IL35151A

IL35151A - Ionization smoke detector

Info

Publication number: IL35151A
Application number: IL35151A
Authority: IL
Original assignee: Nittan Co Ltd
Priority date: 1969-07-24
Filing date: 1970-08-23
Publication date: 1973-08-29
Also published as: NL7010779A; DE2036447A1; CH526178A; IL35151A0; DE2036447B2; GB1282536A; US3688119A; FR2055543A5

Description

IONIZATION SMOKE DETECTOR This invention relates to an Ionization smoke detector , especially to an ionization smoke detector which can. operate accurately with, a pulsating vo ltage whic is insufficiently sraodhened, as well.

In general, an ionization smoke detector according to prior art comprises a closed Ionization chamber having a pair of electrodes and a radioactive source therein, an open Ionization chamber, in which smoke or combustion product can enter , having similarly a pair of 'electrodes and a radioactive source therein and connected in series to said closed ionization ohamber, a voltage source for supplying a voltage to the series connection of the ' .both, ionization chambers* a field- effect transis or, for, detecting a voltage change at the Junction of the bo th ionization chambers resulting from increase of impedance of the open ionization chamber, and a switching element euch as SCR which is driven into conduction by the output of said field effect ' transistor* In the such prior ionization smoke detector , however , i the case of using a pulsatin voltage obtained by only rectifying an a.c . vo ltage from a voltage source as the above , the SCR does not conduct at the zero cross-over points of the Inexpensive pulsating voltage source, by providing 1t with a for keeping the SCR 1n conduction, once 1t conducts, even 1f a pulsating voltage 1s applied to the SCR.

According to this invention, there 1s provided an ionization smoke detector, comprising a pair of terminals, a closed ionization chamber and an open ionization chamber connected in series between said pair of terminals, said ionization chambers each including a pai of electrodes and a radioactive source, a field effect transistor having a gate. electrode connected to the connection point between said both ionization chambers, a drain electrode connected to one of said terminals and a source electrode connected through a load resistor to the other terminal, a silicon controlled rectifier having a control electrode connected through a Zener diode to the source electrode of said field effect transistor and a conduction path connected between said both terminals, characterized by a bipolar transistor having an emitter-collector path connected between the drain electrode of said field effect transistor and said one terminal, a resistor connected between the base electrode of said bipolar transistor and said one terminal and a Zener diode connected between the base electrode of said bipolar transistor and said other terminal.

Other objects and features of this invention will be best understood by reading the following description with reference to the accompanying drawings.

In the drawings: Figure 1 1s a schematic diagram representing a first embodiment of the Ionization smoke detector according to this Invention; Figure 2 1s a diagram representing voltage waveforms used for an aid of explanation of the Ionization smoke detector 1n Figure 1.

Figure 3 1s a characteristic diagram used for an aid of explanation of operation of the Ionization smoke detector 1n Figure 1; and Figure 4 1s a schematic diagram representing a second embodiment of the ionization smoke detector according to this invention.

The description will be first made In conjunction with the Ionization smoke detector shown in Figure 1. A olosed ionization chamber 10 including a pair of eleotrodes 11 and 12 and a radioa-otlve source 13 and an open ionization chamber 20 similarly Including a pair of eleotrodes 21 and 22 and .radioactive source 23 are oonneoted in series between a pair of conductors 1 and 2 which are connected to a voltage source (not shown). A Junction 5 between the both ionization chambers 10 and 20 is connected to the gate elctrode 31 of a field effect translator 30 having a drain electrode 32 connected directly to the conductor 2. The anode electrode 42 of the SCR 40 Is connected through a resisto 7 having a relativel low value and a diode 6 to the conductor 1 and the bathode electrode 43 thereof Is connected directly to the conductor 2. A capacitor 9 Is oonneoted between the Junction 8 of th© resistor 7 and the diode 6 and the conductor 2· The capacitor 9 and the diode 6 form a holding circuit.60 for maintaining the conducting SCR 40 as it is. A resistor 44 and a capacitor 45 are connected between the ¾ gate electrode 41 of the SCR 40 and the conductor 2.

In operation of the abovementloned ionization smoke detector, it is assumed that a pulsating voltage prepared by only rectifying an a. c.voltage Is applied between the conductors 1 and 2. Since the ionization chambers 10 and 20 essentially have electrostatic capacitance and high impedance, the pulsating voltage is smoothened. thereby. Upon observation by an oscilloscope, the voltage at the Junction 5 of the both ionization chambers varies as shown by a full line 51 of Figure 2 when no smoke exists in the open ionization chamber 20.

When smoke enters into the open ionization chamber 20, however, the ionization current flowing therethrough is reduced to raise / regardless of presence and absence of smoke η the open Ionization chamber 20· Since a saturated Ionization current flows always through the closed Ionization chamber 10, the Ionization current flowing through the open Ionization chamber 20 does not change much by a voltage change across the open Ionization chamber 20 caused by an Impedance change of the closed Ionization chamber 10· This situation Is shown b the characteristic diagram of Figure 3 In wh ch the voltage at the Junction 5 Is shown on the abscissa and the Ionization ourrent Is shown on the ordinate. As clearly found from the drawing the voltage across the closed Ionization chamber 10 Is high and varies along a curve 61 when no smoke e^Lsts In the open Ionization chamber 20 and the voltageat the Junction 5 Is low (the full line 51 of Figure 2), but the voltage across the closed Ionization chamber 10 Is low and varies along a curve 62 when smoke enters .into . the open ionization chamber 20 and the voltage at the Junction 5 is high (the dotted line 52 of Figure 2).. In the both cases, however, there is a wide range (0 to V ) wherein the saturated Ionization current in the closed Ionization chamber 10 does not vary substantially.

Though the ionization ourrent In the open Ionization chamber 20 normally varies along a curve 71 In Figure 3, it tends to vary along a curve 72 when smoke enters into the open ionization chamber 20. This ionization current In the open ionization chamber 20 scarecely varies even if a pulsating voltage Is applied between the conductors 1 and 2, because of a lafcge time constant inherently possessed by the ionization chambers. Therefore , the voltage at the Junction 5 of the both ionization chambers varies between and V2 In accordance with presence and absence of smoke % n the open ionization chamber 20, and this voltage change is detected "by the field effect transistor 30 and derived as a signal from the source electrode 33 thereof. When smoke enters into the open ionization chamber 20, the voltage at the Junction 5 rises from to in Figure 3 and the current flowing through the drain*souroe path of the field effect transistor 30 increases* Thue, when the voltage across the resistor 4 exceeds the Zener voltage of the Zener diode 34·· a control signal is applied through the Zener diode 3 to the gate electrode 41 of the SCR 40 to drive the SCR 40 into conduction* However, since a pulsating voltage is applied between the conductors 1 and 2, the SCR which is driven Into conduction Is again turned off when the voltage becomes zero. This undesirable action is prevented by the holding circuit 60i More specifically, since the capacitor 9 of the holding circuit 60 Is discharged through the resistor 7 and the SCR 40 when the voltage between the conductors 1 and 2 becomes zero* the SCR 40 is maintained always In conduction by this discharge current* This conduction of the SCR 40 results in an interconnection of the both conductors 1 and 2 through the resistor 7 having a relatively low value, the current flowing through the conductors 1 and 2 is Increased and this current increase is detected to give an alarm.

Experimental results showed that sensitivity of the above-mentioned ionization smoke detector scarcely varies and is practically usable so long as the root mean square value of the pulsating voltage applied between the conductors 1 and 2 is maintained substantially constantly. It 1B of oourse 1 that the ionization smoke detector aocordlng to this embodiment is also usable with a sufficiently smoothened effective for economy of Installation.

Figure represents the second embodiment of Ioniza on " smoke detector according to this Invention* which Is arranged to be operable more reliably with a non-stabilized source voltage by providin with a protection circuit for exclusive use for the field effect transistor so that the field effect transistor having low breakdown voltage Is never applied with a> voltage higher than a predetermined value. In the drawing, the components Identical with or analogous to those In Figure 1 are given the same reference numerals as in Figure 1, and description of these components is omitted.

The drain electrode 32 of the field effect transistor 30 Is connected through the collector-emitter path of a transistor 15 to the conductor 1. The base electrode of the transistor 15 Is connected to a Junction 18 of a resistor 16 and a Zener diode 17 which, are connected in series between the conductors 1 and 2. Therefore, the base electrode of the translator 15 is maintained at a fixed voltage determined by the Zener diode 17. In other words, the transistor 15» the resistor 16 and the Zener diode 17 constitute a constant voltage protection circuit for the field effect transistor 30.

In the present embodiment of ionization smoke detector, if the voltage between the conductors 1 and 2 rises, the voltage at the drain electrode 32 of the field effect transistor 30, that is, at the emitter electrode of the translator 15 also rises. However, since the voltage at. the base electrode of the transistor 15 Is maintained always constant as described In the above, this xaxtsfcH voltage Increase at the emitter electrode of the transistor 15 results in.an Increase of the Impedance of the emitter-collector path of the transistor 15 and a consequent increase of the voltage across the emitter- collector path. Thus, the voltage at the drain electrode 32 of the field effect transistor 30 can be maintained under a predetermined value, In addition to . the advantages of the ionization smoke detector according to the first embodiment as abovementioned, the present ionization smoke deteotor according to theseoond embodiment has such an advantage as follows. The field effeot transisto having low breakdown voltage is never damaged even by a transient high, voltage from a rela or bell provided In the side of the voltage source. Moreover, the Ionization Hmoke deteotor can be operated without fear of damaging the field effect transistor also by a voltage source of low quality exhibiting large voltage fluctuation.

The above embodiment is merely for an Illustrative purpose and various modifications and changes can be made without leaving the spirit and scope of this invention. For example, though the abovementioned embodiment Is shown as having a single closed Ionization chamber and a single open ionization ohamber respectively Including a single pair of electrodes and a single radioactive source, it is' of course that the practical ionization smoke deteotor according to this invention can be constructed as having any number of elesed and open ionization chambers respectively having any number of electrodes and radioactive sources in accordance with the purpose of utility. The number of the field effect transistors can be arbitrarily selected also.

Claims

35151/2 Claims:

1. An Ionization smoke detector, comprising a pair of terminals, a closed ionization chamber and an open Ionization chamber connected 1n series betweewisald pair of terminals, said Ionization chambers each including a pair of electrodes and a radioactive source, a field effect transistor having a gate electrode connected to the connection point between said both Ionization chambers, a drain electrode connected to one of said terminals and a source electrode connected through a load resistor to the other terminal, a silicon controlled rectifier having a control electrode connected through a Zener diode to the source electrode of said field effect transistor and a conduction path connected between said both terminals, characterized by a bipolar transistor having an emri.t†;er-col lector path connected between the drain electrode of said field effect transistor and said one terminal, a resistor connected between the base electrode of said bipolar transistor and said one terminal and a Zener diode connected between the base electrode of said bipolar transistor and said other terminal.

2. An Ionization smoke detector substantially as hereinbefore described and with reference to the accompanying drawings. For the Applicants Wolff, Bregman and Goller