JP2002122464A - Level sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a level sensor reactive to temperature and capable of being reduced in cost and size. SOLUTION: This level sensor 1 comprises a case 2 having a tapered bottom, a thermistor 3 fixed to the bottom tip 2a, and a control circuit connected to the thermistor 3 to detect its temperature change. The control circuit may be housed in the case 2. The heat conductivity of the bottom tip 2a of the case 2 is desirably higher than the other part of the case 2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a temperature sensitive liquid level sensor. The present invention is particularly suitably used as a sensor for detecting the level of a liquid containing impurities.

[0002]

2. Description of the Related Art As a liquid level sensor, a float type sensor has been most often used. In recent years, optical sensors having a light emitting diode and a phototransistor have become popular because they are light and small. However, when the liquid level is detected in a device such as a toilet, a washing machine, a dishwasher, and the like, the liquid sensor contains impurities such as water scale and cannot be accurately detected by an optical sensor. Therefore, a temperature-responsive sensor is suitable as a liquid level sensor built in these devices.

As shown in FIG. 4, a sensor 11 of this type has a resin case 12, a thermistor 13 fixed to the case, and a control circuit 14 connected to the thermistor 13. In this type of sensor 11, the liquid level 1
When 5 rises and comes into contact with the bottom of case 12, thermistor 13 is cooled by the liquid to lower its temperature, and the electric resistance changes accordingly. Then, the change in the electric resistance value is detected as an electric signal by the control circuit,
Thereby, the liquid level 15 is detected.

[0004]

However, after the case 12 comes into contact with the liquid level 15, water droplets adhere to the surface of the case 12. If a water droplet adheres forever, the thermistor 13 is cooled by the water droplet, and as a result, the temperature of the thermistor 13 does not return even after leaving the liquid surface 15. Therefore, in the conventional liquid level sensor 11, the temperature insensitivity of the thermistor 13 due to the attachment of water droplets must be predicted and corrected, and a complicated control circuit 14 is required. Therefore, in the conventional liquid level sensor 11, the manufacturing cost is high and large. Therefore, an object of the present invention is to provide a liquid level sensor that can be reduced in cost and size.

[0005]

A liquid level sensor according to the present invention comprises a case having a tapered bottom, a thermistor fixed to the tip of the bottom, and a control circuit connected to the thermistor and detecting a temperature change of the thermistor. It is characterized by the following.

In the present invention, since the bottom of the case is tapered, the drainage is good, and water droplets do not continue to adhere to the surface of the bottom of the case. Therefore, when the bottom is separated from the liquid surface, the temperature of the thermistor fixed to the bottom quickly returns. Therefore, there is no need to complicate the control circuit. Therefore, according to the present invention, cost reduction and size reduction are possible. Further, in the present invention, since the control circuit can be small, it can be housed in a case together with the thermistor, thereby making it possible to further reduce the size of the entire sensor.

The thermistor may be fixed to the bottom end of the case by preparing a case having an open bottom end, inserting the thermistor into the opening, and using an adhesive having a higher thermal conductivity than the case. With this configuration, when the bottom end of the case touches the liquid surface, the heat absorbing action by the liquid concentrates on the adhesive and the thermistor in contact with the adhesive without being transmitted to the case very much. Therefore, the thermistor is suddenly deprived of heat and instantaneously lowers the temperature. Therefore, this sensor shows excellent responsiveness. When the case leaves the liquid surface, the adhesive quickly absorbs the surrounding heat and transmits the heat to the thermistor, so that the temperature of the thermistor quickly returns.

Here, a mixture of a resin and a ceramic is preferably used as the adhesive. In this case, for example, the resin is an epoxy resin, and the ceramic is alumina. Even if the thermistor is not fixed by the adhesive, the same effect can be obtained if the thermal conductivity at the tip of the bottom of the case is made higher than that of other portions. Further, in the present invention, the case may be filled with resin to make the case solid.

[0009]

Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a sectional view showing a liquid level sensor according to the embodiment. The liquid level sensor 1 includes a case 2 having a cylindrical shape and a tapered bottom. Case 2 is made of epoxy resin and is solid. Furthermore, at the tip 2a at the bottom,
Alumina is included in addition to the epoxy resin. A thermistor 3 is buried in the bottom end 2a of the case 2, and a board 4 (a cross section in the thickness direction in the figure) on which a control circuit is mounted is buried above the thermistor 3. The control circuit is connected to the thermistor 3 and further to three external terminals via leads 6 penetrating the cap 5 of the case 2. As a result, an electric circuit as shown in FIG. 2 is formed.

The liquid level sensor 1 of the present embodiment is manufactured as follows. First, a cylindrical case 7 made of an epoxy resin as shown in FIG. 3 is prepared. This case 7
Alumina is contained at the tip of the groove, and two grooves 7a (only one on the other side are shown) whose upper ends are open are provided on the inner surface of the side wall so as to face each other. Next, the thermistor 3 and the lead 6 are connected to the substrate 4. Then, the substrate 4 is inserted into the case 7 by sliding the substrate 4 into the groove 7a. At this time, the thermistor 3 is turned downward. Subsequently, an epoxy resin containing alumina is put into the case 7 until the thermistor 3 is filled, and the case 7 is filled with the epoxy resin. Finally,
When the cap 5 is closed, the liquid level sensor 1 of the present embodiment is completed.

The liquid level sensor 1 of the present embodiment detects a liquid level as follows. When the bottom end 2a of the case 2 comes into contact with the liquid surface, the liquid removes the heat of the thermistor 3 through the case 2, and as a result, the temperature of the thermistor 3 decreases. When the temperature of the thermistor 3 decreases, the electric resistance of the thermistor 3 changes accordingly. Then, the change in the electric resistance value is detected as an electric signal through the control circuit. Thereby, it can be known that the bottom end 2a of the case 2 is in contact with the liquid surface.

In the liquid level sensor 1 of this embodiment, since alumina is added to the bottom tip 2a, the bottom tip 2a
Has a higher thermal conductivity than the other parts of the case 2. Therefore,
When the bottom end 2a of the case 2 touches the liquid surface, the heat absorption by the liquid concentrates on the bottom end 2a and the thermistor 3,
Therefore, the temperature of the thermistor 3 instantaneously decreases. Therefore, the liquid level sensor 1 of the present embodiment has excellent responsiveness.

Further, in the liquid level sensor 1 of the present embodiment, the bottom end 2a of the case 2 is sharp. Therefore, when the bottom end 2a separates from the liquid surface, water droplets adhering to the surface almost flow down and do not continue to adhere. Moreover, since the thermal conductivity of the bottom tip 2a is higher than the others, when the bottom tip 2a separates from the liquid surface, the surrounding heat is quickly absorbed. Therefore, the temperature of the thermistor 3 returns quickly. Therefore, this embodiment does not have a correction circuit, and the control circuit is simple and small.
Moreover, the control circuit is housed in a case together with the thermistor 3. Therefore, the liquid level sensor 1 of the present embodiment is smaller than the conventional one and the manufacturing cost is lower.

[0014]

According to the liquid level sensor of the present invention, the cost can be reduced and the size can be reduced. Further, in the present invention, the responsiveness can be improved by making the thermal conductivity at the bottom end of the case higher than other portions.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating a liquid level sensor according to an embodiment.

FIG. 2 is a diagram illustrating a control circuit of the liquid level sensor according to the embodiment.

FIG. 3 is a cross-sectional view showing a case in a manufacturing stage.

FIG. 4 is a diagram showing a conventional liquid level sensor.

[Explanation of symbols]

 1 Liquid level sensor 2 Case 2a Bottom end 3 Thermistor 4 Substrate 5 Cap 6 Lead 7 Cylindrical case 7A groove

Claims (5)

[Claims] 1. A liquid level sensor comprising: a case having a tapered bottom portion; a thermistor fixed to a tip of the bottom portion; and a control circuit connected to the thermistor and detecting a temperature change of the thermistor. 2. The liquid level sensor according to claim 1, wherein the control circuit is housed in a case. 3. A bottom end of the case is open, and the thermistor is fixed to the bottom end by inserting a thermistor into the opening and using an adhesive having a higher thermal conductivity than the case. 3. The liquid level sensor according to 1 or 2. 4. The liquid level sensor according to claim 3, wherein the adhesive is a mixture of a resin and a ceramic. 5. The liquid level sensor according to claim 1, wherein the thermal conductivity at the bottom end of the case is higher than that of other parts of the case.