JPH1030949A - Water-level detector of positive-characteristic thermistor type - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain stabilized output voltage in a broad temperature range by using a synthetic resistance circuit network, which is constituted of only resistors containing a negative-characteristic thermistor as a temperature compensating circuit. SOLUTION: A water-level detector has the circuit constitution, wherein a positive- characteristic thermistor THp , a resistor R1 and a semi-fixed resistor VR are connected to a power supply E in sequence in series, a terminal voltage V1 between the positive- characteristic thermistor THp and the resistor R1 is outputted to a synthetic resistance circuit network Rx comprising resistors R2 and R3 and a negative-characteristic thermistor THn , and an output voltage V0 is outputted by a voltage dividing resistor R4 . When ambient temperature T is changed, the negative-characteristic thermistor THn , by which the voltage V1 obtained by a current I1 of the positive-characteristic thermistor THp becomes R4 .V1 /(R4 +R) (R; resistance value of negative-characteristic thermistor YHn , R4 ; voltage dividing resistor with negative-characteristic thermistor THn ) so that the output voltage V0 becomes constant, is used. by constituting the temperature compensating circuit by the synthetic resistance circuit network including the negative-characteristic thermistor in this way, the measuring errors caused by the ambient temperature are decreased.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positive-characteristic thermistor-type water level detecting device for detecting a water level of a positive-characteristic thermistor generating heat at a constant temperature by converting a difference in heat dissipation between air and liquid into a voltage change. About.

[0002]

2. Description of the Related Art In recent years, a positive-characteristic thermistor-type water level detecting device using a positive-characteristic thermistor having a constant-temperature heating function as a detecting element has been used instead of a water-level detecting device using a hot wire or a negative-characteristic thermistor as a detecting element. It has become. This is because a special drive circuit is not required due to the self-temperature control function which is a feature of the positive temperature coefficient thermistor.

As a water level detection circuit using a positive temperature coefficient thermistor, Japanese Patent Application Laid-Open No. Sho 60-14126 discloses a circuit in which an output voltage of a positive temperature coefficient thermistor is compared with a reference voltage by a comparator. This is because when the sensing element of the positive temperature coefficient thermistor self-heated to the Curie temperature enters the liquid from the air, the heat dissipation constant is large, so the self-heating amount is suppressed by the cooling action, and the self-heating temperature of the positive temperature coefficient thermistor is lowered. It is kept constant at a lower temperature than the air. At this time, since the resistance value of the positive characteristic thermistor decreases, the output voltage of the positive characteristic thermistor increases. The water level is detected based on a comparison value between the voltage change and the reference voltage.

On the other hand, Japanese Patent Laid-Open Publication No. 52-22948 discloses a relay having a structure in which a positive temperature coefficient thermistor is used as a detection element and a negative temperature coefficient thermistor is connected in series with the positive temperature coefficient thermistor as a temperature compensation element. A water level detecting device connected in series to the exciting coil of the above is disclosed. Since the positive characteristic thermistor and the negative characteristic thermistor are connected in series, the correlation that the resistance values are inversely proportional to each other is maintained. Thus, even when the ambient temperature changes, the change in the output voltage of the water level detection unit can be suppressed to a small value. The output voltage when the water level detector is located in the liquid is smaller than that in the air due to the difference in heat dissipation constant. The water level is detected by setting the relay operating voltage in the middle of the output voltage.

[0005]

However, the above-mentioned conventional positive characteristic thermistor type water level detecting circuit has the following problems. The sensing element of the positive temperature coefficient thermistor is made of semiconductor ceramics such as barium titanate and has a self-temperature control function. Ideally, the output characteristics of air and liquid should not change significantly with ambient temperature. It is. FIG. 4 is a graph showing the temperature dependence of the positive temperature coefficient thermistor, where the horizontal axis represents the ambient temperature, and the vertical axis represents the terminal voltage of the voltage dividing resistor connected in series with the positive temperature coefficient thermistor. As described above, there has been a drawback that accurate water level detection can be performed only in a narrow temperature range due to the insulating coating of the sensing element and thermal asymmetrical structure.

The fact that the temperature range in which the water level can be detected is narrow means that, for example, when the output voltage V ₁ = 5.0 V is determined as the air / liquid determination level, the temperature range in which the water level can be detected is 0 ° C.
The temperature is about 45 ° C., and the use environment is considerably limited.
In the latter conventional technique, the water level can be detected up to a certain temperature range by providing a temperature compensating element and correlating the temperature with the detecting element. When the temperature is required to be wider, an erroneous detection occurs. If the characteristics of the element of the positive characteristic thermistor and the characteristic of the negative characteristic thermistor are selected and detailed matching can be performed, erroneous detection should not occur, but it is almost impossible in practice.

The present invention has been made to solve such a conventional disadvantage, and the object thereof is to
It is an object of the present invention to provide a positive temperature coefficient thermistor type water level detecting device having a temperature compensating circuit in which the output characteristics of air and liquid are almost constant even when the ambient temperature changes.

[0008]

SUMMARY OF THE INVENTION In order to solve the above problems, a positive temperature coefficient thermistor type water level detecting device according to the present invention has a water level detecting section comprising a water level detecting sensor and a temperature compensating sensor, and a water level having a temperature compensating circuit. In a water level detection device configured by a combination with a detection circuit, the water level detection unit is a water level detection unit using a positive characteristic thermistor for the water level detection sensor, using a negative characteristic thermistor for the temperature compensation sensor, The water level detection circuit is configured such that, in a state where the temperature-dependent positive temperature coefficient thermistor is self-heated to a constant temperature, a water level connected in series with a combined resistance network composed only of resistors including the negative temperature coefficient thermistor It is a detection circuit. Further, the water level detection section is characterized in that the positive characteristic thermistor and the negative characteristic thermistor have a structure in which they are kept in an electrically insulated state and are installed at substantially the same height.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following can be considered as a temperature compensation circuit in a water level detection circuit of a positive temperature coefficient thermistor type water level detection device of the present invention having the above-described configuration. First,
5, current _{I 1,} the ambient temperature T, the heat dissipation constant in air is C, between these, the general formula ^{_{P = I 1 2 · R =}} C (Tc-T) Tc thermistor The self-heating temperature R and the resistance value of the thermistor R can be considered to have a relationship of I ₁ = f (T, C) = g (a / T) · f (C). Where f, g
Is a function of T and a is a constant.

Further, change of the current I ₁ is inversely proportional to the change in the ambient temperature T, be regarded as a change in resistance to changes in ambient temperature T given that the use of heat-sensitive resistive element for temperature compensation be able to. Here, considering the use of a negative temperature coefficient thermistor exhibiting a resistance value change that is inversely proportional to a temperature change in order to correct the influence of the ambient temperature T, the change in the ambient temperature T is represented by R = Ro exp From the equation [B (1 / T-1 / To)], it can be replaced with a change in the resistance value R of the negative characteristic thermistor.

Therefore, the current I _{1 of the} positive temperature coefficient thermistor
When the effect of the ambient temperature T is replaced with the resistance value R of the negative characteristic thermistor, the following expression is obtained. I ₁ = g (a / T) · f (C) = h (R) · f (C) where h is a function. In order to correct such temperature characteristics in the air, the temperature characteristics can be corrected by the following calculation: I ₁ ′ = [h (R) · f (C)] · h ⁻¹ (R) = f (C) Here, h (R) =
It is obvious at a glance when replacing as R, h ^-1 (R) = 1 / R. Here, 1 / R becomes operational to perform in an electronic circuit, such that the voltages V ₁ obtained by the current I ₁ when the ambient temperature T changes the output voltage Vo is constant, R ₄ ·
It is conceivable to use a negative characteristic thermistor that satisfies V ₁ / (R ₄ + R). Here, R ₄ is a voltage dividing resistance with the negative characteristic thermistor.

Thus, the 1 / R calculation can be easily realized, but the temperature compensation using only the negative characteristic thermistor cannot completely compensate for the fluctuation of the current flowing through the positive characteristic thermistor due to the ambient temperature, and the measurement error increases. . Therefore, by configuring the temperature compensation circuit with a combined resistance network including a negative temperature coefficient thermistor, the measurement error due to the ambient temperature is extremely reduced.

As the structure of the positive temperature coefficient thermistor and the negative temperature coefficient thermistor, a glass-sealed type in which the chip element is sealed with borosilicate glass or the like is preferable because the characteristics are very stable in all environments. .

Since the thermistor is placed in the air or liquid, a waterproof structure is required, and the thermistor must be electrically insulated with a molding material so as not to cause a short circuit. As the molding material, a fluorine-based resin excellent in heat resistance and hydrophobicity is preferable, and at a low temperature, an olefin-based resin is economical. It is also possible to use a metal case such as SUS or Al to make the response faster.

The positional relationship of the thermistor of the water level detecting section is as follows:
In order to obtain good characteristics under all conditions without erroneous detection even if the temperature in the air or liquid is extremely different in a wide temperature range, it is necessary to install them at substantially the same height. For this purpose, it is conceivable to use a holder, but it is desirable to use a thermal insulator so that the negative-characteristic thermistor does not pick up much heat generated by the positive-characteristic thermistor.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in more detail with reference to the drawings. FIG. 2 shows the structure of the water level detection sensor 1 and the temperature compensation sensor 2 of the water level detection unit used as one embodiment of the present invention. Reference numeral 11 denotes a positive temperature coefficient thermistor in which a barium titanate-based ceramic element (Curie temperature: 200 ° C.) is glass-sealed, and reference numeral 21 denotes a glass-sealed material obtained by mixing and sintering a transition metal oxide such as Mn, Co, and Ni. It is a negative characteristic thermistor. A PTFE insulated wire 13 is connected to both ends of the positive temperature coefficient thermistor 11 and the negative temperature coefficient thermistor 21 by caulking terminals 12, and these are molded by a fluororesin molding material 14, respectively. The compensating sensor 2 is configured. The water level detecting sensor 1 and the temperature compensating sensor 2
The holder 3 is fixed at a substantially same height while maintaining a certain distance. In the present embodiment, a fluorine-based resin shrink tube made of PFA inside and PTFE outside is used for the molding material 14.

Next, referring to FIG. 1 and FIG. 4, a description will be given of a positive characteristic thermistor type water level detecting circuit according to the present invention.
FIG. 1 is a circuit diagram of a positive-characteristic thermistor type water level detecting device showing one embodiment of the present invention. A positive-character thermistor THp, a resistor R ₁ , and a semi-fixed resistor VR are connected in series to a power supply E in this order. the terminal voltages _{V 1} between the thermistor THp and the resistor _{R 1,} resistor _{R 2,} resistors _{R 3,} the circuit configuration for outputting the output voltage Vo by combining resistor network Rx and voltage dividing resistors _{R 4} consisting of a negative temperature coefficient thermistor THn It has become.

FIG. 4 is a graph showing the relationship between the temperature change of the positive temperature coefficient thermistor THp in the air and liquid depending on the ambient temperature using the circuit diagram shown in FIG. The operating conditions, power source E DC24V, the resistance _{R 1} is 100 [Omega, 1
/ 2W, the semi-fixed resistor VR is set to Max 200Ω, and the terminal voltage V is applied to the positive temperature coefficient thermistor THp in the air and in the liquid (water depth 30 mm) at an ambient temperature of 0 ° C to 80 ° C at 20 ° C intervals.
₁ was measured. As can be seen from this graph, it is clear that the positive temperature coefficient thermistor THp has temperature dependency.

Here, looking at the terminal voltage V ₁ at each temperature in the air in FIG. 4, V ₁ (0 ° C.) = 4.9 V, V
₁ (40 ° C.) = 4.0 V, V ₁ (80 ° C.) = 3.1 V,
However, in order to make the output voltage Vo constant over the entire temperature range, the combined resistance network Rx (0 ° C.) and Rx (40 ° C.) are obtained from V ₁ × R ₄ / (Rx + R ₄ ) = Vo (constant). , R
If the value of x (80 ° C.) is obtained, the output voltage Vo becomes 0 ° C.
The output is constant with respect to an ambient temperature change between ℃ 80 ° C.

If this is applied to the present embodiment, assuming that the output voltage Vo = 2 V and the voltage dividing resistor R ₄ = 10 kΩ, the value of the combined resistance network Rx is Rx (0 ° C.) = 14.5 kΩ, Rx
(40 ° C.) = 10.0 kΩ, Rx (80 ° C.) = 5.5 k
A value of Ω will be required. When the respective resistance values are calculated in order to set the value of the combined resistance network Rx to the required value, the resistance R ₂ = 1.624 kΩ and the resistance R ₃ = 14.
The value is 6 kΩ. Note that the negative characteristic thermistor THn is
A glass-sealed negative characteristic thermistor with R (70 ° C.) = 7.0 kΩ and B constant = 3510 K was used.

As described above, a positive temperature coefficient thermistor type water level detecting circuit including a temperature compensating circuit is constructed, and the water level detecting section shown in FIG. And the output voltage Vo was measured and shown in FIG. As can be seen from this graph, it was proved that the temperature dependency was extremely small.

In the present invention, as described above, the water level detecting unit has a structure in which the water level detecting sensor 1 and the temperature compensating sensor 2 are installed at substantially the same height, so that the surroundings in the air and liquid are provided. There is no erroneous detection even if the temperature is extremely different.
For example, if the ambient temperature is 0 ° C in air and 80 ° C in liquid,
If the respective sensors are at substantially the same height, no erroneous detection occurs as shown in FIG. 3, but when the temperature compensation sensor 2 is immersed in the liquid and the water level detection sensor 1 is in the air, the output voltage Vo is reduced. It becomes 3.15 V, and the water level detection sensor 1 erroneously detects that the sensor 1 is in the liquid even though it is in the air. Similarly,
The water level detection sensor 1 is immersed in the liquid and the temperature compensation sensor 2
Erroneous detection even if the user is in the air.

In the above embodiment, the water level detecting sensor and the temperature compensating sensor are installed at substantially the same height by the holder,
The positive temperature coefficient thermistor and the negative temperature coefficient thermistor are kept at a certain distance so as not to be affected by heat. However, even if the two sensors are integrated, only the operating point on the circuit changes, and the circuit constant and the Similar characteristics can be obtained by changing the specifications of the temperature compensation sensor.

The resistance of the combined resistance network is preferably mounted on a substrate and connected to the lead wire of the temperature compensation sensor. However, as shown in FIG. And miniaturization is possible by connecting it directly to the lead wire of the temperature compensation sensor in a relay manner.
At this time, the resistance is protected by the insulating contraction tube 15.

[0025]

As described above in detail, according to the present invention, the ambient temperature dependence of the positive temperature coefficient thermistor is obtained by using a temperature compensating circuit as a composite resistance network composed only of resistors including a negative temperature coefficient thermistor. A positive-characteristic thermistor-type water level detection circuit that can significantly reduce the water resistance and obtain a stable output voltage over a wide temperature range can be realized, and at the same time, the water level detection sensor and the temperature compensation sensor of the water level detection section should be approximately the same height Thus, a positive-characteristic thermistor-type water level detecting device capable of accurately detecting the water level can be obtained.

[Brief description of the drawings]

FIG. 1 is a view showing one embodiment of the present invention, and is a circuit configuration diagram of a positive characteristic thermistor type water level detecting device.

FIG. 2 is a view showing one embodiment of the present invention, and is a structural diagram of a water level detecting unit of the positive characteristic thermistor type water level detecting device.

FIG. 3 is an ambient temperature-output voltage characteristic diagram in one embodiment of the present invention.

FIG. 4 is a graph showing the temperature dependency of a positive temperature coefficient thermistor used in one embodiment of the present invention, and is a graph showing ambient temperature-terminal voltage characteristics.

FIG. 5 is a graph showing the temperature dependence of a positive temperature coefficient thermistor used in one embodiment of the present invention, and is a graph showing an ambient temperature-terminal current characteristic.

FIG. 6 is a view showing another embodiment of the present invention, and is a structural diagram of a temperature compensating sensor of a water level detecting unit of the positive characteristic thermistor type water level detecting device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Water level detection sensor 2 ... Temperature compensation sensor 3 ... Holder 11 ... Positive characteristic thermistor 12 ... Caulking terminal 13 ... PTFE insulated wire 14 ... Molding material 15 ... Insulation contraction tube 21 ... Negative characteristic thermistor THp ... Positive characteristic thermistor THn ... Negative characteristic thermistor VR... Semi-fixed resistors R _{1 to} R _3. Resistors R ₄ .

Claims (2)

[Claims] 1. A water level detection device comprising a combination of a water level detection unit including a water level detection sensor and a temperature compensation sensor and a water level detection circuit having a temperature compensation circuit, wherein the water level detection unit includes the water level detection unit. A water level detection unit using a positive temperature coefficient thermistor for the detection sensor and using a negative temperature coefficient thermistor for the temperature compensation sensor, wherein the water level detection circuit is configured such that the temperature-dependent positive temperature coefficient thermistor is self-heated to a constant temperature. A positive-characteristic thermistor-type water-level detecting device, wherein the water-level detecting circuit is connected in series with a combined resistance network composed of only resistors including the negative-characteristic thermistor in the closed state. 2. The water level detection unit according to claim 1, wherein the positive temperature coefficient thermistor and the negative temperature coefficient thermistor are in a state of being electrically insulated and installed at substantially the same height. Item 7. A positive temperature coefficient thermistor type water level detecting device according to Item 1.