JPH07244101A - Thermistor for microwave detection - Google Patents

Abstract

(57) [Abstract] [Purpose] By limiting the characteristics of the thermistor element to be used, thermistor output sensitivity in the region where the microwave input is small is improved without increasing the thermistor output in the region where microwave input is large. . The detection sensitivity is high when there is an object to be heated, and there is no risk of thermal shock to the thermistor element when there is no object to be heated. [Structure] A pair of leads 12 and 13 is connected to a thermistor element 11 of a thermistor 10 for microwave detection, and the leads 12 and 13 are provided at positions where microwaves are applied. The leads 12 and 13 are configured to induce a high frequency current when the microwave is applied to cause the thermistor element 11 to self-heat. The thermistor element 11 has a resistance value R _{25 at} 25 ° C. in the range of 100Ω to 10 kΩ. Further, it is preferable that the thermistor element 11 has a B constant of 3000 to 5000 K or a heat dissipation constant δ of 1 to 3 mW / ° C., or both.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermistor for detecting microwaves equipped with an NTC thermistor element having a lead. More specifically, the present invention relates to a microwave detection thermistor suitable for detecting microwave intensity for automatic control of a microwave heating device such as a microwave oven.

[0002]

2. Description of the Related Art The inventors of the present invention connect a pair of leads 12 and 13 to a thermistor element 11 as shown in FIGS. 7 to 9 and provide the leads 12 and 13 at positions where microwaves are applied. In addition, the inventors have invented and applied for a patent for the microwave detection thermistor 10 configured to induce a high-frequency current during the microwave irradiation to cause the thermistor element 11 to self-heat (Japanese Patent Application No. 5-126765). This microwave detecting thermistor has a feature that it can detect the microwave intensity quickly with a small size and a simple structure.

[0003]

However, the above microwave detecting thermistor has, for example, two of the thermistor elements.
Resistance value at 5 ℃ is 15kΩ, B constant is 2950K
In addition, when the heat dissipation constant has a characteristic of 4 mW / ° C., and is attached to a microwave oven as shown in FIGS. As shown, the thermistor output is small even if the output of the microwave exceeds 400W. Further, as shown by the symbol T in FIG. 10, when the microwave is loaded with 200 g of water as a substitute for food, most of the microwaves are absorbed by the water and the microwaves incident on the thermistor are relatively reduced. The detection sensitivity is low. Therefore, both had points to be improved. In order to solve these points, it is possible to increase the thermistor output by enlarging the slit 51 shown in FIGS. 7 and 9 to increase the incident amount of the incident microwaves. The output in the strong area is also inevitably large. Therefore, when the microwave oven is operated in a no-load state, that is, when the microwave oven is cooked, there is a possibility that the thermistor element is subjected to thermal shock.

It is an object of the present invention to provide a thermistor for microwave detection which improves the thermistor output sensitivity in the region where the microwave input is small without increasing the thermistor output in the region where the microwave input is large. Another object of the present invention is to provide a thermistor for microwave detection which has a high detection sensitivity when there is an object to be heated and does not give a thermal shock to the thermistor element when there is no object to be heated.

[0005]

As shown in FIGS. 1, 7 and 9, according to the present invention, a pair of leads 12 and 13 are connected to a thermistor element 11, and the leads 12 and 13 are irradiated with microwaves. A thermistor 10 for microwave detection, which is provided at a position where the thermistor element 11 self-heats by inducing a high-frequency current at the time of microwave irradiation. Hereinafter, R ₂₅ ) is in the range of 100Ω to 10 kΩ. R ₂₅ is preferably 500Ω to 2kΩ. R ₂₅ is not high detection sensitivity small variation of resistance value caused by temperature changes is less than 100 [Omega, also R ₂₅ is 1
If it exceeds 0 kΩ, the detection sensitivity when the microwave oven output is small becomes low, which is not good.

As shown in FIG. 2, the thermistor element 11 preferably has an R ₂₅ in the range of 100Ω to 10 kΩ and a B constant in the range of 3000 to 5000K.
If the B constant is less than 3000K, the detection sensitivity is not high because the amount of change in resistance value due to temperature change is small, and the value is 5000K.
If it exceeds, the resistance value changes abruptly with a slight change in temperature, so that both are not preferable. Further, the resistance value of the thermistor and the B constant generally have a positive correlation, and when the B constant is large, the resistance value becomes too large and is not suitable. As shown in FIG. 3, the thermistor element 11 has R ₂₅ in the range of 100Ω to 10 kΩ and further has a heat dissipation constant (hereinafter, referred to as δ) of 1.
It is preferably in the range of 3 mW / ° C. δ is 1 mW
If the temperature is less than / ° C, the output of the microwave detection thermistor changes greatly due to a slight increase in the electric power of the microwave heating device, and the detection accuracy tends to decrease. Further, when δ exceeds 3 mW / ° C., it is difficult to obtain high detection sensitivity because the output change of the microwave detection thermistor becomes small even if the power of the microwave heating device increases. As shown in FIGS. 4 to 6, the thermistor element 11 has R ₂₅ in the range of 100 Ω to 10 kΩ, B constant in the range of 3000 to 5000 K, and δ in the range of 1 to 3 mW / ° C. It is preferable because the advantages of the above characteristics are combined.

[0007]

When the leads 12 and 13 are irradiated with the microwave M, a high frequency current is induced in the leads 12 and 13. The high frequency current causes the thermistor element 11 to self-heat,
The resistance value of the microwave detection thermistor 10 changes.
The microwave intensity is detected by converting this resistance value into temperature. Here, if an element with R ₂₅ in the range of 100Ω to 10 kΩ is used as the thermistor element 11, microwaves can be detected with high sensitivity. Further, as the thermistor element 11, R ₂₅ is in the range of 100Ω to 10 kΩ and the B constant is 3
Is in the range of 000-5000K, and δ is 1-3mW
If an element in the range of / ° C is used, microwaves can be detected with high sensitivity, and there is no risk of thermal shock to the thermistor element when there is no object to be heated.

[0008]

Embodiments of the present invention will now be described in detail with reference to the drawings. As shown in FIGS. 7 and 9, the microwave detector 15 includes two thermistors 10 and 20 for detecting microwaves with a lead, and the window 3 of the housing 30 of the microwave oven.
It is attached to 1. The housing 30 is a rectangular parallelepiped and has a width (x direction in the figure) of 280 mm and a depth (y direction in the figure) of 28.
It has an inner dimension of 0 mm and a height (z direction in the figure) of 180 mm. The microwave detection thermistor 10 is located at a position where the microwave M is irradiated, and the microwave detection thermistor 2 is provided.
0 is for measuring the environmental temperature and is arranged at a position where the microwave M is not irradiated. Specifically, a printed circuit board 4 of a glass cloth epoxy board having a length of 40 mm, a width of 40 mm, and a thickness of 1.6 mm, in which a horizontally long square hole 41 is formed in the center.
The microwave detection thermistors 10 and 20 are arranged so as to vertically cross the square hole 41.

These microwave detecting thermistors 10
And 20 are small-sized NTC bead type thermistors, which have the same structure. Bead type thermistor element 1
Leads 12, 13 and 22, 23 made of a pair of dumet wires are connected to both ends of 1 and 21, respectively. In this example, these are all G manufactured by Mitsubishi Materials Corporation.
A13 series thermistor, R ₂₅ is 1kΩ,
The B constant is 3950 K, and δ is 2 mW / ° C. Each of the thermistor elements 11 and 21 has a diameter of 1.6 mm and a length of 2.5 mm, and the total length of the thermistor including the leads is 20 mm.

The thermistor elements 11 and 21 are arranged in the center of the square hole 41 in the z direction, and the leads 12, 13 and 22,
Each end of 23 is soldered to the electrode pads 42 and 43 on the substrate 40, respectively. These leads 12, 13
22 and 23 are in the height direction of the microwave oven, that is, z in the figure.
It is arranged parallel to the direction (direction E of the electric field of the microwave M). The portion of the leads 12 and 13 exposed from the square hole 41 is longer than the length of the square hole 51 of the box 50 described later in the z direction. Four enamel-coated leads 44 are soldered to the two sets of electrode pads 42 and 43 perpendicularly to the substrate 40.

The printed circuit board 40 on which the thermistors 10 and 20 are mounted is adhered to the bottom surface of the box 50 with an adhesive. The inside dimension of the box 50 is 40 mm in length, 40 mm in width, and 10 mm in thickness. The box 50 is set on the left side surface of the housing 30 of the microwave oven such that the center of the box is 190 mm deep and 90 mm high. The box 50 is selected from copper, stainless steel, aluminum or the like having good electric conductivity in order to have a function as a microwave shielding plate for the thermistor 20. In this example, it is made of aluminum. The bottom surface of the box 50 faces the inside of the microwave oven, and a square hole 51 for exposing only the thermistor 10, which is smaller than the square hole 41 of the printed circuit board 40, is formed therein. The rectangular hole 51 has a length in the z direction (lead direction) of 5 to 15 mm and a length in the y direction of 10 mm to 30 mm. In this example, the size of the square hole 51 is 10 mm (z direction) × 15 mm (y direction).

The bottom surface of the box 50 is covered with a plate 60 from the outside. The plate 60 is a heat-resistant thin plate that transmits microwaves, and is selected from mica, Teflon, polystyrene and the like. In this example, it is mica. The plate 60 prevents water vapor, oil drops, etc. generated in the microwave oven from adhering to the thermistor 10. The opening of the box 50 is covered with a lid 70 made of an insulating material so that dust or the like does not enter the box and adhere to the thermistors 10 and 20. The lid 70 is the lead 44 described above.
Has four holes 71 with a diameter of 1 mm, which have a function of a microwave reflection plate.

The microwave detector 15 is assembled by passing four bolts 81 through the four corners of the plate 60, the box 50 and the lid 70 and screwing them with nuts 82. The microwave detector 15 thus assembled is fitted into the window hole 31 of the housing 30 of the microwave oven having the same area as the outer shape of the detector. Specifically, the upper flange 34 and the lower flange 35 are fixed to the hole edge of the window hole 31 with screws 32 and 33. After inserting the box 50 into these flanges 34 and 35, the upper flange 34 and the lower flange 35 are attached to the upper surface and the lower surface of the box 50. Screws 36 and 3 through flanges 34 and 35, respectively
By tightening 7, the microwave detector 15 becomes a window hole 3
It is fixed to 1.

Next, the two pairs of leads 44, 44 drawn from the hole 71 of the lid 70 were connected to an ohmmeter (not shown). And a state where the object to be heated is not put in the housing 30,
The microwave M was applied to the microwave detector 15 from a magnetron of a microwave oven (not shown). Here, the output of the microwave oven was switched stepwise from 0 W to 500 W and the microwave M was irradiated for 1 minute each. A high-frequency current was induced in these leads 12 and 13 by the irradiation of microwaves, and the thermistor element 11 of the thermistor 10 self-heated according to the output of the microwave oven and its resistance value was lowered. on the other hand,
The thermistor element 21 of the thermistor 20 is the thermistor 10.
The resistance value decreased due to the increase of the ambient temperature in the vicinity. The resistance of the thermistors 10 and 20 was measured with a resistance meter, and the temperature was determined from the thermistor characteristics. The output of the detector is a value obtained by subtracting the temperature of the thermistor 20 from the temperature of the thermistor 10. The result is shown in the characteristic curve of symbol P in FIG. Then, in a state where 200 g of water as an object to be heated was put in the housing 30, the microwave M was irradiated in the same manner as described above. The result is shown in the characteristic curve of the symbol Q. From these characteristic curves, the microwave thermistor 10 of this embodiment has a high detection sensitivity, and the thermistor output does not increase abnormally when there is no object to be heated, so that thermistor element 11 is not subjected to thermal shock.

In the above example, in order to check the output of the detector, resistance meters are connected to the two pairs of leads 44, 44, but the output of the detector may be measured by the bridge circuit shown in FIG. In this circuit, the thermistor 10 has a resistor R
₁ , the resistor R ₂ is connected to the thermistor 20 in series, respectively, and these are connected in parallel to use both terminals a and b as input terminals. The terminal c is connected to the connection point B between the thermistor 10 and the resistor R ₁ , and the connection point C between the thermistor 20 and the resistor R ₂ is connected.
The terminal d is connected to and both terminals c and d are used as output terminals. By applying the DC voltage V _IN to the input terminals a and b and measuring the output voltage V _OUT of the output terminals c and d at this time, the difference in resistance between the two thermistors 10 and 20, that is, the substantial microwave. The temperature rise value due to is calculated.

[0016]

As described above, according to the present invention, by limiting the characteristics of the thermistor element to be used, the thermistor output in the region where the microwave input is large is not increased, and the region where the microwave input is small is provided. The output sensitivity of the thermistor can be improved. Especially, R ₂₅ is 100Ω ~ 1
By using a thermistor element in the range of 0 kΩ for microwave detection, high detection sensitivity can be obtained when there is an object to be heated, and in addition, the B constant is 3000 to 5000.
By using an element in the range of K and δ in the range of 1 to 3 mW / ° C., a thermistor for microwave detection can be obtained in which there is no risk of thermal shock to the thermistor element when there is no object to be heated.

[Brief description of drawings]

FIG. 1 shows that B constant is less than 3000K and δ is 3 m.
FIG. 5 is an output characteristic diagram of a microwave detector that uses four types of thermistor elements that exceed W / ° C. and have R ₂₅ of 1 kΩ, 5 kΩ, 10 kΩ, and 15 kΩ.

FIG. 2 shows that R ₂₅ is 10 kΩ, δ exceeds 3 mW / ° C., and B constants are 2950K, 3450K, 3950.
FIG. 7 is an output characteristic diagram of a microwave detector using four types of thermistor elements of K and 4450K.

[Fig. 3] In both cases, R ₂₅ is 10 kΩ and B constant is 3000.
Less than K and δ is 1 mW / ° C, 2 mW / ° C, 3 mW /
FIG. 5 is an output characteristic diagram of a microwave detector using four types of thermistor elements of 4 ° C. and 4 mW / ° C.

FIG. 4 shows a B constant of 3950 K and δ of 2 mW /
C, and R ₂₅ is 1 kΩ, 5 kΩ, 10 kΩ and ₁₅ k
FIG. 4 is an output characteristic diagram of a microwave detector using four types of Ω thermistor elements.

FIG. 5: R ₂₅ is 10 kΩ and δ is 2 mW / ° C.
And B constant is 2950K, 3450K, 3950K
And 4450K are output characteristic diagrams of a microwave detector using four types of thermistor elements.

FIG. 6 shows that R ₂₅ is 10 kΩ and B constant is 2950.
FIG. 4 is an output characteristic diagram of a microwave detector using four types of thermistor elements having K and δ of 1 mW / ° C., 2 mW / ° C., 3 mW / ° C., and 4 mW / ° C.

FIG. 7 is a vertical cross-sectional view of the microwave detector according to the embodiment of the present invention.

FIG. 8 is a detection circuit diagram including the microwave detector.

FIG. 9 is an assembled perspective view of the microwave detector.

FIG. 10 is an output characteristic diagram of a microwave detector using a thermistor element in which R ₂₅ is 15 kΩ, B constant is less than 2950 K, and δ is 4 mW / ° C.

[Explanation of symbols]

 10,20 Microwave detection thermistor 11 Thermistor element 12,13 Lead

Claims (4)

[Claims] 1. A thermistor element (11) having a pair of leads (12,
13) is connected, the leads (12, 13) are provided at positions where microwaves are irradiated, and are configured to induce a high-frequency current during microwave irradiation to cause the thermistor element (11) to self-heat. A thermistor (10) for microwave detection, wherein the thermistor element (11) has a resistance value of 10 at 25 ° C.
A thermistor for microwave detection, which is in the range of 0Ω to 10 kΩ. 2. The thermistor for microwave detection according to claim 1, wherein the thermistor element (11) has a resistance value at 25 ° C. in the range of 100Ω to 10 kΩ and a B constant in the range of 3000 to 5000K. 3. The thermistor for microwave detection according to claim 1, wherein the thermistor element (11) has a resistance value at 25 ° C. in the range of 100Ω to 10 kΩ and a heat dissipation constant in the range of 1 to 3 mW / ° C. 4. The thermistor element (11) has a resistance value in the range of 1000Ω to 10 kΩ at 25 ° C. and a B constant of 3
The microwave detection thermistor according to claim 1, wherein the thermistor is in the range of 000 to 5000K and has a heat dissipation constant in the range of 0.5 to 3 mW / ° C.