US3662177A

US3662177A - Ionization type smoke detectors with fet protected against open circuit input

Info

Publication number: US3662177A
Application number: US830817A
Authority: US
Inventors: Koju Sasaki; Akihiro Kobayashi
Original assignee: Nittan Co Ltd
Current assignee: Nittan Co Ltd
Priority date: 1968-07-11
Filing date: 1969-06-05
Publication date: 1972-05-09
Anticipated expiration: 1989-05-09
Also published as: BE735386A; DE1933938A1; GB1277091A; DE1933938B2; NL6910256A; CH512121A

Abstract

In an ionization type smoke detector wherein smoke is introduced in an ionization chamber through a mesh electrode and the change in the ionization current caused by the smoke is detected by the potential difference between the gate and source electrodes of a field effect transistor, means is provided to short circuit the gate and source electrodes in response to the removal of the mesh electrode.

Description

United States Patent [151 3,662,177 Sasaki et al. 1 May 9, 1972 [54] IONIZATION TYPE SMOKE [56] References Cited DETECTORS WITH FET PROTECTED AGAINST OPEN CIRCUIT INPUT UNTED STATES PATENTS 3,447,137 5/1969 Feuer ..307/279 X [72] Inventors: Koju Sasaki, Tokyo; Akihiro Kobayashi, 3 500 3 3 1970 150 1 X Fujlsawa; both oflapan 3,514,603 5 1970 Klein... ..250/44 x 'tt I. k [73] Asslgnee NI an Company muted To yo Japan Primary Examiner-James W. Lawrence [22] Filed: June 5, 1969 Assistant E.\aminerDavis L. Willis Armrne\'-Chittick, Pfund. Birch, Samuels & Gauthier [21] Appl.No.: 830,817

ABSTRACT Foreign Application Priority Data In an ionization type smoke detector wherein smoke is in July 1 l [968 Japan 43/585 18 troduced in an ionization chamber through a mesh electrode and the change in the ionization current caused by the smoke is detected by the potential difference between the gate and [52] i source electrodes ofa field effect transistor, means is provided [51 1 C n to short circuit the gate and source electrodes in response to [58] Field of Search 50/44, 83.6, 83.6 FT, the removal Ofthe mesh electroda 3 Claims, 4 Drawing Figures FET 25 3* i' G 23 PATENTEUMAY 9 I972 3,662,177

INVENTORS KOJU SASAKI AKIHIRO KOBAYASHI BACKGROUND OF THE INVENTION This invention relates to an ionization type smoke detector and more particularly to the improvent of an ionization type smoke detector utilizing a field effect transistor wherein damage of the transistor caused by the removal of a mesh electrode can be prevented.

FIG. 1 of the accompanying drawing is a connection diagram to explain the principle of an ionization type smoke. detector 1 comprising an enclosed ionization chamber 3 defined by an electroconductive isolating plate 2 and an open ended ionization chamber 5 defined by a mesh electrode 4. A radioactive substance 6 is mounted on the bottom of the isolating plate 2 to irradiate ionizing chambers 3 and 5 with radiations. A field effect transistor FET is provided having a gate electrode G connected to the isolating plate 2, a drain electrode D connected to an electrode 7 disposed in the enclosed ionization chamber 3 and a source electrode S connected to the mesh electrode 4. When a fire hazzard occurs and smoke enters into the open ended ionization chamber 5 through openings of mesh electrode 4, the ionization current flowing through the ionization chamber 5 will be decreased by the disturbance caused by the smoke. This results in the variation of the potential between the gate electrode G and the source electrode S of the field effect transistor. The potential variation is applied to the gate electrode of a silicon controlled rectifier element SCR via a Zener diode D to alarm the occurrence of the fire hazard.

In the detector of the type outlined above it is frequently necessary to remove the mesh electrode 4 to clean the surface contamination of the radioactive substance 6 or to clear inner and outer surfaces of the mesh electrode 4. Removal of the mesh electrode 4 means the interruption of the connection between the gate electrode G and the source electrode S of the field effect transistor. Since the input impedance of the field effect transistor is extremely high, that is of the order of about 10 ohms, upon interruption of the connection between the gate electrode G and the source electrode S, a high voltage is generated between these electrodes thus damaging the field effect transistor FET. Thus, the static electricity generated when the radioactive substance is cleaned causes to apply a high voltage to the field effect transistor owing to said high input impedance, such voltage often exceeding the insulation breakdown voltage of the field effect transistor which is of the order of 150 volts.

SUMMARY OF THE INVENTION It is therefore an object of this invention to protect the field effect transistor against the high voltage generated when the mesh electrode is removed.

This invention contemplates to automatically short circuit the gate electrode G and the source electrode S when the mesh electrode is removed.

According to this invention there is provided an ionization type smoke detector comprising an enclosed ionization chamber defined by an electroconductive isolating plate and adapted to contain a radioactive substance, an open ionization chamber defined by said isolating plate and a mesh electrode, an electrode disposed in the enclosed ionization chamber, and a field effect transistor including a gate electrode connected to the isolating plate, a drain electrode connected to the electrode and a source electrode connected to the mesh electrode characterized by means interlocked with the removal of the mesh electrode to electrically short circuit the source electrode and the gate electrode.

BRIEF DESCRIPTION OF THE DRAWING This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawing in which:

FIG. 1 is a connection diagram to explain the principle of an ionization type smoke detector;

FIG. 2 is an enlarged cross-sectional view of a portion of an ionization type smoke detector embodying this invention;

FIG. 3 shows a portion of the detector shown in FIG. 2 when the mesh electrode is removed and FIG. 4 is a view similar to FIG. 3 illustrating a modified embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 2 in an ionization type smoke detector of the type described above, the field effect transistor FET and the enclosed ionization chamber 3 are usually disposed in a cylindrical housing 20 and the mesh electrode 4 is secured to an annular ring 21 which is threaded into the lower end of the inner wall of the casing 20. The annular ring 21 and the mesh electrode 4 are electrically connected to the source electrode S of the field effect transistor F ET. The drain electrode D and the gate electrode G of the field effect transistor are connected to the electrode 7 and the isolating plate 2, respectively.

According to this invention an electroconductive resilient piece 22 is secured to the inner wall of the casing 20 and the assembly of the mesh electrode 4 and the annular ring 21 is provided with a upright rod 23 cooperating with the resilient piece 22. The upper end of a screw 25 for securing the electroconductive isolating plate 2 to a support 24 is projected upwardly for a sufficient length beyond the upper surface of the support 24.

With this construction, when the mesh electrode 4 and the annular ring 21 are secured to the lower end of the casing 20, the rod 23 will urge upwardly the resilient piece 22 against the resiliency thereof as shown in FIG. 2 An equivalent circuit under this condition is shown in FIG. 1. When the ring 21 is disengaged from the casing 20 to remove the mesh electrode 4, the resilient piece 22 comes to engage the upper end of the screw 25 as shown in FIG. 3. As the screw 25 is electrically connected to the isolation plate 2, the gate electrode G and the source electrode S of the field effect transistor FET are short circuited. Thus, when the mesh electrode is removed for cleaning, this short circuit prevents the field efiect transistor from being damaged by static electricity applied to the gate electrode.

In a modified embodiment shown in FIG. 4 rod 23 is integrally formed with the resilient piece 22 to urge the lower end of the rod 23 against the upper surface of the annular ring 21. Thus, when the ring 21 is removed, the resilient piece 22 engages the screw 25 to effect the desired short circuiting. The rod 23 may be substituted by a cylindrical operating member (not shown) concentric with the casing 20. Such an operating member may be slidably disposed to be pushed upwardly when the annular ring 21 is threaded into the casing 20. The electrical connection between the source electrode of the field effect transistor F ET and the mesh electrode may be provided either through the engagement between the casing 20 and the annular ring 21, as shown in the drawing or through rod 23 of a conductor. Further instead of electrically interconnecting the resilient piece 22 and the isolating plate 2 by means of screw 25, the upper end of the isolating plate 2 may be bent upwardly above the support 24 to establish the desired electrical connection.

As has been described hereinbefore, in accordance with this invention, when the mesh electrode is removed the source electrode of the field effect transistor and the isolating plate to which the gate electrode is connected are electrically connected to short circuit the source and gate electrodes and when the mesh electrode is again mounted in position, the electroconductive resilient piece opens said short circuit, whereby damage of the field effect transistor inherent to the prior art ionization type smoke detector can be effectively prevented. It is to be understood that this invention can equally be applied to the protection of other elements having high input impedances than the field effect transistor, for example unijunction transistors and the like.

What is claimed is:

1. In an ionization type smoke detector comprising an enclosed ionization chamber defined by an electroconductive isolating plate and adapted to contain a radioactive substance, an open ionization chamber defined by said isolating plate and a mesh electrode, an electrode disposed in said enclosed ionization chamber, and a field effect transistor including a gate electrode connected to said isolating plate, a drain electrode connected to said electrode disposed in said enclosed ionization chamber and a source electrode connected to said mesh electrode, the improvement which comprises switch means connected between said source and gate electrodes, and actuator means associated with said mesh electrode for open circuiting said switch means when said mesh electrode is in place on said open ionization chamber and for closing said switch means when said mesh electrode is removed to short circuit said source electrode and said gate electrode.

2. The ionization type smoke detector according to claim 1 wherein said enclosed ionization chamber and said field effect transistor are disposed in a cylindrical casing, said mesh electrode is removably connected to the lower end of said cylindrical casing by means of an annular ring and said switch means comprises an electric contact connected to said gate electrode, a resilient member connected to said source electrode and normally urged against said electric contact and said actuator means comprises an extension mounted on said annular ring adapted to engage said resilient member to cause it to disengage from said electric contact.

3. The ionization type smoke detector according to claim 1 wherein said enclosed ionization chamber and said field effect transistor are disposed in a cylindrical casing, said mesh electrode is removably connected to the lower end of said cylindrical casing by switch means of an annular ring and said means comprises an electric contact connected to said gate electrode, a resilient member connected to said source electrode and normally urged against said electric contact and said actuator means comprises an extension mounted on said resilient member depending toward said annular ring and adapted to be engaged thereby for opening said electric contact.