JPH0530735U - Liquid level sensor - Google Patents

Abstract

(57) [Summary] [Purpose] When detecting the liquid surface level, no moving parts are required to provide vibration resistance and a long service life can be obtained, and a switching signal at the time of level detection can be obtained with high accuracy. To be able to [Structure] A positive temperature coefficient thermistor is connected to a constant current circuit, and a constant current is supplied from the constant current circuit so that a heat generation temperature of the positive temperature coefficient thermistor becomes a Curie temperature, and the positive temperature coefficient thermistor is arranged in a level detection part of a liquid container. Made possible

Description

[Detailed description of the device]

[0001]

[Industrial applications]

 The present invention relates to a liquid level sensor suitable for detecting the liquid level of various liquids such as water, oil and kerosene.

[0002]

[Prior Art]

 As a general liquid level sensor, one using the buoyancy of a float is known. As shown in FIG. 4, a guide shaft 2 is attached to a housing 1, and a float 3 made of foamed resin is attached so as to be vertically movable along the guide shaft 2. A magnet 4 is embedded inside the float 3, and a reed switch 5 that is operated by a magnetic action when the magnet 4 comes closest is built in the tip of the guide shaft 2. The guide shaft 2 is submerged in the liquid so that the reed switch 5 is located at the detection level, and the reed switch 5 is turned ON / OFF by the displacement of the float 3 which changes with the change of the liquid level. The surface level is detected.

However, in such a float type level sensor, there are disadvantages that the number of parts is large, the structure is complicated, and the detection liquid surface varies due to the specific gravity of the liquid to be detected. .. In addition, the float and the guide shaft are mechanically in contact with each other, so they are weak against vibration, and the reed switch has a contact structure, so there is a drawback in that the number of times of detection is long.

In order to improve such a problem, a level sensor using an NTC (Negative Temperature Coefficient) thermistor has been devised and put into practical use. The principle of this is that, as shown in Fig. 5 (1), a current is supplied to the thermistor 6 to cause it to self-heat, and a difference in the temperature of the thermistor itself occurs due to the difference in the heat dissipation constants in air and liquid. Are using. Therefore, this temperature change can be detected as a change in the resistance R, and the liquid level can be detected.

[0005]

[Problems to be solved by the device]

 However, in the method using the NTC thermistor, (1) the characteristics of the thermistor have a small coefficient of resistance change with respect to temperature change as shown in Fig. 5 (2), and the characteristic line changes gently, so it is used as a switching signal. Not suitable for rubbing. (2) When the temperature of the liquid itself rises, it becomes impossible to distinguish it from self-heating, so the level of the high temperature liquid cannot be detected. (3) As described in (1) above, since an analog signal is output as the detection signal, a signal processing circuit is required to have a switching function. However, the current situation is that it is not widely used.

The present invention focuses on the above-mentioned conventional problems, eliminates any moving parts at the time of detecting the liquid level to provide vibration resistance, has a long life, and is highly accurate. It is an object of the present invention to provide a liquid level sensor capable of obtaining a switching signal at the time of level detection.

[0007]

[Means for Solving the Problems]

 In order to achieve the above-mentioned object, the liquid level sensor according to the present invention connects a positive temperature coefficient thermistor to a constant current circuit, and outputs a constant current from the constant current circuit where the heat generation temperature of the positive temperature coefficient thermistor becomes the Curie temperature. The positive temperature coefficient thermistor can be supplied to the level detecting portion of the liquid container.

[0008]

[Action]

 A positive temperature coefficient thermistor is called a PTC (Positive Temperature Coefficient) thermistor, and its temperature-resistance characteristic changes drastically around the Curie temperature as shown in FIG. Therefore, the PTC thermistor may be energized to generate Joule heat so as to reach the Curie temperature, and this may be installed in the level detection unit of the liquid container. As a result, the heat dissipation constant differs between when the PTC thermistor is in a liquid wet state and when it is exposed to the air above the liquid surface. By detecting this as a change in the resistance value of the thermistor, the liquid surface level Can be reliably detected. Since the resistance of the detection signal changes rapidly with the Curie temperature as the transition temperature, it can be detected as a so-called ON / OFF signal and the liquid level can be output as a switching signal.

[0009]

【Example】

 Hereinafter, a specific embodiment of the liquid level sensor according to the present invention will be described in detail with reference to the drawings.

FIG. 1 is a circuit configuration of a liquid level sensor according to an embodiment. As shown in this figure, a PTC thermistor 10 is used as the sensor body, and a constant current source 12 is connected to its terminal. The constant current source 12 is composed of a general circuit composed of a combination of a Zener diode 14, an operational amplifier 16 and a transistor 18, and a constant current i (= VZ / R0) is passed through the PTC thermistor 10. Further, an operational amplifier 20 is connected to this circuit in parallel with the PTC thermistor 10, whereby the resistance change of the PTC thermistor 10 is detected and displayed as a liquid level signal.

In such a circuit configuration, the sensor main body including the PTC thermistor 10 is attached to the detection level position of the liquid container 22 and is placed in the liquid or the air depending on the fluctuation of the liquid level 24.

By the way, when the current i is passed from the constant current source 12 to the PTC thermistor 10, the heat generated in the PTC thermistor 10 is proportional to the power consumption P (= Ri 2). Therefore, the current i is supplied as a constant current value such that the PTC thermistor 10 reaches its Curie temperature due to heat generation. In this case, since the temperature-resistance characteristic of the PTC thermistor 10 used is known from the rating table, the current value to be supplied from the constant current source 12 is uniquely determined.

The operation of the liquid level sensor having such a configuration is as follows. The PTC thermistor 10 mounted at the detection position on the wall surface of the liquid container 22 is supplied with the current i supplied from the constant current source 12, and thereby adjusted to generate heat to the Curie temperature TC. When such a PTC thermistor 10 is located in the air higher than the liquid surface due to the position of the liquid surface 24, the heat transfer coefficient of the air is extremely small, so that the heat dissipation becomes poor, so that the temperature of the PTC thermistor 10 becomes low. It rises in proportion to power consumption P (TH) and balances at point H in FIG. Conversely, when the PTC thermistor 10 is located in the liquid, the heat transfer coefficient of the liquid is large, so the heat generated by the PTC thermistor 10 is dissipated in the liquid and becomes lower than the Curie temperature TC (TL). Balance at point L. At this time, the resistance of the PTC thermistor 10 becomes RH and RL, respectively, due to the difference in heat dissipation constant between the air and the liquid, but because of the characteristics of the PTC thermistor 10, RH >> RL, so under each condition. The voltage applied to the PTC thermistor 10 is VH >> VL. Therefore, by outputting this detection value, it can be regarded as an ideal liquid level detection switch.

Such a sensor is taken into the operational amplifier 20 as a digital element as a switching signal, and the voltage change of the PTC thermistor 10 is input as a liquid level signal. As a result, the VH output is set to "1" and the VL output is set to "0" and directly output to various devices having a digital element, and it can be easily used as a switching signal without requiring a special A / D conversion circuit or interface. You can

Further, since the PTC thermistor 10 changes between ON and OFF, it is not affected even if the liquid temperature of the detection target is high. Therefore, it is possible to effectively detect the liquid level of the high temperature liquid.

[0016]

[Effect of the device]

 As described above, according to the liquid level sensor of the present invention, a positive temperature coefficient thermistor is connected to the constant current circuit, and a constant current from the constant current circuit to the Curie temperature is generated by the constant temperature circuit. Since the above-mentioned positive temperature coefficient thermistor can be placed in the level detection part of the liquid container, it has no moving parts when detecting the liquid level, and it has vibration resistance and long life. In addition to this, there is an excellent effect that a switching signal at the time of level detection can be obtained with high accuracy.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a liquid level sensor according to an embodiment.

FIG. 2 is a diagram illustrating the behavior of the sensor.

FIG. 3 is a temperature-resistance characteristic diagram of a PTC thermistor.

FIG. 4 is a sectional view of a conventional float type liquid level sensor.

FIG. 5 is an explanatory diagram of a conventional thermistor type liquid level sensor.

[Explanation of symbols]

 10 PTC thermistor 12 Constant current source 22 Liquid container 24 Liquid level

Claims (1)

[Scope of utility model registration request] 1. A constant temperature circuit is connected to a positive temperature coefficient thermistor, and a constant current is supplied from the constant current circuit so that the heat generation temperature of the positive temperature coefficient thermistor becomes a Curie temperature, and the positive temperature coefficient thermistor is used as a level detector of a liquid container. A liquid level sensor that can be placed on