JPS5847647B2 - Ekimen Kenchiki - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a liquid level detector that utilizes a change in resistance value due to self-heating of a thermistor.

A conventional liquid level detector of this type will be explained with reference to FIG.

RB is the first thermistor with a small heat radiation coefficient, RA is the second thermistor with a large heat radiation coefficient, and RA and RB are connected to the power source ■
The connection point P is connected in series to the first transistor TR of the voltage comparator S consisting of a Schmitt trigger circuit.
It is connected to the base of I.

The collector of the second transistor TR2 of the voltage comparator S is connected to the base of a switching transistor TR3, which is connected in series with the alarm L (lamp, buzzer, etc.) to the power supply ■.

The first thermistor RB is placed at a predetermined position indicating the liquid level of the liquid container so that it is always submerged in the liquid, and when the liquid level falls below the predetermined position, the thermistor RB is placed above the liquid level. When exposed, RB accumulates self-heating due to the difference in heat dissipation coefficient between the liquid and the air, the temperature rises, and its resistance value decreases.

Now, if we set TRI to turn on and TR2 to turn off at the voltage at point P when RB is in the liquid, then TR3
Since the emitter and base are at the same potential, there is no conduction, but
When the resistance value of RB decreases and the voltage at point P drops, TRI is turned off and TR2 is turned on.

Therefore, the base voltage of TR3 decreases and becomes conductive, and the alarm L is activated.

In the circuit shown in FIG. 1, the first thermistor RB has a small heat radiation coefficient, and the second thermistor RA has a large heat radiation coefficient.
The reason why these thermistors are connected in series is to compensate for temperature in order to reduce the influence of changes in ambient temperature on the voltage at point P, but these thermistors alone can compensate for the temperature of the voltage at point P. Since it is difficult to
are connected in parallel (or in series) to adjust the voltage change coefficient with respect to temperature changes of RA and RB, thereby performing temperature compensation of the voltage at point P.

By the way, according to this method, due to the fixed resistor γ inserted in parallel or series, the difference in self-heating amount between when RB is in liquid and when it is in air becomes small, and the combined resistance value becomes smaller. The amount of change becomes small, and therefore the amount of change in voltage at point P becomes small, making it difficult for TRI to detect this.

? The object of the invention is to provide a liquid level detector that does not have the above-mentioned drawbacks.

The embodiment will be described below with reference to FIG.

RB is a first thermistor with a small heat dissipation coefficient, and is connected in series with a fixed resistor to the power supply (2), and its contact P is connected to the base of the voltage comparison transistor TR4 via a resistor R2.

The collector of TR4 is connected via a resistor R to the base of a switching transistor TR5, which is connected in series with an alarm L (lamp, buzzer, etc.).

A voltage (2) at a dividing point Q, which is obtained by dividing the power supply voltage (2)CC by a fixed resistor R3 and a second thermistor RO having a large heat radiation coefficient, is applied to the emitter of the TR4 as a reference voltage.

The resistance value of the fixed resistor R3 that determines this reference voltage (2), the B constant of the thermistor RO, and its resistance value at room temperature are determined so as to satisfy the following equation within the temperature range used.

V2='(voltage at point P when RB is in the liquid■12 Voltage at point P when +RB is in the air ■1) +vBE However, VBE is the voltage between the pace emitters of TR4.

The heat dissipation coefficient of the second thermistor Rc is set to be sufficiently large so that its resistance value hardly changes even if the thermistor RQ is in liquid or air. It is placed at a predetermined position in the liquid.

FIG. 3 shows the relationship between the voltage v2 at point Q and the voltage 1 at point P determined by R3 and RO and the ambient temperature.

That is, since both RB and RO are thermistors, their resistance values decrease as the ambient temperature rises, and the voltages 1 and v2 at points P and Q drop in parallel.

In addition, the reference voltage (2) is lower than the voltage (2) at point P when RB is in the liquid, and the voltage at point P (V) when RB is in the air.
taller than.

Therefore, when RB is in the liquid, TR4 is cut off and no voltage is applied to the base of TR5, so TR5
is off and alarm L does not operate.

Now, when the liquid level drops and RB is exposed to the air, the heat dissipation coefficient in air is smaller than in liquid, so RB accumulates self-heating, its temperature rises, and its resistance value decreases. As a result, the voltage at point P drops.

When the voltage v1 at point P drops to V2 VBE, TR4
is conductive, and the voltage of resistor R4 is the only one in TR5? is applied to conductivity, and the alarm L is activated.

Note that when TR4 becomes conductive and current flows to its collector, the voltage drop across R3 increases, causing a change in 2, causing negative feedback, and causing TR4 to perform a switching operation as a voltage comparison transistor. To reduce this effect, it is desirable to choose the current flowing through R3 and RO to be sufficiently larger than the collector current.

FIG. 4 shows another embodiment of the invention.

In this embodiment, the reference voltage ■3 applied to point Q by the thermistor RO, parallel resistor R6, and series resistors R7, R6 is used.
is set.

The B constant of the thermistor Rc that determines the reference voltage ■3, its low resistance value at room temperature, and R, . R7 and R
The resistance value of 8 is determined to satisfy the following equation within the temperature range used.

■3-7 (Voltage at point P when RB is in the liquid v1+R
Voltage at point P when B is in the air V1) VBE However, VBE is the voltage between the pace emitters of TR6.

If the reference voltage ■ is set as above, the voltage at point P when RB is in the liquid is lower than the voltage at point Q3, and TR6, TR7, TR8, and TR9 are all off. Alarm L does not work, but if RB is exposed to air, RB
The resistance value of decreases and the voltage at point P becomes higher than the voltage v3 at point Q, TR6, TR? , TR8, and TR9 are all conductive, and the alarm L is activated.

Resistor R9 and capacitor C4 constitute an integral circuit, and provide an appropriate delay in the start of operation of TR8 so that alarm L does not operate immediately if RB is temporarily exposed to the air due to vibration of the container, etc. .

TR7 has the function of making the voltage for charging C1 constant regardless of the temperature of the liquid.

In general, the resistance value of a thermistor tends to increase with aging, so in the present invention, the aging characteristics of the resistance value of the second thermistor RO, which sets the reference voltage, are
It is made to match that of the thermistor RB.

Therefore, even if the resistance value of RB changes due to aging during long-term use, the resistance value of RO will also change accordingly, so the relationship between the voltages at points P and Q will maintain the state shown in Figure 3. do.

This is extremely effective for liquid level detectors that are used for long periods of time.

As described above, the liquid level detector of the present invention uses the first thermistor RB, which has a small heat radiation coefficient, for liquid level detection, and the second thermistor RO, which has a large heat radiation coefficient, for setting the reference voltage. The value of the fixed resistor R1 connected in series can be set to an optimal value, and by placing R(3) in the liquid together with RB, changes in the ambient temperature of Rc and RB can be prevented over the entire operating range of ambient temperature. Since the resistance change coefficients can be made equal and the voltages at points P and Q can be reliably temperature-compensated, highly accurate oil level gauges and other liquid level detectors can be obtained.

[Brief explanation of the drawing]

Figure 1: Circuit diagram of a conventional liquid level detector, Figure 2: Circuit diagram of an embodiment of the present invention, Figure 3: Voltage characteristics diagram at points P and Q in Figure 2, Figure 4: Main FIG. 3 is a circuit diagram of another embodiment of the invention. Symbol, RB...First thermistor, RO...
...Second thermistor, TRI~RT9...Transistor, R1~R,...Resistor, C0...
...Capacitor, ■...Power supply, L...
・Alarm device.

Claims (1)

[Claims] 1. A first thermistor with a small heat dissipation coefficient is connected in series with a fixed resistor, the connection point of which is connected to the base of a voltage comparison transistor, and the power supply voltage is applied to the emitter of the transistor with a low heat dissipation coefficient. A reference voltage divided by a large second thermistor and a resistor is applied, the first and second thermistors are placed in a position in the liquid container, and the first thermistor is in the liquid. A liquid level detector characterized in that an alarm is operated by an output obtained from the collector of the transistor by utilizing a change in resistance value when the liquid level detector is in air. 2. Claim 1, wherein the first and second thermistors have the same aging characteristics of resistance values.
liquid level detector.