AN IONIZATION-TYPE SMOKE DETECTOR

Abstract: In order to provide an ionization-type smoke detector, wherein the radiation from a radioactive source mounted on the inner electrode disposed within the inner ionization chamber also reaches into the outer ionization chamber through a through-hole formed in the intermediate electrode, which is easy to assemble and which has a constitution to prevent scattering of the radioactive source even though inner and outer ionization chambers are provided with the insulation base board for supporting the inner and inter­ mediate electrodes at a predetermined spacing is connected to the printed circuit board by a mounting rod, and the outer electrode surrounding the intermediate electrode and the in­ sulation base board with smoke inlet openings is connected at its end periphery to the printed circuit board, the printed cir­ cuit board being connected to the base of the detector by con­ necting pins.

Description: BACKGROUND OF THE INVENTION The present invention relates to a smoke detector, and more particularly to an ionization-type smoke detector. An ionization-type smoke detector has been known in which an ionization chamber is provided to detect the presence of smoke, and when smoke enters the ionization chamber the change in the ion current therein is transformed into an alarm signal, issuing a fire alarm. Although it has been known in a smoke detector of this kind to provide only one ionization chamber this has a drawback in that its sensibility is affected by changes in characteristics in regards to change in enviromental temperature, or gradual increases in smoke concentration. To avoid such a drawback in WO-A-85101110 an ionization-type smoke detector has already been proposed in which in addition to an outer ionization chamber into or out of which environmental air is free to enter or exit, an inner ionization chamber is provided which has a hermetically sealed mechanism independent from the environment, the inner electrode within the inner ionization chamber mounting a radioactive source, the radiation of the radioactive source penetrating into the outer ionization chamber through a through-hole formed axially of the intermediate electrode. Further, it has also been proposed that in order to prevent the radioactive source contained in the detector from scattering outwards when the outer case made of synthetic resin is melted due to a fire the radioactive source is incased within a metallic case so that it can fall to the floor together with the case. Thus, it will be understood that hitherto known ionization-type smoke detectors have required an insulation base board having a complicated constitution, and since the main components of the detector such as the inner, intermediate and outer electrodes are mounted on the base board by screws, pins, etc. it has a complicated construction, requiring long assembly time, making for poor productivity. Further, since the shield case and the case for preventing the radioactive source from scattering outside in case of the fire are independently made of metal or other suitable material the cost of the detector becomes more expensive. SUMMARY OF THE INVENTION It is a primary object of the present invention to provide an ionization-type smoke detector which can eliminate substantially all of the constructional drawbacks exemplified above inherent to conventional smoke detectors of this kind. It is another object of the present invention to provide an ionization-type smoke detector in which the means for preventing the radioactive source from scattering outside in case of fire and the means for electrical protection of the electrical circuit such as an electrical shield have a simple constitution, allowing easy assembly even though inner and outer ionization chambers are provided. It is a further object of the present invention to provide an ionization-type smoke detector in which the radioactive source can be retained within the shield case even if the detector falls for example from the ceiling to which it is mounted onto eg. g. the floor, so that the scattering of the radioactive source outside is effectively prevented. In accordance with the present invention an ionisation-type smoke detector is proposed in which the mounting rod also connects the insulation base board to the printed circuit board, the outer electrode at one of its ends is connected to the printed circuit board, connecting pins rigidly connect the printed circuit board to the base of the smoke detector and mount a cap-like shield plate made of metal and a cup-shaped shield case surrounds the outer electrode and the printed circuit board and is held at the periphery of its open end to the shield plate. In the new detector the printed circuit board instead of the insulation base board is the central supporting element to which both the inner electrode and the insulation base board are connected together by one single element, i. e. the mounting rod. The form of the insulation base board can be simplified. When mounting the electrodes on the printed circuit board its electrical function can be tested before the detector is completed and defective parts eliminated in an early state of assembly. Finally, the same means, i. e. the connecting pins connect the printed circuit board to the detector base and to the metal shield case and plate the latter besides its electrical shielding function having the further function of preventing scattering of radioactive material when the detector is partly melted and burnt and falls down from a wall or ceiling. BRIEF DESCRIPTION OF THE DRAWINGS The above brief description, as well as further objects, features and advantages of the present invention will be more fully understood by reference to the following detailed description of the presently preferred, but nonetheless illustrative embodiment, in accordance with the present invention, when taken in conjunction with the accompanying drawings, wherein: Fig. 1 is a side elevational view of one embodiment of the present invention in section, and Figs. 2 and 3 illustrate schematically the relationships between the outer electrode and the shield casing shown in Fig. 1 after the detector shown in Fig. 1 falls from the ceiling onto the floor in case of fire. These figures exemplify the two typical configuration conceivable. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to Fig. 1 of the drawings, reference character A designate an ionization-type smoke detector, reference numeral 1 a printed circuit board provided with detecting circuits containing a field effect transistor, the circuit board also having an insulation base board 2 to support the inner electrode 5 and the intermediate electrode 8 at a predetermined spacing. The insulation base board 2 is formed centrally thereof with a through-hole 3 and peripherally with a plurality of supporting rods 4 for supporting the intermediate electrode 8, the inner electrode 5 being screwed to the printed circuit board 1 by a mounting rod 7 passing through the through-hole 3 formed in the insulation board 2. The intermediate electrode 8 is secured to the insulation board 2 by screwing the electrode 8 to the supporting rods 4. In this case the central through-hole 9 formed in the intermediate electrode 8 is caused to confront the radioactive source 6 mounted on the inner electrode 5. The intermediate electrode 8 and the insulation base board 2 are surrounded by a cylindrical outer electrode 11 which has a small diameter portion a formed with smoke inlet openings 12 and a large diameter portion b which has a number of engaging pieces 13 protruding from its end periphery, said pieces being adapted to be introduced into the corresponding openings 14 formed in the printed circuit board 1 and clamped or soldered thereto. Defined between the inner electrode 5 and the intermediate electrode 8 is an inner ionization chamber 15, the air within this chamber 15 being ionized by the radioactive source 6, and in this case the air within the outer ionization chamber 16 which is defined between the intermediate and outer electrodes 8, 11 is also ionized by the radiation penetrating into it through the central through-hole 9 formed in the intermediate electrode 8. The printed circuit board 1 is secured to a detector base board 19 by connecting pins 17 provided with a cap-like shield plate 18 made of metal, one end of the pins 17 passing through the printed circuit board 1, and the other ends connected to connecting terminal members 20 at the opposite side of the detector base board 19 to be secured to the connecting terminal members 20 through such means as soldering. The connecting terminal members 20 serve to connect themselves mechanically and electrically to the corresponding connecting terminal members (not shown) of the base mounted on the ceiling of a room within which the smoke detector is to be installed. As the fixing means, passing-through condensers may be utilized instead of the connecting pins 17. The cap-like shield plate 18 is provided at its periphery with a number of spring means 23 which serve to firmly elastically hold the outer periphery of a cup-shaped shield case 21 which has a shape to surround the large diameter portion b of the outer electrode 11 and the printed circuit board 1, and which is formed with an opening 22 to loosely fit the small diameter portion a of the outer electrode 11. The height c of the shield case 21 is so selected that it is substantially equal to or lower than the height d of the outer electrode 11. The shield case 21 and the base board of the detector 19 are further surrounded by a detector cover 24 provided with smoke inlet openings 25, and the cover 24 is integrally secured to the base board 19 by engaging the hook-like engaging pieces 26 protruded from the upper end periphery with the stepped engaging portions 27 formed in the inner periphery of the base board 19. The outer electrode 11 is provided at its large diameter portion b with a number of protruding pieces 29 the height of each of which is so selected that even if the shield plate 18 is deflected the contact of the shield plate 18 with the electrical parts on the printed circuit board 1 is still effectively prevented. Since the smoke detector according to the present invention has such a constitution as described above, on assembly the printed circuit board 1 is first secured to the insulation board 2 by fastening the mounting rod 7 of the inner electrode 5 between them, the intermediate electrode 8 being screwed to the insulation base board 2, and the outer electrode 11 then being secured to the printed circuit board 1 by inserting the engaging pieces 13 of the former into the corresponding openings 14 of the latter to be rigidly fastened together. Thus the intermediate electrode 8 and the insulation base board 2 are surrounded by the outer electrode 11. Thereafter the printed circuit board 1 is mounted to the base board 19 of the detector by the connecting pins 17 provided from shield plate 18, the cup-shaped shield case 21 being trapped by the spring means 23 integrally formed around the shield plate 18 after the opening 22 formed centrally of the shield case 21 passed over the small diameter portion a of the outer electrode 11. The following describes the operation of the smoke detector according to the present invention after having thus been assembled. The radioactive source 6 mounted on the inner electrode 5 ionizes the air within the inner ionization chamber 15 as well as the outer ionization chamber 16 through the central through-hole 9 formed in the intermediate electrode 8. In this condition, should there be a fire and the resulting smoke having a predetermined concentration enters the outer ionization chamber 16 through the smoke inlet openings 25 and 12 formed in the cover 24 and the outer electrode 22, respectively, the ionization current flowing between the outer and intermediate electrodes 11 and 8 decreases by a predetermined value. This change in electrical current is detected by the detecting circuit of the printed circuit board 1, and transmitted to the receiver not shown through the connecting terminal members 20 and electrical wires to report the occurrence of a fire. After the outbreak of the fire has been reported, if the fire further develops and increase in temperature the detector cover 24 will start to melt and the solder holding the connecting pins 17 and the connecting terminal members 20 together will also eventually melt, so that the detector will drop from the ceiling onto the floor. In this case, even if the outer cover 24 of the smoke detector has substantially melted, since the shapes and heights of the outer electrode 11 and the shield case 21 are suitably selected the shield casing comprising the shield plate 18 and the shield case 21 is in a state as indicated in Fig. 2 or 3 after it has dropped onto the floor. Thus, it will be appreciated that even if the shield casing has dropped onto the floor the radioactive source 6 will still be retained within the metallic shield casing and not be scattered outside. Further, even if the detector case 28 becomes deformed, or the shield casing 18, 21 itself becomes deformed, due to deteriolation of material with time, resultant slacking of the shield plate 18 downwards is blocked by the protruding pieces 29 of the outer electrode 11, thus preventing contact of the shield plate 18 with the electrical parts of the printed circuit board 1. Although such deformation may also be caused by vibration carried through the surface of the ceiling on which the smoke detector is mounted the possible contact of the shield plate 18 with the electrical parts of the printed circuit board 1 can be similarly prevented by the protruding pieces 29 formed on the outer electrode 11. From the foregoing it will be appreciated that the present invention reveals such various excellent effects as exemplified below: i) The quantity of screws, pins, etc. which parts require a relatively long time for assembly can be reduced a great deal; ii) The assembly time is therefore, shortened, improving productivity; iii) Since (a) the printed circuit board is mounted on the detector base by connecting rods provided with a cap-like shield plate and (b) the cup-shaped shield case which has a height lower than the height of the outer electrode and is formed with an opening to loosely fit over the small diameter portion of the outer electrode surrounds the large diameter portion of the outer electrode and the printed circuit board and (c) they are all surrounded by the cup-shaped shield case formed with smoke inlet openings, even if the detector may drop from the ceiling onto the floor due to the melting of the solder connecting the connecting pins to the detection base board by a fire the radioactive source remains within the shield casing, to effectively prevent scattering of the radioactive source outside; and iv) An inexpensive ionization type smoke detector with less height or smaller size and well protected electrical circuits can be obtained.

Claims: An ionisation-type smoke detector wherein radiation from a radioactive source (6) mounted on an inner electrode (5) disposed within an inner ionisation chamber (15) also reaches an outer ionisation chamber (16) through a through-hole (9) formed in an intermediate electrode (8), said smoke detector comprising an insulation base board (2) supporting the inner electrode (5) and the intermediate electrode (8) in a predetermined space, a printed circuit board (1), a mounting rod (7) fixing the inner electrode (5) to the printed circuit board (1) and an outer electrode (11) surrounding the intermediate electrode (8) and the insulation base board (2) and formed with smoke inlet openings (12), characterized in that the mounting rod (7) also connects the insulation base board (2) to the printed circuit board (1), the outer electrode (11) at one of its ends is connected to the printed circuit board (1), connecting pins (17) rigidly connect the printed circuit board (1) to the base (19) of the smoke detector and mount a cap-like shield plate (18) made of metal and a cup-shaped shield case (21) surrounds the outer electrode (11) and the printed circuit board (1) and is held at the periphery of its open end to the shield plate (18). An ionisation-type smoke detector as claimed in claim 1 wherein the outer electrode (11) comprises two integral concentric cylindrical bodies (a, b) having small and large diameters, respectively, and is adapted to surround the intermediate electrode (8) and the insulation base board (2), said large diameter cylindrical body (b) being connected to said printed circuit board (1) by inserting free end portions of engaging pieces (13) protruding therefrom into corresponding openings formed in the printed circuit board (1), the shield case (21) having a cup-shaped configuration with a central hole (22) to loosely fit the small diameter cylindrical body (a) and the height of the shield case (21) being less than the height of the outer electrode (11), whereby the shield case (21) surrounds the large diameter cylindrical body (6) of the outer electrode (11) and the printed circuit board (1). An ionisation-type smoke detector as claimed in claim 2 wherein the outer electrode (11) is provided at the periphery of its open end with protruding engaging pieces (13) to be engaged with the printed circuit board (1) and protruding pieces (29) to keep a space between the shield case (21) and the printed circuit board (1).

Inventors: IGARASHI, YOSHINORI NOHMI BOSAI KOGYO CO. LTD., SASAKI, TORU NOHMI BOSAI KOGYO CO. LTD.
Applicants: NOHMI BOSAI LTD.
Application year: 1986
Main classification: G08B17
IPC codes: G08B17