JPH0213966Y2 - - Google Patents

Description

[Detailed Description of the Invention] This invention relates to a sensor circuit in which the resistance value of a series resistor changes in response to a change in the resistance value of a semiconductor gas sensor.

FIG. 1 shows the basic circuit configuration of a sensor circuit that detects gas using changes in resistance.

In Figure 1, S is the resistance value R due to gas adsorption.
Semiconductor gas sensor that changes, R _S is a series resistor,
E is the applied voltage and V is the output voltage.

Next, the operation will be explained.

The resistance value of the series resistor R _S is constant, but
Since the semiconductor gas sensor S changes its resistance value R depending on the state of the detection target, a change in the detection target can be detected from a change in the output voltage V. However, depending on the purpose of use, it may be desirable to change the value of the series resistor R _S in accordance with the change in the resistance value R of the semiconductor gas sensor S. The reason for this will be explained below.

Generally, the semiconductor gas sensor S is usually used at a high temperature of 300 to 400° C., and the temperature at which it is used largely controls the performance of the gas sensor.

By the way, since the semiconductor gas sensor S spends most of its time in a gas waiting state, that is, in a state where no target gas is present, the temperature in this gas waiting state affects the long-term stability of the semiconductor gas sensor S, such as its sensitivity. decide. Especially if the temperature is too high,
causing a decrease in the gas sensor resistance value R over time,
Since this makes it difficult to guarantee long-term performance, it is important to ensure that the gas sensor temperature does not rise above the design value while waiting for gas. On the other hand, the sensitivity (resistance change rate) of the semiconductor gas sensor S to gas largely depends on temperature, and usually needs to be sufficiently higher than the temperature in the gas waiting state.

Due to the above two demands, a two-system electrical heating method as shown in FIG. 2 is used as a general heating method for semiconductor gas sensors.

In Figure 2, E _S is the power supply voltage, E _H is the heater voltage, H is the heater, S is the semiconductor gas sensor, and R _S
is a series resistor, and V is the output voltage.

In this configuration, (a) the heater H is set to the heater voltage
Two systems of heating are performed: (b) heating by E _H , and (b) heating by Joule heating of the current flowing through the semiconductor gas sensor S by the power supply voltage E _S. The heating in (a) is constant regardless of the resistance of the semiconductor gas sensor S, but the heating in (b) is because when there is a gas to be detected in the atmosphere, the resistance value of the semiconductor gas sensor S decreases and the Joule calorific value increases. To increase.

According to the configuration shown in FIG. 2, the semiconductor gas sensor S is put into practical use as a gas alarm because the temperature does not exceed a certain level in the gas waiting state and reaches a relatively high temperature in the gas atmosphere.

However, with the spread of alarms, the number of environments in which they are used is increasing, such as in commercial kitchens, where gases other than those intended for detection, such as combustion exhaust gas and high-temperature water vapor, are constantly present, albeit in small amounts. In such an environment, the resistance value of the semiconductor gas sensor S decreases due to a small amount of atmospheric gas while waiting for gas.
As a result, the sensor temperature increases beyond the limited range, and the sensitivity of the semiconductor gas sensor S changes (sensitization) over time.

One effective way to avoid such increased sensitivity is to suppress Joule heat generation by increasing the value of the series resistor R _S , but even if the series resistor R _S is fixed, For example, even if the concentration of the gas to be detected in the atmosphere reaches a sufficient concentration to be detected, the resistance value of the series resistor R _S is still large.
Due to the small Joule heat generation, the temperature is kept low, making it difficult to obtain the necessary gas sensitivity. Therefore, in order to maintain long-term stability in an atmosphere containing trace gases such as exhaust gas, it is important to appropriately increase the value of the series resistor R _S in the gas waiting state and to appropriately decrease it in the gas atmosphere to be detected. This is a necessary condition to simultaneously satisfy both the requirements of high sensitivity and high sensitivity.

This idea was developed in consideration of the above points, and by using a Zener diode, when the sensor resistance value exceeds a certain value, the resistance value of the series resistor is a constant value, and the resistance value of the series resistor is a constant value. The purpose of the following is to provide a sensor circuit in which the sensor resistance value decreases approximately linearly. This idea will be explained below.

Figure 3 shows an example of this invention.
R ₁ , R ₂ , R ₃ are fixed resistors, D _Z is a Zener diode, and its Zener voltage is V _Z. E is an applied voltage, which equivalently constitutes the series resistor R _S shown in FIGS. 1 and 2. For convenience, the fixed resistor _R1 is referred to as a series resistor. Other details are the same as in Figure 1. The fixed resistor R ₂ is connected in series with the Zener diode D _Z , and the series connection body of this Zener diode D _Z and the fixed resistor R ₂ is connected in parallel with the series resistor R ₁ .
R ₃ is a parallel connection body of a series resistor R ₁ , a fixed resistor R ₂ and a series connection body of a Zener diode D _Z ,
connected in series. Furthermore, as a matter of course,
The resistance values of each resistor are R ₁ ≠0, R ₂ ≠∞, and R ₃ ≠∞.

Next, the operation will be explained.

In the circuit shown in Figure 3, the effective series resistance R _S
can be expressed by the equivalent circuit shown in FIG. 4, where R _S =V/I (1) where I is the current flowing through the semiconductor gas sensor S. According to Figure 3, if I and V are solved and substituted into equation (1), R _S =aR+b...(2) However, a≧0, b≧0 Here, a, b are the following equations. expressed.

R _C = (E/V _Z -1) R ₁ - R ₃ , and when R < R _C , a=γ・V _Z /E−γ・V _Z (3.1) b=(R′+R ₃ )E /E−γ・V _Z (3.2) Here, when R ₂ ≠ 0, R′=1/1/R ₁ +1/R ₂ , γ=R′/R ₂ When R ₂ =0, R′=0 , γ=1 When R>R _C , a=0 (4.1) b=R ₁ + R ₃ (4.2) From equation (2), the effective series resistance R _S is the value when the sensor resistance value R is larger than R _C takes a constant value when ,
It can be seen that when R is smaller than R _C , it decreases linearly as R decreases. The values of a and b can be arbitrarily determined by appropriately selecting the resistance values of the fixed resistors R ₂ and R ₃ and the Zener voltage V _Z of the Zener diode D _Z. In this case, it goes without saying that the value of the series resistor _R1 may also be changed.

FIGS. 5 and 6 are diagrams showing other embodiments of this invention, respectively.

In FIG. 5, the applied voltage E is set to alternating current, and a fixed resistor R ₂ is connected in series to a series body in which a pair of Zener diodes D _Z1 and D _Z2 are connected in series.
This pair of Zener diodes D _Z1 , D _Z2 and the fixed resistor R ₂ connected in series are connected in parallel to the series resistor R ₁ , and the pair of Zener diodes D _Z1 , D _Z2 and the fixed resistor are connected in parallel to the series resistor R 1 . Fixed resistor in series connection body of R ₂ and parallel connection body of series resistor R ₁
R ₂ are connected in series. In this example,
Although this embodiment differs from the embodiment shown in FIG. 3 in that the applied voltage is alternating current, the effects are almost the same.

The embodiment shown in Fig. 6 is a Zener diode D _Z
This embodiment differs from the embodiment shown in FIG. 5 in that there is only one, but the rest is the same as that of FIG. That is, while the embodiment shown in FIG. 5 uses a full wave of the AC applied voltage E, the embodiment shown in FIG. 6 uses a half wave of the AC applied voltage E.

In addition, in the semiconductor gas sensor circuit of the above embodiment, to explain the terminals for taking out the output, the connection point between the semiconductor gas sensor S and the series resistor R ₁ is A, the series resistor R ₁ is fixed, the fixed resistor R ₂ is fixed. The connection point of resistor R ₃ is B, the connection point of fixed resistor R ₃ and applied voltage E is C, the connection point of Zener diode D _Z and fixed resistor R ₂ or the connection point of Zener diode D _Z2 and fixed resistor R ₂ . The connection point is D, and the connection points A and C are the basic connection points, but any two other points, namely A and B, B and C, and B
Any combination of and D can be used as an output terminal.

As explained in detail above, this invention consists of a semiconductor gas sensor whose resistance value changes depending on the state of the detection target, a series low antibody connected in series with this semiconductor gas sensor, and a series resistor connected in parallel with this series resistor. a series connection body of the Zener diode and the first fixed resistor; a second fixed resistor connected in series to the parallel connection body of the series resistance body and the series connection body; and one end of the semiconductor gas sensor. a power source that applies a voltage between one end of the second fixed resistor;
Since we adopted a configuration consisting of a series resistor and an output terminal connected to any two points of each end of the first and second fixed resistors, the effective series resistance is sufficient when the semiconductor gas sensor is in the standby state. to a large value,
In an operating state where the resistance value of the semiconductor gas sensor is smaller than a certain value, the effective series resistance can be reduced linearly. Therefore, it has the advantage of being extremely effective when used in a sensor circuit using a semiconductor gas sensor.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the basic circuit configuration of a sensor circuit using a conventional semiconductor gas sensor, Figure 2 is a circuit diagram showing a general heating configuration of a semiconductor gas sensor, and Figure 3 is an example of an embodiment of this invention. Schematic diagram shown,
FIG. 4 is a diagram showing the equivalent circuit of FIG. 3, FIG.
FIG. 6 is a circuit diagram showing another embodiment of this invention. In the figure, S is a semiconductor gas sensor, R ₁ is a series resistor, R ₂ and R ₃ are fixed resistors, R is the resistance value of the semiconductor gas sensor, D _Z , D _Z1 , and D _Z2 are Zener diode,
I is the current flowing through the semiconductor gas sensor, E is the applied voltage, and V is the output voltage.

Claims (1)

[Scope of utility model registration request] A semiconductor gas sensor whose resistance value changes depending on the state of the detection target, a series low antibody connected in series with the semiconductor gas sensor, a Zener diode and a first fixed resistor connected in parallel to the series resistor. a series connected body, and a second series connected body connected in series to the parallel connected body of the series resistor and this series connected body.
a fixed resistor; a power source for applying a voltage between one end of the semiconductor gas sensor and one end of the second fixed resistor; A sensor circuit comprising: an output terminal connected to any two points among the output terminals.