JPH08128902A - Temperature detecting device - Google Patents

Abstract

PURPOSE: To provide a detecting device with its safety protection function by selecting reference voltages to be input to a pair of voltage comparators, respectively, related with the temperature-resistance characteristics of positive characteristic thermisters, respectively, and outputting the logical product of the comparator outputs. CONSTITUTION: The output from a positive characteristic thermister 1 and the reference voltage corresponding to that resistance value are produced by means of resistors R2 , R3 , and R4 , and the voltage between the resistors R3 and R4 is defined as a reference voltage V2 , and the voltage between the resistors R2 and R3 a reference voltage V3 . The voltage V2 and V3 are input to the respective inversion input and non-inversion input of voltage comparators 2 and 3, and the voltage V1 between the resistor R1 and the thermister 1 is input to the non-inversion input and inversion input of the comparators 2 and 3. Thus, the logical product of the comparators 2 and 3 outputs is output to a terminal V0 , which serves as a safety protection function. The voltage V2 selects a voltage which is a resistance value (approx. 300Ω) below the resistance minimum value on the temperature and resistance characteristics, and the voltage V3 is selected as a voltage which makes the resistance value (approx. 5kΩ) of a temperature below the maximum heating temperature of a heated body on the same characteristics. The voltages V2 and V3 are reference voltages for detecting short-circuit, preventing overheating, and detecting disconnection, respectively.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a temperature detecting device having a safety protection function used for preventing overheating of a gas table, for example. More specifically, the present invention relates to a temperature detecting device using a positive temperature coefficient thermistor (hereinafter abbreviated as PTC thermistor) and having a function of detecting temperature and detecting a disconnection or short circuit of the temperature detecting element itself.

[0002]

2. Description of the Related Art In recent years, in cooking oil such as tempura and deep-fried food performed on a gas table, fires have frequently occurred due to overheating of the tempura oil, and prevention thereof has become an urgent task. As a countermeasure against this, a temperature detecting element housed in a metal case is pressed against the bottom of the pan, and the spontaneous ignition temperature of tempura oil is 30.
Fire is prevented by detecting overheating of the tempura oil in the temperature range of 250 to 270 ° C, which is slightly lower than 0 ° C, and cutting off the gas supply.

Generally, a negative characteristic thermistor (hereinafter abbreviated as NTC thermistor) is used as the temperature detecting element. FIG. 4 is a connection diagram for detecting the characteristic of the NTC thermistor Th, and FIG. 5 is its output characteristic diagram. Here, the output V ₄ from the NTC thermistor Th is input to a voltage comparator, compared with a reference voltage corresponding to the overheat prevention temperature, and the output thereof is configured to cut off the gas supply via a power switch. There is.

[0004]

However, the above-mentioned conventional structure has the following problems. First, considering the disconnection detection level of the thermistor itself of -10 ° C or lower, the operating temperature range is about 300 ° C, which is very wide. Secondly, most gas tables operate with two batteries, and up to about 70% of them must be operated, so the power supply voltage is 1.5 V x 2 lines x 0.7 ≈ 2 V, which is a fairly small value. Is.

On the other hand, an appropriate characteristic of the NTC thermistor Th in this temperature range is that the B constant (200/300 ° C.) = 51.
33K, resistance value R (200 ° C.) = About 4 kΩ, and with such characteristics, the resistance value is R (−10 ° C.) ≈1.
It becomes very large, 0 MΩ. Under the above conditions, the rate of change of V ₄ at both ends is approximately several mV / 10 deg, and the difference between V ₄ and the power supply potential or ground is 20 to 30 mV.
The degree is very small. On the other hand, in an IC voltage comparator, the variation in offset voltage is about ± 5 mV, which is a large error factor.

Under these characteristics, the end on the high temperature side is the most important temperature region for safety function used for overheat prevention and short circuit detection of the NTC thermistor itself, and the end on the low temperature side is the most important temperature region for safety protection. There is very little voltage change in that region. Therefore, the difference between the actual temperature of the object to be heated and the detected temperature becomes large due to an error in the parts used, which leads to malfunction of the overheat prevention device. Therefore, it is necessary to precisely adjust the reference voltages to the voltage comparators one by one in order to suppress variations in the parts used. In practice, the resistance for generating the reference voltage is a thin film resistance, and it is realized only by using an advanced manufacturing technique called precision trimming, and the temperature detection device as a whole is expensive.

Incidentally, referring to the resistance value error of the NTC thermistor, if the error is ± 2.5%, it corresponds to an output voltage V ₄ on the high temperature side of about ± 0.5 mV, which corresponds to that of the voltage comparator. Much smaller than the offset voltage,
The error of the NTC thermistor may be considered individually. The phenomenon described above is due to the large B constant of the NTC thermistor. Converting the B constant into a temperature coefficient α, α (0 to 270 ° C.) = − 1.34 to −1.75%
/ Deg, which is a considerably large value.

Therefore, it is conceivable to use a platinum thin film resistor having a small temperature coefficient (temperature coefficient α≈0.4% / deg) in place of the NTC thermistor as the temperature detecting element. The unit price is very high, which is several times higher than that of the NTC thermistor, and it is extremely poor in practicality.

Further, there is a CTR thermistor having a characteristic similar to that of the NTC thermistor, but since this characteristic lacks stability, it is the fact that it is rarely used.

The present invention has been made to solve the above-mentioned conventional drawbacks, and its purpose is to:
An object of the present invention is to provide an inexpensive temperature detection device having a safety protection function of detecting a disconnection or a short circuit of the temperature detection element itself without using an advanced manufacturing technique.

[0011]

In order to solve the above-mentioned problems, in the temperature detecting device according to the present invention, the voltage obtained from the connection point between the resistor connected in series to the power source and the positive temperature coefficient thermistor is the first voltage comparison. Is input to the non-inverting input terminal of the voltage comparator and the inverting input terminal of the second voltage comparator, and the first reference voltage is input to the inverting input terminal of the first voltage comparator.
A second reference voltage is input to a non-inverting input terminal of the second voltage comparator, which is a temperature detecting device that obtains an output by being input and compared, wherein the first reference voltage has a positive characteristic. A voltage corresponding to the minimum resistance value or less on the temperature-resistance characteristic of the thermistor is set, and the second reference voltage corresponds to a region showing a positive resistance characteristic and a region showing a negative resistance characteristic on the characteristic. As a larger voltage, the logical product of the outputs of the two voltage comparators that have been compared is connected so as to be the device output.

As the PTC thermistor, any structure and material suitable for the operating temperature may be used. In particular, the glass-sealed type is preferable because the characteristics are very stable in any atmosphere.

Since the first reference voltage is a reference voltage for detecting a short circuit, the temperature-resistance characteristic (hereinafter referred to as T
It is preferable to set the voltage such that the resistance value is a little smaller than the smallest resistance value in the (-R characteristic).

Since the second reference voltage is a reference voltage for preventing overheat or detecting disconnection, the resistance value at a temperature slightly lower than the maximum heating temperature of the object to be heated on the TR characteristic. The voltage is preferably such that

[0015]

The temperature detecting device of the present invention having the above-described structure is shown in FIG.
In the TR characteristic of the PTC thermistor as shown in
It consists of negative resistance characteristics and positive resistance characteristics. In the negative resistance region, the B constant is about 1000 K (temperature coefficient α = −0.4 to
Since it is about -1.3% / deg), the change width of the output voltage is relatively small even in a wide temperature range such as -10 to 270 ° C, and the output voltage can be changed even when driven by a low voltage such as a battery power source. Since it can be separated from the power supply voltage and the ground level, it can be made less susceptible to variations in the offset voltage of the voltage comparator.

Since the output of the temperature detecting device is connected so as to be the logical product of the outputs of the two voltage comparators, it functions as a safety protection function only when an abnormality such as overheating or disconnection occurs.

Here, the TR characteristic of the PTC thermistor shows a valley characteristic as shown in FIG. Generally, in various sensing elements having a mountain-valley characteristic, the physical quantity corresponding to a certain sensing quantity is observed as two values with the apex as a boundary, so it is essential to specify the physical quantity when operating as any sensing element. Is impossible. However, as shown in FIG. 3, the T-R characteristic of the PTC thermistor is remarkably asymmetric, and if the entire region overlapping the negative resistance characteristic on the low temperature side is not used for temperature measurement, the region is black. It can be regarded as a box-shaped continuous temperature detecting device.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in more detail below with reference to the drawings. FIG. 2 is a sectional view of the PTC thermistor 1, P
TC thermistor chip element 11 is a slag lead wire 13,
13 and the glass tube 14 are sealed in a thermocompression bonded state. This is typically a small signal diode DO-3
It is similar to the shape called by the package name of 5. In the PTC thermistor chip element 11, after barium titanate-based porcelain semiconductor is fired into a wafer to form silver-based paste electrodes 12 and 12 on both sides, 0.3
It is made into chips by dicing into 5 mm × 0.35 mm × 0.2 mm. In this example, a PTC thermistor having a Curie temperature Tc (temperature at which the resistance value is twice the resistance value at room temperature (25 ° C.)) is Tc≈272 ° C. is used.

FIG. 3 shows the TR characteristics of the PTC thermistor 1 used in this embodiment. As you can see from this figure,
The low temperature side shows negative resistance characteristics with respect to temperature, and the high temperature side shows positive resistance characteristics. Point A showing the smallest resistance value is about 2
Point B, which is 50 ° C. and does not overlap with the negative resistance region, is practically equivalent to a resistance value of −10 ° C. Table 1 shows main TR characteristic values of the PTC thermistor 1.

[0020]

[Table 1]

Here, a reference temperature and a resistance value as a safety protection function are set. In FIG. 3, if the resistance value is at the point B or higher, overheating can be prevented or disconnection can be detected, so this resistance value is set to 5 kΩ with a margin. Moreover, since the resistance value at the point A is the lowest, the margin is set as the short circuit detection level when the resistance value of the PTC thermistor is 300Ω.

The circuit diagram shown in FIG. 1 designed in this manner is a temperature detecting device using the PTC thermistor of the present invention. This temperature detecting device has a PTC as shown in FIG.
A reference voltage corresponding to the output from the thermistor 1 and the resistance value (5 kΩ, 300 Ω) of the PTC thermistor is created by the resistors R ₂ , R ₃ and R ₄ , and the voltage between the resistors R ₃ and R ₄ is the first reference. The voltage between the voltage V ₂ and the resistors R ₂ and R ₃ is the second reference voltage V ₃ . The first reference voltage V ₂ is input to the inverting input terminal of the first voltage comparator 2, and the second reference voltage V ₃ is input to the non-inverting input terminal of the second voltage comparator 3. Resistance R
₁ and the PTC measured voltages V ₁ between the thermistor 1 non-inverting input terminal of the first voltage comparator 2, are input to the second inverting input terminal of the voltage comparator 3. The voltage comparators 2 and 3 are open collector outputs, and as shown in the figure, load resistance R ₅
It is obvious that the logical product output Vo can be obtained by connecting together. The circuit of FIG. 1 is a window comparator, and when the resistance value of the PTC thermistor is in the range of 300Ω to 5 kΩ, all are normal and the output Vo is ON, which is a continuous but black box-like operating region.

Here, the power supply voltage E is 2 V and the resistance R ₁ = 1 k
As Ω, Table 2 shows the values calculated by the measured voltage V ₁ regarding the discrimination margins of the resistance values of 300 Ω and 5 kΩ for the safety protection.

[0024]

[Table 2]

From this table, it can be seen that the black box width between A and B is suppressed to 0.917 V, which has a sufficient discriminative difference from other state levels. Also, V
₁ is a value that is sufficiently distant from the power supply potential and the ground level, and the variation in the offset voltage of the voltage comparator is ±
It is a value that does not matter even at 5 mV. Further, even if the resistance value error of the PTC thermistor is about ± 10%, it does not cause a large detection error.

Next, R ₂ = 620Ω + 39Ω, R ₃ = 2.
4kΩ, R ₄ = 910Ω, put the connected PTC thermistor into the insulating liquid thermostatic chamber, sweep the temperature, measure V ₁ output while watching the output Vo of the detector, and calculate the equivalent PTC thermistor resistance backwards. , Standard resistance value 300
When compared with Ω and 5 kΩ, the error was within ± 1.5% in the average of two reciprocations of heating and cooling.

For comparison, the connection diagram shown in FIG.
A simple trial calculation using a thermistor shows that the resistance value is R (−10 ° C.) ≈10 MΩ, R (270 ° C.) =
0.99346 kΩ, R ₂ = 100 kΩ, and power supply voltage E = 2 V, the output V ₄ is V _{4 as} shown in FIG.
(−10 ° C.) = 19.8 mV, V ₄ (270 ° C.) = 1.
It becomes 9803V. Considering the variation of the circuit part including the offset voltage of the voltage comparator, it can be seen that the resistance for generating the reference voltage must be very precise.

In the present embodiment, the PTC thermistor is exposed and used, but it goes without saying that the PTC thermistor is actually incorporated in a metal case or the like for use. Based on the above, it is clear that the problem when the NTC thermistor is used in the temperature detecting device for safety protection can be sufficiently solved by using the PTC thermistor.

[0029]

As described above in detail, according to the present invention, by using the PTC thermistor as the temperature detecting element, the safety protection function such as overheat detection, disconnection detection and short circuit detection, which has been difficult with the NTC thermistor, is provided. It is possible to provide a temperature detecting device for a PTC thermistor, which has the same performance without using a thin-film resistor that requires advanced manufacturing technology and is highly economical.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a temperature detecting device of the present invention using a PTC thermistor.

FIG. 2 is a sectional view of a PTC thermistor used in an example of the present invention.

FIG. 3 is a temperature-resistance characteristic diagram of a PTC thermistor used in an example of the present invention.

FIG. 4 is a connection diagram for obtaining characteristics of an NTC thermistor.

FIG. 5 is an output characteristic diagram of an NTC thermistor used in a conventional temperature detecting device.

[Explanation of symbols]

1 ... PTC thermistor 2 ... first voltage comparator 3 ... second voltage comparator V 1 _... measured voltage V 2 _... first reference voltage V 3 _... second reference voltage E ... supply voltage R ₁ to R ₅ ... resistor 11 ... PTC thermistor chip element 12 ... silver-based paste electrode 13 ... slag lead wire 14 ... glass tube

Claims (1)

[Claims] 1. A voltage obtained from a connection point between a resistor connected in series to a power source and a positive temperature coefficient thermistor is inverted at a non-inverting input terminal of the first voltage comparator and at a second voltage comparator. The first reference voltage is input to the inverting input terminal of the first voltage comparator, and the second reference voltage is input to the non-inverting input terminal of the second voltage comparator. In the temperature detecting device, the output of which is obtained by comparison with the first reference voltage is the voltage corresponding to the minimum resistance value or less on the temperature-resistance characteristic of the positive temperature coefficient thermistor, and the second reference voltage. The reference voltage is a larger voltage corresponding to a region showing a positive resistance characteristic and a region showing a negative resistance characteristic on the characteristic, and the logical product of the respective outputs of the two voltage comparators compared is the device output. Temperature characterized by being connected so that Detection device.