Patent Publication Number: US-2017352510-A1

Title: Thermal response switch

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a National Stage Entry into the United States Patent and Trademark Office from International PCT Patent Application No. PCT/JP2014/084082, having an international filing date of Dec. 24, 2014, the entire content of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a thermally responsive switch used as a protection device for motors or the like. 
     BACKGROUND OF THE INVENTION 
     Many suggestions have been made for this type of thermally responsive switch which utilizes a thermally responsive element such as a bimetal. An example of such thermally responsive switch will be described with reference to  FIGS. 10 and 11 . A thermally responsive switch  101  is provided with a metal housing  102  and a lid plate  103 . The lid plate  103  is fixed to an opening of the housing  102  by welding to form an airtight container. Through holes are formed through the lid plate  103 . Metal conductive terminal pins  104 A,  104 B are inserted into the through holes. The conductive terminal pins  104 A,  104 B are airtightly fixed by an electrically insulating material  105  such as glass. A fixed contact  106  is fixed to a portion of one (conductive terminal pin  104 A) of the conductive terminal pins located inside the airtight container. One end of a heater  107 , being one example of a heating member, is connected to a portion of the other (conductive terminal pin  104 B) of the conductive terminal pins located inside the airtight container. The other end of the heater  107  is connected to the lid plate  103 . 
     A thermally responsive plate  109  configured by a bimetal or the like is connected to the inner side of the housing  102  via a connecting body  110 . A movable contact  108  is provided on a movable end of the thermally responsive plate  109 . The thermally responsive plate  109  is molded into a shallow dish shape. The thermally responsive plate  109  reverses its curving direction when it reaches a predetermined operating temperature and recovers its original curving direction when it reaches a predetermined recovering temperature. Normally, the movable contact  108  of the thermally responsive plate  109  is placed in contact with the fixed contact  106  as shown in  FIG. 10 . 
     The thermally responsive switch  101  is used for example in a sealed electric compressor that compresses refrigerant used in an air conditioner. In such case, the thermally responsive switch  101  is arranged inside a sealed housing of the compressor not shown, so that the conductive terminal pins  104 A,  104 B are series connected to a motor. During operation of the air conditioner, an operating current of the electric compressor flows through the thermally responsive switch  101  connected in the aforementioned manner, by the following route: the conductive terminal pin  104 B—the heater  107 —the lid plate  103 —the housing  102 —the connecting body  110 —the thermally responsive plate  109 —the movable contact  108 —the fixed contact  106 —the conductive terminal pin  104 A. The current flowing in this manner causes the heater  107  and the thermally responsive plate  109  of the thermally responsive switch  101  to be heated. However, current flowing during normal operation of the air conditioner keeps the temperature of the thermally responsive plate  109  to be less than the operating temperature. Hence, the motor stays energized. 
     However, when the rotation of the motor is somehow restricted for example, an overcurrent which is several times larger than normal operation current flows through the motor. Thus, when left unattended, components such as motor coils may become burned. 
     When the heating value of the heater  107  and the thermally responsive plate  109  largely exceed the normal state due to overcurrent, the temperature of the thermally responsive plate  109  rises to the predetermined operating temperature and the curving direction of the thermally responsive plate  109  becomes reversed. As a result, the movable contact  108  fixed to the tip of the thermally responsive plate  109  is moved away from the fixed contact  106  to release the connection between the movable contact  108  and the fixed contact  106  and thereby cut off the electric circuit. The thermally responsive switch  101  releases the connection between the contacts in the above described manner when the compressor behaves abnormally to ensure that current is cut off from the motor before the motor coils reach a burning temperature. 
     Japanese Patent Publication No. 2005-240596 is representative of the prior art. 
     SUMMARY OF THE INVENTION 
     For example, when the size of the electric compressor to be protected is small, its energization current is small. Thus, it is not possible for components such as the heater and the thermally responsive plate to be sufficiently self-heated under the structure of the conventional thermally responsive switch  101 . Hence, measures need to be taken to increase the heating value of the heater and the thermally responsive plate. However, since limited types of metals are used as bimetal and tri-metal of the thermally responsive plate for example, resistivity can only be increased to a limited level. Hence, there is a limit to increasing the heating value by modifying the materials of the thermally responsive plate. Another conceivable approach for increasing the heating value is thinning the heat reactive plate to thereby reduce its cross-sectional area and increase the resistance value. However, since drive force for opening and closing the movable contact needs to be secured for the thermally responsive plate, there is also a limit to thinning the thermally responsive plate. Further, types of metal used as the material of the heater is also limited by the required physical properties such as weldability and by cost requirements. Hence, there is substantially a limit to replacing the material of the heater with a material having high resistivity. Thus, the most effective way to increase the heating value of the thermally responsive switch is to reduce the cross-sectional area of the heater while increasing the overall length of the heater. 
     Through creative efforts, the applicant has endeavored to reduce the cross-sectional area of the heater while extending its overall length. The applicant has conceived of the following configuration in the endeavor. According to the thermally responsive switch conceived by the applicant, a heating element of the heater is provided with multiple meandering portions formed of a strip-shaped metal plate. The multiple meandering portions are disposed so as to face one another with a conductive terminal pin disposed therebetween and a portion of the meandering portions are bent with respect to a predetermined reference axis. 
     According to the thermal responsive switch configured in the above described manner, it is possible to reduce the cross sectional area of the heater and further extend the overall length of the heater. As a result, it is possible to increase the heating value of the heater. 
     However, because the heater is provided with meandering portions and is bent within a small space inside the sealed container, there is a risk of a so-called heat bank being formed in the heater where straining is prone to occur and heat is prone to accumulate. Therefore, there is a concern that the heater may become fused at unexpected locations due to excessive heat generated by overcurrent. Thus, a technology is being conceived for controlling the location where fusing occurs due to excessive heat generated by overcurrent by intentionally providing a fusing portion to the heater which is easily fused compared to other portions of the heater. Such fusing portion is formed by providing a portion having a narrower width compared to other portions to the heater. 
     When such fusing portion is fused, droplets of melt known as sputter and formed of metal pieces and metal particles produced by the fusing are scattered. Current is discharged from the fusing portion to components such as the housing and the lid plate by the scattering of the sputter, thereby causing arc to continue. Thus, it may not be possible to completely cut off current flow even if the fusing portion is fused. 
     According to the thermally responsive switch of the present invention, a heating element of the heater has a meandering portion formed of a strip-shaped metal plate. The meandering portion is bent twice, namely with respect to a first reference axis and a second reference axis both extending in a longitudinal direction of a housing to thereby form an outer vertical portion located in an outer side of the first reference axis and being perpendicular to an inner surface of a lid plate, an inner vertical portion located in an inner side of the second reference axis and being perpendicular to the inner surface of the lid plate, and an middle vertical portion located between the first reference axis and the second reference axis so as to be disposed between the outer vertical portion and the inner vertical portion and being perpendicular to the inner surface of the lid plate. The middle vertical portion has a narrow portion narrower than a width of said middle vertical portion. The narrow portion is provided on an end portion located in one side of the middle vertical portion where no other heating element exists among two widthwise end portions of the middle vertical portion. 
     According to the thermally responsive switch of the present invention, the narrow portion serving as a fusing portion is provided on the end portion located in one side of the middle vertical portion where no other heating element exists among the two widthwise end portions of the middle vertical portion. According to such configuration, the sputter generated when the narrow portion is fused scatters toward a relatively wide space where no other heating elements of the heater exists. Thus, even if arc is generated by the sputtering, it is possible to extinguish the arc before it is transferred to other portions and thereby allowing current flow to be cut off. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front view of a thermally responsive switch of one embodiment. 
         FIG. 2  is a vertical cross-sectional view of the thermally responsive switch. 
         FIG. 3  is a transverse cross-sectional view of the thermally responsive switch. 
         FIG. 4  is a perspective view of a heater. 
         FIG. 5  is a development of the heater. 
         FIG. 6  is a plan view of the heater. 
         FIG. 7  is a perspective view of a main portion of the heater. 
         FIG. 8  is an enlarged view of a narrow portion located at a middle vertical portion and its periphery. 
         FIG. 9A  is a vertical cross-sectional side view of the heater taken along line A-A of  FIG. 6 . 
         FIG. 9B  is a vertical cross-sectional side view of the heater taken along line B-B of  FIG. 6 . 
         FIG. 9C  is a vertical cross-sectional side view of the heater taken along line C-C of  FIG. 6 . 
         FIG. 9D  is a side view of the heater. 
         FIG. 10  is a vertical cross-sectional view of a conventional thermally responsive switch. 
         FIG. 11  is a transverse cross-sectional view of a conventional thermally responsive switch. 
     
    
    
     DESCRIPTION OF EMBODIMENT(S) OF THE INVENTION 
     A description will be given hereinafter on one embodiment of a thermally responsive switch to which the present invention is applied with reference to the drawings. As shown in  FIGS. 1 and 2 , a thermally responsive switch  1  is an airtight container configured of a metal housing  2  and a lid plate  3 . The housing  2  is formed into a long-dome shape having an open end. The lid plate  3  is airtightly fixed to the open end of the housing  2  by welding or the like. Conductive terminal pins  4 A,  4 B made of metal are inserted into two through holes provided in the lid plate  3 . These conductive terminal pins  4 A,  4 B are fixed by an electrically insulating filler such as glass. Thus, the conductive terminal pins  4 A,  4 B are airtightly fixed in an electrically insulated state. 
     A fixed contact  6 A is fixed, through a conductive fixed contact support  6 B, to a portion of the conductive terminal pin  4 A located inside the airtight container. Also, a thermally responsive plate  9  configured of bimetal or tri-metal, for example, is fixed to the inner side of the housing  2  through a connecting body  10 . The thermally responsive plate  9  is formed into a dish shape by drawing and has one end connected to an inner surface of the housing  2  through the connecting body  10 . The thermally responsive plate  9  reverses its curving direction when it reaches a predetermined temperature. Also, a movable contact  8  is fixed to a movable end which is the other end of the thermally responsive plate  9 . 
     When the thermally responsive plate  9  is reversed, the movable contact  8  moves away from the fixed contact  6 A. This releases the connection between the movable contact  8  and the fixed contact  6 A, and cuts off an electric circuit formed of: the conductive terminal pin  4 B—a heater  7 —the lid plate  3 —the housing  2 —the connecting body  10 —the thermally responsive plate  9 —the movable contact  8 —the fixed contact  6 A—the fixed contact support  6 B—the conductive terminal pin  4 A. Note that in a normal state in which the thermally responsive plate  9  is not reversed, the movable contact  8  is placed in contact with the fixed contact  6 A and forms the above electric circuit. Thus, the movable contact  8  opens and closes the electric circuit by being driven by the thermally responsive plate  9  to come into contact with and separate from the fixed contact  6 A. 
     As also shown in  FIG. 3 , one end of the heater  7  is connected to a portion of the conductive terminal pin  4 B located inside the airtight container. The other end of the heater  7  is connected to an inner surface of the lid plate  3 . The shape of the heater  7  will be described with reference to  FIGS. 4 and 5 . The heater  7 , taking a three-dimensionally meandering shape as shown in  FIG. 4 , is manufactured by bending a strip-shaped heater forming material meandering as illustrated in  FIG. 5  with respect to predetermined references axes  7 Ha and  7 Hb serving as folding lines. The heater forming material shown in  FIG. 5  is obtained, for example, by blanking a planar metal plate having a predetermined resistivity. The heater  7  has meandering portions, and the meandering portions are bent. That is, the heater  7  is configured of multiple heater units including a linear portion  7 A being a linear heating element and a semicircular portion  7 B being a semicircular heating element. Multiple heater units are alternately connected by joining the linear portion  7 A of one heater unit to the semicircular portion  7 B of another heater unit to form the heater  7 . Thus, the heater  7  has multiple meandering portions  7 C,  7 D in which multiple linear portions  7 A are provided adjacent to one another with the semicircular portion  7 B interposed therebetween. 
     The structure of the heater  7  adopts the meandering heating element so that a longer electric circuit can be obtained in a limited space. The meandering portions  7 C,  7 D are connected by a connecting portion  7 E. In this case, the connecting portion  7 E is a strip-shaped element extending in a straight line. However, the connection portion  7 E may be configured as a meandering portion. Further, fixing portions  7 F,  7 G are provided on the two end portions of the heater  7 . 
     The meandering portions  7 C,  7 D are bent twice with respect to a predetermined first reference axis  7 Ha and a second reference axis  7 Hb illustrated in  FIG. 5 . The first reference axis  7 Ha and the second reference axis  7 Hb are each an axis extending along a longitudinal direction of the housing  2  shaped like a long dome. Further, the first reference axis  7 Ha is set so as to be located on the outer side of the second reference axis  7 Hb as viewed in the width direction of the heater  7  and the second reference axis  7 Hb is set so as to be located on the inner side of the first reference axis  7 Ha as viewed in the width direction of the heater  7 . More specifically, the second reference axis  7 Hb is set on the outer sides of the two ends of the connecting portion  7 E so as to sandwich the connecting portion  7 E and the first reference axis  7 Ha is set further on the outer side of the second reference axis  7 Hb. 
     The first reference axis  7 Ha and the second reference axis  7 Hb extend in a direction perpendicular to the direction in which the linear portion  7 A extends and the direction in which the connecting portion  7 E connecting the meandering portions  7 C,  7 D extends. In the meandering portion  7 D, the linear portion  7 A of the heater unit located in the portion facing the fixing portion  7 F (the portion facing the conductive terminal pin  4 B when mounted inside the airtight container) is shorter than the linear portions  7 A of other heater units  7 A. In the meandering portion  7 C, the linear portion  7 A of the heater unit located in the portion facing the fixing portion  7 F (the portion facing the conductive terminal pin  4 B when mounted inside the airtight container) is shorter than the linear portions  7 A of other heater units  7 A. 
     The meandering portions  7 C,  7 D are bent with respect to the first reference axis  7 Ha and the second reference axis  7 Hb such that a first surface of the two surfaces of the linear portion  7 A faces the same first surface. In other words, the meandering portions  7 C,  7 D are bent 180 degrees at two locations, namely with respect to the first reference axis  7 Ha and with respect to the second reference axis  7 Hb. In the meandering portions  7 C,  7 D bent in this manner, a predetermined gap is formed between opposing planes of the first surface of the same linear portion  7 A, that is, between the surfaces located on the inner side in the bent state. Further, the meandering portions  7 C,  7 D are configured such that the strip-shaped planar portions constituting the linear portions  7 A face each other. Also, the meandering portions  7 C,  7 D are bent such that the linear portions  7 A extend in the direction perpendicular to the connecting portion  7 E. The heater  7  is arranged inside the airtight container such that the connecting portion  7 E is parallel to the inner surface of the lid plate  3 . Accordingly, the heater  7  is arranged inside the airtight container such that the linear portions  7 A extend in a direction perpendicular to the inner surface of the lid plate  3 . 
     By bending the meandering portions  7 C,  7 D in this manner, it is possible to reduce the dimension of the heater  7  in the width direction which is the direction perpendicular to the first reference axis  7 Ha and the second reference axis  7 Hb and which is the extending direction of the connection portion  7 E. Hence, the heater  7  can be accommodated in a smaller space and the heater  7  having a longer overall length can be arranged inside a conventional-sized airtight container. Also, the heater  7  having the meandering portions  7 C,  7 D bent in this manner is arranged inside the airtight container such that the linear portion  7 A of one meandering portion  7 C faces the linear portion  7 A of the other meandering portion  7 D. Additionally, the heater  7  is arranged inside the airtight container such that the linear portion  7 A of one meandering portion  7 C is parallel to the linear portion  7 A of the other meandering portion  7 D. 
     Also, when arranged inside the airtight container, the heater  7  surrounds the periphery of the conductive terminal pin  4 B with the fixing portion  7 G—the meandering portion  7 C—the connecting portion  7 E—the meandering portion  7 D—the fixing portion  7 F. That is, the heater  7  is arranged around the conductive terminal pin  4 B so as to form a spiral. Further, the heater  7  is arranged so that the meandering portions  7 C,  7 D oppose each other with the conductive terminal pin  4 B interposed therebetween. Also, the heater  7  is arranged such that the meandering portions  7 C,  7 D are parallel to the inner surface of the lid plate  3 . The heater  7  is also arranged such that the side surfaces on the outer sides of the meandering portions  7 C,  7 D are aligned with an inner peripheral surface of the housing  2 . The fixing portion  7 G being an end portion of the heater  7  on the circumferential edge side is fixed to the inner surface of the lid plate  3  by welding, for example. On the other hand, the fixing portion  7 F being an end portion of the heater  7  on the center side is fixed to an end portion of the conductive terminal pin  4 B inside the airtight container by welding, for example. 
     Further, the heater  7  is arranged inside the airtight container such that the connection portion  7 E is on the thermally responsive plate  9  side, a bent portion closest to the connection portion  7 E is on the lid plate  3  side, and the next bent portion is on the thermally responsive plate  9  side. Hence, when the heater  7  is arranged inside the airtight container, its area is larger on the thermally responsive plate  9  side than on the lid plate  3  side which is opposite of the thermally responsive plate  9  side. 
     Further creative efforts are put in to the shape of the heater  7  which will be described hereinafter. As shown in  FIG. 4 , the meandering portions  7 C,  7 D are bent twice with respect to the first reference axis  7 Ha and the second reference axis  7 Hb extending in the longitudinal direction of the housing  2  so that each of the meandering portions  7 C,  7 D form multiple outer vertical portions  71 , multiple inner vertical portions  72 , and multiple middle vertical portions  73 . The outer vertical portion  71  is located in the outer side of the first reference axis  7 Ha and is vertical with respect to the inner surface of the lid plate  3 . The inner vertical portion  72  is located in the inner side of the second reference axis  7 Hb and is vertical with respect to the inner surface of the lid plate  3 . The middle vertical portion  73  is located between the first reference axis  7 Ha and the second reference axis  7 Hb so as to be interposed between the outer vertical portion  71  and the inner vertical portion  72  and is vertical with respect to the inner surface of the lid plate  3 . 
     The middle vertical portions  73  formed in the heater  7  can be categorized into two types, namely, type A in which other middle vertical portion  73  exists on both widthwise end portions of the middle vertical portion  73  and type B in which other middle vertical portion  73  does not exist on one of the widthwise end portions of the middle vertical portion  73  as shown in  FIG. 6 . In this example, three type B middle vertical portions  73 B are formed in a single heater  7 ; that is, one middle vertical portion  73 B formed in immediate proximity of the fixing portion  7 F and two middle vertical portions  73 B formed on both ends of the connecting portion  7 E. In the thermally responsive switch  1  of the present embodiment, special creative efforts are put in to the shape of the middle vertical portion  73 B formed in immediate proximity of the fixing portion  7 F. 
       FIG. 7  illustrates a part of the heater  7  and in particular, the part near the fixing portion  7 F. A narrow portion  74 , serving as a fuse portion that melts more easily compared to other portions of the heater  7 , is formed to the middle vertical portion  73 B formed in immediate proximity of the fixing portion  7 F. The narrow portion  74  is located at one of the two widthwise end portions of the middle vertical portion  73 B in which other heating element does not exist and has a narrower width compared to the middle vertical portion  73 B. As shown in  FIG. 3 , the thermally responsive switch  1  is arranged inside the airtight container so that relatively wide space is secured in the side of the end portion where the narrow portion  74  is provided among the two end portions of the middle vertical portion  73 B. 
     As also shown in  FIG. 8 , the narrow portion  74  is provided so as to be shifted toward the end portion located in the free side of the middle vertical portion  73 B where no other heating element exists (the end portion in the right side as viewed in  FIG. 8 ) as viewed in the width direction of the middle vertical portion  73 B. That is, the narrow portion  74  is provided in a position shifted toward the free side of the middle vertical portion  73 B relative to a center line CL as viewed in the width direction of the middle vertical portion  73 B. On the end portion of the middle vertical portion  73 B located in the opposite side of the narrow portion  74 , that is, on the end portion located in the side where other heating element exists, a recess  75  is formed which caves in the shape of a circular arc toward the end portion located in the side where no other heating element exists. The shape of the recess  75  is not limited to a circular arc. 
     Further, the heater  7  is provided with a thin portion  76  between the fixing portion  7 F which is an end portion connected to the conductive terminal pin  4 B and the inner vertical portion  72  which faces the middle vertical portion  73 B provided with the narrow portion  74 . The width of the thin portion  76  is at least thinner than the width of the inner vertical portion  72 . Thus, the heater  7 , when starting from the fixing portion  7 F becomes temporarily thin at the thin portion  76  and thereafter is widened at the inner vertical portion  72 . Then, the narrow portion  74  is provided at the middle vertical portion  73 B which comes after the inner vertical portion  72 . 
     Further, as illustrated in  FIGS. 9A to 9D , a vertical dimension H 1  of the inner vertical portion  72  is shorter than a vertical dimension H 2  of the middle vertical portion  73 . Though not shown, the heater  7  may be configured so that the vertical dimension of the outer vertical portion  71  is shorter than the vertical dimension of the middle vertical portion  73  or the vertical dimensions of both the outer vertical portion  71  and the inner vertical portion  72  are shorter than the vertical dimension of the middle vertical portion  73 . In other words, the heater  7  may be configured so that at least either of the outer vertical portion  71  and the inner vertical portion  72  is shorter than the middle vertical portion  73 . 
     According to the thermally responsive switch  1  of the present embodiment, the heating elements of the heater  7  are provided with meandering portions  7 C,  7 D formed of a strip-shaped metal plate. Each of the meandering portions  7 C,  7 D is bent twice with respect to the first reference axis  7 Ha and the second reference axis  7 Hb extending in the longitudinal direction of the housing  2 . As a result, the meandering portions  7 C,  7 D are each provided with the outer vertical portion  71  disposed in the outer side of the first reference axis  7 Ha so as to be perpendicular to the inner surface of the lid  3 , the inner vertical portion  72  disposed in the inner side of the second reference axis  7 Hb so as to be perpendicular to the inner surface of the lid  3 , and the middle vertical portion  73  disposed between the first reference axis  7 Ha and the second reference axis  7 Hb and between the outer vertical portion  71  and the inner vertical portion  72  so as to be perpendicular to the inner surface of the lid  3 . Among the multiple middle vertical portions  73 , the middle vertical portion  73 B provided in immediate proximity of the fixing portion  7 F is provided with the narrow portion  74  being narrower than the width of the middle vertical portion  73 B on the end portion located in the free side of the middle vertical portion  73 B where no other heating element exists among the two widthwise end portions of the middle vertical portion  73 B. 
     According to the thermally responsive switch  1 , the narrow portion  74  serving as the fusing portion is provided on the end portion located in the free side of the middle vertical portion  73 B where no other heating element exists among the two widthwise end portions of the middle vertical portion  73 B. Relatively wide space is provided beside the end portion in the free side of the middle vertical portion  73 B. According to such configuration, the sputter generated when the narrow portion  74  is fused scatters toward the relatively wide space where no other heating elements of the heater  7  exists. Thus, even if arc is generated by the sputtering, it is possible to extinguish the arc before it is transferred to other portions such as the housing  2  and the lid  3  and thereby allowing current flow to be cut off. 
     As illustrated in  FIG. 6 , the heater  7  is provided with three type-B middle vertical portions  73 B in which no other middle vertical portion  73  exists on one of the two widthwise end portions of the middle vertical portion  73 . Among them, the two middle vertical portions  73 B other than the middle vertical portion  73 B located in immediate proximity of the fixing portion  7 F are disposed near the thermally responsive plate  9 . Thus, when the narrow portion  74  having a relatively large heating value is formed at these two middle vertical portions  73 B, heat transfer may occur in a concentrated manner between the heater  7  and the thermally responsive plate  9  and possibly affect the stability of operation. In the thermally responsive switch  1  according to the present embodiment, the narrow portion  74  is formed at the middle vertical portion  73 B located most distant from the thermally responsive plate  9 . It is thus, possible to prevent the operation of the switch from becoming unstable by the formation of the narrow portion  74 . 
     Further according to the thermally responsive switch  1 , the heater  7  forms meandering portions  7 C,  7 D configured of meandering strip-shaped metal plates. The meandering portions  7 C,  7 D are bent twice with respect to two reference axes  7 Ha and  7 Hb to provide a complicated shape. According to such structure, heat tends to accumulate especially in the middle vertical portion  73  disposed between the outer vertical portion  71  and the inner vertical portion  72 . According to the thermally responsive switch  1 , the heater  7  is configured so that the vertical dimension of the inner vertical portion  72  is shorter than the vertical dimension of the middle vertical portion  73 . It is thus, possible to reduce the area of the inner vertical portion  72  facing the middle vertical portion  73 . In other words, it is possible to increase the area of the middle vertical portion  73  for releasing heat. As a result, it is possible to improve heat dissipation from the middle vertical portion  73  and prevent excessive temperature elevation at the middle vertical portion  73  to thereby provide a homogenous temperature distribution. 
     Further according to the thermally responsive switch  1 , the heater  7  is configured so that thin portion  76  thinner than the inner vertical portion  72  is provided between the fixing portion  7 F connected to the conductive terminal pin  4 B and the inner vertical portion  72 . According to such configuration, it is possible to prevent the temperature of the inner vertical portion  72  from becoming too low by the heat escaping towards the conductive terminal pin  4 B side from the fixing portion  7 F. The heater  7  is required to generate amount of heat which is correlated with the size of current flowing through the heater  7 . When heat escapes towards the conductive terminal pin  4 B side from the fixing portion  7 F, the temperature of the inner vertical portion  72  may become too low. Especially because the narrow portion  74  exhibiting relatively large heating value is located close to the fixing portion  7 F in the present embodiment, it may not be possible to obtain the desired fusing performance when subjected to overcurrent. Thus, according to the thermally responsive switch  1  of the present embodiment, the thin portion  76  is provided to increase the heating value in the vicinity of the fixing portion  7 F. As a result, heat at the inner vertical portion  72  including the narrow portion  74  does not easily escape toward the conductive terminal pin  4 B side. It is thus, possible to maintain the capacity of the heater  7  to generate amount of heat which is correlated with the size of current flowing through the heater  7 . 
     The present invention is not limited to the embodiment described above but may be modified or expanded within the gist of the invention. For example, the number of meandering portions provided to the heater is not limited to two but may be increased or decreased as required.