provided with a reset signal by the inverter 27 and will be turned into the OFF state on condition that the output signal of the lowest power level line changes from the &#34;1&#34; logic level to the &#34;0&#34; logic level. 
     As the relay 32 will then be deenergized the normally open switch 32a will be opened and the power supply will be cut off. 
     It should be understood, however, that the magnetron 4 can at any time resume generating microwave if the cook-start switch 30 is given a push. 
     Referring now to FIG. 2, the comparator used in the invention is shown, in which a thermistor 9 2  is provided to sense the temperature of the food in the heating oven. The thermistor 9 2  is preferably inserted into the food for higher precision. 
     This can be achieved by mounting the thermistor on the acute sensing probe and by deeply piercing the food with the thermistor. 
     A lead wire connects the thermistor with an amplifier 13. A resistor 12 connected in series with the thermistor 9 2  is in turn connected to the DC power source +V. The junction of the resistor 12 and the thermistor 9 2  is connected to the positive terminal of the amplifier 13, while the output terminal of the amplifier 13 is fed back to its negative input terminal. 
     In response to the balance of the electric resistance between the resistor 12 and the thermistor 9 2 , the amplifier 13 sends amplified signals to the positive terminals of a plurality of comparator elements 14 1 , 14 2 , and 14 3 . 
     For the purpose of staggering the threshold temperature level, it is necessary that each comparator element be weighted differently in terms of electric voltage relative to ground. 
     In accordance with the particular embodiment of the present invention for each comparator element of 14 1 , 14 2  and 14 3 , there is provided a set of relatively weighted resistors generally designated at 15, 16 and 17. 
     The first comparator element 14 1  operates as the lowest temperature level comparator, so that the standard voltage Vr 1  should be higher than the other standard voltages Vr 2  and Vr 3 . 
     The resistor 16 1  and the variable resistor 17 1  are connected between ground and the negative terminal of the comparator element 14 1 . 
     The resistor 15 1  is also connected between the junction of the resistor 16 1  and the positive DC supply +V. 
     The added resistance value of the resistor 16 1  and the variable resistor 17 1  should be comparatively high so that the standard voltage Vr 1  is set at a high value. 
     The similar connections are employed for the rest of the comparator elements 14 2  and 14 3 , namely the resistors 15 2 , 16 2  and the variable resistor 17 2  are connected in series between ground and the positive DC power supply +V, with the junction between the resistor 15 2  and the resistor 16 2  connected to the negative terminal of the comparator element 14 2 . 
     The same arrangement of elements is applied to the third comparator element 14 3  with the exception that different values are conferred on each element. 
     Referring now to FIG. 3, the temperature-voltage characteristic of the thermistor to be used in this invention is presented. 
     The standard voltages Vr 1 , Vr 2  and Vr 3  correspond to the values at -10° C., -5° C. and 0° C. respectively. Since the thermistor voltage does not follow a linear characteristic, the standard voltages are not in a proportional relationship. 
     It should be noted that in case a thermistor having a different temperature-voltage characteristic is employed as a temperature sensing element a new set of standard voltages should be obtained in accordance with the desired temperatures by adjusting the variable resistors 17 1 , 17 2  and 17 3 . 
     The output signals of the comparator elements 14 1 , 14 2  and 14 3  are indicated in FIG. 4 as TH 1 , TH 2  and TH 3 , respectively. 
     The function of the comparator element is now described. When a signal from the amplifier 13 proves greater than a respective reference signal, which means the temperature of the food is still lower than the one defined by the reference voltage Vr i , the comparator element continues to deliver high level signals. However, once a signal from the amplifier 13 exceeds the respective Vr i , which means that the temperature of the food exceeds the one defined by the reference voltage Vr i , the comparator element ceases to send a high level output and instead begins to send a low level output. 
     The above-described condition can be better appreciated by reference to FIG. 4. Because the first comparator element 14 1  is set to operate at the lowest temperature -10° C., its output TH 1  is the first to change to a low state. Then comes the second comparator element output TH 2  which turns low at -5° C., and then the third comparator element output TH 3  at 0° C. 
     As discussed above, when the third comparator element output TH 3  turns low, the low level signal is given to the flip-flop reset block 29, thereby opening the power supply circuit. 
     Referring now to FIG. 5, a circuit for controlling the gate of a thyristor 7 using a logical circuit is presented. 
     A set of comparator element outputs TH 1 , TH 2  and TH 3  is brought to a logical gate circuit having three AND gates 19 1 , 19 2  and 19 3 , and a couple of NOT gates 20 1  and 20 2 . 
     Into the AND gate 19 1  come the comparator outputs TH 1  and TH 3 , while the AND gate 19 2  is supplied with the comparator output TH 2  and the inverted TH 1  by way of the NOT gate 20 1 . Similarly the AND gate 19 3  is provided with the comparator output TH 3  and the inverted TH 2  through the interposed NOT gate 20 2 . 
     The output signals of the AND gates 19 1 , 19 2  and 19 3  are sent to a set of analog switches 21 1 , 21 2  and 21 3 , respectively. 
     Additionally connected with the analog switches 21 1 , 21 2  and 21 3  is a shift register 22 for generating kinds of duty cycle pattern signals, such as Q 2 , Q 4  and Q 6 , and sending them to the analog switches 21 1 , 21 2  and 21 3 . The shift register 22 is resettable to a particular output configuration to produce outputs of varying duty cycle. 
     The shift register 22 in this embodiment can produce ten different duty cycle patterns of ten seconds, namely Q 1  through Q 10 , each continuing to send high level signals for a period corresponding to the accompanying numeral and then sending low level signals for the rest of the cycle, for example as shown in FIG. 6, the duty cycle pattern Q 2  sends high level signals for two seconds followed by the light seconds of low level signals alternatively. As for the duty cycle pattern Q 4 , this pattern exhibits a high level keeps for four seconds and a low level for the rest of the six seconds. The duty cycle pattern Q 6  has six seconds of high levels and the subsequent four seconds of low levels. 
     To enable the requisite outputs to be supplied, shift register 22 is provided with a signal supplying circuit 23, which keeps supplying a high level signal, and with a pulse generating circuit 24 which produces pulses at one second intervals, in order to shift the high level from one stage to another. 
     Still referring to FIG. 5, the duty cycle pattern terminal Q 6  and the AND gate 19 1  are connected to the analog switch 21 1 , the duty cycle pattern terminal Q 4  and the AND gate 19 2  are connected to the analog switch 21 2  and similarly the duty cycle pattern Q 2  and AND gate 19 3  terminal are connected to the analog switch 21 3 . 
     All the analog switches 21 1 , 21 2  and 21 3  are connected to the OR gate 25. 
     The gate of the thyristor 7 can be controlled through the gate control circuit 26 which is interposed between the OR gate and the thyristor 7. 
     It will be readily appreciated that the thyristor 7 is thus energized and deenergized according to the duty cycle patterns Qi in response to the food temperature. 
     From the food temperature below -10° C., as is shown in FIG. 7, the magnetron performs with the duty cycle pattern Q 6 , the highest capability of the present embodiment, with the analog switch 21 1  closed. 
     Within the temperature range from -10° C. to -5° C. the magnetron power is decreased to the duty cycle pattern Q 4 . The thyristor 7 is triggered via the closed analog switch 21 2 . 
     When the temperature increases above -5° C., the duty cycle pattern changes into Q 2 , the magnetron power being decreased to another lower rank, and after the temperature becomes higher than the freezing point, the magnetron automatically suspends operation. 
     It should be understood that in this way the microwave of the invention controls its duty cycle operation in response to the directly sensed food temperature as the temperature increases with the heating time, so that it becomes unnecessary to set in advance the heating time and heating power required to thaw the frozen food. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended Claims, the invention may be practiced otherwise than as specifically described herein.