KR800000400B1 - Smoke sensing fire alarm circuit - Google Patents

Abstract

Alarm cct. having detection cct. affected by no variation of supply voltage and low power dissipation is disclosed. The open and closed ionization chambers(18)(20) of which impedance is changed when the smoke enters the open chamber, are connected in series with the power supply source-battery. The N-channel J-FET(42) and P-channel MOS-FET(44) are biased in a nonconducting state ordinarily and a couple of complementary type voltage response switch(26) - the MOS and junction FETs- is conducted when the sensing voltage(Vc) reaches the preset alarm voltage(Va).

Description

Smoke Detection Fire Alarm Circuit

Schematic diagram of a smoke detection fire alarm in accordance with the present invention.

The present invention relates to a smoke detection fire alarm. In particular, the present invention relates to a smoke detection fire alarm having a battery as a power source and having a detection circuit.

Smoke detection fire alarms that detect a change in impedance when smoke enters the ionization chamber are well known. In particular, the open ionization chamber connects the power supply to the voltage distribution circuit, so the change in impedance is represented by the change in voltage here. This sensitive voltage is detected by the detection circuitry, and if it exceeds the predetermined alarm limit, the detection circuitry operates the appropriate alarming circuit.

In the battery power fire alarm of this type, it is known that a battery monitoring circuit is provided to generate a low battery signal when the battery voltage drops below a level at which the alarm circuit can be operated successfully.

Known detection circuits have one or more disadvantageous characteristics.

For example, the sensed voltage between an open ionization chamber and a closed ion cell connected in series via a power supply acts on an impedance matched field effect transistor amplifier across a Zener diode.

An alarm will sound if the sense voltage exceeds the zener diode limit voltage. Inconveniently, field effect transistors operate in a linear range, with a constant current draw in the power supply.

The stricter tolerances are required for zener diodes because the alarm voltage is only generated by the threshold voltage characteristics of the zener diode.

Another problem is that the actual alarm voltage acting through the zener diode does not change due to the change in supply voltage, whereas the sense voltage varies with supply voltage, so the sense level stability requires adjustment of the voltage supply.

In another example, the current problem in the alarm circuit is usually off, but it is solved by using a matched field effect transistor that turns on only when the response voltage generated by the ion chamber exceeds the limit voltage.

However, the threshold voltages cannot be adjusted, which in turn requires the selection of stringent field effect transistors for the characteristics of their limit voltages, and lack of means for changing the sensitivity. For the same reason as in the alarm circuit of the first example mentioned above, the detection circuit needs a unstable regulated power supply voltage to operate successfully.

It allows the use of active devices in detection circuits with widely varying characteristics, and other circuits that can be adjusted for sensitivity lack power supply voltage stability and consume an unacceptable amount of reserve power.

SUMMARY OF THE INVENTION The present invention has been made to eliminate the closed end in view of such a point, and a first object of the present invention is to provide a smoke detection fire alarm having a detection circuit that is essentially unaffected by supply voltage fluctuations.

It is a second object of the present invention to provide a smoke sensitive fire alarm having a lower power consumption than a known fire alarm.

One embodiment designed to achieve these objectives is a pair of complementary complements connected through the output of a Wheaten bridge to compare the response voltage of the ionization chamber at one time with a preselected alarm voltage identified by a potentiometer on the other side. (Phase-type) field effect transistors are used.

The positive field effect transistor is non-conducting until the response voltage in the ionization chamber is essentially equal to a predetermined alarm voltage.

The discharge rate of the battery during the stop period with the supply voltage independent is practically zero.

These field effect transistors are connected so as to cancel each gate-source threshold voltage with each other so that they actually respond to a zero voltage difference.

This occurs when the impedance of the ionization chamber element has an alarm condition regardless of the change in the supply voltage. When an alarm condition is detected, both field-effect transistor switches are energized to activate the audible alarm circuit.

Another advantageous feature of the detection circuit is that its output is connected to its input via one side of the Wheatstone bridge, which has a hysteretic characteristic that tries to keep the field effect transistor conducting after the reactive voltage returns to non-alarm conditions. To provide a switch.

The present invention according to the accompanying drawings in detail as follows.

In the figure, the smoke detection fire alarm of the present invention includes a detection circuit 10, an adjustable horn circuit 12 for generating an audible alarm, a battery 14 providing a power supply voltage Vb, and a battery monitoring circuit 16; have.

The detection circuit 10 detects the fire alarm condition and causes the horn circuit 12 to emit a continuous audible alarm.

The battery monitoring circuit 16 monitors the fire alarm condition and causes the horn circuit 12 to emit a continuous audible signal.

The battery monitoring circuit 16 senses the selected low voltage condition of the battery 14 and simultaneously causes the horn circuit 12 to emit an hepatic audible signal.

In the detection circuit 10, a pair of general ionization chambers 18 and 20 connected in series through the resistor 22 in the battery 14 are used. Ionization chambers 18 and 20 thus create a voltage divider that provides a sensitive voltage Vc at the junction between both ionization chambers 18 and 20, which is a function of the associated impedance value there.

While the ionization chamber 20 is closed, the ionization chamber 18 is open to the surrounding atmosphere, whereby smoke and other gases generated from a fire can enter and exit.

When smoke enters the open ionization chamber 18, its impedance increases, while the impedance of the closed ionization chamber 20 is not affected.

The positive electrode 25 of the battery 14 is connected to the ionization chamber 18, and the negative electrode 27 is connected to the ionization chamber 20. As a result, an increase in the impedance of the open ionization chamber 18 induced by the fire reduces the response voltage Vc.

The ionization chambers 18 and 20 have different characteristics, such as their temperature coefficients, so that other changes in the surrounding environment affecting the ionization chambers do not cause an offset change in these ionization chambers and thus change the response voltage Vc. Competing.

Since the impedances of the ionization chambers 18 and 20 are in hundreds of thousands of megohms, the battery 14 is almost never consumed. It has been found that relative values for the ionization chambers 18 and 20 work, but relative values that produce a response voltage Vc equivalent to 0.6 times the cell voltage Vb in the absence of smoke.

The sensitive voltage Vc is compared with the preselected alarm voltage Va generated by the voltage responding switching circuit 26 at the movable terminal 28 of the potentiometer 30. When the response voltage Vc is actually reduced to a value equivalent to the alarm voltage Va, the switching circuit drives the horn circuit 12 to generate a continuous alarm.

The potentiometer 30 is connected in series with two fixed resistors 32 and 34 from the battery 14 to form a variable voltage divider 36.

The voltage divider has a resistance in the range of MΩ to minimize the drain of the battery 14. The variable voltage divider 36, together with the voltage divider formed by the ionization chambers 18 and 20, constitutes a whitish bridge circuit.

As a result, the change in the response voltage Vc due to the change in the battery voltage Vb is canceled by the equivalent change in the alarm voltage Va.

This minimizes the sensitivity of the switch circuit 26 to variations in the battery voltage Vb.

The fact that the voltage adjusting switch 26 is in a conductive state when the bridge is almost in equilibrium adds stability by minimizing the supply voltage sensitivity.

The movable terminal 28 is adjusted in position so that the value of the alarm voltage Va is smaller than the direct action value of the sensitive voltage Vc.

By changing the position of the movable terminal 28, the sensitivity can be selectively changed. When the direct operation value of the sensitive voltage Vc was 0.6 Vb for the battery voltage of 9 Volt, it was found that 0.5 Vb is appropriate for the alarm voltage Va.

The switching circuit 26 is composed of an N-channel silicon junction field effect transistor, i.e., a J-FET 42, and a P-channel increased silicon MOS field effect transistor, i.e., a MOS-FET 44.

Gate 46 of MOS FET 44 is connected to junction 24. The output of the switch 26 is extracted from the drain 48 of the MOS FET 44 and is connected to the gate 51 of the SCR 53 of the adjustable horn circuit 12.

The engine 50 is connected to the positive electrode 25 of the battery through the current limiting resistor 22.

The power supply terminal 52 of the MOS FET 44 is connected to the power supply terminal 54 of the J-FET 42. The J-FET 42 has a source follower whose gate 56 is connected to the movable terminal 28 of the variable voltage divider 36 and its drain 58 is connected to the positive electrode 25 of the battery. ) Form.

As an important aspect of the present invention, both the MOS FET 44 and the J-FET 42 are biased to minimize the current consumption of the battery l4, and the reactive voltage Vc is equal to the alarm voltage Va. Only when reduced to nearly equal state will the state of conduction be achieved. In this case, both the MOS-FET 44 and the J-FET 42 are in a conductive state, and the drain current from the MOS-FET 44 is in a conductive state accordingly, so that the gate 51 of the SCR 53 is in a conductive state. Is applied to.

The MOS FET 44 acts as a high impedance buffer between the SCR 53 and the sensitive circuits of the ionization chambers 18 and 20 and also acts as a limit switch device. The MOS FET 44 is in a conductive state when the gate-source threshold voltage is exceeded.

However, unlike known circuits, the response voltage at which this occurs can be selectively changed by adjusting the movable terminal 28.

The position of the movable terminal 28 determines the magnitude of the sensitive voltage Vc exceeding the limit voltage of the MOS FET 44 by forming the voltage at the source 52. As a result, MOS-FETs 44 with different limit characteristics can be used for alarms that signal at the same voltage and reduced manufacturing cost.

The high input low output impedance of J-FET 42 provides an impedance match between voltage divider 36 and MOS-FET 44. In addition, the change in the gate-source voltage Vgs of the MOS-FET 44 due to the temperature change is canceled by the change in the response characteristic at (Vgs) of the J-FET, and vice versa.

The complementary arrangement of MOS-FET 44 and J-FET 42 also maximizes the independence of the supply voltage of the detection circuit.

This is because the MOS-FET 44 and the J-FET 42 are conductive when the bridge is nearly balanced and the balance point is made only by the relative impedance value of the sides of the bridge, and is completely independent of the supply voltage.

In another aspect of the present invention, the MOS-FET 44 drain 48 is connected to the junction 49 of the voltage divider 36 to provide hysteresis to the switch circuit 26.

When the MOS-FET 44 and the J-FET 42 are conducting, a relatively low impedance shunt is applied through the resistors 32 and 34 so that the alarm voltage Va at the movable terminal 28 becomes MOS-FET. The 44 and J-FET 42 are slightly increased when they become non-conducting. Therefore, the sensitive voltage Vc at the junction 24 must be increased to a value larger than the value which becomes the conduction state before the MOS-FET 44 and the J-FET 42 become non-conduction state.

When the drain current is applied to the gate 51 of the SCR 53 as described above, the SCR 53 is brought into a conductive state to operate the horn circuit 12.

The filter capacitor 70 is connected between the gate 51 of the SCR 53 and its cathode, and the resistor 34 provides an appropriate gate bias for the SCR 53. The horn circuit 12 is common and includes a relay coil 60 connected in series between an anode of the SCR 53 and a pair of relay contacts 62 operated by it.

One of the contacts 62 is connected to a horn (not shown) which vibrates to emit an audible alarm signal. The horn circuit 12 also includes a series connection of a resistor 64 and a capacitor 66 connected across the coil 60 and also includes a starter capacitor 68 connected to the battery 14.

After the gate drive is removed from the SCR 53, when the contact 62 is opened, the SCR 53 immediately becomes non-conductive. The gate drive is connected between the gate 51 and the cathode of the SCR 53 and is removed by closing the normally open switch 72.

MOS-FET 44 and J-FET 42 also become non-conductive when the impedance of ionization chamber 18 returns to a level low enough to reduce its voltage below pinch-off, Remove gate drive.

The test circuit is provided by a normally open switch 74 connected between the negative cell terminal 27 and the junction between the resistor 22 and the ionization chamber 18 via a resistor 76.

The closing of the switch 74 lowers the response voltage Vc to a level lower than the alarm voltage in order to simulate the alarm condition.

Unexplained circuit 16 is a battery monitoring circuit that notifies when the battery is depleted.

In addition to the special operating characteristics of the circuit fabricated by the present invention, the various devices used also depend on the particular characteristics and values. Thus, while other devices can be successfully applied, it has been found that the circuit fabricated according to the drawing works properly using the devices shown below.

Because of the various advantageous features of the present invention mentioned above, the total cell current in the quiescent state is 10 μA or less, and the circuit will be fully operational for one time with a single cell of the type specified above before the low cell state is indicated.

Claims (1)

It is connected to the open ionization chamber 18 and the closed ionization chamber 20 via the battery 14, which is a power source, and a pair of pairs of each change in impedance, which increases when smoke is introduced, causes a response voltage Vc at the output. The N-channel field effect transistor (J-FET) 42 and the P-channel field effect transistor (MOS-FET) 44 having the conversion terminal and one regulating terminal are biased in a non-conductive state so that the response voltage ( A smoke detection fire alarm circuit comprising a pair of complementary voltage response switches (26) which are in a conductive state when Vc) is actually equal to a preselected alarm voltage (Va).

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