JP5045259B2 - Liquid level and thermistor ambient temperature detection device - Google Patents

Description

The present invention relates to a liquid surface and a thermistor ambient temperature detection device for detecting the presence or absence of a liquid and detecting the ambient temperature of the thermistor by utilizing the fact that the heat dissipation constants in gas and liquid due to self-heating of the thermistor are different. .

A thermistor used as this type of detection device has a large difference in heat dissipation constant between gas and liquid and must not cause thermal runaway due to self-heating within the operating temperature range. In order to eliminate this point, for example, a liquid level meter disclosed in Patent Document 1 is disclosed. In the liquid level meter disclosed in Patent Document 1, a resistor is connected in series to each of the liquid level detection thermistor and the temperature compensation thermistor, and these series circuits are connected to a power source, and each series circuit is connected. Is connected to the positive input terminal and the negative input terminal of the first operational amplifier, and the operational amplifier is configured to turn off and on when the liquid level detection thermistor is in the liquid and in the air.

  In the liquid level meter constructed in this way, when the ambient temperature is below a certain level, the temperature is detected and the load resistance value of the thermistor for detecting the liquid level is automatically reduced, and the resistance value of the temperature compensating thermistor is further reduced. Is made extremely larger than the resistance value of the thermistor for detecting the liquid level. As a result, the fluctuation of the reference voltage due to ON / OFF of the operational amplifier is eliminated. Further, the reference voltage does not fluctuate due to the temperature fluctuation of the base-emitter voltage such as a transistor. Further, the operating temperature range can be expanded to a low temperature while suppressing the element temperature, and the temperature compensating element is difficult to self-heat, so that a smaller element can be achieved.

Here, “thermal runaway” will be described in more detail. When a voltage is applied to an NTC thermistor whose resistance value decreases as the temperature rises, a temperature rise determined by the resistance value and the heat dissipation constant according to the ambient temperature at that time occurs. Usually, the temperature is measured to such an extent that this temperature rise can be ignored (0.01 to 0.1 ° C.). On the other hand, in the so-called power thermistor used to detect the liquid level, and used in conditions such as heating of the thermistor is 40 to 150 ° C., exothermic difference depending on a heat dissipation constant in the liquid and gaseous, i.e. reaches a temperature The difference is detected. Here, when a predetermined voltage is applied to the thermistor, heat is generated when the resistance value decreases due to self-heating and the current value increases, and further heat generation occurs when the resistance value further decreases and the current value further increases. This is repeated until a predetermined temperature is finally reached. However, if the voltage applied to the thermistor is too high, the temperature of the thermistor rises above its heat resistance before the above loop converges, causing the thermistor to break (damage). This phenomenon is called thermal runaway.

JP-A-10-176944 (Claim 1, paragraph [0018])

The following problems remain in the conventional technology.
In the liquid level meter shown in the above-mentioned conventional patent document 1, since a thermistor having a small heat dissipation constant is used, the response time is as long as 180 to 400 seconds, and it is used for an empty display of a gasoline tank or the like that does not require much responsiveness. Although it was possible, it was unsuitable for detection of hot water such as water heaters and baths where responsiveness was required. If the thermistor is further downsized to increase the response of the thermistor, the heat dissipation constant increases, making it difficult to generate sufficient heat as a liquid level gauge from the viewpoint of thermal runaway. It is known that depending on the ambient temperature, the lower the ambient temperature, the more thermal runaway occurs with a small current. For this reason, it has been difficult to realize a liquid level gauge with good response in a wide temperature range.

Also, in order to cause self-heating, the thermistor is used with a current higher than normal. However, as described above, there is a problem that thermal runaway occurs above a certain current value, and this current value is lower as the ambient temperature of the thermistor is lower. Since the current value is small, the current value of the thermistor needs to be a current value that does not cause thermal runaway in the gas at the lowest temperature in the operating temperature range. For this reason, if a thermistor with excellent response and a large heat dissipation constant is used, there is a problem in that sufficient self-heating does not occur, particularly in the high temperature range during liquid level detection, and sufficient sensitivity as a liquid level meter cannot be obtained. there were.

  Further, in a system using a recent computer, the maximum input voltage of an AD converter that converts a sensor output voltage into a digital signal has become as low as about 5 V, and it is necessary to set the sensor output voltage within this range. . For this reason, it is difficult to accurately detect the level. That is, the sensor output voltage at each temperature for obtaining sufficient level detection accuracy is about 1 V or less to about 10 V, and may be out of the AD converter input voltage range.

The present invention has been made in view of the above-mentioned problems of the prior art, and its purpose is to quickly and without causing thermal runaway in a wide temperature range even when using a small thermistor having a large heat dissipation constant. An object of the present invention is to provide a device for detecting the liquid level and the thermistor ambient temperature that can detect the liquid level with high accuracy.

The present invention employs the following configuration in order to solve the above problems. That is, the liquid level and thermistor ambient temperature detection device of the present invention detects the liquid level by utilizing the fact that the thermistor connected to the power supply has different heat dissipation constants in the gas and liquid due to self-heating. In addition, in the device for detecting the ambient temperature of the thermistor, temperature detection is performed by detecting an output voltage corresponding to the ambient temperature of the thermistor connected between the power source and the thermistor via a temperature detection resistor. A constant current circuit connected between the circuit, the power source and the thermistor to change the reference current of the thermistor driven at a constant current in order to change the characteristics of the output voltage difference with respect to the temperature in the gas and liquid of the thermistor. The current switching circuit, the temperature detection circuit, and the constant current switching circuit can be switched, and the output is based on the detection output of the temperature detection circuit. Comprising a controller for controlling the constant current switching circuit toward the voltage difference becomes larger, and the controller is built-in memory, temperature and to the memory, for the output voltage of the thermistor detected by said temperature detecting circuit The temperature which becomes the boundary of the change of the reference current by the constant current switching circuit, the output voltage difference based on the difference of the heat dissipation constant in the gas and liquid corresponding to each reference current, and the thermistor is in the gas Or a predetermined value of the output voltage difference that is a boundary for determining whether or not the liquid is in the liquid is stored as a map, and the controller compares the output voltage of the thermistor detected by the temperature detection circuit with the map. The reference temperature corresponding to the detected ambient temperature is obtained from the ambient temperature and the temperature that becomes the boundary of the change of the reference current by the constant current switching circuit. At the time of selection, the reference current is selected so that the difference in output voltage of the thermistor with respect to the temperature in the gas and the liquid becomes larger, and then the switching is made to the constant current switching circuit, and the thermistor is switched with the selected reference current. The output voltage difference is calculated by subtracting the output voltage at the ambient temperature in the liquid stored in the memory from the output voltage when the self-heat is generated, and the output voltage difference is calculated as a predetermined value stored in the memory. It is characterized in that it is determined whether the thermistor is in a gas or a liquid as compared with an output power difference .

  In this liquid level and thermistor ambient temperature detection device, a constant current switching circuit for changing a reference current of a thermistor driven by a constant current in order to change a characteristic of an output voltage difference with respect to temperature in gas and liquid, and temperature A controller that controls the constant current switching circuit so that the output voltage difference becomes larger based on the detection output of the detection circuit, so that the current that causes thermal runaway by switching the constant current drive reference current according to the ambient temperature The output voltage difference is increased while avoiding the value, and the output voltage difference can be set to a sufficient level as a level sensor on the liquid level and measured. In addition, since constant current driving is used, the current value does not increase as the temperature rises due to self-heating, so that there is little risk of deterioration of the thermistor, and the current value for stable operation can be set large. Furthermore, when the ambient temperature is detected, the ambient temperature can be accurately detected by the same thermistor by switching to the temperature detection resistor.

  In the liquid level and thermistor ambient temperature detection device of the present invention, the temperature detection circuit and the constant current switching circuit include an AD converter that converts an output voltage of the thermistor from an analog signal to a digital signal, and the controller The constant current switching circuit is controlled so that the output voltage of the thermistor is lower than the maximum input voltage of the AD converter. That is, in this liquid level and thermistor ambient temperature detection device, the constant current switching circuit is controlled so that the output voltage of the thermistor is lower than the maximum input voltage (for example, 5 V) of the AD converter. Level detection can be performed.

  Furthermore, the liquid level and thermistor ambient temperature detection device according to the present invention is such that the controller switches the constant current so that the output voltage difference exceeds a minimum voltage difference set according to a reference voltage and resolution of the AD converter. The circuit is controlled. That is, in this liquid level and thermistor ambient temperature detection device, the constant current is set so that the output voltage difference is greater than the minimum voltage difference set according to the reference voltage (eg, 5 V) and resolution (eg, 10 bits) of the AD converter. Since the switching circuit is controlled, it is switched to a sufficient output voltage difference according to the performance of the AD converter, and can be detected with sufficiently practical sensitivity as a liquid level sensor.

  Also, in the liquid level and thermistor ambient temperature detection device of the present invention, the controller controls the constant current switching circuit so that the thermistor becomes less than half of the current value at which the thermistor starts thermal runaway. And That is, in this liquid level and thermistor ambient temperature detection device, the constant current switching circuit is controlled so that the thermistor is less than one half of the current value at which the thermal runaway starts, so the variation and stability of the thermistor are taken into account. Thus, thermal runaway in the operating temperature range can be reliably suppressed and used.

The present invention has the following effects.
That is, according to the device for detecting the liquid level and thermistor ambient temperature according to the present invention, a constant current switching circuit for changing the reference current of the thermistor driven at a constant current in order to change the output voltage difference characteristic of the thermistor, and the temperature And a controller that controls the constant current switching circuit so that the output voltage difference becomes larger based on the detection output of the detection circuit, so that the output voltage difference is a level sufficient as a liquid level sensor according to the ambient temperature. It can be switched to and measured. In addition, the temperature detection resistor can accurately detect the ambient temperature with the same thermistor, and thermal runaway can be suppressed by constant current driving. Therefore, even if a small thermistor having a large heat dissipation constant is used, the liquid level can be detected quickly and accurately without causing thermal runaway in a wide temperature range. Thereby, the detection device of the present invention is suitable as a device for preventing hot cooking such as a water heater or a bath that needs to detect the liquid level at high speed in a range from low temperature to high temperature.

  Hereinafter, an embodiment of a liquid level and thermistor ambient temperature detection device according to the present invention will be described with reference to FIGS.

The liquid level and thermistor ambient temperature detection device according to the present embodiment is used for, for example, detection of emptying of a water heater or a bath, detection of empty of a gasoline tank, and the like. As shown in FIG. This is a device for detecting the liquid surface and the ambient temperature of the thermistor TH by taking out the difference in heat dissipation constant between the gas and the liquid due to self-heating of the connected thermistor TH as an output voltage difference.

This detection device is connected between the power source 1 and the thermistor TH and detects the ambient temperature of the thermistor TH, and connected between the power source 1 and the thermistor TH in the gas and liquid of the thermistor TH. The constant current switching circuit 3 that changes the reference current of the thermistor TH that is driven by constant current to change the characteristic of the output voltage difference with respect to the temperature of the current, the temperature detection circuit 2, and the constant current switching circuit 3 can be switched. And a controller C that controls the constant current switching circuit 3 so that the output voltage difference becomes larger based on the detection output of the temperature detection circuit 2.
The temperature detection circuit 2 and the constant current switching circuit 3 share an AD converter 4 that converts the thermistor output voltage from an analog signal to a digital signal.

The thermistor TH is a so-called NTC thermistor (negative temperature coefficient thermistor) whose resistance decreases as the temperature rises. This thermistor TH has, for example, a heat dissipation constant of 0.1 to 8 mW / ° C., preferably 1 to 4 mW / ° C. in gas, and 0.2 to 10 mW / ° C., preferably 2 to 8 mW in liquid. It is a single fast response thermistor having a heat dissipation constant of / ° C.

Examples of the gas include air, water vapor, oxygen, nitrogen, and carbon dioxide, and examples of the liquid include water, alcohol, gasoline, and lubricating oil.
The power source 1 is a DC power source and is grounded after being connected to the thermistor TH via the three-terminal regulator 5. The power source 1 is connected to the input terminal 5a of the three-terminal regulator 5, and the thermistor TH is connected to the output terminal 5b of the three-terminal regulator 5 via the first switch SW1 or the third switch SW3.

  The adjustment terminal 5c of the three-terminal regulator 5 is grounded after being connected to the fifth resistor (temperature detection resistor) R5 via the second switch SW2. The fifth resistor R5 functions as a voltage setting resistor for setting the output voltage to a constant voltage in combination with the first resistor R1 to the fourth resistor R4. The second switch SW2 is controlled by the controller C so as to connect the three-terminal regulator 5 and the fifth resistor R5 when temperature is detected (temperature measurement mode).

  On the other hand, a third switch SW3 is provided between the thermistor TH and the three-terminal regulator 5. The third switch SW3 connects the thermistor TH and the three-terminal regulator 5 via a sixth resistor (temperature detection resistor) R6 at the time of temperature detection (temperature measurement mode), and detects the liquid level (level). In the detection mode), the controller C is controlled to connect the thermistor TH and the three-terminal regulator 5 via the first switch SW1 and any one of the first resistor R1 to the fourth resistor R4. The

  The thermistor TH is connected to the input terminal of the AD converter 4, and the reference voltage of the AD converter 4 is set to be the same as the reference voltage of the thermistor TH. The output terminal of the AD converter 4 is connected to the control input (I / O port) of the controller C, and the control output of the controller C is connected to the first switch SW1, the second switch SW2, and the third switch SW3. .

  The first switch SW1 is a current value change switch for switching the connection resistance to any one of the first resistor R1 to the fourth resistor R4 in order to change the reference current of the thermistor TH driven by constant current. . For example, switching to the first resistor R1 to the fourth resistor R4 enables switching to reference currents of 10 mA, 5 mA, 2 mA, and 1 mA, respectively. Therefore, the constant current switching circuit 3 includes the three-terminal regulator 5, the first switch SW1, the first resistor R1 to the fourth resistor R4, and the AD converter 4.

  The second switch SW2 and the third switch SW3 are switches for setting the reference current by measuring the ambient temperature of the thermistor TH as described above. Therefore, the temperature detection circuit 2 includes a three-terminal regulator 5, a second switch SW2, a third switch SW3, a fifth resistor R5, a sixth resistor R6, and an AD converter 4.

The controller C has a function of controlling the constant current switching circuit 3 so that the thermistor output voltage is lower than the maximum input voltage of the AD converter 4.
Further, the controller C has a function of controlling the constant current switching circuit 3 so that the output voltage difference becomes greater than the minimum voltage difference set according to the reference voltage and resolution of the AD converter 4.
The controller C has a function of controlling the constant current switching circuit 3 so that the thermistor TH is less than half of the current value at which the thermal runaway starts.

The controller C includes a memory (not shown). The memory includes a temperature corresponding to the thermistor output voltage detected by the temperature detection circuit 2 and a temperature serving as a boundary for changing the reference current by the constant current switching circuit 3. The difference between the output voltage difference based on the difference between the heat dissipation constant in the gas and the liquid corresponding to each reference current and the output voltage difference that is a boundary for determining whether the thermistor TH is in the gas or in the liquid The predetermined value is stored as a map.

  Next, a method for detecting the liquid level and thermistor ambient temperature by the liquid level and thermistor ambient temperature detection device of the present embodiment will be described.

  First, as shown in the flowchart of FIG. 2, the controller C switches the second switch SW2 and the third switch SW3 to the temperature detection side (Temp side) and configures the first switch SW1 to configure the temperature detection circuit 2. A connection position to the first resistor R1 at which the reference current is 10 mA is set in advance (step S1). Next, the ambient temperature of the thermistor TH is detected (step S2). That is, the thermistor output voltage is input to the AD converter 4, and the AD converter 4 converts the input voltage into a digital signal and takes the digital signal into the controller C through the control input of the controller C. And the controller C calculates | requires ambient temperature compared with the map memorize | stored in memory.

  Next, the controller C determines whether the ambient temperature detected based on the detection output of the temperature detection circuit 2 is 60 ° C. or higher based on the map in the memory (step S3). A reference current corresponding to the detected ambient temperature is selected from the ambient temperature and the temperature that becomes the boundary for changing the reference current by the constant current switching circuit 3 (step S4). At this time, the controller C selects the reference current so that the thermistor output voltage difference with respect to the temperature in the gas and in the liquid becomes larger. In addition, when the ambient temperature is 60 ° C. or higher, the controller C keeps the first switch SW1 connected to the first resistor R1 (step S5).

  In this way, the resistance value corresponding to the selected reference current is selected from the first resistor R1 to the fourth resistor R4, and the first switch SW1 is controlled and connected to the corresponding resistor. For example, the temperature that becomes the boundary of the change of the reference current is set to 20 ° C., 40 ° C., and 60 ° C. If the temperature is less than 20 ° C., the fourth resistor R4, and if it is 20 ° C. or more and less than 40 ° C., the third resistor R3, 40 ° C. The reference current is changed to a different reference current by connecting the first switch SW1 to the second resistor R2 when the temperature is lower than 60 ° C. or higher and the first resistor R1 when the temperature is 60 ° C. or higher. These settings are such that the output voltage difference is greater than the minimum voltage difference set according to the reference voltage and resolution of the AD converter 4. Further, the thermistor TH is set to be less than half of the current value at which the thermal runaway starts.

  Next, the controller C switches the second switch SW2 and the third switch SW3 to the liquid level detection side (Level side) in order to switch from the temperature detection circuit 2 to the constant current switching circuit 3. Then, the thermistor TH is self-heated and the liquid level is detected (step S5). At this time, the thermistor output voltage is input to the AD converter 4 and converted into a digital signal and then taken into the controller C. The controller C calculates an output voltage difference by subtracting the output voltage at the ambient temperature in the liquid stored in the memory from the output voltage.

Next, the controller C compares the output voltage difference with a predetermined output voltage difference stored in the memory, and if the digital output value of the AD converter 4 is less than a certain value, the thermistor TH is in the liquid. to decide. Further, the controller C compares the output voltage difference with a predetermined output voltage difference stored in the memory, and determines that the thermistor TH is in the gas when the digital output value of the AD converter 4 is equal to or larger than a certain value. (Step S6). At this time, the constant value of the digital output value of the AD converter 4 is set to 5 LSB (Least Significant Bit), for example, and when the output voltage difference is less than 5 LSB, it is determined that the liquid is in the liquid. Judged to be in gas.
Here, when it is determined that the liquid is in the liquid, the controller C performs control to repeat the above operation again after a predetermined time has elapsed. If it is determined that the gas is in the gas, a warning is displayed or an alarm is generated.

  Next, a simulation result of the method for detecting the liquid level and the thermistor ambient temperature by the liquid level and thermistor ambient temperature detection device of the present embodiment will be described.

  First, for comparison, the case where the thermistor TH detects the temperature with the conventional general circuit configuration shown in FIG. 3 will be described. In this case, the reference resistance Rref is preferably about the same as the resistance value of the thermistor in the main operating temperature range, and the reference voltage Vref is desirably a voltage that does not cause the thermistor to self-heat. Here, in a thermistor TH having a resistance value of 2.186 kΩ at 25 ° C., a B constant of 3386 K, a heat dissipation constant in liquid of 3.2 mW / ° C., and a heat dissipation constant in gas of 1.5 mW / ° C., the reference resistance Rref FIGS. 4 to 6 show simulation examples of the output in the gas and in the liquid due to the difference in the reference voltage Vref.

  These figures show the output voltage with respect to the ambient temperature when the reference voltage Rref is set to 12 V, 8 V, and 5 V, respectively, and the reference resistor Rref is changed to 0.3 kΩ, 0.5 kΩ, and 1 kΩ in the circuit configuration of FIG. Yes. In each figure, the points indicated by ◯ are equal to or higher than the current value causing thermal runaway, and the points indicated by □ are current values equal to or more than one half of the current value causing thermal runaway. From the viewpoint of deterioration of the thermistor TH, it is desirable to use less than half of the current value that causes thermal runaway.

  In these drawings, in order to have practical sensitivity as a liquid level sensor, a suitable level of the output voltage difference between the liquid and the gas depends on the reference voltage and resolution of the AD converter 4, For example, in the AD converter 4 with the reference voltage Vref: 5 V and the resolution of 10 bits, it is desirable that the voltage is about 0.1 V or more. When the reference voltage Vref is 12 V, the reference resistance Rref is 1 kΩ, and even when the reference voltage Vref is 8 V and the reference resistance Rref is 1 kΩ, the output voltage difference is 0.1 V or more, but the output voltage is 5 V or more on the low temperature side. Yes, the input range of the AD converter 4 is exceeded.

  On the other hand, at the reference voltage Vref: 5V, the output voltage is less than 5V, but the output voltage difference is significantly lower than 0.1V, which is not practical as a liquid level sensor. In order to cope with this, it is possible to cope with this by using a thermistor TH having a low resistance value in the operating temperature range, but it is not general and can be obtained by using a chip thermistor. When the thermistor TH having such a resistance value is used for normal temperature measurement, the output voltage becomes small and the temperature detection accuracy deteriorates. Although the reference voltage Vref of the AD converter 4 is 5 V and the resolution is 10 bits here, this value is changed according to the control device to be used.

  On the other hand, in the detection device according to the embodiment of the present invention, detection is performed by supplying a constant current to the thermistor TH as described above. FIG. 7 shows a simulation example of the thermistor output voltage (both-end voltage) and the output voltage difference with respect to the ambient temperature when a constant current is passed through the thermistor TH similar to the comparison simulation. This figure shows the output voltage when the constant current value is changed to 1 mA, 2 mA, 5 mA, and 10 mA.

  In FIG. 7, the points marked with ◯ are equal to or greater than the current value causing thermal runaway, the points marked with □ are points where the simulation results are vibrational and unstable, and the points marked with △ are The current value is one half or more of the current value causing the runaway. However, in the present embodiment, unlike the case of using the reference resistance and the constant voltage, the current value does not increase with the temperature rise due to self-heating, and therefore the thermistor TH is less likely to be deteriorated. The result is also stable. From these results, when the thermistor TH is used at a constant current, the current value for stable operation can be increased as compared with the use at the reference resistance + constant voltage.

In addition, the values indicated by total_liq and total_air in FIG. 7 change by changing the current value so that the thermistor output voltage is 5 V or less and the difference between the thermistor output voltage in the liquid and the gas becomes large at each ambient temperature. In this plot, the output voltage difference at that time is shown as ΔV.
From this result, the thermistor TH is used at a constant current, and the current value is changed according to the ambient temperature, so that the thermistor output voltage is set to 5 V or less while using the thermistor TH having a normal resistance value. The output voltage difference among them can also be used as a liquid level sensor. Further, by using a thermistor TH having a normal resistance value, it is possible to accurately detect the ambient temperature with the same thermistor TH. Here, the reference voltage of the AD converter 4 is assumed to be 5 V, but when the voltage changes, it can be dealt with by changing the ambient temperature of current switching.

  Thus, in the detection apparatus of the present embodiment, the constant current switching circuit 3 that changes the reference current of the thermistor TH that is driven by constant current in order to change the characteristics of the output voltage difference with respect to the temperature in the gas and in the liquid, And a controller C that controls the constant current switching circuit 3 so that the output voltage difference becomes larger based on the detection output of the temperature detection circuit 2, so that the reference current for constant current driving is switched according to the ambient temperature, The output voltage difference is increased while avoiding a current value that causes thermal runaway, and the output voltage difference can be set to a sufficient level as a liquid level sensor for measurement.

In addition, since constant current driving is used, the current value does not increase as the temperature rises due to self-heating, so that the thermistor TH is less likely to deteriorate, and the current value for stable operation can be set large. Furthermore, when the ambient temperature is detected, the ambient temperature can be accurately detected by the same thermistor TH by switching to the temperature detection resistor.
Further, since the constant current switching circuit 3 is controlled so that the output voltage of the thermistor TH is lower than the maximum input voltage (for example, 5 V) of the AD converter 4, the liquid level can be detected with high accuracy.

Further, since the constant current switching circuit 3 is controlled so that the output voltage difference is larger than the minimum voltage difference set according to the reference voltage (for example, 5 V) and the resolution (for example, 10 bits) of the AD converter 4, the AD converter The output voltage difference is switched to a sufficient level according to the performance of No. 4, and can be detected with sufficiently practical sensitivity as a liquid level sensor.
In addition, since the constant current switching circuit 3 is controlled so that the thermistor TH is less than one half of the current value at which the thermal runaway starts, the thermal runaway in the operating temperature range is considered in consideration of the variation and stability of the thermistor TH. It can be used with certainty.

Next, an example in which the liquid level is actually detected by the liquid level and thermistor ambient temperature detection device of the present embodiment will be described.
In this embodiment, the resistance values of the first resistor R1 to the sixth resistor R6 are 120Ω, 240Ω, 620Ω, 1.2 kΩ, 360Ω, and 10 kΩ, respectively. The thermistor TH having the same characteristics as the above simulation was used.
Further, when the first switch SW1 is switched and connected to the resistors of the first resistor R1 to the fourth resistor R4, 10.4 mA (first resistor connection) and 5.2 mA (second resistor connection), respectively. , 2.0 mA (third resistor connection) and 1.0 mA (fourth resistor connection) were set to energize the thermistor TH.

Further, when the second switch SW2 and the third switch SW3 are connected to the temperature detection side (Temp side) and switched to the temperature detection circuit 2, the temperature measurement mode with the reference voltage of 5.0 V and the reference resistance of 10 kΩ is set. Set.
In the embodiment set in this way, when the liquid level was actually detected, the detection time was 180 to 400 seconds in the conventional circuit configuration, but the detection time was 10 to 60 seconds, and the speed was greatly increased. I found out. Further, the thermistor output voltage at that time can be adjusted to the maximum input range of the AD converter 4, and the temperature can be detected with high accuracy by the same thermistor TH.

  In addition, this invention is not limited to the said embodiment, A various change can be added in the range which does not deviate from the meaning of this invention.

1 is a circuit diagram showing a circuit configuration in an embodiment of a liquid level and thermistor ambient temperature detection device according to the present invention. FIG. In this embodiment, it is a flowchart in the detection method of a liquid level and thermistor ambient temperature. It is a circuit diagram which shows a circuit structure about the prior art example which concerns on this invention. It is a graph which shows the thermistor output voltage by the reference resistance with respect to ambient temperature and a reference voltage (12V) about the prior art example which concerns on this invention. It is a graph which shows the thermistor output voltage by the reference resistance with respect to ambient temperature and a reference voltage (8V) about the prior art example which concerns on this invention. It is a graph which shows the thermistor output voltage by the reference resistance with respect to ambient temperature and a reference voltage (5V) about the prior art example which concerns on this invention. It is a graph which shows the output voltage difference between the thermistor output voltage (both-ends voltage) by which constant current drive was carried out with respect to ambient temperature, and air and liquid about this embodiment.

  DESCRIPTION OF SYMBOLS 1 ... Power supply, TH ... Thermistor, R1 ... 1st resistor, R2 ... 2nd resistor, R3 ... 3rd resistor, R4 ... 4th resistor, R5 ... 5th resistor (resistance for temperature detection), R6 ... 6th resistor (temperature detection resistor), 2 ... temperature detection circuit, 3 ... constant current switching circuit, C ... controller, 4 ... AD converter, 5 ... 3-terminal regulator

Claims (4)

In a device for detecting the liquid level of the thermistor by utilizing the fact that the thermistor connected to the power source has different heat dissipation constants in gas and liquid due to self-heating, and detecting the ambient temperature of the thermistor,
A temperature detection circuit connected via a temperature detection resistor between the power source and the thermistor to detect an output voltage corresponding to the ambient temperature of the thermistor as a detection output ;
A constant current switching circuit that is connected between the power supply and the thermistor and changes the reference current of the thermistor driven at a constant current in order to change the characteristics of the output voltage difference with respect to the temperature in the gas and liquid of the thermistor. When,
A controller that is capable of switching between the temperature detection circuit and the constant current switching circuit and that controls the constant current switching circuit in such a manner that the output voltage difference is increased based on a detection output of the temperature detection circuit ;
The controller has a built-in memory,
In the memory, the temperature with respect to the output voltage of the thermistor detected by the temperature detection circuit, the temperature at which the reference current is changed by the constant current switching circuit, and the gas and liquid corresponding to each reference current An output voltage difference based on the difference between the heat dissipation constants and a predetermined value of the output voltage difference serving as a boundary for determining whether the thermistor is in gas or liquid, are stored as a map,
The controller compares the output voltage of the thermistor detected by the temperature detection circuit with the map to determine the ambient temperature;
When the reference current corresponding to the detected ambient temperature is selected from the ambient temperature and the temperature that becomes the boundary of the change of the reference current by the constant current switching circuit, the thermistor with respect to the temperature in the gas and the liquid is selected. Select the reference current for the one with the larger output voltage difference,
Next, switching to the constant current switching circuit and subtracting the output voltage at ambient temperature in the liquid stored in the memory from the output voltage when the thermistor self-heats with the selected reference current. Calculate the difference,
Liquid level and thermistor ambient temperature, wherein the output voltage difference is compared with a predetermined value of output power difference stored in the memory to determine whether the thermistor is in gas or liquid. Detection device. In the liquid level and thermistor ambient temperature detection device according to claim 1,
The temperature detection circuit and the constant current switching circuit include an AD converter that converts an output voltage of the thermistor from an analog signal to a digital signal,
The liquid level and thermistor ambient temperature detection device, wherein the controller controls the constant current switching circuit so that the output voltage of the thermistor is lower than the maximum input voltage of the AD converter. In the liquid level and thermistor ambient temperature detection device according to claim 2,
The controller controls the constant current switching circuit so that the output voltage difference is equal to or greater than a minimum voltage difference set according to a reference voltage and resolution of the AD converter. Detection device. In the detection device of the liquid level and thermistor ambient temperature according to any one of claims 1 to 3,
The liquid level and thermistor ambient temperature detection device, wherein the controller controls the constant current switching circuit such that the thermistor becomes less than half of a current value at which the thermistor starts thermal runaway.

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