JPH0122922B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a combustion control device that detects an abnormality in a thermistor for temperature measurement.

Conventionally, in combustion control devices for gas, etc., the temperature of water, which is the load, is constantly converted into voltage using a thermistor.
This value is compared with the value corresponding to the set temperature using a differential amplifier, and combustion is controlled so that the difference is always zero, making it possible to accurately maintain the outlet temperature at a predetermined temperature. , there is no way to detect if the thermistor were to short-circuit or fall off.
Combustion control becomes impossible and overheating occurs.

The present invention solves the above problems and relates to a combustion control device that can easily detect abnormalities in a thermistor.

First, before explaining one embodiment of the present invention, an example of converting a change in the resistance value of a thermistor into a voltage will be explained first.
This will be explained using figures. If you connect resistor R ₂ in series to the parallel circuit of resistor R ₁ and thermistor R _H , apply a predetermined positive potential +V and measure the potential at the midpoint, you will find that the thermistor has negative temperature characteristics as the temperature increases. The resistance value decreases, the division ratio with the resistor _R2 changes, and the potential at the midpoint increases as shown in the figure. However, this resistance change only occurs within a certain temperature range (e.g. 10°C).
~80℃) and will not change outside of that range. In other words, the potential at the midpoint hardly changes outside the predetermined temperature range.

However, if the thermistor R _H is short-circuited or removed, the potential at the midpoint will become a positive potential +V or close to zero (earth potential), indicating a value outside the normal temperature range. It is possible to determine whether or not the thermistor R H is present, and as a result, it is possible to detect an abnormality in the thermistor R _H.

FIG. 2 is a combustion control circuit diagram showing an embodiment of the present invention. In the figure, 1 is a counter, which always performs counting operation based on the clock signal CLK.
For example, in the case of an 8-bit configuration, outputs corresponding to decimal numbers 0 to 255 are output to the latch circuit 2 and the digital-to-analog (abbreviated as D/A) converter 3. For example, as the clock signal CLK to counter 1,
When using a 2KHZ pulse signal, 2000÷256=7.8
Output occurs at a cycle of (times/second).

The D/A converter 3 converts the 8-bit output of the counter 1 mentioned above into an analog voltage value _.
It consists of _R18 and resistors _R21 to _R28 that connect it. One end of the resistor _R28 is connected to the positive potential +V,
Corresponding to the output of counter 1 from 0 to 255, for example, a potential change of 0 to 5 V is output. The contacts of the resistors R ₁₁ and R ₂₁ are connected to the positive input of the first operational amplifier op ₁ .

A resistor _R4 is connected to the positive input of the first operational amplifier _op1 via a resistor _R3 and an analog switch AS. The output of the D/A converter 3, as shown by the dashed line in Fig. 3, generates an analog output A for the digital input D, but when resistor R ₃ is connected, the entire output is shifted up to the position shown by the solid line, and the analog output is Set the output A to the output range of the thermistor circuit 5 (10~
(80℃). Furthermore, when the analog switch AS is made conductive by applying the signal SE, it is shifted down to the position indicated by the dotted line by the resistor R4, so that the analog output A corresponds to the output of the thermistor circuit 5 in the range below room temperature. Therefore, by turning on or off the analog switch AS, it is possible to select whether the positive input of the first operational amplifier op ₁ is a solid line or a dotted line, thereby forming an offset circuit 4.

The output of the first operational amplifier op ₁ is the output of the second operational amplifier
parallel circuit of thermistor R _H and resistor R ₂ connected to the positive input of op ₂ and the negative input of the other
It is compared with the potential of the thermistor circuit 5 consisting of _R2 ,
Output occurs when the positive input side is large. Therefore, when the output of the first operational amplifier op ₁ changes as shown by the solid line in Fig. 3, the output range (10 to 80°C) of the thermistor circuit 5 shown in Fig. 1 is included within the change range, and the initial The output of the first operational amplifier op ₁ is small,
As the size gradually increases, there is always a point where the magnitude relationship reverses. The point in time is detected by the differentiating circuit 6,
This is the write signal for the latch circuit 2. Since the latch circuit 2 is always supplied with the input of the counter 1, the output value of the counter 1 at that time is written into the latch circuit 2 in synchronization with the write signal. For example, if the output of the second operational amplifier op ₂ occurs when the counter 1 is 200, then 200 is written into the latch circuit 2 by the output of the differentiating circuit 6. In this manner, during the repetition of the output of the counter 1, it is always determined whether the output of the thermistor circuit 5 and the output of the first operational amplifier op ₁ , that is, the shifted value of the D/A converter 3 are inverted.

However, the thermistor R _H is installed at the hot water outlet point via the connector CT to measure the outlet temperature, so if the connector CT is defective or installed incorrectly, the thermistor R _H may be short-circuited or fall off. be. When it is dropped, the negative input of the second operational amplifier op ₂ becomes the potential at point a in FIG. 3 due to the division ratio between the resistors R ₁ and R ₂ and is outside the range of change indicated by the solid line. Further, when short-circuited, the negative input becomes the same as the positive potential +V and similarly falls outside the range of change indicated by the solid line, making it impossible to match. Therefore _, if the analog switch AS is turned on by the signal SE and the output of the first operational amplifier op ₁ is shifted down to the level indicated by the dotted line in Fig. 3, the second
The output of operational amplifier op ₂ occurs. At this time, the AND gate AND has the signal SE and the second operational amplifier op ₂
Since the output of is given, an output is generated, indicating that the output of the thermistor circuit is at point a, that is, thermistor R _H is out. If an alarm is generated by this output, it is possible to easily know if the connector CT is improperly installed, and depending on the settings of the thermistor circuit 5, it is also possible to detect low temperatures (below 5 degrees Celsius).
The detection circuit 7 is constituted by an AND gate.

On the other hand, when the thermistor R _H is short-circuited, there is no coincidence in either the solid line or the dotted line range, and the negative input of the second operational amplifier op ₂ is always greater than the positive input, and no output is generated. Therefore, if the output of the second operational amplifier OP ₂ does not occur for a predetermined period of time, an alarm is generated.
It is possible to warn of a short circuit accident of thermistor R _H.

The output of the latch circuit 2 is a digital value indicating the temperature of the load measured by thermistor R _H , and is compared with the output of a set temperature (not shown), and the combustion amount is controlled according to the result. .
The output of the set temperature can be generated by analog-to-digital conversion of the change in the division ratio between the variable resistor and the resistor, or it can be directly output as a digital value using the keyboard.

The above signal SE may be output in advance when starting the circuit, and the dropping of the thermistor R _H may be monitored using an AND gate. Therefore, it is sufficient to detect it during or before or after the pre-purge operation in the gas combustion device.

Further, a short circuit accident of the thermistor R _H may be monitored at the time of starting, but since it takes some time, it may be constantly monitored.

Further, in the above example, the load temperature is converted into a digital value by the counter 1, the D/A converter 3, and the latch circuit 2, but a combination of a sawtooth signal generator and an analog memory can also be used.

As described above, the present invention provides a thermistor that measures the temperature of a load that changes above room temperature, an output generation circuit that generates an output voltage corresponding to the output range of this thermistor, and It consists of an offset circuit that shifts down to include the following output range, and a detection circuit that compares the output of the output generation circuit and the output of the thermistor and detects the state of the thermistor output based on the comparison result. It is characterized by detecting whether the output of the thermistor is normal or not based on the output of the detection circuit during the shift-up operation, and it is possible to prevent accidents due to installation or defective thermistors.

[Brief explanation of drawings]

Figure 1 is a diagram explaining the operation of the thermistor, Figure 2 is a diagram explaining the operation of the thermistor.
The figure is a circuit diagram showing one embodiment of the present invention, and FIG. 3 is a diagram explaining the operation of FIG. 2. In the figure, 1 is a counter, 2 is a latch circuit, and 3
4 is a D/A converter, 4 is an offset circuit, 5 is a thermistor circuit, and 6 is a differential circuit.

Claims (1)

[Claims] 1. A thermistor that measures the temperature of the load that changes above room temperature, an output generation circuit that generates an output voltage corresponding to the output range of this thermistor, and an output range of this output generation circuit that changes the output range of the thermistor below room temperature. The shift-down operation by the offset circuit consists of an offset circuit that shifts down to include the above-mentioned output, and a detection circuit that compares the output of the output generation circuit with the output of the thermistor and detects the state of the output of the thermistor based on the comparison result. A combustion control device characterized in that an abnormality in the output of a thermistor is detected based on the output of a time detection circuit.