JPH0410984B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a gas sensor that detects contaminated gas, and in particular to a power supply unit that supplies DC voltage to a gas detection element and a detection unit that detects changes in its characteristics. Regarding improvements.

[Technical background and problems of the invention]

Generally, many of what are called gas sensors detect contaminated gas using a gas detection element, and send an alarm signal to an external device, also called a main controller, when the concentration of the contaminated gas exceeds a certain value.

FIG. 2 is a circuit diagram showing the overall configuration of this type of conventional gas sensor, in which a power supply unit 10 receives the voltage of an AC power supply 1 and generates two types of DC voltage, high and low.
The lower DC voltage of the power supply unit 10 is applied to the heater coil, and the partial voltage of the higher DC voltage is applied to the resistor. a detection unit 30 that detects that the voltage across the resistor of this gas detection element has fallen below a set value, and a determination unit that sends an alarm signal to the outside based on the output signal of this detection unit. It is composed of an output circuit 40.

Here, the power supply unit 10 includes a constant voltage circuit 11 that stabilizes a higher DC voltage and a constant voltage circuit 12 that stabilizes a lower DC voltage, and the detection unit 30 includes a well-known comparator 31. Further, the discrimination output circuit 40 has a light emitting diode 41 that lights up when polluted gas is detected, a connector 42 also called an outlet that can supply the same voltage as the AC power supply 1 when polluted gas is detected, and has substantially the same function as the above-mentioned comparator. The photocoupler 44 is provided in the output circuit of the comparator.

In the above configuration, the power supply unit 10 has a DC circuit that rectifies and smoothes an AC voltage to generate two types of DC voltages, high and low.
The lower DC voltage is stabilized by the constant voltage circuit 12 and applied to the resistor of the gas detection element 20, the subsequent detection section 30, and the discrimination output circuit 40, while the lower DC voltage is stabilized by the constant voltage circuit 12 and applied to the heater coil of the gas detection element 20.

The gas detection element 20 has a characteristic that the resistance value of the resistor decreases when a contaminated gas is detected, and when a substantially constant voltage is applied to the resistor through the resistor, the voltage across both ends and the reference voltage are A comparison is made by a comparator 31, and when a contaminated gas is detected, the output of the comparator 31 is inverted to "H" level.

At this time, the discrimination output circuit 40 lights up the light emitting diode 41 and supplies an AC voltage to a device (not shown) connected to the connector 42. Furthermore, the comparator 43 also determines the presence or absence of contaminated gas, and detects the presence of the contaminated gas. When it is determined that there is an alarm, an alarm signal is sent to external equipment via the photocoupler 44.

FIG. 3 is a circuit diagram showing the detailed configuration of the power supply section 10 including the constant voltage circuits 11 and 12. A diode 14 and a capacitor 15 are connected in series between the output terminals of the power transformer 13 to form a high-voltage rectifier circuit. Further, a diode 16 and a capacitor 17 are connected in series between the intermediate tap of the power transformer 13 and one output terminal to form a low voltage rectifier circuit.

Of these, a resistor 111 as a constant voltage circuit is connected to both ends of the capacitor 15 forming a high voltage rectifier circuit.
and a Zener diode 112 are connected in series, and the voltage V _C across the Zener diode 112 is supplied to the resistor of the gas detection element, the subsequent detection section, and the like. Note that a capacitor 113 is connected to both ends of the Zener diode 112 in order to suppress sudden changes in the voltage V _C due to disturbances or the like.

On the other hand, capacitor 1 forming a low voltage rectifier circuit
The collector of the power transistor 121 is connected to the positive end of the power transistor 121.
A heater coil 21 is connected between the emitter of the capacitor 17 and the negative terminal of the capacitor 17. Further, in order to supply a constant base current to the power transistor 121, a series circuit of a resistor 122, a semi-fixed resistor 123 and a resistor 124 is connected across the Zener diode 112, and a series circuit of a semi-fixed resistor 123 and a resistor 1
24 mutual junctions are connected to the base of power transistor 121. Note that between the base of the power transistor 121 and the negative terminal of the capacitor 17, and between the emitter of the power transistor 121 and the negative terminal of the capacitor 17, in order to control sudden changes in the base current due to noise, etc. capacitors 125 and 126 are connected. Then, a power transistor 121, a resistor 1
A constant voltage circuit is formed by 22, a semi-fixed resistor 123, and the like.

In FIG. 3, since it is necessary to flow a constant current through the heater coil 21 of the gas detection element 20, the current is adjusted by a power transistor 121. In this case, the DC voltage generated across the capacitor 15 is transferred to the resistor 111 and the Zener diode 1.
12 series circuits, and a stabilized DC voltage V _C is generated across the Zener diode 112 . This DC voltage V _C is divided by a resistor 122, a semi-fixed resistor 123, and a resistor 124, and the voltage generated across the resistor 124 is applied between the base and emitter of the power transistor 121.
Therefore, even if the voltage applied to the heater coil 21 via the power transistor 121, that is, the voltage across the capacitor 17 fluctuates, the base current of the power transistor 121 is kept constant, so the current in the heater coil 21 also changes. is kept constant.

By the way, the power transistor 121 has considerable manufacturing variations, and the semi-fixed resistor 123 is provided to compensate for this variation.

However, the semi-fixed resistor 123 has a relatively large shape and requires space for adjustment, which makes the structure of the device complicated accordingly.

On the other hand, when the base current of the power transistor 121 is adjusted by the semi-fixed resistor 123, the DC voltage V _C for obtaining the reference voltage also changes, which causes the calculated value and the actual value to differ when detecting pollutant gas. There were some differences.

〔Effect of the invention〕

The present invention has been made in order to solve the above-mentioned problems, and it is possible to significantly simplify the configuration, and also to suppress DC voltage fluctuations caused by keeping the current of the heater coil constant, thereby making it possible to The object of the present invention is to provide a gas sensor that can significantly improve the accuracy and reliability of detecting contaminated gas.

[Summary of the invention]

In order to achieve this object, the present invention provides a gas detection element having a resistor sensitive to pollutant gas and a heater coil for improving sensitivity, a detection section that detects a change in the resistance value of the resistor, and a gas detection element that detects a change in the resistance value of the resistor. a DC circuit that supplies a DC voltage for operation to the unit and the gas detection element; a first constant voltage circuit that stabilizes the operating voltage of the resistor and the detection unit; and a first constant voltage circuit that stabilizes the operating voltage of the heater coil. a second constant voltage circuit, wherein the second constant voltage circuit includes a first transistor connected in series with the heater coil to the DC circuit; and a first transistor connected in series with the heater coil to the DC circuit; As well as being connected to the circuit,
A second transistor whose junction point with the resistor is connected to the base of the first transistor, and a plurality of resistors connected in series between the output terminals of the first constant voltage circuit; The voltage dividing circuit is characterized in that the mutual junction point is connected to the base of the second transistor.

[Embodiments of the invention]

FIG. 1 is a circuit diagram showing the configuration of an embodiment of the present invention, and the same elements as those of the conventional gas sensor shown in FIG. 3 are given the same reference numerals, and the explanation thereof will be omitted. Then, the semi-fixed resistor 12 shown in FIG.
The biggest difference is that 3 is removed and a transistor 128 is provided in its place. Here, the emitter of the transistor 128 is connected to the positive end of the capacitor 15 via the resistor 127, and
connected to the base of the power transistor 121;
The collector is connected to the negative terminal of capacitor 15, and the base is connected to the mutual junction of resistors 122 and 124, respectively. Note that the capacitors 113, 125, and 126 shown in FIG. 3 are for suppressing voltage fluctuations due to noise, etc., but these are omitted in FIG. 1 to avoid complication of the drawing.

The operation of this embodiment configured as described above will be explained below, focusing in particular on the points that are different from the conventional device.

First, the resistors 122 and 124 form a voltage divider circuit that divides the voltage across the Zener diode 112, and together with the resistor 127 inserted in the emitter circuit of the transistor 128, the resistors 122 and 124 can be adjusted by selecting an appropriate resistance value. A predetermined base current is then supplied to the transistor 128. Therefore, the transistor 128 becomes conductive, and the voltage generated between the collector and emitter of the transistor 128 causes the power transistor 1 to become conductive.
21 is supplied with a base current, and the heater coil 21
A desired current is applied to the

Next, when the voltage across the capacitor 15 increases due to an increase in the AC voltage applied to the power transformer 13, the base current of the transistor 128 also increases.
On the other hand, when the voltage across the capacitor 17 increases, the current flowing through the heater coil 21 through the power transistor 121 also tends to increase. However, as the base current of transistor 128 increases, the resistance between the collector and emitter of transistor 128 decreases, so the base current of power transistor 121 decreases slightly, and as a result, the current flowing through power transistor 121 remains constant. be done.

In this case, the current flowing through the transistor 128,
Furthermore, since the current flowing to the base of the power transistor 121 is extracted from the front stage of the constant voltage circuit that obtains the DC voltage V _C , the voltage V _C hardly changes even if the current of the heater coil 21 is controlled to be constant. become.

In this way, the semi-fixed resistor 21 used in the conventional device is no longer necessary, so the configuration is simplified, and since the DC voltage V _C hardly changes, the difference between the calculated value and the actual value is also extremely small.

〔Effect of the invention〕

As is clear from the above explanation, according to the present invention, a control transistor is provided in order to keep the current of the heater coil constant, and the current of this transistor and the base current of the power transistor are controlled at the stage before the constant voltage circuit. Since it is made to take out more, the semi-fixed resistor can be removed and the configuration can be simplified.
Moreover, by minimizing fluctuations in the DC voltage that creates the reference voltage, etc., detection accuracy and reliability can be significantly improved.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing the configuration of the main parts of an embodiment of the present invention, Fig. 2 is a circuit diagram showing the overall structure of a conventional gas sensor, and Fig. 3 shows the detailed structure of the main parts. FIG. 10...Power supply unit, 11, 12... Constant voltage circuit,
13... Power transformer, 14, 16... Diode, 15, 17... Capacitor, 20... Gas detection element, 21... Heater coil, 30... Detection section, 40... Discrimination output circuit, 111, 122, 1
24, 127...Resistor, 121...Power transistor, 128...Transistor.

Claims (1)

[Claims] 1. A gas detection element having a resistor sensitive to pollutant gas and a heater coil for improving sensitivity, a detection unit that detects a change in the resistance value of the resistor, and an operating unit for this detection unit and the gas detection element. A DC circuit that supplies a DC voltage, a first constant voltage circuit that stabilizes the DC voltage applied to the resistor and the detection unit, and a second constant voltage circuit that stabilizes the DC voltage applied to the heater coil. In the gas sensor, the second constant voltage circuit includes a first transistor connected to the DC circuit in series with the heater coil, and a first transistor connected to the DC circuit via a resistor. A second transistor whose junction point is connected to the base of the first transistor and a plurality of resistors are connected in series between the output terminals of the first constant voltage circuit, and the mutual junction point of these resistors is connected to the base of the first transistor. and a voltage dividing circuit connected to the base of the second transistor.