JPH021667Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a gas alarm that corrects sensitivity that changes in accordance with fluctuations in power supply voltage.

It is well known that the sensitivity of semiconductor gas sensors used in gas leak alarms, gas concentration detectors, etc. changes due to fluctuations in power supply voltage. For example, as shown in Figure 4, the comparison voltage that determines the alarm concentration of a gas alarm is set to a fixed value R, and the power supply voltage is set to
When the voltage is 100V, the alarm point is a gas concentration of 0.3%. Here, if the power supply voltage fluctuates to 110V and 90V, the alarm points will be 0.1% and 0.9% concentration, respectively.
becomes. In other words, as the power supply voltage increases or decreases, the sensitivity of the semiconductor gas sensor also increases or decreases. For this reason, conventional methods include stabilizing the voltage applied to the sensor. However, this method requires a complicated constant voltage circuit and is for high voltage, so it is very disadvantageous in terms of cost.

Another conventional method is a self-bias method in which a comparison voltage source that outputs a signal corresponding to the alarm concentration is varied in accordance with fluctuations in the power supply voltage. This method will be explained with reference to FIG.

In FIG. 1, a semiconductor gas sensor 1 is directly connected to an AC commercial power source via a load resistor 2, and its heater is supplied from a transformer 3. The low voltage secondary winding of the transformer 3 is connected to a smoothing circuit including a diode 4 and a capacitor 5 to supply a DC voltage.
Variable resistor 6 for adjusting alarm sensitivity in parallel with load resistor 2
is connected, and its output is connected to the inverting input of comparator 9 via a smoothing circuit consisting of diode 7 and capacitor 8. A non-inverting input of comparator 9 is connected to a comparison voltage source consisting of resistors 10 and 11. The output of the comparator 9 is connected to a buzzer 12 for gas leak alarm.

In this conventional alarm, the voltage signal applied to the non-inverting input of the comparator 9 is obtained by dividing the voltage obtained by stepping down and rectifying the power supply voltage by the transformer 3 by using the resistors 10 and 11. Therefore, when the power supply voltage fluctuates, it fluctuates at the same rate of fluctuation as the power supply voltage, and the alarm point is automatically adjusted. For example, if the power supply voltage starts from 100V
If it fluctuates by 10% to 110V or 90V, the comparison voltage that sets the alarm concentration will also fluctuate up or down by 10% around voltage R (see Figure 4), so the alarm point range will be the same as when the comparison voltage is fixed. The case can be smaller than 0.1-0.9% (0.15-0.5%).

However, it is known that the characteristic of the power supply voltage (plate voltage) versus sensitivity of the semiconductor gas sensor 1 exhibits not a linear but a secondary change. Therefore,
The self-biasing method in which the comparison voltage source of the comparator 9 is changed in proportion to fluctuations in the power supply voltage cannot sufficiently correct fluctuations in the comparison voltage with respect to fluctuations in the power supply voltage.

The present invention was developed in view of the above circumstances, and is a gas alarm system that can correct the comparison voltage for fluctuations in the power supply voltage without a significant increase in cost, and has an effect similar to that achieved when using a constant voltage circuit. The purpose is to provide

Hereinafter, a preferred embodiment of the present invention illustrated in FIG. 2 will be described in detail. In FIG. 2, elements that are equivalent to those in FIG. 1 are designated by the same reference numerals.

The gas alarm according to the present invention has a comparator 9.
The comparison voltage source is replaced by a Zener diode 13 and a resistor 11.

Here, the fluctuation rate of the voltage applied to the non-inverting input of the comparator 9 will be explained. When the power supply voltage on the secondary side _{is Vcc} , the voltage of the Zener diode 13 is E, and the comparison voltage of the comparator 9 is v, this voltage v is expressed as v= _Vcc -E. Here, if the voltage before variation is v ₁ and the voltage after variation is v ₂ , then v ₁ =V _cc −E v ₂ =aV _cc −E. Here, a is the fluctuation rate of the DC voltage. Therefore, the fluctuation rate α of the comparison voltage can be expressed as α=v ₂ −v ₁ /v ₁ = (a−1)V _cc /V _cc −E, and from this, the fluctuation rate α of the comparison voltage can be expressed as It can be seen that the voltage can be freely changed depending on the value of the voltage E of the diode 13. This relationship is shown in FIG. 3, and can be set to any rate of variation by appropriately selecting the voltage E of the Zener diode 13. This voltage E can be obtained from the following equation.

E=V _cc (1-a-1/α) Here, let us give a numerical example. DC voltage
V _cc is 10V, and the fluctuation rate a of this voltage is 10% (=
1.1), the comparison voltage fluctuation rate (correction factor) α is 20
% (=0.2), E=10×(1-1.1-1/0.2)=5, and the Zener diode 13 needs only to have a voltage of 5V.

Applying this to the conventional self-bias method under the same conditions, if the comparison voltage before fluctuation is v ₁ = 5V, the values of resistors 10 and 11 must be the same (R ₁₀ = R ₁ ), so v ₁ = R ₁₁ /R ₁₀ +R ₁₁ V _cc = 1/2V _cc = 5, and the voltage v ₂ after fluctuation is v ₂ = a1/2V _cc = 5.5. Here, the fluctuation rate a of voltage V _cc is 10% (=
1.1), the comparison voltage fluctuation rate α is α=v ₂ −v ₁ /v ₁ =5.5−5/5=0.1 (=10%).

Therefore, under the example conditions, the comparative voltage fluctuation rate α can be corrected twice as much as 20% in the present invention compared to 10% in the conventional case.

According to the present invention, as shown in FIG. 4, even if the power supply voltage fluctuates, the comparison voltage of the comparator 9 is R.
to Q or S to set the alarm point to 0.3, for example.
% can be kept constant. Furthermore, although the cost is somewhat higher than the conventional self-bias method, the cost is significantly reduced compared to the constant voltage method, and yet quality close to that of the constant voltage method can be obtained. Moreover, the fluctuation rate (correction rate) can be freely changed by appropriately selecting the voltage of the Zener diode.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram illustrating a conventional gas alarm;
Figure 2 is a circuit diagram showing a gas alarm according to the present invention;
FIG. 3 is a diagram showing the relationship between Zener voltage and comparative voltage fluctuation rate, and FIG. 4 is a diagram showing the relationship between gas concentration and sensor output. 1... Semiconductor gas sensor, 3... Transformer, 9...
... Comparator, 11 ... Buzzer, 13 ... Zener diode.

Claims (1)

[Scope of utility model registration request] Gas equipped with a comparator that receives a detection signal from a semiconductor gas sensor at one input, receives a comparison signal corresponding to the concentration to be alarmed at the other input, and generates an alarm signal when the detection signal exceeds the comparison signal. In the alarm, the device that generates the comparison signal is composed of a resistor connected to the reference potential side of the detection signal and a Zener diode connected to an unstable power supply, and the comparison signal is generated by the resistance and the Zener diode. A gas alarm characterized in that the gas alarm is obtained from the connection point of the gas alarm.