JPS5878B2 - ionization smoke detector - Google Patents

Description

[Detailed description of the invention] In an ionization type smoke detector, the air in the room is ionized by radiation from a radiation source installed in the ionization chamber, and the ionization chamber is ionized by the voltage applied to the ionization chamber. A weak current flows between the constituent electrodes, and the increased voltage caused by the decrease in ion current due to the inflow of smoke activates the switch circuit and sends a signal to the receiver, which generates an alarm. It is.

In such ionization type smoke detectors, the impedance between the electrodes is generally about 1011 ohm to 1013 ohm, so insulation deterioration due to dirt, moisture, etc. cannot be ignored, and stable alarm generation is impossible. It is something that does not work.

From this point of view, the present invention aims to provide an ionization type smoke sensor that does not generate false alarms even if the creeping surface of the insulator holding the electrodes deteriorates.

That is, the present invention provides an ionization type smoke sensor in which a radiation source is provided on one side of a pair of electrodes constituting an ion chamber, and the two electrodes are communicated with the outside air. An electrode is installed, and the guard electrode is connected to the input terminal of the field effect transistor for impedance conversion of a current amplifier constituted by a field effect transistor for impedance conversion and a differential amplifier. .

DESCRIPTION OF THE PREFERRED EMBODIMENTS An ionization type smoke detector according to the present invention will be explained below with reference to the drawings.

FIG. 1 shows a current amplification type sensor circuit, which includes an ion chamber 1 and a field effect transistor 2 for impedance conversion.
, 2 and a differential amplifier 3, a threshold circuit, and a switching circuit, which is installed between the anode electrode A and the cathode electrode of the ion chamber 1. The guard electrode G is connected to the positive input terminal of the field effect transistor 2.

Further, FIG. 2 shows a voltage amplification type sensor circuit, which is connected to a negative input terminal other than the input terminal of the differential amplifier 3.

Both the circuits in Figures 1 and 2 configure a high-gain differential amplifier with high input impedance by combining two field-effect transistors and a high-gain differential amplifier. The width filter amplifies the potential difference between the positive input and the negative input.

For example, when the negative input is OV1 and the positive input is 0.1 mV, and the gain of the differential amplifier is 10,000 times, an output of 1V is produced.

In other words, if there is a gain of tens of thousands of times with an output of several volts, the difference between the positive input and the negative input can be regarded as OV.

Looking at the circuits in Figures 1 and 2 from this perspective,
■If the potential difference between a positive input described as an input terminal and a negative input described as an input terminal is, for example, 0.1 mV, either the + or - input terminal is connected to the cathode electrode or the guard electrode G. Therefore, the potential difference between the cathode electrode and the guard electrode G is less than 0.1 mV.

Figure 3 shows the circuit when the insulation of the ion chamber has deteriorated.As shown in this figure, the insulation has deteriorated and the resistance R1,
It is assumed that R2 is connected in parallel between the electrodes.

Ro is the equivalent resistance of the ion chamber 1, and its value is 101
There is about 1 ohm, and when voltage Vc = 10V, 100P
Assume that an ionic current of A flows.

If the guard electrode G is not fixed to a specific electrode and is floating, then Ro and R1+R2 are connected in parallel.In other words, this is a phenomenon of insulation deterioration, so sufficient stability is required. In order to obtain RO≪R1+R2.

However, in the present invention, the guard electrode G is located at a low impedance point V1 with a potential difference of 0.1 mV from the cathode electrode.
Therefore, a current of 100 PA is flowing through the resistor R0, and if we allow an error of up to 1 PA due to the resistors R1 and R2, the current flowing through the resistor R2 is 1 PA.

Therefore, when the voltage across the resistor R2 is 0.1 mV, the allowable value is as follows, and when R2=100 megohms, an error occurs in the IPA current.

This is an improvement of 104 times compared to the conventional method which caused an error of 1 PA at 1018.

FIG. 4 shows a specific example of the configuration of the ion chamber of the ionization type smoke sensor of the present invention, but if the provision of the guard electrode G deteriorates the creepage distance between the anode electrode A and the cathode electrode, , always passes through the guard electrode G.

In this figure, A is an insulator that holds the electrode, the mouth is a smoke inlet, and C is a radiation source attached to the cathode electrode K.

An ionization type smoke sensor according to the present invention is an ionization type smoke sensor in which a radiation source is provided on one side of a pair of electrodes constituting an ion chamber, and the two electrodes are communicated with outside air. A guard electrode with low impedance is installed in between, and the guard electrode is connected to a point close to the potential of the input terminal of the amplifier. This has the effect of preventing the occurrence of false alarms caused by this.

[Brief explanation of drawings]

Figures 1 and 2 are electrical circuit diagrams showing examples of the ionization smoke detector according to the present invention, Figure 3 is an equivalent circuit diagram when the insulation of the ion chamber deteriorates, and Figure 4 is an electric circuit diagram of the ion chamber. FIG. 3 is a cross-sectional view showing a configuration example. 1 is an ion chamber having an anode electrode A1 and a guard electrode G as a cathode electrode; 2 is a field effect transistor; 3 is a differential amplifier; and 4 is a current amplifier.

Claims (1)

[Claims] 1. In an ionization type smoke detector in which a radiation source is provided on one side of a pair of electrodes constituting an ion chamber, and both electrodes are communicated with the outside air, a guard electrode with low impedance is installed between the pair of electrodes. , wherein the guard electrode is connected to an input terminal of a field effect transistor for impedance conversion of a current amplifier constituted by a field effect transistor for impedance conversion and a differential amplifier.