JPH0519945B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a CO gas detection device using a hot wire type semiconductor sensor as a gas sensor to detect CO gas.

[Conventional technology]

Conventionally, various gas sensors for gas detection have been proposed. Among these, the catalytic combustion type and semiconductor type are commonly used. The former is a platinum coil with a sintered catalyst, and when air containing flammable gas comes into contact with the catalytically active surface (maintained at approximately 300°C), it becomes flammable even at concentrations below the lower explosive limit. The reactive gas and oxygen react and generate heat of reaction, which raises the temperature of the platinum coil and increases its resistance value. This change in resistance value is detected by a bridge circuit or the like.

Although this catalytic combustion type gas sensing element has linear sensitivity characteristics, it is inferior to semiconductor type gas sensors in terms of long-term stability.

On the other hand, a semiconductor gas sensor is made by sintering a metal oxide semiconductor, such as SnO ₂ or ZnO, between a pair of electrodes, one of which is formed into a coil shape and is usually used as a heater electrode.

Then, the temperature is maintained at 300° C. to 400° C. using an electrode that also serves as a heater, and gas detection is performed by detecting that the conductivity of the semiconductor increases due to gas adsorption and the resistance value between both electrodes decreases.

This semiconductor type gas sensor has a longer lifespan and superior long-term stability than the catalytic combustion type, and is also better against poisoning than the catalytic combustion type, but it consumes a lot of power, so the capacity of the power supply part such as the power transformer is limited. It gets bigger and costs more.

Additionally, semiconductor-type gas sensors are generally capable of capturing specific gases during low-temperature heating;
The drawback is that false alarms occur due to gas accumulation.

In order to remove this, a semiconductor type gas sensor is temporarily heated to a high temperature and gas purge is performed before gas measurement (for example, see Japanese Patent Publication No. 53-43320).

In such a known example, heating is performed alternately in pulses to high and low temperatures, and a rectangular wave generator such as a multivibrator is used to apply pulsed voltage to adjust the peak value. applies large and small pulses to a semiconductor gas sensor.

Therefore, not only does the mechanism for generating pulses become large-sized, but the pulse-like nature causes sudden thermal distortion in the gas sensor, leading to problems such as cracks.

The present invention was made in order to solve the above-mentioned problems, and an object of the present invention is to provide a CO gas detection device that has an extremely simple configuration and can prevent the occurrence of cracks in the gas sensor.

[Means for solving problems]

Using the hot wire type semiconductor sensor according to this invention
The CO gas detection device includes a sine wave applying means for applying a sine wave voltage to the hot wire type semiconductor sensor, and a measuring means for detecting the resistance value of the hot wire type semiconductor sensor when the voltage is near the zero crossing point of the sine wave voltage. It is something.

Moreover, the second invention of this invention further includes:
It is equipped with a direct current applying means for keeping the hot wire type semiconductor sensor at a low temperature.

[Effect]

In this invention, a sine wave voltage is applied to a hot wire type semiconductor sensor, gas purge is performed at a high voltage part near the peak value of this sine wave voltage, and a hot wire type semiconductor sensor is applied to a hot wire type semiconductor sensor at a low voltage part near the zero cross point. The resistance value of the semiconductor sensor is measured, and the gas concentration is detected based on this value.

Further, in the second aspect of the present invention, the hot wire type semiconductor sensor is always maintained at a constant low temperature by the direct current applying means.

〔Example〕

First, a hot wire type semiconductor sensor (hereinafter simply referred to as a sensor) used in the present invention will be explained.

FIG. 2 shows the configuration of the sensor used in the present invention, where 1 serves as a heater and an electrode for detecting changes in electrical resistance. For example, a coiled metal wire made of platinum wire with a diameter of 15 μm, and 2 are metal oxide semiconductors such as SnO ₂ and ZnO that are brought into close contact with the metal wire 1, and the sensor 3 is constituted by these. Reference numeral 4 denotes a metal pin that serves both as a lead and as a support, to which both ends of the metal wire 1 are welded and supported.

This sensor 3 has an extremely fast response speed and an extremely short initial stabilization time. It also has features such as low power consumption, high sensitivity, and no catalyst deterioration.

An embodiment of the present invention constructed using the sensor 3 shown in FIG. 2 is shown in FIG.

In FIG. 1, R _b is a resistor and 3A is a compensation element, which together with the sensor 3 constitute a bridge circuit. The compensating element 3A has the same shape as the sensor 3 shown in FIG. 2, but its surface is covered with glass or the like so that it does not react with gas. E is a power source such as a battery, R _l is a variable resistor for signal detection,
These components constitute a resistance value change detection circuit 10. 11 is a transformer, with variable AC power supply 1 on the primary side.
2 are connected, and the secondary side is connected to both ends of the sensor 3, and these transformers 11 and the variable AC power supply 12 constitute an AC voltage applying means. Reference numeral 13 denotes a zero cross point detector, which detects whether the output of the variable AC power supply 12 has reached the vicinity of the zero point. Reference numeral 14 denotes an electronic switch, which is closed for a predetermined period of time after the output from the zero-crossing point detector 13 rises. Reference numeral 15 denotes a voltmeter as a resistance value measuring means, which indicates the output voltage, that is, the gas concentration, when the electronic switch 14 is closed.

Next, the operation will be explained with reference to the waveform diagram in FIG.

When a sinusoidal AC voltage as shown by curve A shown in FIG.
It will look like curve B in figure b.

That is, when the voltage is as high as the peak value E _p in a of FIG. 3, the heat generation also becomes high as in b of FIG. 3, and the gas is purged.

Then, the voltage gradually decreases until it reaches a temperature near the zero-crossing point, for example, between the thresholds _L1 and _L2 of a in Figure 3, and reaches a temperature suitable for gas detection, that is, b in Figure 3.
When the temperature drops to the temperature level line _L3 , the zero cross point detector 13 outputs an output and switches the electronic switch 1.
Close 4. As a result, the detection circuit 1 at that time
Measure the output of 0. This output essentially indicates the internal resistance of the sensor 3, and since this internal resistance depends on the gas concentration, it means that the gas concentration is measured after all.

FIG. 4 shows a second embodiment of the present invention. The difference from the embodiment shown in FIG. 1 is that a DC power source 16 and a resistance value 17 constitute a DC applying means, and a constant current is constantly applied to the sensor 3 to heat the sensor 3 to a temperature slightly lower than the gas concentration measurement temperature. The other points are the same as in Fig. 1.

In the case of this embodiment, the waveforms a and b in FIG. 5, that is, the 0 point for both voltage and temperature has shifted to the 0' point. This is because a current always flows from the DC power supply 16 to the sensor 3, and the sensor 3 is heated to a constant low temperature, so that the thermal shock applied to the sensor 3 is further alleviated.

〔Effect of the invention〕

In the present invention, as described above, since an alternating current voltage is applied to the sensor, a smoothly changing voltage is applied to the sensor, which reduces the amount of thermal shock and prevents the occurrence of cracks.
Moreover, since the resistance value of the sensor is measured after the gas is purged at the peak value of the AC voltage, error-free measurement can be performed.

In addition, the second aspect of the present invention has the advantage that the sensor is heated to a predetermined low temperature by constantly passing current through the sensor using the direct current applying means, which further reduces thermal shock when an alternating current voltage is applied. be.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing an embodiment of the present invention, Fig. 2 is a diagram showing an example of the configuration of a sensor 3 used in the invention, and Figs. 3a and b are for explaining the operation of the embodiment of Fig. 1. 4 is a circuit diagram showing an embodiment of the second invention of the present invention, and FIGS. 5a and 5b are diagrams for explaining the operation of the embodiment of FIG. 4. It is a figure which shows a voltage waveform and a heat generation waveform. In the figure, 1 is a metal line, 2 is a metal oxide semiconductor, and 3
is a sensor, 3A is a compensation element, 4 is a metal pin, 10
is a detection circuit, 11 is a transformer, 12 is a variable AC power supply, 13 is a zero cross point detector, 14 is an electronic switch, 15 is a voltmeter, 16 is a DC power supply, 17,
R _b is a resistor, R _l is a variable resistor, and E is a power supply.

Claims (1)

[Scope of Claims] 1. A hot wire type semiconductor sensor whose electrical resistance changes by adsorbing gas; a sine wave applying means for applying a sine wave voltage of a required peak value to the hot wire type semiconductor sensor; 1. A CO gas detection device using a hot wire type semiconductor sensor, comprising a measuring means for detecting a resistance value of the hot wire type semiconductor sensor when the voltage is near the zero crossing point of a sine wave voltage. 2. A hot wire type semiconductor sensor whose electrical resistance changes by adsorbing gas, a direct current applying means for applying a DC voltage to constantly maintain this hot wire type semiconductor sensor at a predetermined temperature, and this hot wire type semiconductor sensor. and a measuring means for detecting the resistance value of the hot wire semiconductor sensor when the voltage is near the zero-crossing point of the sine wave voltage. CO gas detection device using a hot wire semiconductor sensor.