Humidity detecting circuit

Abstract: A humidity detecting circuit for detecting the ambient humid­ ity, which inludes a constant current source, a first temperature-­ detecting resistor self-heated by the constant current source to detect the ambient humidity and having one terminal connected to an output terminal of the constant current source, a second temperature-detecting resistor having one terminal connected to said one terminal of the first temperature-detecting resistor to detect the ambient temperature, an operational amplifier having an inverting input terminal, a noninverting input terminal and an output terminal, said inverting input terminal being connected to the other terminal of the second temperature-detecting resistor so that the terminal voltage of the first temperature-detecting resistor may be inputted into the inverting input terminal through the second temperature-detecting resistor, and a feedback resis­ tor associated with the operational amplifier and having one terminal connected to the output terminal of the operational amplifier and the other terminal connected to the inverting input terminal of the operational amplifier.

Description: The present invention generally relates to an electronic circuit and more particularly, to a humidity detecting circuit for detecting the finish of cooking through humidity in a cooker such as a microwave oven or the like. Such humidity detecting circuit is incorporated in a humidity detecting device employed in the microwave oven or the like. The conventional humidity detecting circuit is constructed as shown, for example, in Fig. 1. More specifically, with employment of a first temperature-detecting resistor H′ and a second temperature-detecting resistor N′, the first temperature-detecting resistor H′ draws out an output voltage VH, through self-heating by a large and constant current circuit IH, with both of the temperature-detecting resistors H′ and N′ being linearly varied in their resistance values with positive temperature coefficients through the temperature by the use of metallic films. On the other hand, the second temperature-detecting resistor N′ draws out a voltage VN proportional to the ambient temperature by a very small and constant current circuit IN. When a constant current value is set so that a voltage difference between the two temperature-detecting resistors H′ and N′ i.e., the value of VN-VH, becomes zero in the dry condition, a difference voltage also becomes zero independent of the ambient temperature. When air becomes to contain steam through progress of cooking in the microwave oven, the first temperature-detecting resistor H′ self-heated to 150°C through 200°C is deprived of heat by the steam, thereby to lower the temperature thereof, thus resulting in that the voltage on the side of the first temperature-detecting resistor H′ is lowered. As the output voltage VN on the side of the second temperature-detecting resistor N′ does not change, the VN-VH does not become zero as a result. This voltage is amplified by Rf′/RS′ magnification by an operational amplifier CP3 to detect the existence of the humidity. An operational amplifier CP1 and an operational amplifier CP2 are voltage followers for transmitting the output voltages VN and VH to the operational amplifier CP3. The following equation is established, where resistance values at 0°C of the first and the second temperature-detecting resistors H′ and N′ are respectively RH and RN, temperature coefficients thereof are respectively αH and αN, and the resistance values thereof at the temperatures tH and tN are respectively rH and rN. rH = RH(1+αH·tH) rN = RN(1+αN·tN) Meanwhile, a temperature rise (tH-tN) through the self-heating is linearly related to an electric power consumed by the first temperature-detecting resistor H′. As tN is equal to the ambient temperature, rH·IH² = hm(tH-tN)S Hereupon, hm is a heat transfer coefficient and S is a surface area of the first temperature-detecting resistor H′. From the equations of (1) and (3), the following equation can be obtained. rH = RH 1 - αH·IH² hm·S RH (1+αH·tN) As the antecedent of the equation of (4) becomes a constant when hm is constant in the dry condition, the first temperature-detecting resistor H′ on the side of the self-heating is as follows in the resistance value at 0°C. RH 1- αH·IH² hm·S RH Accordingly, the first temperature detecting-resistor H′ in this case is equivalent to the temperature-detecting resistor having the temperature coefficient of αH. The output voltages of the operational amplifiers CP1 and CP2 in Fig. 1 are respectively as follows. VH = rH·IH = RH·IH 1 - αH·IH² hm·S RH = (1+αH·tN) VN = rN·IN = RN·IN(1+αN·tN) An output Vout of the operational amplifier CP3 is as follows. Vout = Rf′ RS′ (VN-VH) = Rf′ RS′ {RN·IN(1+αN·tN) - RH·IH 1 - αH·IH²·RH hm·S (1+αH·tN)} When IN and IH are set so that αH = αN, and RN·IN = RH·IH 1 - αH·IH²·RH hm·S may be established to set a constant, Vout of the equation of (7) normally becomes zero as hm is constant under the dry condition. The following equation can be obtained from the equation of (7), when it is rewritten. Vout = Rf′ RS′ {RN·IN RH·IH 1- αH·IH²·RH hm·S }(1+αN·tN) Although hm is constant in the dry condition, hm increases as the steam starts to come out of the cooked in the course of the cooking operation. At this moment, Vout increases rapidly from zero so that the humidity detection may be performed. A change of Vout with time is shown in Fig. 2. In the conventional humidity detecting circuit of Fig. 1, however, it has been required to provide a large number of circuit elements such as two of the constant-current sources IH and IN, three of the operational amplifiers CP1, CP2 and CP3, and the like. Furthermore, it has been very difficult to incorporate IH and IN of the two constant-current sources into the circuit so as to meet the conditions of the equation of (9). In Patent Abstracts of Japan, Vol. 9, No. 203 (P-381), 1926, 21st August 1985, a humidity detecting circuit is disclosed, wherein two thermistors are provided in a bridge circuit. The bridge circuit has two branches, one branch containing a series connection of both thermistors, and the other branch containing a series connection of two resistors. The balancing voltages of the bridge circuit are fed to a differential amplifier whose output signal is delivered to a control circuit. The control circuit receives at another input thereof the voltage of the junction of the two resistors, for providing a value of the absolute humidity, even if the temperature fluctuates during measurement. This circuit needs a bridge circuit and its exactness depends from the tolerances and variations of the bridge resistors. Accordingly, the present invention has been developed with a view to substantially eliminating the above described disadvantages inherent in the prior art humidity detecting circuit, and has for its essential object to provide an improved humidity detecting circuit which is not required to jointly incorporate two constant-current sources thereinto. Another important object of the present invention is to provide a humidity detecing circuit of the above described type which is simple in construction and stable in functioning, and can be readily manufactured at low cost. This object is solved, according to the invention, with a humidity detecting circuit having the features of claim 1. These and other objects and features of the present invention will become apparent from the following description taken in conjunction with the preferred embodiment thereof with reference to the accompanying drawings, throughout which like parts are designated by like reference numerals, and in which: Fig. 1 is an electronic circuit diagram of the conventional humidity detecting circuit for detecting the finish within a microwave oven (already referred to); Fig. 2 is a graphical diagram showing the outline of the characteristics with time of an output voltage produced from the conventional humidity detecting circuit of Fig. 1 (already referred to); Fig. 3 is a fundamental electronic circuit diagram of a humidity detecting circuit according to the present invention for detecting the finish within the microwave oven; Fig. 4 is a graphical diagram showing the outline of the characteristics with time of the output voltage produced from the humidity detecting circuit of Fig. 3; Fig. 5 is an electronic circuit diagram showing a preferred embodiment of the humidity detecting circuit of Fig. 3; Fig. 6 is a graphical diagram showing the outline of the characteristics with time of the output voltage produced from the electronic circuit of Fig. 5; Fig. 7 is a fundamental electronic circuit diagram of the humidity detecting circuit according to another embodiment of the present invention for detecting the finish within the microwave oven; Fig. 8 is a perspective view of the microwave oven to which the present invention is applied; Fig. 9 is a side elevational view of an exhaust duct having thereon a humidity sensor incorporated in a humidity detecting device; Fig. 10 is a perspective view of the humidity sensor of Fig. 9, which particularly shows a first example thereof; Fig. 11 is a side elevational view of one of temperature-detecting resistors on an enlarged scale, showing an uneven temperature produced thereon in a case where no rectifier box is provided; Fig. 12 is a graphical diagram showing an output voltage from the humidity sensor of Fig. 10 in a case where no rectifier box is provided; Fig. 13 is a perspective view of the humidity sensor, which particularly shows a second example thereof; Fig. 14 is a perspective view of one of the temperature-detecting resistors mounted in the humidity sensor of Fig. 13, on an enlarged scale, showing a state in which an exhaust air is brought into collision against the temperature-detecting resistor; Fig. 15 is a side elevational view of Fig. 14 similar to Fig. 11, showing the flow of the exhaust air; Fig. 16 is a graphical diagram similar to that in Fig. 12, which particularly shows the output voltage from the humidity sensor of Fig. 13; Fig. 17 is an exploded perspective view of the humidity sensor, which particularly shows a third example thereof; Fig. 18 is a graphical diagram showing the output voltages from the humidity detecting circuit of Fig. 5 in a case where no rectifier box is provided and in a case where the rectifier box is provided, with a cooling fan provided in the microwave oven having been operated; Fig. 19 is a graphical diagram showing the characteristics of the output voltages from the humidity detecting circuit of Fig. 5, in a case where the service voltage of the cooling fan has been varied when the output voltage is kept constant; Fig. 20 is a view similar to Fig. 13, which particularly shows a fourth example of the humidity sensor; Fig. 21 is a front elevational view of the humidity sensor of Fig. 20; Fig. 22 is a perspective view of the temperature-detecting resistor on an enlarged scale, similar to Fig. 14, showing the uneven temperature of a temperature detecting portion thereof in a case where the exhaust air is brought into collision against lead leg portions thereof; Fig. 23 is a view similar to Fig. 17, which particularly shows a fifth example of the humidity sensor; Fig. 24 is a graphical diagram showing the characteristics with time of the output voltages from the humidity detecting circuit of Fig. 5, in cases where two kinds of different materials are applied to the rectifier box, with the cooling fan having been operated; Fig. 25 is a view similar to Fig. 17, which particularly shows a sixth example of the humidity sensor; Fig. 26 is a view similar to Fig. 17, which particularly shows a seventh example of the humidity sensor; Fig. 27 is a sectional view of the temperature-detecting resistors of Fig. 26; Fig. 28 is a view similar to Fig. 10, which particularly shows a eighth example of the humidity sensor; and Fig. 29 is a graphical diagram showing an influence upon the output voltages from the humidity detecting circuit of Fig. 5 by a time constant of the temperature-detecting resistor with respect to the heat. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The principle of a humidity detecting circuit for detecting the finish of the cooking in a microwave oven through the humidity in accordance with the present invention will be described hereinafter with reference to Figs. 3 and 4. Although an electronic circuit according to the present invention as shown in Fig. 3 is substantially the same as the conventional electronic circuit as shown in Fig. 1 in a point that a first temperature-detecting resistor H is self-heated by a constant-current source Io, the former is different from the latter in that a second temperature-detecting resistor N is interposed between an output from the first temperature-detecting resistor H and an inverting input into an operational amplifier OP. Namely, the electronic circuit according to the present invention includes the first temperature-detecting resistor H which performs a self-heating operation for detecting the humidity, the second temperature-detecting resistor N for detecting the ambient temperature, the constant-current source Io, the operational amplifier OP and a feedback resistor Rf associated with the operational amplifier OP. More specifically, one terminal of the first temperature-detecting resistor H is directly connected to the output terminal of the constant-current source Io so that the first temperature-detecting resistor H is self-heated by the constant-current source Io, while the other terminals of the former and the latter are grounded. The terminal voltage of the first temperature-detecting resistor H is applied to the inverting input terminal (i.e., (-) terminal) of the operational amplifier OP through the second temperature-detecting resistor N. The output from the operational amplifier OP is negatively fed back to the inverting input terminal thereof through the feedback resistor Rf, with a noninverting input terminal (i.e., (+) terminal) thereof being grounded. Accordingly, components provided in the humidity detecting circuit of the present invention can be extremely reduced in number as compared with the conventional humidity detecting circuit. On the assumption that the relation is rH « rN in Fig. 3, the terminal voltage VH of the first temperature-detecting resistor H on the side of the self-heating becomes as follows as in the equation of (5). VH = RH·Io² 1 - αH·Io²·RH hm·S (1+αH·tN) As the gain of the operational amplifier OP is Rf rH+rN ≒ Rf rN under the condition of rH«rN, the output Vout can be obtained as follows in the selection into αH = αN. Vout = Rf -rN ·VH = RH·Io·Rf -(1- αH·Io²·RH hm·S )·RN Since the heat transfer coefficient hm is constant in the dry condition, the output Vout becomes a negative constant value. As the hm increases because of the steam produced in the course of the cooking, the absolute value of the Vout becomes small so that a time lapse of the output as shown in Fig. 4 may be obtained. Fig. 5 shows one preferred embodiment of the present invention. Although both of the positive and the negative power sources are required for the operational amplifier OP in the electronic circuit as shown in Fig. 3, the electronic circuit shown in Fig. 5 is simplified so that only the positive power source will be required. In Fig. 5, the constant-current circuit is composed of a first operational amplifier OP1 , a transistor Q, a resistor RS and a reference power source Vref to feed the constant current Io=Vref/RS to the first temperature-detecting resistor H. More specifically, the output voltage from the reference power source Vref is applied to the noninverting input terminal of the first operational amplifier OP1, and one terminal of the first temperature-detecting resistor H is directly connected to the emitter of the transistor Q which amplifies the output current from the first operational amplifier OP1, with the output from the first operational amplifier OP1 being applied to the base of the transistor Q. The other terminal of the first temperature-detecting resistor H is grounded through the resistor RS and also, it is connected to the inverting input terminal (i.e., (-) terminal) of the first operational amplifier OP1 so that upon amplification of the output from the first operational amplifier OP1, it is negatively fed back to the inverting input terminal of the first operational amplifier OP1 through the first temperature-detecting resistor H. Furthermore, one terminal voltage of the first temperature-detecting resistor H is applied to the inverting input terminal of the second operational amplifier OP2 through the second temperature-detecting resistor N which is a temperature-detecting resistor for detecting the ambient temperature as in Fig. 3. The other terminal voltage of the first temperature-detecting resistor H is directly applied to the noninverting input terminal of the second operational amplifier OP2. The output from the second operational amplifier OP2 is negatively fed back to the inverting input terminal thereof through a feedback resistor Rf. A junction between the second temperature-detecting resistor N and the inverting input terminal of the second operational amplifier OP2 is grounded through a resistor RB. In addition, another power source Vcc is provided in this electronic circuit so that the output voltage from the power source Vcc is applied to the first operational amplifier OP1 , to the second operational amplifier OP2 and to the collector of the transistor Q. The output voltage Vout from the operational amplifier OP2 in Fig. 5 is derived as follows in case of αH = αN. Vout = - RH RN(1- αH·RH·Vref² hm·S·RS² ) × Rf RS ·Vref + (1+ Rf RB )Vref First, the resistor RB which is connected to the negative input terminal of the second operational amplifier OP2 is kept adjusted so that the Vout may become a constant value in the initial condition of the cooking. When the hm increases by the steam of the cooked produced in the course of the cooking operation, the first term in the equation of (10) decreases. As a result, the Vout rapidly increases to provide such characteristics with time as shown in Fig. 6. In the principle shown in Fig. 3, although the selection is required as αH = αN, such selection is hard to be achieved in terms of performance of the temperature detection resistance. In the other embodiment of the humidity detecting circuit of the present invention, therefore, the detection of the humidity is adapted to be performed even if the relation of αH = αN is not required to be established. The other embodiment of the humidity detecting circuit according to the present invention will be described hereinafter with reference to Fig. 7. What is different from the above described electronic circuit shown in Fig. 3 is that the terminal voltage VH of the first temperature-detecting resistor H is applied to the inverting input terminal of the operational amplifier OP through the second temperature-detecting resistor N and a resistor Rc additionally connected in series therewith. More specifically, there are provided a first temperature-detecting resistor H which performs the self-heating operation for detection of the humidity, a second temperature-detecting resistor N for detecting the ambient temperature, a constant-current source Io, an operational amplifier OP, a feedback resistor Rf associated with the operational amplifier OP and a resistor Rc connected in series with the second temperature-detecting resistor N. The first temperature-detecting resistor H is self-heated by the constant-current source Io, and the terminal voltage of the first temperature-detecting resistor H is adapted to be inputted into the inverting input terminal of the operational amplifier OP through the second temperature-detecting resistor N and the resistor Rc. The humidity detecting circuit may be constructed without making the αH equal to the αN through this circuit. The gain of the operational amplifier shown in Fig. 7 is as follows. Rf rH + rN + Rc ≒ Rf rN + Rc Accordingly, the output Vout is derived as follows. Vout = Rf -(rN + Rc) ·VH = Rf·Io -(1- αH·Io²·RH hm·S ) × RH(1 + αH·tN) RN(1 + αN·tN) + Rc Hereupon, when Rc is selected so that the following equation may be established RH(1 + αH·tN) RN(1+ αN·tN) + Rc = constant the output Vout is derived as follows. Vout = Rf·Io -(1- αH·Io²·RH hm·S ) × C In this equation, C is derived as a constant. Since the hm is constant in the dry condition, the output Vout is a constant value. As the hm is increased by the steam produced in the course of the cooking operation, the absolute value of the Vout becomes smaller so that such time lapse outputted as shown in Fig. 7 may be provided. As the humidity detecting circuit of the present invention is constructed as described hereinabove, the finish of the cooking in the microwave oven or the like can be detected by the humidity only through the adjusting operation of the resistors connected in series without making equal to each other the respective humidity coefficients αH and αN of the first and the second temperature-detecting resistors H and N, thus resulting in a remarkable effect in the automatic heating operation of the food. In the next place, various examples in construction of a humidity sensor incorporated in a humidity detecting device, to which the present invention is applied, will be explained hereinafter. Fig. 8 shows the microwave oven having a humidity detecting device incorporating therein the humidity detecting circuit of the present invention. The microwave oven shown in Fig. 8 is provided with a magnetron 1, a high-tension transformer 2, a cooling fan 3 for cooling the magnetron 1 and the high-tension transformer 2, a heating chamber 4 and an exhaust duct 7, with the magnetron 1, the high-tension transformer 2 and the cooling fan 3 being accommodated within a main body 5 outside the heating chamber 4. The radiation from the magnetron 1 itself is discharged to the exhaust duct 7 together with warm air (a) which is produced by an object to be heated, within the heating chamber 4 from a ventilation opening 6 defined in a chamber wall of the heating chamber 4. A humidity sensor 8 is arranged, as shown in Fig. 9, on the exhaust duct 7 through which the steam produced from the heated object within the heating chamber 4 is exhausted outside the heating chamber 4. Fig. 10 is a first example of the humidity sensor 8 for detecting the humidity in the atmosphere within the exhaust duct 7 as a ventilation passage, which sensor 8 is provided with the first temperature-detecting resistor H to be self-heated or to be heated by a heat source, the second temperature-detecting resistor N juxtaposed with the first temperature-detecting resistor H and for detecting the humidity within the atmosphere, and a base plate 11 for supporting thereon the first and the second temperature-detecting resistors H and N. Preferably, the humidity sensor 8 is further provided with a rectifier box as shown in the examples described hereinbelow. Fig. 13 shows a second example of the humidity sensor 8A which is provided with a rectifier box 12A in the form of a rectangular parallelpiped having an opened bottom and surrounding the first and the second temperature-detecting resistors H and N. The rectifier box 12A includes a front plate 12A-a having a rectangular inlet opening 13 and a rear plate 12A-b having a rectangular outlet opening 14, wherein the ventilation air introduced into the rectifier box 12A is controlled in amount by both of the inlet and the outlet openings 13 and 14, with the rectifier box 12A being arranged so that the front and the rear plates 12A-a and 12A-b are caused to cross at a right angle to the flowing direction of the ventilation air within the ventilation passage. By the above described construction, the air C which enters the rectifier box 12A through the exhaust duct 7 flows against the first and the second temperature-detecting resistors H and N. In the atmosphere of 20°C, for example, when the exhaust air containing the steam of 50°C is caused to flow against temperature detecting portions 9 and 10 of the temperature-detecting resistors H and N as shown in Fig. 14 under the condition that no rectifier box is provided, the strong or weak collision of the exhaust air against the surfaces of the temperature-detecting resistors H and N may take place as shown in Fig. 11 to cause uneven temperatures A1 and A2, because the flow speed of the exhaust air C is so high that the rectifying operation is not performed. Accordingly, an unevenness undesirably arises even in the response speed with respect to the temperature of the exhaust air, thus resulting in such major oscillation with respect to output values of the temperature-detecting resistors H and N as shown in Fig. 12. However, when the exhaust air is introduced into the rectifier box 12A to reduce the flow speed thereof and to perform the rectifying operation, the collision of the exhaust air becomes uniform so that the exhaust air flows along the whole surfaces of the temperature-detecting resistors H and N as shown in Fig. 15, thus resulting in that the uneven temperature on the surfaces of the temperature-detecting resistors H and N or the uneven response speed is settled. As a result, the output signal D is stabilized as shown in Fig. 16. Accordingly, the stabilized output signal obtained has an effect that the correct heating time may be calculated. Fig. 17 shows a third example of the humidity sensor 8B. When the inlet opening 13 of the rectifier box 12A is small in Fig 13, the opening 13 may be stopped up by foreign substances such as oil, food garbage or the like contained in the steam which is produced in the course of the cooking. In this case, the air containing the steam never flows against the first and the second temperature-detecting resistors H and N so that the humidity cannot be detected. In this example, therefore, the rectifier box 12B is provided with the front plate 12B-a having a plurality of inlet openings 13a defined therein. The rectifier box 12B is securely mounted on the base plate 11B in a manner that leg portions 12a of the rectifier box 12B are initially extended through respective holes 11a defined in the base plate 11B and are bent thereafter. The other construction is similar to that in the second example. Suppose that one inlet opening 13a is stopped up by the foreign substances in such construction as described hereinabove, the air C is still permitted to flow into the rectifier box 12B through the other inlet openings 13a to make it possible to perform the stable humidity-detection. It is to be noted that the output fluctuation can be reduced in the second and the third examples, because the air speed is lowered even when the voltage to be supplied has been changed. More specifically, in the case where the humidity detecting circuit shown in Fig. 5 is employed, the output voltages Vout are graphically shown in Fig. 18 with respect to the humidity sensor 8 described in the first example thereof and to the humidity sensor 8A or 8B described in the second or the third example thereof, when the cooling fan 3 has been operated from the stop condition. In Fig. 18, the output voltage Vout produced from the humidity sensor 8 in the first example is shown by X and the output voltage Vout produced from the humidity sensor 8A or 8B in the second or the third examples is shown by Y. In the latter, the voltage change is reduced, since the speed of the air flowing against the temperature-detecting resistors H and N is lowered. Furthermore, Fig. 19 graphically shows the characteristics in a case where the voltage supplied to the cooling fan 3 has been changed when the output voltage Vout becomes constant upon rotation of the cooling fan 3. In Fig. 19, X1 shows the case in the first example and Y1 shows the case in the second or the third example. In the latter, the output fluctuation is controlled to an extremely small extent. Figs. 20 and 21 show a fourth example of the humidity sensor 8C, wherein the inlet opening 13b and the outlet opening 14b each elongated horizontally are formed at the lower portion of the rectifier box 12C, with the horizontal width of each elongated opening 13b or 14b being caused to be shorter than a distance between a pair of spaced lead leg portions 9A or 10A of the respective temperature-detecting resistors H or N so that the air C introduced into the rectifier box 12C may not be brought into direct collision against the first and the second temperature-detecting resistors H and N. As a result, the hot air is lowered in its flow speed to perform the rectifying operation and is caused to substantially uniformly collide against the whole surfaces of the temperature-detecting resistors H and N to rectify the temperature variation which tends to occur thereon. The other construction of the humidity sensor 8C in this example is similar to that in the above described second example. In the above described construction, the air C which introduced into the rectifier box 12C turns a turbulent flow to flow along the whole surfaces of the temperature-detecting resistors H and N as shown in Fig. 15 so that the collision of the exhaust air C becomes uniform. It is to be noted here that if the horizontal width of each opening 13b or 14b is caused to be longer than the distance between the pair of lead leg portions 9A or 10A of respective temperature-detecting resistors H or N, the exhaust air passing through the inlet opening 13b directly collides against the leg portions 9A and 10A of the temperature-detecting resistors H and N as shown in Fig. 22 so that the thermal conduction is caused from the leg portions 9A and 10A to raise the temperature initially from both end portions A1 of the temperature detecting portions 9 and 10, thus resulting in that there undesirably occurs the temperature difference between the both end portions A1 and the central portion A2 thereof. Accordingly, the unevenness takes place even at the response speed with respect to the temperature of the exhaust air so that such oscillation as shown in Fig. 12 appears in the output Vout produced from these elements 9 and 10. In this example, however, since the horizontal width of each openings 13b or 14b is caused to be shorter than the distance between the pair of leg portions 9A or 10A, the exhaust air which has entered the rectifier box 12C turns the turbulent flow to flow along the entire surfaces of the temperature-detecting resistors H and N, and the collision of the exhaust air thereagainst becomes uniform to settle the uneven temperature on the surfaces of the temperature detecting portions 9 and 10 or the uneven response speed with respect thereto. As a result, the output signal D is desirably stabilized as shown in Fig. 16. A fifth example of the humidity sensor 8D is illustrated in Figs. 23 and 24, wherein the rectifier box 12D is made of metal such as aluminum or the like having a superior heat conductivity. It is to be noted in this example that a heat insulating packing 15 is provided to prevent the base plate 11D from being subjected to the thermal influence. The other construction of the humidity sensor 8D in this example is similar to that of the aforegoing third example. Fig. 24 shows the characteristics of time required from the initial rotation of the cooling fan 3 to the stabilization of the circuit output in a case where the rectifier box 12D is made of polypropylene resin having a thickness of 1 mm and in a case where it is made of aluminum having a thickness of 0.6 mm. As clearly shown in Fig. 24, although the circuit has required approximately three minutes to be stabilized in output therefrom in the case of the polypropylene resin as shown by W, it advantageously requires only approximately fifty seconds to be stabilized in the case of the metal such as aluminum or the like having the superior thermal conductivity. A sixth example of the humidity sensor 8E is illustrated in Fig. 25, wherein a partition plate 16 is provided between the first temperature-detecting resistor H and the second temperature-detecting resistor N in the rectifier box 12E to partition the two resistors H and N from each other, with one or more openings 16a being formed in the partition plate 16 to enable the exhaust air to pass therethrough. The other construction of the humidity sensor 8E in this example is similar to that in the aforementioned third example. By the above described construction, the air C introduced into the rectifier box 12E can be reduced in its speed variation so that the variation in the hm of the equation of (9) caused thereby can be minimized to provide the correct humidity detection. A seventh example of the humidity sensor 8F is illustrated in Figs 26 and 27, wherein synthetic resin films 17 and 18 are formed on the first and the second temperature-detecting resistors H and N, respectively. The synthetic resin films 17 and 18 are of a heat resistant material such as silicon resin, polyimide or the like, and entirely cover both of the leg portions 9A and 10A and the temperature detecting portions 9 and 10 of the temperature-detecting resistors H and N. The other construction of the humidity sensor 8F in this example is similar to that in the aforegoing third example. By such construction as described hereinabove, the first and the second temperature-detecting resistors H and N being raised in thermal capacity thereof, the variation in the hm caused by the uneven air being reduced to eliminate the influence therefrom, and the output voltage being prevented from its oscillation for the purpose of its stabilization, the humidity detection which is high in accuracy can be performed. Figs . 28 and 29 show an eighth example of the humidity sensor 8G, wherein the temperature-detecting portions 9 and 10 of the first and the second temperature-detecting resistors H and N is spaced by 5 mm or more (S) from the base plate 11. It is to be noted that the other construction of the humidity sensor 8G in this example is similar to that in the above described second example. As the cooling fan 3 does not rotate in a condition where the microwave oven is out of operation, with the temperature-detecting resistors H and N being mounted on the base plate 11 in close contact relation therewith as shown in Figs. 9 and 10, the exhaust air never flow against the temperature-detecting resistors H and N and the base plate 11 so that the base plate 11 is heated by the self-heating temperature-detecting resistor H. Thereafter, upon rotation of the cooling fan 3 in a cooking mode, the exhaust air from inside of the microwave oven flows against the temperature-detecting resistors H and N and the base plate 11. Since each of the temperature-detecting resistors H and N has a small time constant with respect to the heat, it is brought into a thermal equilibrium condition after approximately thirty seconds from the start of rotation of the cooling fan 3. However, since the base plate 11 is large in thermal capacity, it has a time constant of approximately three minutes with respect to the heat. Accordingly, in the case where the temperature-detecting resistors H and N are mounted on the base plate 11 in close contact relation therewith, the circuit output is not stabilized until approximately three minutes have elapsed after the start of rotation of the cooling fan 3, as shown by a solid line W in Fig. 29. On the contrary, when the temperature detecting portions 9 and 10 of the temperature-detecting resistors H and N are kept away by 5 mm or more from the base plate 11 as in this example, the heating of the base plate 11 by the self-heating temperature detecting resistor H is reduced. Furthermore, since heat insulating layers of the air are formed between the base plate 11 and the temperature-detecting resistors H and N with the rotation of the cooling fan 3, the heat stored on the base plate 11 exerts only an extremely small influence upon the temperature-detecting resistors H and N, thus resulting in that the circuit output is stabilized in approximately thirty seconds as shown by a broken line U in Fig. 29, as being substantially dependent on the time constant of the temperature-detecting resistors H and N with respect to the heat. By the above described simple construction of the humidity sensor in which the rectifier box having the inlet and the outlet openings for enabling the air to pass therethrough is only provided so as to surround the temperature-detecting resistors in the humidity detecting device, there is advantageously produced an effect that the uneven temperature of the temperature-detecting portions caused under the influence of the uneven air can be eliminated and the correct humidity detection can be performed.

Claims: A humidity detecting circuit for detecting ambient humidity in a surrounding environment, comprising: a constant current source (Io); a first temperature-detecting resistor (H) self-heated by said constant current source to detect ambient humidity; means for connecting one terminal of said first temperature-detecting resistor (H) to an output terminal of said constant current source (Io); a second temperature-detecting resistor (N) for detecting ambient temperature; an operational amplifier (OP; OP2) having an inverting input terminal, a non-inverting input terminal and an output terminal; and a feedback resistor (Rf) associated with said operational amplifier; means for connecting said feedback resistor (Rf) between said inverting input terminal and said output terminal of said operational amplifier, characterized by means for connecting said second temperature-detecting resistor (N) between said one terminal of said first temperature-detecting resistor (H) and said inverting input terminal of said operational amplifier so that the terminal voltage of said first temperature-detecting resistor is inputted to said inverting input terminal through said second temperature-detecting resistor, wherein the output voltage (Vout) of said operational amplifier changes from a constant value upon the occurrence of humidity in said environment. A humidity detecting circuit as claimed in claim 1, further comprising a third resistor (Rc) and means for connecting said third resistor between said second temperature-detecting resistor (N) and said inverting input terminal of said operational amplifier (OP) so that the terminal voltage of said first temperature-detecting resistor is inputted into said inverting input terminal through said second temperature-detecting resistor (N) and said third resistor (Rc). A microwave oven comprising a humidity detecting circuit as defined in claim 1 or 2.

Inventors: ANDO YUZI, FUJII MASANOBU, MORI YUICHI, NAKATANI MASATO, TAHARA HIROKAZU, TSUDA TATSUYA, ANDO, YUZI, FUJII, MASANOBU, MORI, YUICHI, NAKATANI, MASATO, TAHARA, HIROKAZU, TSUDA, TATSUYA
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1987
Main classification: G01N25
IPC codes: G01N25