JPS6326436B2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to a gas leak alarm.

BACKGROUND ART In recent years, with the increase in the usage of propane gas and city gas, the scale of accidents caused by gas leaks such as explosions has become larger, and efforts are being made in various fields to prevent gas leak accidents. Among them, gas leak alarms are attracting attention as the last line of defense against accidents, and efforts are being made to popularize them.

The present invention aims to further improve the performance and reliability of a gas leak alarm that has the above-mentioned important mission.

The present invention particularly relates to stabilizing the heater voltage of a gas sensor.

A conventional example will be explained based on FIGS. 1 to 3.
First, in Figure 1, from the power supply 101 to the power transformer 10
After the output voltage of the transformer 102 is rectified by the diode bridge 103 and smoothed by the capacitor 104, the series regulator 105 supplies the output voltage of the transformer 102 to the heater 107 of the gas sensor 108, which is composed of a sensitive body 106 and a heater 107. This is to stabilize the voltage as DC. This method has the following drawbacks.

(1) Considering power supply voltage fluctuations, the series regulator 105 has a large power loss and heat generation.

(2) From the above (1), a heat sink etc. of the series regulator 105 is required, which makes the structure complicated and the shape large.

(3) In addition to power loss, since it is converted to DC, the output current of the transformer 102 flows concentrated near the peak voltage, and other transformer outputs become ineffective, resulting in poor efficiency and large transformer 102. It becomes necessary.

In addition, in FIG. 2, a commercial power supply 101 is directly rectified by a diode bridge 103, then switched by a switching circuit 109, and then connected to a high frequency transformer 11.
This method converts the voltage to DC voltage using a diode 111 and a capacitor 112 after converting the voltage at 0. Although this method is efficient and compact and lightweight, it has the following disadvantages.

(1) It is necessary to use ferrite or the like for the core of the high frequency transformer 110, which increases the cost.

(2) Noise from the switching circuit 109 enters the AM band of the radio receiver and radio equipment, necessitating a shield, which complicates the construction method and shape.

Finally, as shown in Figure 3, although the efficiency is good because it is controlled using AC voltage, the peak voltage of AC voltage is only clipped by the constant current circuit 113.
Since it does not correspond to the effective voltage of the heater 107 and has a large error, the voltage dependence of the alarm concentration is large.

The present invention provides a gas leak alarm equipped with a gas sensor heater voltage stabilizing circuit that is free from the problems of the conventional examples.

Hereinafter, one embodiment of the present invention will be described based on FIG. FIG. 4 is a configuration diagram of a gas leak alarm that employs an indirect heater type gas sensor using an N-type metal oxide semiconductor (iron oxide complex oxide semiconductor) as a sensitive body. Now, when the power supply 1 is turned on, an AC voltage is applied to the diode bridge 5 from the output of the power transformer 4, and this diode bridge 5
A DC voltage rectified by a capacitor 6 and smoothed by a capacitor 6 is supplied to the circuit. This DC voltage is stabilized by a stabilizing circuit 40. At this time, while the capacitor 48 is charged via the resistor 47, the gas sensor 25
A timer (hereinafter referred to as the initial timer) that prevents an initial alarm due to a decrease in resistance value that occurs during initial energization operates until the comparator 49 is inverted, and during this time, a light emitting diode (hereinafter referred to as LED) 62 for indicating energization is activated.
is blinked by the oscillation circuit 57 and the LED drive circuit 58 to indicate that the initial timer is in operation, but the alarm buzzer 63 does not operate. and,
When the capacitor 48 is charged and the comparator 49 is inverted, the initial timer ends and a normal monitoring state is entered. In addition, the resistor 45 and diode 46 are powered off.
This is to determine the time to discharge the charge in the capacitor 48.

On the other hand, when the power supply 1 is turned on, AC voltage is also supplied to the secondary side point a of the transformer 4, and the voltage of the heater 24 is divided by the resistors 33a and 33b and input to the square circuit 42 via the capacitor 39. , the output of this squaring circuit 42 is averaged by a capacitor 50 and compared with a reference voltage by an error amplifier 52. The output of this error amplifier 52 is the capacitor 50
It changes depending on the difference between the voltage and the reference voltage. Furthermore, the AC voltage at point a is divided by resistors 15 and 16, and input to a zero-cross circuit 41 that generates a synchronizing signal via a capacitor 38. This zero-cross circuit 41 outputs an output when the input AC signal passes through zero. 0, discharge the voltage of the capacitor 56, and then connect the resistor 55 to the capacitor 56 from the stabilized power supply.
is charged via. A comparator 60 compares the charging/discharging voltage c of the capacitor 56 and the output i of the error amplifier 52, and when the voltage at point c is greater than i, the output of the comparator 60 becomes "L" level, and the resistors 10, 37 The PNP transistors 8 and 14 are driven through the PNP transistors 8 and 14. At this time, when the voltage at point a is positive, the transistor 8 is turned on and voltage is applied to the heater 24 of the gas sensor 25. At this time, the base current is supplied to the NPN transistor 11 from the transistor 8 via the resistor 9, but it does not turn on because of the presence of the diode 12. The diode 12 and the diode 13 are transistors 11 and 1.
It has the role of protecting the base emitter when 4 is OFF. On the other hand, when the voltage at point a is negative, the transistor 14 is turned off, the transistor 11 is turned on, and a voltage is applied so that the point d of the heater 24 becomes negative. In this way, the heater voltage
Each circuit operates to keep the average value constant, and heater 2
This is a phase control type voltage stabilizing circuit that keeps the effective voltage value of 4, that is, the power constant. Figure 5a
- d, each part a to d during voltage stabilization in Fig. 4
The voltage waveform of is shown. In Figures 5a to 5d, A indicates the case where the power supply voltage is high, B indicates the case where the power supply voltage is low,
The shaded area is the case where voltage is applied to the heater 24.

Note that the diagram at point b in FIG. 5 is the capacitor 50.
shows the waveform without averaging.

In this way, since the effective voltage is stabilized and supplied to the heater 24 of the gas sensor 25, the sensitive body 23 of the gas sensor 25 is always maintained at a stable temperature. Now, the alarm reference level is determined by the temperature compensation circuit consisting of resistors 19, 20 and thermistor 21, and resistor 22, and when the gas sensor 25 is exposed to high concentration gas, the sensitive body 23 causes thermal runaway due to self-heating and deteriorates. Therefore, when the resistance value of the sensitive body falls below a certain value, the heater voltage is turned off, and the high concentration protection level that prevents deterioration is similarly determined by the resistors 17 and 18, and the respective reference level setting resistors and the sensitive body 23 The resistor 26 constitutes a bridge circuit.
The AC voltage at point a is applied to this bridge circuit, but the gas sensor output l, alarm concentration reference k and high concentration protection reference j are connected to diodes 27, 28, 29.
The signals are rectified by resistors 30, 31, 32 and capacitors 34, 35, 36, respectively, and then input to comparators 43, 44. Since the voltage at each point j, k, and l is a bridge circuit, although it is an alternating voltage, it does not change due to fluctuations in the power supply voltage.

Now, when combustible gas touches the gas sensor 25, the resistance value of the sensitive element 23 decreases, the AC voltage at point l and the rectified and smoothed output voltage increase, and when the alarm reference voltage output n is exceeded, the comparator 44 is inverted. ,
Activate the oscillation circuit 57 and the LED drive circuit 58,
The LED 62 starts blinking, activates the delay timer circuit 61, and changes the external output circuit 64 from a normal state of about 6V to a gas detection state of 12V or more. When the operation of the delay timer circuit 61 is completed, the buzzer drive circuit 59 operates to drive the buzzer 63 and issue an alarm. Here, the delay timer circuit 61
is a timer for reducing false alarms caused by transient gas.

Further, when the gas concentration increases and the voltage at the point becomes higher than the voltage at point m of the high concentration protection standard, the comparator 43 is reversed, the operation of the comparator 60 for controlling the heater voltage is stopped, the heater voltage is turned off, and the sensitive 23 to prevent deterioration. Then, when the resistance value of the sensitive body 23 increases due to the heater voltage being turned off, the system automatically returns to normal operation and repeats the above operation. Also,
Since the heater voltage is lower in the high concentration protection state, a high voltage is applied to the heater voltage when the heater is restored.As a countermeasure, when the high concentration protection circuit is activated, the output b of the square circuit 42 is lowered by the resistor 51 so that the heater voltage is high. condition, and the heater voltage is gradually returned from a low voltage upon recovery.

Now, if an abnormality such as a failure occurs in the heater voltage control system, it cannot be expected to function normally as a gas leak alarm, and in this alarm, the heater control system has a large current and there is a possibility of failure. If the voltage at the output point i of the error amplifier 52 exceeds the voltage range set by g and h, it is detected by the comparators 53 and 54, and the LED drive circuit 58 is turned off and the LED 62 is turned off.
to notify the failure. This circuit works even if the power supply voltage drops abnormally and the heater voltage stabilization circuit is normal but does not reach the reference voltage. However, it will not work if the high concentration protection circuit is activated and the heater voltage is turned off.

Further, the varistor 3 and the diode 7 are for absorbing abnormal voltage such as lightning surge, and 2 is a fuse.

The present invention is constructed as described above, and has the following effects.

(1) Since AC stabilization of the heater voltage is performed using a transistor, a diode is not required when the input heater voltage is low, resulting in low saturation voltage and high efficiency.

(2) Since the switching speed of the transistor is slow, no noise is generated in radio receivers and radio equipment.

(3) Since transistors are used for switching, small and inexpensive transistors can be used.

[Brief explanation of drawings]

1 to 3 are circuit diagrams of conventional gas leak alarms, FIG. 4 is a circuit diagram showing an embodiment of the gas leak alarm according to the present invention, and FIGS.
b, c, and d are voltage waveform diagrams at various parts in FIG. 4. 8...Second PNP transistor, 9...Resistor, 1
0...Resistor, 11...NPN transistor, 12,1
3...Diode, 14...First PNP transistor, 23...Sensor, 24...Heater, 25...Gas sensor, 37...Resistor.

Claims (1)

[Claims] 1 The resistance value changes due to the presence of flammable gas,
The gas concentration is determined by the change in resistance of a sensitive body made of an N-type metal oxide semiconductor kept at a high temperature by a heater, and an alarm is issued.One end of the heater is connected to an earth line, and the other end of the heater is The collectors of the first PNP transistor and the NPN transistor are respectively connected to the transistors, and the base of the NPN transistor is connected via a resistor to the collector of the second PNP transistor whose emitter is connected to a sufficiently high DC positive voltage power supply. , the first PNP above
The emitters of the transistor and the NPN transistor are connected directly or through a diode connected to the base and emitter in the forward direction, respectively, and an AC voltage for the heater voltage is applied between this connection point and the ground line, and the above first
A gas leak alarm device characterized by having an AC heater voltage control circuit that drives the bases of a PNP transistor and a second transistor with a phase control signal that serves as a ground line through a resistor, respectively.