JPH06105234B2 - Gas detector - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas detection device in which the temperature of a metal oxide semiconductor gas sensor is periodically changed between a high temperature region and a low temperature region. This invention is particularly
The present invention relates to driving a heater of a gas sensor with duty ratio control to reduce power consumption and simplify a power supply circuit.

[0002]

2. Description of the Related Art JP-B-53-43320 discloses a gas sensor that utilizes a change in resistance of a metal oxide semiconductor to periodically change the temperature between a high temperature region and a low temperature region, and output a gas from the output of the gas sensor in the low temperature region. Is disclosed. The feature of this technique is that a specific gas can be detected very selectively, and the gas to be detected is, for example, CO or N 2.
There are H3, H2S, EtOH, NOx, etc.

The inventors have tried to put this technology to practical use,
The problem was that there was a large power loss when dropping the power supply voltage to the heater voltage of the gas sensor. In the gas sensor developed by the inventors, the power consumption of the sensor is, for example, 600 mW on the high temperature side and 60 mW on the low temperature side. The heater resistance is about 4Ω and the heater voltage is 1.6 on the high temperature side.
V, 0.5V on the low temperature side. If the power supply output is set to 5V in consideration of the operation of the auxiliary circuit, the power of 3.4V for the high temperature side is 1360mW, and the power of 4.5V for the low temperature side is 5V.
60 mW is lost without being used. The utilization efficiency of electric power is only 32% on the high temperature side and 10% on the low temperature side. If the high temperature side is 60 seconds and the low temperature side is 90 seconds, the average utilization efficiency of electric power is 24% and the average electric power is 1160 mW. Of these, 276 mW is only used in the gas sensor. This is from a 5V power supply to a 1.6V heater voltage (for high temperature range)
This is for taking out the heater voltage of 0.5 V and the heater voltage (for low temperature range).

Next, in order to take out the heater voltage of 1.6 V and the heater voltage of 0.5 V, two power supply circuits (actually, output variable power supplies of 1.6 V and 0.5 V) are required. Further, since the voltage drop (voltage drop) in the power supply circuit is large, the heat generation in the power supply circuit is large. In this example, the rated heat generation capacity is 2W
An att class power supply circuit is required, and a constant voltage I
If realized by C, an expensive IC with a large chip area and considering heat dissipation from the package in order to prevent thermal destruction
Is required. In this way, the power supply circuit requires two systems,
Since the power loss is large, a power supply circuit with a large heat generation capacity is required, which increases the cost of the power supply circuit.

Here, the problem has been described by taking a gas sensor of a specific specification of 1.6 V / 0.5 V as an example, but this is the same for gas sensors of other specifications. If the gas sensor is heated cyclically to a high temperature region and a low temperature region, two power sources for the high temperature region and the low temperature region are required, and the power consumption in the process of reducing the power supply voltage by the power supply circuit is large. .

JP-A-57-184961 discloses
It shows that the heater of the gas sensor is duty ratio controlled. However, this technique does not relate to periodically changing the temperature of the gas sensor between a high temperature region and a low temperature region, and the temperature of the gas sensor is constant. Furthermore, the need for two power sources, one for the high temperature region and one for the low temperature region, is a peculiar problem associated with periodically changing the temperature of the gas sensor between the high temperature region and the low temperature region. Further, a low heater voltage is required in a low temperature range, and the power supply voltage must be lowered to waste most of the electric power wastefully, which is a peculiar problem associated with the temperature change of the gas sensor.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to reduce power consumption and power supply circuit in a gas detection device in which the temperature of a gas sensor is periodically changed between a high temperature region and a low temperature region. Collected in a single power supply circuit using switches,
Moreover, it is to reduce heat generation in the power supply circuit so that a power supply circuit having a small heat generation capacity can be used. In addition to the above, an object of the invention of claim 2 is to monitor the fluctuation of the power supply voltage and prevent the influence of the power supply voltage fluctuation on the gas sensor.

[0008]

According to the present invention, a gas sensor having a power source, a metal oxide semiconductor whose resistance value changes with gas, and a heater for heating the semiconductor, and a heater for the gas sensor are provided at a predetermined duty ratio. A switch for connecting to the power supply, a gas detection circuit for detecting a specific gas from the resistance value of the metal oxide semiconductor in the gas sensor, and a timer are provided, and the duty ratio of the switch is set by the timer. The temperature of the gas sensor is cyclically changed to a high temperature region and a low temperature region, and a sampling signal is generated from the timer to operate the gas detection circuit at a predetermined timing to detect the gas. The gas detector is adapted to do so.

Here, preferably, the power supply voltage is monitored, and the duty ratio of the switch is changed according to the power supply voltage fluctuation to prevent the influence of the power supply voltage fluctuation.

Still more preferably, the gas sensor is heated by the heater in both the high temperature region and the low temperature region, and the duty ratio of the heater is controlled by the switch in the low temperature region and the high temperature region. With this configuration, the two heater powers of the low temperature region and the high temperature region can be taken out from the switch, and the power of one system can heat to the two temperatures of the high temperature region and the low temperature region. Further, it is possible to eliminate waste of power consumption when the power supply voltage is lowered to the heater voltage in the low temperature range.

[0011]

According to the first aspect of the invention, the temperature of the gas sensor is periodically changed between a high temperature region and a low temperature region to obtain gas selectivity, and the heater of the gas sensor is driven by the duty ratio control by the switch. By doing so, it is not necessary to reduce the power supply voltage to the required heater voltage, and the power consumption can be reduced. Further, when the power consumption of the power supply circuit is reduced, the power supply circuit can be realized by a transistor or a small IC having a small heating capacity. As a result, the power supply circuit can be simplified.

According to the second aspect of the invention, the fluctuation of the power supply voltage is monitored, and the duty ratio of the switch is controlled according to the fluctuation of the power supply voltage. As a result, the influence of power supply voltage fluctuation can be prevented.

[0013]

【Example】

[0014]

First Embodiment FIGS. 1 to 4 show a first embodiment. Figure 1
2 is, for example, a battery power source whose output Vcc is 5V, and N
An i-Cd battery, a dry battery, a lead storage battery or the like is used. 4 is a timer, which is composed of an oscillation circuit 6 and a counter 8
The on-duty ratio of the switching transistors Tr1 and Tr2 is periodically changed via R and the pulse width variable monostable multivibrator MM. Further, the sampling signal S is generated in the low temperature range, and the D-type flip-flop circuit FF
1 clock pulse. The multi-vibrator MM has a variable pulse width in order to prevent a change in the heating temperature of the gas sensor 10 due to a change in the output of the battery power source 2 or a change in the temperature. That is, the output Vcc of the power source 2 is divided by the NTC thermistor Th, the resistor R1 and the like, the difference from the reference potential determined by the Zener diode TZ1 is amplified by the differential amplifier OP1, and the output pulse width of the multivibrator MM is changed. The pulse width to the transistors Tr1 and Tr2 is changed according to changes in the output Vcc of the power source 2 and the temperature.

Reference numeral 10 is a gas sensor, for example, 1 wt% of SnO2
% In each of the metal oxide semiconductors 12 to which Pd is added.
The heater / electrodes 14 and 16 are embedded. The operating conditions of the gas sensor 10 are, for example, in the case of CO detection,
The maximum temperature in the high temperature range is about 300 ℃ and the heater power is about 640.
mW, the minimum temperature in the low temperature range is about 80 ° C, and the heater power is about 6
It is 1 mW. In order to heat the gas sensor 10 uniformly,
The two heater / electrodes 14 and 16 are both used as heaters, and the heaters 14 and 16 are connected to the power supply 2 through the two switching transistors Tr1 and Tr2 to control the duty ratio. The multi-vibrator MM turns on and off the transistors Tr1 and Tr2 to set the duty ratio of ON to 16% by the pulse signal H in the high temperature region and 1.6% in the low temperature region by the pulse signal L. Heater power control is performed by turning on and off the transistors Tr1 and Tr2, so that power loss due to the drop of the power supply output Vcc does not occur.
A variable output power supply is not required, and a single-system power supply circuit including transistors Tr1 and Tr2 may be used. The switch may be other than a transistor.

The resistance value of the gas sensor 10 is, for example, about 100 KΩ in clean air and about 1 KΩ in CO of about 200 ppm. The change in the resistance value is detected by the gas detection circuit including the load resistance RL, the comparison circuit C2, and the D-type flip-flop circuit FF1, the output of the comparison circuit C2 is read by the sampling signal S into the flip-flop circuit FF1, and the transistor Tr3 is read. Causes the buzzer 18 to ring when CO is generated.

FIG. 2 shows the timer 4. The timer 4 includes, for example, an oscillation circuit 6 of 10 MHz and 1/1000 each.
It is composed of frequency dividing circuits 20, 22, 24 having a frequency dividing ratio of about
For example, it operates in one cycle of 180 seconds. Divider circuit 20, 2
Outputs 2 and 24 are processed by AND circuits AND1 and AND2, and a pulse signal H having a short pulse width of 160 Hz for the first 60 seconds of the cycle, and 1 for the remaining 120 seconds.
A pulse signal L having a short pulse width of 6 Hz is obtained. The AND circuit AND3 generates the sampling signal S at intervals of, for example, 30 seconds in 120 seconds in the latter half of the cycle. When the transistors Tr1 and Tr2 are turned on, the load resistance RL is grounded. Therefore, the sampling signal S is made asynchronous with the pulse signal L, and the voltage to the load resistance RL is detected when the transistors Tr1 and Tr2 are off. The pulse signals H and L are duty ratio control signals.

FIG. 3 shows the multivibrator MM.
The multivibrator MM includes NOR circuits NOR1 and NOR2.
And a comparison circuit C1, amplifiers OP2 and OP3, a transistor Tr4, a capacitor 26, and the like. When the pulse signals H and L are input from the input A through the OR circuit OR, the multivibrator MM is turned on and an output pulse is generated from the output C. At the same time when the output pulse is generated, the signal from the NOR circuit NOR1 causes the transistor Tr.
4 is turned on, the capacitor 26 is discharged, and the input to the comparison circuit C1 is lowered. The reference potential of the comparator circuit C1 is the signal of the amplifier OP1 via the input B plus the power source output Vcc divided by a resistor. When the power source voltage increases or the temperature increases, the reference potential decreases. Thus, the pulse width from output C is reduced. As a result, the on-time of the transistors Tr1 and Tr2 is determined so as to cancel the influence of the power supply voltage fluctuation and the temperature fluctuation.

FIG. 4 shows the operation of the embodiment. Timer 4
Generates the duty ratio control signals H and L as in 1). Further, as in 2), the sampling signal S is generated from the timer 4 in the low temperature range. The duty ratio control signals H and L are sent to the multivibrator MM, and the output pulse from the multivibrator MM causes
As in 3), the switching transistors Tr1 and Tr2 are turned on, and the temperature of the gas sensor 10 is changed by the change of the duty ratio. Note that changes in the power supply output Vcc and changes in temperature are compensated by changing the width of the output pulse of the multivibrator MM. The output of the gas sensor 10 (output to the load resistance RL) changes as shown in 4) as the temperature changes. Gas sensor 1
The output of 0 is detected by the comparison circuit C2, and the sampling signal S
Is read into the flip-flop circuit FF1, and the buzzer 18 is operated when gas is generated.

In this embodiment, electric power was applied to the heaters 14 and 16 of the gas sensor 10 in both the high temperature range and the low temperature range. However, in the low temperature range, the duty ratio of the transistors Tr1 and Tr2 is set to 0, and the heating by the heaters 14 and 16 is performed. You can discontinue.

[0021]

Example 2 In the example of FIG. 5, the metal oxide semiconductor 32 is used.
Gas sensor 30 in which the heater 34 is indirectly heated
To use. In this embodiment, the metal oxide semiconductor 32 and the heater 34 are separated and one transistor Tr5 is connected to the heater 34. Further, in this gas sensor 30, the heater 3
Since the resistance value of 4 can be made large, the ON duty ratio of the transistor Tr5 can be made large, and in an extreme case, the duty ratio on the high temperature side can be made 100%.

[0022]

According to the first aspect of the invention, it is possible to reduce the power consumption of the gas detection device in which the temperature of the gas sensor is periodically changed between a high temperature region and a low temperature region. Also, simplify the power supply circuit to a single power supply using switches,
Moreover, heat generation in the power supply circuit can be reduced, and the cost of the power supply circuit can be reduced. In the invention of claim 2, in addition to the above, it is possible to monitor the fluctuation of the power supply voltage and prevent the influence of the power supply voltage fluctuation on the gas sensor.

[Brief description of drawings]

FIG. 1 is a block diagram of a gas detection device according to an embodiment.

FIG. 2 is a circuit diagram of a timer used in the gas detection device of the embodiment.

FIG. 3 is a circuit diagram of a pulse width variable monostable multivibrator used in the gas detector of the embodiment.

FIG. 4 is a waveform diagram showing the operation of the gas detection device of the embodiment.

FIG. 5 is a block diagram of a gas detection device according to another embodiment.

[Explanation of symbols]

2 Battery power source 4 Timer 6 Oscillation circuit 8 Counter 10,30 Gas sensor 12,32 Metal oxide semiconductor 14,16,34 Heater 18 Buzzer 20,22,24 Dividing circuit AND1-AND3 AND circuit MM Pulse width variable monostable multivibrator Th thermistor Tr1 to Tr5 transistors

Claims (2)

[Claims] 1. A gas sensor having a power source, a metal oxide semiconductor whose resistance value changes with gas, and a heater for heating the semiconductor, and a heater of the gas sensor is connected to the power source at a predetermined duty ratio. Switch, a gas detection circuit for detecting gas from the resistance value of the metal oxide semiconductor in the gas sensor, and a timer, and the duty ratio of the switch is periodically changed by the timer. , The temperature of the gas sensor is periodically changed between a high temperature region and a low temperature region, a sampling signal is generated from the timer, and the gas detection circuit is operated at a predetermined timing to detect the gas. Detection device. 2. The gas detection device according to claim 1, further comprising means for detecting a voltage fluctuation of the power supply and changing a duty ratio of the switch.