JPS5828625A - Liquid level sensor - Google Patents

Abstract

PURPOSE:To detect liquid level stably over a wide range of temp., by making voltage applied to a sensor change in accordance with a change of temp. CONSTITUTION:Voltage VS is outputted through a constant-voltage circuit 2 from a power supply 1 and is applied to a series circuit of a coil 3a of a lead switch 3 and a thermister 4 for a liquid surface detection. The resistance of the thermister 4 becomes small as a temp. elevates and the collector-emitter voltage of a transistor TR1 is changed so that an output voltage increases. While, in compliance with an operation of a level sensor, when the thermister 4 in oil is exposed to the air, a heat dissipation constant becomes small and a resistance becomes small and electric current flowing in the coil 3a becomes large and a contact 3b of the switch 3 is turned on and an alarming lamp 5 is turned on a light. The voltage applied to the thermister 4 is increased as the atmospheric temperature comes down.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a suitable voltage application method for a liquid level sensor using a thermistor whose resistance value changes due to the difference in heat dissipation time constant between liquid and air. The present invention relates to a liquid level sensor using a method.

Conventionally, liquid level sensors using thermistors have been widely used. However, this type of sensor utilizes the difference in heat dissipation time constant between the thermistor in the air and in the liquid, and therefore has the drawback that the ambient temperature cannot be adjusted over a wide range.

An object of the present invention is to provide a liquid level sensor that can perform stable detection over a wide temperature range.

The present invention attempts to perform stable detection over a wide temperature range by providing means for changing the voltage applied to the level sensor for liquid level detection in accordance with temperature changes.

An embodiment of the present invention will be described below.

FIG. 1 shows an embodiment of the present invention.

In the figure, a constant voltage circuit 2 is connected to a power source 1. This constant voltage circuit 2- has a resistor R1.

R2, R3, R4 and 'transistors'rg1, TR,
2, a Zener diode ZD, and a temperature compensation thermistor Th. A coil 3a of a reed switch 3 is connected to this constant voltage circuit 2. This coil 38K# is grounded via the liquid level detection thermistor 4. Further, a lasig 5 is connected to the Y point 3b of the reed switch 3, and resistors J- and R2 of the constant voltage circuit 2 are connected to the other terminals of this contact 3b.

Since it is constructed in this way, the voltage vs is output from the power supply 1 through the constant voltage circuit 2, and this voltage is applied to the series circuit of the coil 3a of the reed switch 3 and the thermistor for liquid level detection. . The constant voltage circuit 2 has a circuit configuration as shown in the figure, and since the pace potential of the transistor TR2 is kept constant by the Zener diode ZD, the output voltage VB is given by the following equation.

R, h, temperature compensation thermistor resistance value Vz; V to Zener diode voltage! 12: ) Range Meta TR2 Base-Emitter Voltage As shown in equation (1), the resistance of the temperature compensation thermistor decreases as the temperature rises, and the collector-emitter voltage of the transistor TFtl changes so that the output voltage increases.

On the other hand, in the operation of the level sensor, when the liquid level detection thermistor is exposed to air rather than oil, the heat dissipation constant becomes smaller, and the resistance of the thermistor becomes smaller due to self-heating of the thermistor, and the current flowing through the coil of reed switch 3 increases. Then, contact 3b of the reed switch is turned on and the alarm lamp is turned on.

However, the point at which a thermistor generates heat by itself in the air and its resistance suddenly changes is affected by the outside temperature, and if the voltage applied to the thermistor is constant, it may not work if the outside temperature is low.

For this reason, in this embodiment, the voltage applied to the thermistor is increased as the outside air temperature becomes lower, so that it can operate even when the outside air temperature is low, as shown in FIG.

FIG. 3 shows an example of mounting a liquid level detecting thermistor, in which a case 6 is mounted on the side surface of the oil pan 6, and the thermistor is mounted inside the case.

Therefore, according to this embodiment, the conventional operating temperature range is 1.
The level sensor that was 0°C to 80°C was changed to -20°C to +80°C.
As explained above, according to the present invention, stable detection can be performed over a wide temperature range.
The power to do something; be able to do it.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing an embodiment of the present invention, Fig. 2 is a diagram showing the relationship between resistance value change of a thermistor for liquid level detection and outside temperature,
FIG. 3 is a diagram showing the mounting state of the liquid level detection thermistor. 1...Power supply, 2...Constant voltage circuit, 3...Lead switch No. 10

Claims (1)

[Claims] 1. A heat-sensitive resistance element whose resistance value changes due to a difference in heat dissipation time constant in a liquid and in air, a fourth means for detecting a current flowing through the heat-sensitive ML resistance element, and a heat-sensitive resistance element. A liquid level sensor characterized by comprising: second means for changing the voltage applied to the series circuit with the first means according to temperature.