AU645767B2

AU645767B2 - Heat sensor

Info

Publication number: AU645767B2
Application number: AU68414/90A
Authority: AU
Inventors: Isao Asano; Shuzou Minowa
Original assignee: Hochiki Corp
Current assignee: Hochiki Corp
Priority date: 1989-12-25
Filing date: 1990-12-21
Publication date: 1994-01-27
Anticipated expiration: 2010-12-21
Also published as: AU6841490A; GB9027822D0; DE4040639A1; FI906243A0; FI906243A; GB2239709B; GB2239709A

Description

Our Ref: 349027 6451

AUSTRALIA

Patents Act FORM COMPLETE SPECIFICATION

(ORIGINAL)

Application Number: Lodged: Complete Specification Lodged: Accepted: Published: c a.

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Priority: Related Art: Applicant(s): Address for Service: a o a.

*a S Hochiki Corporation 10-43, Kamiohsaki 2-chome Shinagawa-ku

TOKYO

JAPAN

ARTHUR S. CAVE CO.

Patent Trade Mark Attorneys Level 10, 10 Barrack Street SYDNEY NSW 2000 Complete specification for the invention entitled "Heat sensor".

The following statement is a full description of this invention, including the best method of performing it known to me:- 1 5020 HEAT SENSOR BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a heat sensor employing a heat sensitive electronic element such as a thermistor to detect a temperature rise due to the occurrence of a fire.

Description of the Prior Art A heat sensor of the type employing an electronic element such as a thermistor as its heat sensitive element has itself been known in the art. Reference is had for example to Japanese Patent Laid-Open No. 31697/1985.

The problem with this type of heat sensor is the sealing between a heat sensitive element arranged to project from a sensor housing and the housing so that if the sealing is not

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perfect, external gas or steam is introduced into the housing during its use thus causing damages to the electronic circuit devices within the housing.

C. Typical two examples of the heat sensitive element mounting structure in the conventional heat sensors of this type are shown in Figs. 4 and 5, respectively.

In Fig. 4, numeral 41 designates a sensor housing which is composed of an integrally molded plastic member formed therein with a cavity 42 for accommodating electronic circuitry. While the upper surface wall of the housing 41 can be opened to permit access to the inside of the cavity 42 la during the assembly or adjustment of the sensor, it is hermetically closed as shown in the Figure during the use of the sensor as a product. Accommodated within the cavity 42 is a printed plate board 43 having electronic components mounted thereon. Formed through a lower surface wall 44 of the cavity 42 is a hole 45 having an inner diameter slightly greater than the diameter of a lead wire 47 of a heat sensitive element 46 and the lower surface wall 44 is substantially hermetically closed excepting the hole 45. The *M 0 heat sensitive element 46 such as a thermistor is mounted in 0*40 such a manner that its head heat sensitive portion is .o positioned at an external central position of the lower S surface wall 44, and in this case the lead wire 47 of the heat sensitive element 46 is electrically connected through the hole 45 of the lower surface wall 44 to the printed plate *000 board 43 inside the cavity 42 and simultaneously the heat 0 00 0 sensitive element 46 is mechanically supported by the printed a s: plate board 43. In addition, a whole surface coating of a ~paint or the like is applied to the printed plate board 43, the heat sensitive element 46 and the lead wire 47 to protect .Z them from steam, corrosive gases and the like.

With this type of conventional heat sensor construction, the quality of the coating determines the reliability of the sensor against the steam and corrosive gases so that the coating operation of the components must be effected under a satisfactory quality control and this complicates the manufacture and processing operations of the sensor and hence increases. the manufacturing cost.

On the other hand, referring to the conventional structure of Fig. 5, numeral 51 designates a sensor housing which is composed of an integrally molded plastic material formed therein with a cavity 52 for accommodating electronic circuitry. A lower surface wall 54 of the housing 51 is formed with a relatively large opening 55 in the central portion thereof and a rubber-like plastic packing 58 is hermetically fitted in the opening 55 such that its upper edge reaches a printed plate board 53. A heat sensitive ~element 56 has a lead wire 57 which is connected to the 0 0 printed plate board 53 through the packing 58 and thus the

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head heat sensitive portion of the heat sensitive element 99 56 is maintained on the outer side of the lower surface wall 54.

While this conventional construction requires that it is necessary to apply a coating for protection from steem and corrosive gases to only the head of the heat sensitive element 56 and the lead wire 57 with the resulting 'i simplification of the coating operation, the fine lead wire 57 is simply exterded through the rubber-like plastic packing 58 with the result that a gap t ends to be caused in the portion through which the lead wire is extended, and particularly when the heat sensitive element 56 is displaced relative to the packing 58 for position adjusting purposes, the gap between the lead wire 57 and the packing 58 is increased and it is impossible to ensure the hermetic and corrosion resisting properties of the circuit accommodating cavity 52 during a long period of use.

The previously mentioned Japanese Patent Laid-Open No.

31697/1985 shows a similar heat sensor employing a thermister as a heat sensitive element. In this sensor, the thermustor portion projected through the sensor housing or the head heat sensitive portion of the thermistor and the lead wire following the head portion are respectively sealed into a glass material and a ceramic heat insulating material.

However, this sensor has the possibility of the presence of a gap between the sealing portions and a gap between the lead wire sealing portion and the housing wall surface and thus there is the disadvantage that these gaps are increased and the sealing effects are deteriorated during a long period of use.

SUMMARY OF THE INVENTION 9 o0 It is the primary object of the present invention to provide a heat sensor which overcomes the foregoing -P deficiencies in the prior art and which has a highly corrosion resistive construction capable of permanently and highly satisfactorily maintaining the hermetic sealing properties of a housing for electric circuitry including a heat sensitive electronic element without complicating the assembling :nd adjusting operations.

In accordance 1lith the basic idea of the present invention, a heat sensor includes a substantially hermetic sensor housing having an electronic component accommodating cavity and a hole formed through its wall surface at a given position thereof for lead wire insertion purposes, electronic circuitry carrier means arranged inside cavity of the housing and a heat sensitive electronic element supported on the electronic circuitry carrier means through its lead wire inserted through the hole such that its heat sensitive head is placed in a position outwardly projected from the wall surface of the sensor housing, and the heat sensor features that a pin-shaped ploe is integrally molded with the housing wall surface to project outwardly from the portion of the wall surface including the hole and the pole contains the 6 whole projected portion including therein the heat sensitive Shead of the heat sensitive electronic element in a completely hermetic manner against the outside of the housing.

In accordance with a preferred aspect of the present invention, the pin-shaped pole is in the form of a hollow cylindrical member having the closed forward end with the inside of the cylindrical mamber being communicated with the cavity through the hole, and the heat sensitive electronic element connected and supported on the electronic circuitry carrier means has its heat sensitive head inserted into the a 0 cylindrical Tnember.

In accordance with another preferred aspect of the present invention, a heat conductive filler is filled between the heat sensitive electronic element inside the cylindrical member and the inner wall surface of the cylindrical member.

In accordance with still another preferred aspect of the present invention, the pin-shaped pole is composed of a solid plastic component having the heat sensitive electric element embedded therein and the plastic component is integrally bonded to the wall surface of the housing by mold forming.

In accordance with still another preferred aspect of the present invention, a heat insulating material is embedded in the housing wall surface so as to surround the hole in the root portion of the pin-shaped pole.

In accordance with still another preferred aspect to the present invention, an annular depression is formed in the outer surface of the housing wall surface to surround the root potion of the pin-shaped pole.

In accordance with still another aspect of the present invention, the pin-shaped pole is made from a heat conductive plastic material which is different from the housing wall surface.

In accordance with still another aspect of the present ~invention, the pin-shaped pole is made from a plastic material mixed with a filler having heat conductivity improving properties such as metal powder.

In accordance with still another aspect of the present invention, the pin-shaped pole is provided with a metal coating at least on the outer surface at the position corresponding to the heat sensitive head.

In accordance with still another aspect of the present invention, the pin-shaped pole includes at least one integrally formed fin on the outer surface at the position corresponding to the heat sensitive head.

In accordance with still another aspect of the present inventibn, the pin-shaped pole includes at least one metallic heat collecting member on the outer surface in the vicinity of the position corresponding to the heat sensitive head.

In accordance with the heat sensor having such construction of the present invention, the hole formed through the housing wall surface is completely closed by the pin-shaped pole so that the electronic circuit carrier means such as a printed plate board arranged in the cavity inside the housing is completely isolated from the outside and the heat sensitive electronic element such as a thermistor connected by its lead wire to the electronic circuitry r carrier means is also accommodated in an externally closed manner in the pin-shaped pole integrally molded with the housing wall, thereby completely eliminating the need for any coating treatment of the electronic components, enhancing the hermetic sealing properties of the housing for its portion goo* accommodating the electronic circuit components including the heat sansitive electronic element and improving the corrosion resisting properties of the housing.

The above and other constructions, features and "goo.: advantages of the present invention will become more apparent from the following description of typical embodiments of the invention shown in the accompanying drawings by way of examples and not for the purpose of limitation.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a longitudial sectional view showing schematically the construction of a heat sensor according to an embodiment of the present invention.

Fig. 2a to 2g are partial sectional views respectively showing schematically the construction of only the principal part of heat sensors according to modified embodiments of the present invention.

Fig. 3 is a partial sectional view showing schematically the construction of only the principal part of a heat sensor according to still another embodiment of the present invention.

Figs. 4 and 5 are longitudinal sectional views respectively showing schematically the construction of S. conventional heat sensors.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 1, numeral 1 designates a sensor housing composed of an integrally molded plastic material having therein an electronic circuitry accommoding cavity 2.

'Provided on an upper surface wall 8 of the housing 1 are a pair of engaging terminals 11 which are adapted to be

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mechanically and electrically connected to a base member not shown fixed to a mounting surface such as the ceiling of a building. While the upper surface wall 8 can be opened to permit access to the cavity 2 during the assembling or adjusting of the sensor, it is hermetically closed as shown in the Figur-e during the use as a product.

A printed plate board 3 having electric components mounted-thereon is accommodated within the cavity 2 and it is fastened to the housing 1 with small screws 16. The printed plate board 3 is electrically connected to the engaging terminals 11 on the back side through electrically conductive members 17 provided on the side of the sensor housing i.

Formed through a lower surface wall 4 of tLe cavity 2 at substantially the central position thereof is a hole 5 having an inner diameter slightly greater than the diameter of the head heat sensitive portion of a heat sensitive element 6 9:60 such as a thermistor, and a hollow pin-shaped pole 10 having the closed forward end is integrally molded with the portion S of the lower surface wall 4 enclosing the hole 5 so as to project toward the outer side and to thereby to completely se"" hermetically close the cavity 2. In other words, in accordance with the present embodiment, when forming the housing 1 by injection molding a plastic material such as °mro

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polyethylene terephthalate, the pole 10 is integrally molded by a preliminarily prepared injection mold in such a manner that the pole 10 projects outwardly from around the hole 5 in the lower surface wall 4 and in this case the forward end S..o area of the pole 10 has a wall thickness which should preferably be as thin as about 0.3 to 0.8 mm.

Also molded integrally with the outer surface of the lower surface wall 4 are a plurality of radial fins 14 which provide protection against mechanical interference on the outer side of the pole 10, and a hollow portion 15 which tends to prevent the entry of a finger or the like is formed in the central area surrounded by the fins 14, thereby positioning the pole 10 at substantially the center of the aollow portion 15. This protective structure may be provided by integrally molding a cage-like structure with the housing 1 in place of the fins 14.

The heat sensitive element 6 comprising a thermistor or the like is mounted in such a manner that its head heat sensitive portion is positioned at the forward end inside the pole 10. In this case, a lead wire 7 of the heat sensitive element 6 is electrically connected by soldering or the like to the printed plate board 3 within the cavity 2 through the hole 5 in the lower surface wall 4 and simultaneously the heat sensitive element 6 is mechanically supported by the printed plate board 3.

With the heat sensor of the present embodiment, the heat sensitive element 6 comprising a thermistor or the like is received in the blinded hollow pole 10 integrally molded with the lower surface wall 4 and it is connected by soldering to the printed plate board 3 with the lead wire 7, thereby hermetically incorporating all the electric components including the heat sensitive element 6 and the lead wire 7 oo within the cavity 2. Thus, since the heat sensitive element h. 6 is not positioned so as to be exposed to the outside through the lead wire inserting hole as in the past, there is no need to provide a hermetic structure by the coating of the electric components, a packing and the like and it is possible to construct the heat sensor which practically completely protects all the electric components mounted on the printed plate board 3 including the heat sensitive element 6 from determental effects due to the external steam 1 0 and corrosive gases and which is extremely high in corrosion resisting performance.

Figs. 2a to 2g show respectively the principal parts of various modified embodiments of the present invention.

Firstly, in the modified embodiment of Fig. 2a, when accommodating the heat sensitive element 6 in the pin-shaped pole 10 which is integrally molded with the lower surface wall 4 by mold forming, a filer 12 having a good heat o° ~conductivity is used to fill in the gap between the inner surface of the pole 10 and the head of the heat sensitive

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9.9* element 6. As the materials for the filler, it is possible 9 *9

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to use for example the ordinary grease for heat sinking

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purposes as well as a soft plastic material such as polyethylene. In other words, in the embodiment of Fig. 1, there is the possibility of air being present in the gap at .9.9 the head of the heat sensitive element 6 received in the *09* pin-shaped pole 10 and therefore there is the danger of decreasing the efficiency of heat conduction from the pole to the heat sensitive element 6. Thus, in the modified embodiment of Fig. 2a, the filler 12 of a good heat •6 conductivity is used to fill in the gap at the forward end of the heat sensitive element 6 so that the response of the external conduction of heat to the heat sensitive element 6 through the partition wall of the pole 10 is made quicker and the detection response is improved.

In the modified embodiment of Fig. 2b, a ceramic heat insulating material 9 is embedded into the lower surface wall 4 of the housing 1 at the root portion of the pin-shaped pole 1 1 so as to surround the hole 5. In this case, the material 9 is divided into a plurality of small pieces and they are embedded into the lower surface wall 4 so as to be apart from each other and surround the hole 5. The material 9 acts so as to positively impede the heat conduction between the pole and the lower surface wall 4 and to prevent a temperature change caused in the pole 10 from decreasing due to the escape of the heat to the lower surface wall 4.

In the modified embodiment of Fig. 2c, an annular .f depression 18 is formed in the outer surface of the lower 4 surface wall 4 of the housing 1 so as to surround the root •o r portion of the pin-shaped pole 10. The depression 18 also r., i' ~acts so as to positively impede the heat condition between the pole 10 and the lower surface wall 4 and to prevent a temperature change caused in the pole 10 from being decreased 0999 4 90*4 due to the escape of the heat to the lower surface wall 4.

o In the modified embodiment of Fig. 2d, the pin-shaped e• pole 10d is made of a plastic material which is different in heat conductivity from the lower surface wall 4 of the housing i. In this case, in order to improve the heat 4 conductivity of the pole 10d portion, it is only necessary to mold the pole 10d portion by using singly or in the mixed form of two or more of plastic materials which are different from and .which can be made integral with the plastic material of the housing I, such as, polyethylene, polybutylene terephthalate, nylon and polyester. Alternatively, the pole portion may be made of a composite plastic material preparedby mixing a plastic material which is the same or 1 2 different and which can be integrally molded with the plastic of the housing 1 and a heat conductivity improving filler composed of powder of various metals or metal oxide or fiber.

In the modified embodiment of Fig. 2e, the pin-shaped pole 10e includes a metal coating 19 formed by plating or evaporation on at least its outer surface at the position corresponding to the head of the heat sensitive element. The metal coating 19 improves the heat receiving properties of the forward end area of the pole

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In the modified embodiment of Fig. 2f, the pin-shaped pole 10f includes a plurality of radial fins 20 at least on too the outer surface at the positions corresponding to the head of the heat sensitive element and the fins 20 are in the form of flat plates parallel to the axial direction of the pole The fins 20 are integral injection molded plates made *Sol ~of the same material as the pole 10f and they have any

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arbitrary shape provided that the heat response characteristic of the pole 10f is improved.

9 In the modified embodiment of Fig. 2g, the pin-shaped pole log includes a metallic heat collector plate 21 on the S: outer surface in the vicinity of the position corresponding to the head of the heat sensitive element. While, in this example, the single metallic heat collector plate 21 is used, if necessary, a plurality of such plates may be mounted one upon another with a space therebetween or alternatively, although not shown, a metal sleeve having a plurality of vertical fins on its outer surface may be fitted on the pole Fig. 3 shows the principal part of another embodiment of the present invention, and in this embodiment a solid pin-shaped pole 30 is inserted into a hole 5 formed substantantially through the central portion of a lower surface wall 4 so as to be bonded together. The pin-shaped pole 30 is one molded preliminarily separately from the molding of a housing and it contains an embedded heat sensitive element 36 having its lead wire 37 projected through its tail end. The preliminarily prepared pin-shaped

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pole 30 is preliminarily set in place within an injection mold as a separate member during the injection molding of the re o housing and it is made integral with the housing by a

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technique of insert molding. When formed in this manner, the housing includes the heat sensitive element 36 with its lead wire 37 projected into the cavity from substantially the central position of the lower surface wall. Also, it is eleq S. S possible to use another forming method so designed that a housing having a perforated lower surface wall and a pin-shaped pole 30 containing an embedded heat sensitive element re respectively prepared and the pole 30 is inserted a into the hole in the lower surface wall, thereby bonding them together closely by a ultrasonic processing.

In the respective cases of Figs. 1 and 2a to 2g, the pin-shaped pole for receiving the heat sensitive element is the,cylindrical member having the closed forward end and it is desirable to mold its cylindrical portion to have a thin wall thickness of 0.3 to 0.8 mm. If it is difficult to form such thin-walled cylindrical portion, however, it is 14 desirable that a pin-shaped pole 30 is molded as a separate member containing an embedded heat sensitive element and it is then made integral with the lower surface wall 4 by an insert molding procedure.

It is to be noted that such pin-shaped pole 30 which is to be produced as a separate niember can be made by mold forming by selecting and using a material having a good heat conductivity such as polyethylene with a heat sensitive element 6 being incorporated in a mold. As a result, a housing itself can be designed and produced indepenedently of o ,c G0 the heat conduction characteristic of a pin-shaped pole and a aO a housing made of a material having a sufficiently low heat Sor conductivity can be bonded with a pin-shaped pole made of a material having an excellent heat conduztivity as a separate member by insert molding. By so doing, the heat conductivity ,,o of the housing lower surface wall can be reduced considerably room

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"40:0 relative to the pin-shaped pole so that an external or temperature rise due to a fire or the like can be efficiently transmitted to the heat sensitive element in the pin-shaped pole and the response characteristic of the heat sensor to *r a the temperature rise can be improved.

The foregoing embodiments and modifications are illustrative only for purposes of description and the present invention is in no way limited by these embodiments and modifications as far as the technical scope defined by the following claims is concerned.

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Claims

1. A heat sensor including a substantially sealed housing defining a cavity accommodating electronic circuit carrier means, a hole being formed in one wall of the housing, a heat sensitive element supported on the electronic circuit carrier means by an electrical lead which passes through the hole in such a manner that the heat sensitive element is disposed externally of the said one wall and an elongate projection extending outwardly from the said one wall and sealed to the wall round the edge of the hole, the projection being provided with a projected end which is integrally closed such that the heat sensitive element is hermetically sealed inside the projection.

2. A sensor as claimed in claim 1 in which the projection is integrally moulded with the said one wall.

3. A sensor as claimed in claim 1 or claim 2 in which the projection comprises a hollow cylindrical member having a closed forward end, the interior of the cylindrical member communicating with the cavity through the hole and accommodating the heat sensitive element.

4. A sensor as claimed in claim 3 in which there is a space between the heat sensitive element and the inner surface of the cylindrical member which is occupied by a heat conductive filler. A sensor as claimed in claim 1 or claim 2 in which the .projection comprises a solid plastic component having the heat sensitive element embedded therein, the plastic component being integrally bonded to the said one wall of o: the housing by moulding.

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6. A sensor as claimed in any one of claims 1 to 4 in which heat insulating means is embedded in the said one wall of the housing surrounding the hole in the region of the root of the projection.

7. A sensor as claimed in any one of claims 1 to 3 in which an annular depression is formed in the outer surface of the said one wall of the housing around the hole in the region of the root of the projection.

8. A sensor as claimed in any one of claims 1 to 3 in which the projection is made of a plastic material having a AMD/0820a 17 different heat conductivity to that of the said one wall.

9. A sensor as claimed in any one of claims 1 to 3 in which the projection is made of a plastic material mixed with a filler having heat conductivity improving properties.

A sensor as claimed in any one of claims 1 to 3 in which the projection includes a metal coating on its outer surface at least at a position corresponding to that of the heat sensitive element.

11. A sensor as claimed in any one of claims 1 to 3 in which the projection includes at least one integrally moulded external fin at a position corresponding to the heat sensitive element.

12. A sensor as claimed in any one of claims 1 to 3 in which the projection includes at least one external metallic heat collector member near the position corresponding to the heat sensitive element.

13. A heat sensor substantially as specifically herein described with reference to any one of the Figures 1, Figures 2a to 2g and Figure 3. DATED this llth day of November 1993. HOCHIKI CORPORATION "By Its Patent Attorneys DAVIES COLLISON CAVE *In