JPH0580990B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a resistance value detection circuit that is improved so as to be able to stably detect resistance values that vary over a wide range.

(Prior technology) For example, many gas sensors whose main component is SnO2 are highly temperature dependent, but when detecting the concentration of a specific gas in the exhaust gas of a water heater, etc., the temperature condition is kept constant. In many cases, this is not possible. In such cases, the resistance value of the sensor may vary over a very large range from several megaohms to several tens of ohms, exhibiting negative characteristics as shown in Figure 3, depending on the temperature and gas concentration (detected amount). change over time. on the other hand,
When using a sensor with such negative characteristics to determine the gas concentration above a certain level, if the circuit containing the sensor is disconnected, it becomes impossible to determine the concentration, and the water heater Dangerous conditions may arise when controlling the combustion of Therefore, conventionally, in order to avoid such a dangerous situation, as shown in FIG. 2, a set voltage Vset, which serves as a reference for the detection level, and an open detection reference voltage Vopen, which is a higher voltage, are set. The voltage dividing circuit 1 is connected to the power supply 3 in parallel with the sensor voltage detection circuit 2, and the set voltage Vset and the sensor voltage Vsense are compared, and when the latter exceeds the former, the set detection output increases or decreases in response to the sensor voltage. A setting detection circuit 4 such as an operational amplifier is provided, and a comparator, an operational amplifier, etc. that compares the open detection reference voltage Vopen and the sensor voltage Vsense and outputs an open detection signal when the latter exceeds the former. A circuit in which an occurrence detection circuit 5 is provided to detect the occurrence of an open circuit in the sensor circuit 2 is in practical use.

(Problem to be Solved by the Invention) However, when using the open circuit detection circuit 5 that simply uses a comparator or an operational amplifier, it is handled when an open circuit occurs and when it does not occur (during concentration detection). Since the voltages are significantly different, there are parts in the circuit that require a very large voltage setting and parts that require a very small voltage setting, creating a very difficult problem in circuit design.

Taking the above-mentioned circumstances into consideration, the present invention provides an inexpensive resistance value detection circuit that can stably detect resistance values that change over a very wide range, both in low resistance value ranges and high resistance value ranges, by simply adding a small number of components. The purpose is to provide the following.

(Example) Hereinafter, the present invention will be specifically described based on illustrated examples.

FIG. 1 is a circuit diagram of a gas concentration detection circuit according to an embodiment of the present invention. In the figure, parts having the same functions as those of the conventional example shown in FIG. 2 are given the same symbols and names. has been done.

What should be noted is that the voltage divider circuit 1
A voltage dividing resistor 1a that sets Vset, a voltage dividing resistor 1d that sets the open detection reference voltage Vopen, and an intermediate voltage divider connected between these to obtain an intermediate voltage Vfb between the set voltage Vset and the open detection reference voltage Vsense. An operational amplifier 6 is composed of resistors 1b and 1c, and outputs an output when the intermediate voltage Vfb and sensor voltage Vsense are compared and the latter exceeds the former, and the output of this operational amplifier 6 is connected to its sensor side input terminal (positive phase input A feedback circuit 7 for negative feedback at the end) and a capacitor 8 for preventing oscillation of the operational amplifier 6 are added. The return circuit 7 includes a current limiting resistor 7a connected to the output terminal of the operational amplifier 6.
, a transistor 7b whose base is connected through the transistor 7b, and a return resistor 7c connected between the collector of the transistor 7b and the positive-phase input terminal of the operational amplifier 6 to limit the return current. The emitter of the transistor 7b is connected to a power supply line with a predetermined voltage Vcc.

The resistance value of the pull-up resistor 2a of the sensor circuit 2 is several MΩ, the resistance value of the sensor 2b is several MΩ when open is detected, and several hundred KΩ or several when normal.
MΩ, tens to hundreds of Ω during concentration detection, and resistance value of the return resistor 7c to several hundreds to several KΩ. When the sensor 2b is normal and has a resistance value of several MΩ, the resistance values of the resistors 1a to 1d and 2a are set so that the relationship Vopen>Vsense>Vfb>Vset is established.

Next, the operation of the gas concentration detection circuit configured as described above will be explained.

When the sensor 2b is disconnected, the sensor voltage Vsense becomes approximately equal to the power supply voltage Vcc, and
The relationship Vsense>Vopen>Vfb>Vset holds true.
Therefore, the open detection signal is output from the operational amplifier 5 that constitutes the open occurrence detection circuit, and the setting detection signal is not output from the operational amplifier 4 that constitutes the setting detection circuit.

When sensor 2b is normal, its resistance value is several MΩ.
At this time, the relationship of Vopen>Vsense>Vfb>Vset is established, and the operational amplifier 5 does not output an open detection signal. At this time, the output of the operational amplifier 6 turns off the transistor 7b. The resistance value of the sensor 2b is Vopen>Vfb≧Vsense>
When the voltage drops to the level of Vset, the transistor 7b is off, but its base and collector become conductive, and negative feedback begins to be applied to the operational amplifier 6. This state is maintained until transistor 7b is turned on (saturated). Further, when the sensor resistance decreases and reaches several kilohms, even if the transistor 7b is saturated, the amount of feedback is limited by the feedback resistor 7c, and the relationship Vopen>Vfb>Vsense>Vset is established. The sensor resistance further decreases from this state to the detection level, and Vopen>Vfb>Vset>
When the Vsense relationship is established, the operational amplifier 4 outputs a setting detection signal, indicating that the sensor 2b has detected a predetermined concentration.

(Effects of the Invention) As described above, the present invention uses the intermediate voltage between the open detection reference voltage and the set voltage that is the reference for the set level as the reference voltage, and when the voltage across the resistor to be detected is higher than this voltage, the resistor side By providing an operational amplifier to which negative feedback is applied to the input terminal, when the resistance value of the detected resistor is in the open detection range, the voltage division ratio of the pull-up resistor connected to the detected resistor and the detected resistor is When the resistance value of is low and the voltage across it is below the set voltage, the potential to be detected is determined by the voltage division ratio between the feedback resistance of the operational amplifier and the resistance to be detected, so detection can be performed under normal usage conditions with low resistance values. Even for a resistor to be detected with a very large slope of the quantity-resistance to be detected characteristic, high and low resistance values can be detected stably without handling minute voltage levels. Further, an adjustment means for handling minute voltages is not required, and the number of added parts is small, so it can be implemented at low cost.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a gas concentration detection circuit showing an embodiment of the present invention, FIG. 2 is a circuit diagram of a gas concentration detection circuit showing an example of a conventional resistance value detection circuit, and FIG. 3 is a circuit diagram of a gas concentration detection circuit according to the present invention. FIG. 3 is a detected amount-detected resistance characteristic diagram showing an example of resistance value characteristics of a resistor. 1...Voltage dividing circuit, 2...Sensor voltage detection circuit,
2a...Pull-up resistor, 2b...Sensor, 4...
...Setting detection circuit, 5...Open occurrence detection circuit,
6... operational amplifier, 7... return circuit.

Claims (1)

[Claims] 1. A resistor whose resistance value changes significantly due to external stimulation, and a setting detection circuit that outputs a setting detection signal when the resistance value of the resistor is at a set level;
The resistance value detection circuit includes an open detection circuit that detects a break in the resistor and outputs an open detection signal, and a set voltage that serves as a reference for the setting level of the resistance value, and an open detection circuit that serves as a reference for the disconnection detection. An operational amplifier is provided to compare the resistance value detection voltage of the resistor with a voltage intermediate between the reference voltage and the reference voltage as the reference voltage, and a nonlinear element is provided in a feedback circuit from the output terminal of the operational amplifier to the resistance value detection voltage side. resistance value detection circuit.