IL34849A - Ionization type fire detector - Google Patents

Description

Ionization type detector x This invention relates to a fire especially to an Ionization type fire An Ionization detector according to prior art has two ionization ohambers respectively including therein a pair of electrodes and a radioactive One of the ionization chambdr is tightly closed with respect to the external air and the other chamber is opened to the external The both ionization ohambers are connected in series and a constant voltage applied across The atmospheres in the both Ionization chambers are ionized partly by radiant rays from the radioactive sources respectively included the chambers and a specific ionization current flows through the both chambers at a specific If smoke oomes in the open ionization the ionization varies and a potential at the Junction of the both inosation chambers also varies potential variation is detected and converted into a ou ent dhange by means of an element such as field effect transistor to drive an such a prior Ionization smoke detector has had such disadvantage that it can not give a fire alarm when no smoke grows yet even though the ambient temperature has rlsed so remarkably that it has become a state Just before a fire breaks since it is not actuated unless smoke the open an object of this Invention to propose an Improved fire detector which oan give alarm not only by sensing but also by sensing increase of the ambient temperature if any even if no smoke The fire detector according to invention comprises a closed ionization chamber inoludlng a pair of electrodes and a radioactive source an open ionization chambe including field effect transistor for detecting a voltage change across said both Ionization chambers and converting a current change and a switching element by field effect A Junction type field effect transistor whose gate leakage current varies response to temperature variation adopted as the abovementloned field effect and the transistor is located In a position at which it can be easily Other objects and feature of this invention will be clearly understood from the description with referenoe to the accompanying In the Figure 1 a circuit diagram representing an embodiment of the fire detector according Figure 2 through 4 are characteristic diagrams for explaining operation the fire detector shown Figure and Figure 5 a partly broken away side view representing an example of embodied structure of the detector according to Referring now to Figure there shown is a circuit diagram Of a detector according to this including a closed ionization chamber 2 having therein a source 21 and an open Ionization chamber 4 having therein a radioactive source and arranged to allow smoke to come The closed chamber 2 has pair of electrodes 23 and 25 and the open Ionization chamber 4 similarly a pair of electrodes 43 and The Ionization chamber 2 and 4 are connected series between the positive and negative terminals 6 and Θ of a power supply and the Junction point of the both Ionization chamber connected to the gate electrode 14 a Junction type field effect translator The type field effect transistor 12 has a drain electrode the field effect transistor 12 is also connected through a diode 22 to the gate electJnode 26 of a silicon controlled rectifier referred as 24 which has an anode electrode 30 connected to the positive terminal 6 of the power supply and a cathode electrode 32 connected to the negative terminal 8 of the power The gate electrode 26 of the SCR 24 also connected through a bias resistor 28 to the negative terminal 8 of the power Now the operation of the abovementioned fire detector will be described conjunction with the following two where a smoke while the ambient temperature is maintained tially at normal and where the ambient temperature rises smoke CASE where smoke while the ambient temperature maintained substantially at normal this the fire detector operates as an ionization smoke This operation will be described hereunder with reference to Figure When a constant voltage is applied between the source terminals 6 and an current flows through the both ionization chambers 2 and Figure 2 represents a relation between the ionization current flowing through the both ionization chambers and potential differences between the electrodes of the respective Ionization The between the current flowing through the closed ionization chamber 2 and the potential difference the electrodes 23 and 25 always constant as by a curve 51 in the drawing regardless of presence or absence of On the other the relation between the ourrent flowing through the open ionization chamber 4 and the potential difference between the electrodes 43 and 45 when no smoke grows as shown by curve 61 in the Since the both ionization chambers are connected in series Junction point 10 of the Doth corresponds to a voltage at the Intersection of the curves 61 and circuit parameters are so selected when the at the Junction point 10 of the both ionization that the gate voltage of the transistor 12 a current flowing through the path of the transistor 12 substantlaly zero or a voltage drop across the load resistor 20 resulted from this current any does not exceed the Zener voltage of the Zener diode no gate signal is applied to the gate electrode 26 of the SCR 24 and the main conduction path between the anode and cathode of the SCR 24 is in its Of in this no fire alarming signal generated from the fire When smoke grows and comes the open ionization chamber the Ionization current flowing through the open Ionization chamber is trapped by this smoke This means an increase of the internal lmedance of the open ionization chamber smoke comes in the open ionization Chamber the relation the current flowing through the open chamber 4 and the intereleotrode voltage thereof as shown by a curve 62 in Figure With the curreit characteristic of the open ionization chamber 4 as shown by the curve the voltage at the Junction point of the both ionization chambers corresponds to a voltage at the Intersection of the curves 62 and This results in an increase of the gate voltage of the type field effect transistor 12 by a difference of the voltages and Due to this voltage the current flowing through the path of the type field effect transistor 12 This current makes the voltage drop across the load resistor 20 greater than the Zene voltage of the Zener diode the gate signal is applied through the Zener diode 22 As described in the the fire detector according to this invention can sense a start of fire by sensing smoke if it even if the ambient temperature is yet near normal CASE where the ambient temperature rises no smoke a junction type field effect in the leakage current Ig flowing through the gate electrode varies with variation of the ambient a most used Junction type field effect the gate leakage current is nearly doubled at each temperature increment o It is convenient to consider that the gate leakage current Ig flows through a phantom resistor the gate electrode 14 and the drain electrode and the value of or 34 varies due to effect of the ambient The leakage current Ig varies as shown Figure That the gate leakage current begins to when the voltage V at the gate electrode 14 junction point of the both ionization is Vg which is slightly lower than the source voltage and becomes maximum when the potential at the gate electrode 14 reaches othe when the full source voltage applied between the gate electrode 14 and the drain electrode In the the curve 71 represents variatio of the gate leakage current Ig at a temperature which is slightly higher than the normal and this characteristic varies with Increase of the ambient as shown by curves and in the direction of increase the gate leakage In the case of observing the increase of the gate leakage current from the open ionization chamber it can be deemed that the ionization current flowing through the chamber 2 the 2 and the gate leakage Figure According to this Characteristics 53 and 54 Figure 4 can deemed to be the characteristics of equivalent currents flowing through the closed ionization 2 the respective themperaturee higher than the normal While the current characteristic of the opan ionization 4 is represented by the curve 61 or in Figure the 61 is to be adopted i case where no smoke As clearly found in Figure the voltage at the Junction 10 o the both ionization chambers is when the ambient temperature In this the current flowing through the path of the Junction type ί field effect transistor 12 is subotnatlally zero or very small if and no gate signal is applied to the gate electrode 26 of the when the ambient temperature rises and the apparent current flowing through the closed ionization chamber 2 varies as shown by the curve 53 or the at the Junction 10 becomes corresponding to the voltage at the intersection of the curves 61 and 53 or corresponding to the voltage at the intersection of the curves 61 and This sults a large current flowing through the path the Junction type field effect transistor 12 and a large drop across the load resistor thereby supplying a gate signal through the Zener diode 22 to the gate electrode 26 of theSCR the SCR 24 conducts and a fire alarming signal can be transmitted toward the su As described in the if the ambient temperature rises extraordinarily above a predetermined the fire detector When the ambient temperature rises at the as growth of smoke the current characteristic of the open Ionization chamber 4 is as shown by the curve 62 Figure and the curren characteristic of the closed Ionization chamber 2 varies as shown by the curves 53 and 54 in Figure Therefore the voltage at the Junction 10 of the both ionization chambers varies from to to naturally drive the SCR 24 into conduction to a fire alarming signal to be transmitted toward the power An of practical structure of the abovementioned detector according to this invention will be described hereunder reference to Figure wherei the structural elements corresponding to those in Figure 1 are indicated by the same reference numerals as those in Figure In the drawing an electrode 45 such as of cylindrical shape which serves also as is provided with a number of windows 5 for allowing smoke to come In the electrode 45 provided a base member 36 formed of insulating material such modified aromatic polyether for example trade by General Electric Co The bas member 36 is composed of a flat central portion 37 an electrode supporting portio 33 formed around the flat portion 37 and pro jecting toward the electrode 45 and an indented peripheral portion 39 around the electrode supporting portion An electrode 43 fixed to the electrode supporting portion 38 and a active source 41 is provided o a surface of the electrode Thus an open Ionization chamber 4 is formed between the electrodes 45 Since the electrode 43 also serves one electrode 25 of the closed ionization chamber 2 it is referred as in the Another electrode 23 corresponding of the base member 36 and a radioactive source 21 provided on the electrode The closed Ionization chamber 2 formed between the electrodes 23 and The peripheral portion 39 plurality of ridges and grooves order to prevent deterioration of insulation between the electrodes and 45 even if the surface of the base member 1B A depression 40 is formed adjacent to the flat portion 37 of the Insulating base member and a Junction type field effect transistor 12 embedded The field effect transistor 12 has a gate electrode 14 connected to the electrode a drain electrode connected to the electrode 23 and a source electrode 18 oonnected to the electrode Other structural elements in Figure 5 have no connect with the subject matter of this invention and further description of these components According to the present since the gate electrode 14 directly to the electrode which is heated relatively the temperature of the gate electrode rises relatively repSdly with of the ambient temperature and the gate leakage current can be effectively the Junction type field effect transistor 12 may be attached on the electrode a directly thermally coupled as shown by dotted lines Figure insufficientOCRQuality

Claims (4)

34849/2 What 1s claimed are:

1. A fire detector comprising a closed Ionization chamber air- tightly surrounded by first and second electrodes supported by an Insulating base member and Including a radioactive source located on a surface of said first electrode facing to said second electrode, an open Ionization chamber surrounded by said second electrode and a third electrode fixed to said Insulating base member so as to serve as a housing of the device and Including a radioactive source located on a surface of said second electrode facing to said third electrode, and a junction type field effect transistor disposed adjacent said third electrode and having a gate electrode connected to said second electrode and a source-drain path connected between said first and third electrodes.

2. A fire detector, 1n accordance with claim 1, characterized 1n that said junction type field effect transistor 1s embedded 1n said Insulating base membet.

3. A fire detector, 1n accordance with Claim 1, characterized 1n that said junction type field effect transistor 1s attached to said third electrode 1n thermally coupled state.

4. A fire detector substantially as hereinabove described with reference to Figure 5 of the accompanying drawings. For the Applicants Wolff, Bregman and Goll