JPH0120363B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a liquid level detection device using a temperature-sensitive semiconductor element.

Conventionally, as this type of liquid level detection device, one shown in FIG. 1 is known. Codes 1, 2, in the figure
3 and 4 are resistors, 5 and 6 are temperature-sensitive semiconductor elements such as thermistors, 5 is a self-heating element, and 6 is a temperature measurement element. These elements 5, 6 are arranged in a casing (not shown). The above resistance 1,
2, 3, and 4 correct the exponentially changing temperature/resistance characteristics of the self-heating element 5 and the temperature measuring element 6 so that they change in direct proportion. 7 is a voltage comparator, which connects the resistors 1 and 3, i.e., the non-grounded end of the self-heating element 5 (indicated by point a), and the resistors 2 and 4, i.e., measures the temperature. The voltage with the non-grounded end (indicated by point b) of the control element 6 is compared, and the control circuit 8 is operated based on this output.

In the liquid level detection device configured as described above, if negative resistance elements are used as the self-heating element 5 and the temperature measurement element 6, the voltage with respect to the temperature at the points a and b will be as shown in Fig. 2. become. In this figure, if the temperature is constant, the heat radiation coefficient differs depending on whether the self-heating element 5 is in liquid or air, so the resistance value also differs, and this is shown as the difference in voltage at point a. Furthermore, when the voltage at point a is in the liquid, the voltage at point b changes as shown in FIG. Furthermore, since the voltage is low when in air, when the voltage at point a is detected as high by the above comparison of the voltage at point a and point b, it is known that the voltage at point b is high. When it is detected as high, it is known that it is in the air. still,
The difference in the heat dissipation coefficients is due to the fact that the self-heating element 5 is self-heated in the air and its resistance value decreases.

The voltage comparator 7 can detect whether the liquid is in the air or in the air by the output of the voltage comparator 7, so the voltage comparator 7 can detect the change from being in the liquid to the air or vice versa and detect the liquid level. can.

By the way, in a negative resistance temperature-sensitive semiconductor element, the resistance increases exponentially as the temperature decreases, so the applied voltage remains constant in the low temperature range.
Its power consumption is significantly reduced and sufficient heat generation cannot be obtained. If this heat generation is small, the resistance value of the self-heating element 5 may not decrease and the liquid level may not be detected. In order to compensate for this lack of power consumption, it is possible to keep the voltages at points a and b in the liquid close to each other on the low temperature side, but in this case, the self-heating element 5
Due to the difference between the thermistor constants of the temperature measuring element 6 and the temperature measuring element 6, the voltage at point a approaches the voltage at point b, resulting in a drawback that malfunction occurs even though there is a liquid level. To prevent this malfunction, set point b in advance.
If the gain of the voltage is lowered, the voltage at point a will increase due to the difference in the thermistor constant between the self-heating element 5 and the temperature measurement element 6 in the liquid, and the difference in peripheral circuit components, as shown in Figure 3. In high-temperature ranges, the voltage at point b could approach this and cause malfunctions.

In addition, in order to prevent these malfunctions, in order to increase the voltage difference at point a between the liquid and the air, power is consumed to sufficiently heat the self-heating element 5 even when the temperature drops. In some cases, high voltages were applied to ensure that the However, when a special small glass-encapsulated thermistor is used as the self-heating element 5 and temperature measurement element 6, the power consumption of this thermistor is limited to about 60 mW, and its resistance value decreases exponentially in the high temperature range. However, there was a drawback that the thermistor was destroyed due to a significant increase in power consumption.

This invention eliminates the above-mentioned conventional drawbacks, and changes the voltage applied to the self-heating element 5 according to the temperature detected by the temperature measurement element 6, thereby reducing the power consumption of the self-heating element. The present invention provides a liquid level detection device that maintains a constant liquid level.

An embodiment of the present invention will be described below with reference to FIG. In the figure, 1, 2, 4, 10 are resistances,
5 is a self-heating element, and 6 is a temperature measuring element, and these elements 5 and 6 are the same as those explained with the same reference numerals in connection with FIG. 7 is a voltage comparator, and 8 is a control circuit, which are the same as those described with the same reference numerals in connection with FIG. 9 is a three-terminal regulator, and 11 is an impedance conversion circuit.

In the liquid level detection device configured as described above, when the temperature of the surrounding medium is in a low temperature range, the resistance value of the temperature measuring element 6 becomes high, and the voltage at point b increases.
This rise causes the voltage at the ground terminal of the three-terminal regulator 9 to rise via the impedance conversion circuit 11, and the output voltage of the three-terminal regulator to rise. Therefore, a high voltage is applied to the self-heating element 5 to which the power supply current is supplied via the three-terminal regulator 9. Contrary to the above, when the temperature of the surrounding medium is in a high temperature range, the resistance value of the temperature measuring element 6 becomes low, the voltage at point b becomes low, and the voltage is transferred to the three-terminal regulator via the impedance conversion element 11. The voltage of the ground terminal of 9 becomes low, and the voltage of the ground terminal of 3
The output voltage of the terminal regulator drops. Therefore, the voltage applied to the self-heating element 5 is limited.

As described above, the voltage applied to the self-heating element 5 is changed according to the temperature change of the surrounding medium, so that the power consumption of the self-heating element 5 is kept constant regardless of the temperature change. It is composed of When this power consumption becomes constant, the potential difference between point a in the figure in the air and points a and b in the liquid is kept constant, so that the malfunction described in the conventional example can be prevented. Points a and b are connected to a voltage comparator 7, which is connected to a control circuit 8 to detect the liquid level, as in FIG. 1.

In the above embodiment, a negative resistance temperature sensitive semiconductor element is used, but a positive resistance temperature sensitive semiconductor element may be used.

This invention changes the voltage applied to the self-heating element according to the temperature detected by the temperature measurement element, thereby keeping the power consumption of the self-heating element constant,
This makes it possible to maintain a constant difference between the output voltage of the self-heating element and the temperature measurement element in liquid and air, regardless of fluctuations in the temperature of the surrounding medium. Surfaces can be detected accurately.

[Brief explanation of drawings]

Figure 1 is a circuit diagram of a conventional liquid level detection device, Figures 2 and 3 are temperature/voltage characteristic diagrams of the device in Figure 1,
FIG. 4 is a circuit diagram of a liquid level detection device showing an embodiment of the present invention. 1, 2, 4, 10: Resistor, 5: Self-heating element, 6: Temperature measurement element, 7: Voltage comparator, 8:
Control circuit, 9: 3-terminal regulator, 11: impedance conversion circuit.

Claims (1)

[Claims] 1. Two temperature-sensitive semiconductor elements arranged in a casing, one of which is used for measuring the temperature of the surrounding medium, and the other element is used for measuring the temperature of the surrounding medium. A liquid that is used for self-heating and detects the difference in resistance value caused by the difference in heat radiation coefficient between the liquid and the air in the other element by comparing it with the resistance value of the one element at that temperature. A liquid level detection device, characterized in that the voltage applied to the other element is controlled based on the temperature detected by the one element, so that the power consumption of the other element is made constant.