Patent Publication Number: US-2023142477-A1

Title: Breaker, safety circuit and secondary battery pack

Description:
TECHNICAL FIELD 
     The present invention relates to a breaker for use in a safety circuit of an electric device, and the like. 
     BACKGROUND ART 
     There has been disclosed a breaker provided with a pair of fixed contacts and a pair of movable contacts (see, for example, Patent Document 1). 
     PRIOR ART DOCUMENT 
     Patent Document 
     
         
         Patent Document 1: Japanese Patent Application Publication No. 2006-331705 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Resolved by the Invention 
     In the breaker disclosed in Patent Document 1, a pair of contacts are connected in series. Therefore, it has been difficult to reduce the resistance between terminals. 
     The present invention has been devised in view of the above circumstance, and a primary objective of the present invention is to provide a breaker in which the resistance between terminals can be reduced. 
     Means for Solving the Problems 
     The present invention is a breaker provided with
         a first terminal piece on which a fixed contact is formed,   a movable piece with a movable contact for pressing the movable contact against the fixed contact so as to contact therewith,   a thermally-actuated element deforming with a change in temperature and moving the movable piece so that the movable contact is separated from the fixed contact, and   a second terminal piece electrically connected to the movable piece, wherein
 
the movable contact includes
   a first movable contact disposed in a first end portion of the movable piece in its longitudinal direction, and   a second movable contact disposed in a second end portion of the movable piece in the longitudinal direction, and
 
the fixed contact includes
   a first fixed contact disposed at a position where the first movable contact can contact therewith, and   a second fixed contact disposed at a position where the second movable contact can contact therewith.       

     In the breaker according to the present invention, it is preferable that the movable piece has, between the first movable contact and the second movable contact, a first contact region contacting with the second terminal piece. 
     In the breaker according to the present invention, it is preferable that a first portion of the movable piece on the first end portion side of the first contact region, and a second portion of the movable piece on the second end portion side of the first contact region, have different natural vibration frequencies. 
     In the breaker according to the present invention, it is preferable that the thermally-actuated element has a top portion protruding toward the movable piece, and the first contact region is provided in an area overlapping with the top portion when viewed from a thickness direction of the movable piece. 
     In the breaker according to the present invention, it is preferable that the movable piece has a first bent portion which is bent so that the first movable contact approaches the first fixed contact. 
     In the breaker according to the present invention, it is preferable that the movable piece has, between the first movable contact and the first bent portion, a first protrusion protruding toward the thermally-actuated element. 
     In the breaker according to the present invention, it is preferable that the second terminal piece has
         a second contact region contacting with the first contact region, and   a first convex portion protruding toward the movable piece, between the first protrusion and the second contact region.       

     The present invention is a safety circuit for an electric device provided with the above-said breaker. 
     The present invention is a secondary battery pack provided with the above-said breaker. 
     Effect of the Invention 
     In the breaker according to the present invention, the first terminal piece is provided with the first fixed contact and the second fixed contact, and the movable piece is electrically connected to the second terminal piece. 
     When the first movable contact of the movable piece contacts with the first fixed contact, and the second movable contact of the movable piece contacts with the second fixed contact, then parallel circuits are formed between the first terminal piece and the second terminal piece. Thereby, the resistance between the first terminal piece and the second terminal piece is reduced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a perspective view before assembling, showing a schematic configuration of a breaker according to an embodiment of the present invention. 
         FIG.  2    is a cross-sectional view showing the breaker in a normal charge/discharge state. 
         FIG.  3    is a cross-sectional view showing the above breaker in an overcharged state or an abnormal state. 
         FIG.  4    is a cross-sectional view showing a path formed in the breaker under the conductive state, through which an electric current flows. 
         FIG.  5    is a cross-sectional view enlargedly showing a connection portion of the second terminal piece, the first contact region of the movable piece, and a periphery thereof. 
         FIG.  6    is a perspective view before assembling, showing a modified example of the breaker shown in  FIG.  1   . 
         FIG.  7    is a perspective view before assembling, showing another modified example of the breaker shown in  FIG.  1   . 
         FIG.  8    is a plan view showing the configuration of a secondary battery pack provided with the breaker according to the present invention. 
         FIG.  9    is a circuit diagram of a safety circuit provided with the breaker according to the present invention. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     A breaker according to an embodiment of the present invention will now be described with reference to the drawings. 
       FIG.  1    to  FIG.  3    show the structure of the breaker  1 . 
     As shown in  FIG.  1    and  FIG.  3   , the breaker  1  is provided with a first terminal piece  2 , a second terminal piece  3 , a movable piece  4 , a thermally-actuated element  5 , a PTC (Positive Temperature Coefficient) thermistor  6 , etc. 
     By connecting the first terminal piece  2  and the second terminal piece  3  to an external circuit, the breaker  1  constitutes an essential part of a safety circuit of an electric device. 
     The first terminal piece  2 , the second terminal piece  3 , the movable piece  4 , the thermally-actuated element  5 , and the PTC thermistor  6  are housed in a case  10 . The case  10  is composed of a case main body (first case)  7  and a lid member (second case)  8  mounted on the upper surface of the case main body  7 . 
     The first terminal piece  2  is formed into a plate shape by pressing a metal plate containing, as a main component, copper or the like for example (in addition, a metal plate such as a copper-titanium alloy, nickel silver, or brass). 
     The first terminal piece  2  has its longitudinal direction. The first terminal piece  2  is embedded in the case main body  7  by insert molding. 
     The first terminal piece  2  has a pair of fixed contacts  21  and a first terminal  22 . 
     The second terminal piece  3  has a second terminal  32 . 
     The second terminal piece  3  is electrically connected to the movable piece  4 . 
     The movable piece  4  has a pair of movable contacts  41 , and presses the movable contacts  41  against the fixed contacts  21  to bring the movable contacts into contact with the fixed contacts. 
     The thermally-actuated element  5  is deformed with a temperature change, and actuates the movable piece  4  so that the movable contacts  41  are separated from the fixed contacts  21 . 
     The fixed contacts  21  include a first fixed contact  21   a  and a second fixed contact  21   b . The first fixed contact  21   a  and the second fixed contact  21   b  are formed by clad, plating or coating of a highly conductive material such as silver, nickel, nickel-silver alloy, copper-silver alloy, gold-silver alloy and the like. 
     The first fixed contact  21   a  and the second fixed contact  21   b  are formed on one of surfaces of the first terminal piece  2 . 
     In the present application, unless otherwise noted, in the first terminal piece  2 , the surface on the side where the fixed contacts  21  are formed (that is, the surface on the upper side in  FIG.  1   ) is described as A side, and the bottom surface on the opposite side thereto is descried as B side. 
     The same is applied to other parts such as the second terminal piece  3 , the movable piece  4 , the thermally-actuated element  5 , the case  10 , and the like. 
     The first fixed contact  21   a  is formed in one end portion of the first terminal piece  2 . 
     The first fixed contact  21   a  is exposed to an accommodating recess  73  of the case main body  7  through a part of an opening  73   a  formed inside the case main body  7 . 
     The first terminal  22  is formed at the other end portion of the first terminal piece  2 . The first terminal  22  projects from a side wall of the case main body  7  and is connected to an external circuit. 
     The second fixed contact  21   b  is formed near the first terminal  22 , that is, in the other side of the first terminal piece  2 . The second fixed contact  21   b  is exposed to the accommodating recess  73  of the case main body  7  through a part of an opening  73   b  formed inside the case main body  7 . 
     Between the first fixed contact  21   a  and the second fixed contact  21   b , there is formed a support portion  23  for supporting the PTC thermistor  6 . 
     The support portion  23  is exposed to the accommodating recess  73  of the case main body  7  through a part of the opening  73   d  formed inside the case main body  7 , and is electrically connected to the PTC thermistor  6 . 
     As shown in  FIG.  2   , the first terminal piece  2  is bent in a stepped shape (a crank shape in a side view), and has a first-step bent portion  25   a  and a second-step bent portion  25   b.    
     The first-step bent portion  25   a  connects the first fixed contact  21   a  and the support portion  23 , and arranges the first fixed contact  21   a  and the support portion  23  at different heights. 
     The second-step bent portion  25   b  connects the second fixed contact  21   b  and the support portion  23 , and arranges the second fixed contact  21   b  and the support portion  23  at different heights. Thereby, the PTC thermistor  6  can be housed compactly. 
     The second terminal piece  3  is formed in a plate shape by pressing a metal plate containing, as a main component, copper or the like similarly to the first terminal piece  2 . The second terminal piece  3  has its longitudinal direction. 
     The second terminal piece  3  is embedded in the lid member  8  by insert molding. The second terminal piece  3  is arranged so that the longitudinal direction of the second terminal piece  3  is parallel to the longitudinal direction of the first terminal piece  2 . 
     The second terminal piece  3  has a connection portion  31  to which the movable piece  4  is electrically connected, and the second terminal  32  to be electrically connected to an external circuit. The connection portion  31  is arranged at a position facing a first contact region  42 , which will be described later. 
     The second terminal  32  is formed on one side of the second terminal piece  3  (that is, on the side of the first fixed contact  21   a  in the first terminal piece  2 ). The second terminal  32  projects from the side wall of the case main body  7  and is connected to an external circuit. 
     The second terminal  32  is exposed, in the inside of the lid member  8 , to the internal space of the case  10  for accommodating the movable piece  4  and the like. 
     The second terminal piece  3  functions to reinforce the lid member  8 . That is, the second terminal piece  3  increases the rigidity and strength of the lid member  8  and thus the case  10  as the housing, and contributes to the miniaturization of the breaker  1 . 
     The movable piece  4  is formed in a plate shape by pressing a metal material containing, as a main component, copper or the like. The movable piece  4  has its longitudinal direction. The movable piece  4  is formed in an arm shape symmetrical with respect to its center line in the longitudinal direction. 
     At both ends of the movable piece  4 , the movable contacts  41  are formed. 
     The movable contacts  41  are formed on the B side of the movable piece  4  with the same material as the fixed contact  21 , and are joined to the movable piece  4  through a technique such as welding, clad, crimping or the like. 
     The movable contacts  41  include a first movable contact  41   a  and a second movable contact  41   b . The first movable contact  41   a  is arranged at a first end portion  40   a  of the movable piece  4  in its longitudinal direction, and the second movable contact  40   b  is arranged at a second end portion  40   b  of the movable piece  4  in the longitudinal direction. 
     The movable piece  4  is arranged so that the longitudinal direction of the movable piece  4  is parallel to the longitudinal direction of the first terminal piece  2 . Further, the movable piece  4  is arranged so that the first movable contact  41   a  is positioned to be in contact with the first fixed contact  21   a , and the second movable contact  41   b  is positioned to be in contact with the second fixed contact  21   b.    
     In other words, the first fixed contact  21   a  faces the first movable contact  41   a  and is arranged at a position where they can contact each other, and the second fixed contact  21   b  faces the second movable contact  41   b  and is arranged at a position where they can contact each other. 
     Between the first movable contact  41   a  and the second movable contact  41   b , there is formed a first contact region  42  electrically connected to the second terminal piece  3 . Thereby, the second movable contact  41   b  is arranged on the opposite side to the first movable contact  41   a  with respect to the first contact region  42  located therebetween. 
     The second terminal  32  of the second terminal piece  3  and the first movable contact  41   a  and the second movable contact  41   b  are conductive via the first contact region  42 . 
     Through the first contact region  42 , electrical continuity is established between the second terminal  32  of the second terminal piece  3  and the first movable contact  41   a  and the second movable contact  41   b.    
     The movable piece  4  has a first elastic portion  43   a  between the first movable contact  41   a  and the first contact region  42 . The first elastic portion  43   a  extends from the first contact region  42  toward the first movable contact  41   a . Further, the movable piece  4  has a second elastic portion  43   b  between the second movable contact  41   b  and the first contact region  42 . The second elastic portion  43   b  extends from the first contact region  42  toward the second movable contact  41   b.    
     The first contact region  42  is formed wider than the first elastic portion  43   a  and the second elastic portion  43   b . Thereby, the contact resistance between the connection portion  31  and the first contact region  42  is reduced. 
     The thermally-actuated element  5  shifts the state of the movable piece  4  from a conductive state in which the movable contact  41  is in contact with the fixed contact  21  to a cutoff state in which the movable contact  41  is separated from the fixed contact  21 . 
     In the conductive state, the first movable contact  41   a  is in contact with the first fixed contact  21   a , and the second movable contact  41   b  is in contact with the second fixed contact  21   b.    
     The thermally-actuated element  5  has an initial shape curved in an arc shape, and is formed by laminating thin plates having different coefficients of thermal expansion. 
     The curved shape of the thermally-actuated element  5  is reversely warped with a snap motion when it reaches an operating temperature by overheating, and is reset when it becomes below the reset temperature by cooling. 
     The initial shape of the thermally-actuated element  5  can be provided through a press working. 
     The material and shape of the thermally-actuated element  5  are not particularly limited as long as the first elastic portion  43   a  and the second elastic portion  43   b  of the movable piece  4  are pushed up at a desired temperature owing to the reversely warping motion of the thermally-actuated element  5 , and returns to the original by the elastic force of the first elastic portion  43   a  and the second elastic portion  43   b . But, a rectangular shape is desirable in view of the productivity and the efficiency of the reversely warping motion. Further, a rectangular shape close to a square is desirable in order to push up the first elastic portion  43   a  and the second elastic portion  43   b  effectively while being compact. 
     As the materials of the thermally-actuated element  5 , two kinds of materials having different thermal expansion rates such as various alloys, for example, copper-nickel-manganese alloy and nickel-chromium-iron alloy on the high expansion rate side, and iron-nickel alloy, nickel silver, brass, stainless steel on the low expansion rate side, are used by being combined and laminated according to the required conditions. 
     The PTC thermistor  6  is placed on convex protrusions (DABO)  24  formed at three positions on the support portion  23  of the first terminal piece  2 , and is supported by the protrusions  24 . When the movable piece  4  is in the cutoff state, the PTC thermistor  6  makes the first terminal piece  2  conductive with the second terminal piece  3  and the movable piece  4 . 
     The PTC thermistor  6  is disposed between the support portion  23  of the first terminal piece  2  and the thermally-actuated element  5 . When the current between the first terminal piece  2  and the movable piece  4  is cut off by the reversely warping motion of the thermally-actuated element  5 , the current flowing through the PTC thermistor  6  is increased. 
     As long as the PTC thermistor  6  is a positive temperature coefficient thermistor which can limits its current by its resistance increasing with the temperature rise, its type can be arbitrary selected according to the requirements such as the operating current, operating voltage, operating temperature, and reset temperature. And its material and shape are not particularly limited as long as they do not impair these characteristics. In this embodiment, a ceramic sintered body containing barium titanate, strontium titanate, or calcium titanate is used. Aside from the ceramic sintered body, a so-called polymer PTC in which conductive particles such as carbon are contained in a polymer may be used. 
     The case main body  7  and the lid member  8  constituting the case  10  are molded from a thermoplastic resin, e.g. flame-retardant polyamide, polyphenylene sulfide (PPS) having excellent heat resistance, liquid crystal polymer (LCP), polybutylene terephthalate (PBT) and the like. 
     An insulating material other than the resin may be applied as long as the characteristics equal to or higher than those of the above-mentioned resin can be obtained. It may be possible to employ an insulating material other than resins if properties compatible or higher than the above-mentioned resins can be obtained. 
     The case main body  7  is provided with an accommodating recess  73 , which is an internal space accommodating the movable piece  4 , the thermally-actuated element  5 , the PTC thermistor  6 , and the like. 
     The accommodating recess  73  has
         openings  73   a  and  73   b  for receiving the first movable contact  41   a  and the second movable contact  41   b  of the movable piece  4 ,   an opening  73   c  for receiving the movable piece  4  and the thermally-actuated element  5 , and   an opening  73   d  for receiving the PTC thermistor  6 , and the like.       

     The edge of the thermally-actuated element  5  mounted in the case main body  7  is appropriately contacted by a frame formed inside the accommodating recess  73 , and is guided when the thermally-actuated element  5  is reversely warped. 
     As shown in  FIG.  1   , the lid member  8  is attached to the case main body  7  so as to close the openings  73   a ,  73   b ,  73   c , etc. of the case main body  7  accommodating the fixed contact  21 , the movable piece  4 , the thermally-actuated element  5 , the PTC thermistor  6 , and the like. 
     The case main body  7  and the lid member  8  are joined by ultrasonic welding for example. At this time, the case main body  7  and the lid member  8  are joined continuously over the entire circumference of the outer edge portion of each of the case main body and the lid member, and the airtightness of the case  10  is improved. Thereby, the internal space of the case  10  provided by the accommodating recess  73  is sealed, and the parts such as the movable piece  4 , the thermally-actuated element  5 , and the PTC thermistor  6  can be shielded and protected from the atmosphere outside the case  10 . 
       FIG.  2    shows the operation of the breaker  1  in a normal charging/discharging state. In the normal charge/discharge state, the thermally-actuated element  5  maintains its initial shape (before reversely warped). In this state, the first fixed contact  21   a  is in contact with the first movable contact  41   a , and the second fixed contact  21   b  is in contact with the second movable contact  41   b . Since the second terminal piece  3  and the first contact region  42  of the movable piece  4  are electrically connected to each other, the first terminal piece  2  and the second terminal piece  3  of the breaker  1  are conducted with each other through the movable piece  4 . 
     It may be possible that the movable piece  4  and the thermally-actuated element  5  are in contact with each other, and the movable piece  4 , the thermally-actuated element  5 , the PTC thermistor  6  and the first terminal piece  2  are electrically connected as a circuit. 
       FIG.  3    shows the operation of the breaker  1  under an overcharge condition, an abnormal state and the like. 
     When the temperature becomes high due to overcharging or abnormality, the thermally-actuated element  5  reached to the operating temperature, warps reversely, and the first elastic portion  43   a  of the movable piece  4  is pushed up, and thereby the first movable contact  41   a  is separated from the first fixed contact  21   a . Further, the second elastic portion  43   b  of the movable piece  4  is pushed up, and the second movable contact  41   b  is separated from the second fixed contact  21   b.    
     The operating temperature of the thermally-actuated element  5 , at which the thermally-actuated element  5  is deformed in the inside of the breaker  1  and pushes up the movable piece  4 , is from 70 degrees C. to 90 degrees C., for example. At this time, the current flowing between the first fixed contact  21   a  and the first movable contact  41   a  and the current flowing between the second fixed contact  21   b  and the second movable contact  41   b  are cut off, and a slight leakage current will flow through the thermally-actuated element  5  and the PTC thermistor  6 . 
     As far as such leakage current flows, the PTC thermistor  6  continues to generate heat and keeps the thermally-actuated element  5  in the reversely warped state to greatly increase the resistance. Therefore, the current does not flow through the paths between the first fixed contact  21   a  and the first movable contact  41   a  and between the second fixed contact  21   b  and the second movable contact  41   b , and only the above described small leakage current exists (constituting the self-holding circuit). This leakage current can be utilized for other functions of a safety device. 
       FIG.  4    shows a path formed in the breaker  1  through which a current flows in the conductive state. 
     As described above, in the breaker  1 , the first terminal piece  2  is provided with the first fixed contact  21   a  and the second fixed contact  21   b , and the movable piece  4  is electrically connected to the second terminal piece  3 . The movable piece  4  is provided with the first movable contact  41   a  and the second movable contact  41   b.    
     When the first movable contact  41   a  of the movable piece  4  comes into contact with the first fixed contact  21   a , there is formed a first path I 1  from the first terminal piece  2  to the second terminal piece  3  through the first fixed contact  21   a , the first movable contact  41   a , the first elastic portion  43   a , and the first contact region  42 . 
     At the same time, when the second movable contact  41   b  of the movable piece  4  comes into contact with the second fixed contact  21   b , there is formed a second path  12  from the first terminal piece  2  to the second terminal piece  3  through the second fixed contact  21   b , the second movable contact  41   b , the second elastic portion  43   b , and the first contact region  42 . 
     That is, between the first terminal piece  2  and the second terminal piece  3 , there are formed parallel circuits. 
     Thereby, between the movable contact  41  and the first contact region  42 , the cross-sectional area of the conductor becomes doubled, and the electrical resistance is reduced. Further, since the fixed contact  21  and the movable contact  41  come into contact with each other at two positions, their contact resistance is reduced. Thereby, the resistance between the first terminal piece  2  and the second terminal piece  3  is reduced. 
     Meanwhile, an increase of the cross-sectional area of the conductor can be achieved simply by increasing the width and thickness of the movable piece  4 . However, when the width and thickness of the movable piece  4  are increased, the elastic force generated by the movable piece  4  also increases, so it is necessary to increase the thickness of the thermally-actuated element  5  and the like, and it becomes difficult to miniaturize the breaker  1 . 
     Further, according to the breaker  1 , the fixed contact  21  and the movable contact  41  come into contact with each other at two positions, so the conductive state between the first terminal piece  2  and the movable piece  4  is ensured. Therefore, even when a strong impact is applied to the breaker  1  due to a drop of the electric device in which the breaker  1  is incorporated, the conductive state between the first terminal piece  2  and the movable piece  4  is easily maintained, and the instantaneous interruption of the current supplied to the load can be suppressed. 
     It is preferable that the first contact region  42  comes into contact with the second terminal piece  3  between the first movable contact  41   a  and the second movable contact  41   b . Thereby, the total length of the breaker  1  is easily shortened. Further, the contact resistance between the first contact region  42  and the second terminal piece  3  is easily reduced. 
     It is preferable that the movable piece  4  is fixed to the second terminal piece  3  in the first contact region  42 . Thereby, the posture of the movable piece  4  is stabilized, and the contact resistance between the first contact region  42  and the second terminal piece  3  is easily reduced. 
     The fixing structure between the second terminal piece  3  and the movable piece  4  may be, for example, laser welding or resistance welding, bonding with a conductive adhesive, screwing, or the like. 
     It is preferable that the connection portion  31  and the first contact region  42  are formed as flat surfaces. According to such shape, the connection between the connection portion  31  and the first contact region  42  becomes easy. 
     In particular, in the example in which the second terminal piece  3  and the movable piece  4  are fixed to each other by welding, workability in the welding process is enhanced. Further, it is possible to easily secure the contact area between the connection portion  31  and the first contact region  42 . 
     The movable piece  4  may be configured such that a natural vibration frequency of a first portion  45   a  on the first end portion  40   a  side of the first contact region  42  is different from a natural vibration frequency of a second portion  45   b  on the second end portion  40   b  side of the first contact region  42 . 
     By making the natural vibration frequency of the first portion  45   a  different from the natural vibration frequency of the second portion  45   b , even when impacts of various strengths are applied to the breaker  1 , the first movable contact  41   a  and the second movable contact  41   b  are prevented from being simultaneously separated from the fixed contacts  21 , and the instantaneous interruption of the current supplied to the load is prevented. 
     The natural vibration frequencies of the first portion  45   a  and the second portion  45   b  can be easily realized by adjusting the lengths, widths and thicknesses of the first portion  45   a  and the second portion  45   b.    
     The length of the first portion  45   a  is the length of the first portion  45   a  in the longitudinal direction of the movable piece  4 , that is, the distance from the first contact region  42  to the extreme end of the first end portion  40   a.    
     The width of the first portion  45   a  is the length of the first portion  45   a  in the short direction orthogonal to the longitudinal direction of the movable piece  4 . If the length, width and thickness of the first portion  45   a  vary, the average values thereof may be used. 
     The same applies to the second portion  45   b.    
     As shown in  FIGS.  1  and  2   , the thermally-actuated element  5  has a top portion  51  protruding toward the movable piece  4 . The top portion  51  is such a portion where the distance between the second terminal piece  3  and the thermally-actuated element  5  becomes minimum when viewed from the short direction of the movable piece  4 . 
     The first contact region  42  is formed in an area overlapping with the top portion  51  when viewed from the thickness direction of the movable piece  4 . 
     Thereby, the entire length of the breaker  1  can be easily decreased while securing the gap between the first movable contact  41   a  and the first fixed contact  21   a  and the gap between the second movable contact  41   b  and the second fixed contact  21   b  in the cutoff state shown in  FIG.  3   . 
       FIG.  5    enlargedly shows the connection portion  31  of the second terminal piece  3 , the first contact region  42  of the movable piece  4 , and their neighborhood. 
     It is preferable that the movable piece  4  has a bent portion  46   a  located between the first contact region  42  and the first elastic portion  43   a . The bent portion  46   a  is bent so that the first movable contact  41   a  approaches the first fixed contact  21   a . By providing the movable piece  4  with the bent portion  46   a , the contact pressure between the first movable contact  41   a  and the first fixed contact  21   a  is increased, and the contact resistance therebetween is reduced. Further, between the second terminal piece  3  and the first elastic portion  43   a , there is formed a space for the first elastic portion  43   a  to be deformed, and in the cutoff state shown in  FIG.  3   , the gap between the first movable contact  41   a  and the first fixed contact  21   a  is easily secured. 
     Similarly, it is preferable that the movable piece  4  has a bent portion  46   b  located between the first contact region  42  and the second elastic portion  43   b . The bent portion  46   b  is bent so that the second movable contact  41   b  approaches the second fixed contact  21   b.    
     It is preferable that the movable piece  4  has a bent portion  47   a  located between the bent portion  46   a  and the first elastic portion  43   a . The bent portion  47   a  is bent in the direction opposite to the bent portion  46   a , that is, so that the first movable contact  41   a  is separated from the first fixed contact  21   a . By providing the movable piece  4  with the bent portion  46   a  and the bent portion  47   a , the thickness of the breaker  1  can be reduced while increasing the contact pressure between the first movable contact  41   a  and the first fixed contact  21   a.    
     Similarly, it is preferable that the movable piece  4  has a bent portion  47   b  located between the bent portion  46   b  and the second elastic portion  43   b . The bent portion  47   b  is bent in the direction opposite to the bent portion  46   b , that is, so that the second movable contact  41   b  is separated from the second fixed contact  21   b.    
     It is preferable that, as shown in  FIGS.  2  and  3   , the movable piece  4  has a protrusion  44   a  projecting toward the thermally-actuated element  5 . 
     The protrusion  44   a  is disposed in the first elastic portion  43   a  between the first movable contact  41   a  and the bent portion  46   a.    
     It is preferable that the protrusion  44   a  is disposed in the vicinity of the edge of the thermally-actuated element  5 . 
     By providing the protrusion  44   a  in the first elastic portion  43   a , the gap between the first movable contact  41   a  and the first fixed contact  21   a  is easily secured in the cutoff state shown in  FIG.  3   . 
     Similarly, it is preferable that the movable piece  4  has a protrusion  44   b  protruding toward the thermally-actuated element  5 . The protrusion  44   b  is disposed in the second elastic portion  43   b  between the second movable contact  41   b  and the bent portion  46   b.    
     It is preferable that the protrusion  44   b  is provided in the vicinity of the edge of the thermally-actuated element  5 . 
     Only one of the protrusion  44   a  and the protrusion  44   b  may be provided. 
     It is preferable that the second terminal piece  3  has a convex portion  33   a  protruding toward the movable piece  4 . 
     By providing the convex portion  33   a  on the second terminal piece  3 , the contact pressure between the first movable contact  41   a  and the first fixed contact  21   a  in the conductive state shown in  FIG.  2   , is increased. 
     The convex portion  33   a  is disposed between the connection portion  31  and the protrusion  44   a  in the longitudinal direction of the movable piece  4 . 
     The connection portion  31  is a second contact region which comes into contact with the first contact region  42 . 
     By arranging the convex portion  33   a  between the connection portion  31  and the protrusion  44   a , the gap between the first movable contact  41   a  and the first fixed contact  21   a  is easily secured in the cutoff state shown in  FIG.  3   . 
     Similarly, it is preferable that the second terminal piece  3  has a convex portion  33   b  protruding toward the movable piece  4 . The convex portion  33   b  is disposed between the connection portion  31  and the protrusion  44   b  in the longitudinal direction of the movable piece  4 . 
     Only one of the convex portion  33   a  and the convex portion  33   b  may be provided. 
       FIG.  6    is a perspective view of a breaker  1 A, which is a modification of the breaker  1  shown in  FIG.  1   . 
     For the portion of the breaker  1 A not described below, the above described structure of the breaker  1  may be adopted. 
     The breaker  1 A includes the first terminal piece  2 , the second terminal piece  3 , a movable piece  4 A, the thermally-actuated element  5 , the PTC thermistor  6 , and the like. 
     The movable piece  4 A differs from the movable piece  4  shown in  FIG.  1    in that it has two pairs of movable contacts  41   a   1 ,  41   a   2 ,  41   b   1 ,  41   b   2  and two pairs of elastic portions  43   a   1 ,  43   a   2 ,  43   b   1 ,  43   b   2 . 
     Specifically, in the movable piece  4 A, the first movable contact  41   a  includes a movable contact  41   a   1  and a movable contact  41   a   2 , and the first elastic portion  43   a  includes an elastic portion  43   a   1  and an elastic portion  43   a   2 . 
     The movable contact  41   a   1  is formed in the elastic portion  43   a   1 , and the movable contact  41   a   2  is formed in the elastic portion  43   a   2 . 
     Similarly, in the movable piece  4 A, the second movable contact  41   b  includes a movable contact  41   b   1  and a movable contact  41   b   2 , and the second elastic portion  43   b  includes an elastic portion  43   b   1  and an elastic portion  43   b   2 . 
     The movable contact  41   b   1  is formed in the elastic portion  43   b   1 , and the movable contact  41   b   2  is formed in the elastic portion  43   b   2 . 
     The first elastic portion  43   a  is provided with a slit  48   a . By the slit  48   a , the first elastic portion  43   a  is divided into the elastic portion  43   a   1  and the elastic portion  43   a   2 . Thereby, the elastic portion  43   a   1  and the elastic portion  43   a   2  can be deformed independently. 
     Similarly, the second elastic portion  43   b  is provided with a slit  48   b . By the slit  48   b , the second elastic portion  43   b  is divided into the elastic portion  43   b   1  and the elastic portion  43   b   2 . Thereby, the elastic portion  43   b   1  and the elastic portion  43   b   2  can be deformed independently. 
     In the breaker  1 A, the fixed contact  21  and the movable contact  41  are in contact with each other at four positions, and the conductive state between the first terminal piece  2  and the movable piece  4 A is secured. Therefore, even when a strong impact is applied to the breaker  1 A, the conductive state between the first terminal piece  2  and the movable piece  4 A is easily maintained, and the instantaneous interruption of the current supplied to the load is prevented. 
     In the breaker  1 A, it is preferable that the elastic portion  43   a   1  and the elastic portion  43   a   2  have different natural vibration frequencies, and the elastic portion  43   b   1  and the elastic portion  43   b   2  have different natural vibration frequencies. 
     Further, it is preferable that the elastic portion  43   a   1  and the elastic portion  43   b   1  have different natural vibration frequencies, and the elastic portion  43   a   2  and the elastic portion  43   b   2  have different natural vibration frequencies. 
     The adjustment of the natural vibration frequency can be realized in the same manner as the movable piece  4 . 
     By making the natural vibration frequencies of 
     the elastic portion  43   a   1  and the elastic portion  43   a   2  and
 
the elastic portion  43   b   1  and the elastic portion  43   b   2  different,
 
even when an impact of various strengths is applied to the breaker  1 A,
 
the first movable contact  41   a  and the second movable contact  41   b  are prevented from being simultaneously separated from the fixed contact  21 , and the instantaneous interruption of the current supplied to the load is prevented.
 
       FIG.  7    is a perspective view of the breaker  1 B, which is another modification of the breaker  1  shown in  FIG.  1   . 
     For the portion of the breaker  1 B not described below, the structure of the breaker  1  described above can be adopted. 
     The breaker  1 B comprises the first terminal piece  2 , the second terminal piece  3 , a movable piece  4 B, the thermally-actuated element  5 , the PTC thermistor  6  and the like. 
     The movable piece  4 B includes a pair of movable pieces. That is, the movable piece  4 B includes a first movable piece  4   a  and a second movable piece  4   b . The first movable piece  4   a  and the second movable piece  4   b  are arranged side by side in the longitudinal direction of the breaker  1 B. 
     The first movable piece  4   a  is provided with the first movable contact  41   a , a contact region  42   a , and the first elastic portion  43   a . It is preferable that the first elastic portion  43   a  is provided with the protrusion  44   a.    
     The second movable piece  4   b  is provided with the second movable contact  41   b , a contact region  42   b , and the second elastic portion  43   b . It is preferable that the second elastic portion  43   b  is provided with the protrusion  44   b.    
     The natural vibration frequency of the first portion  45   a  of the first movable piece  4   a  and the natural vibration frequency of the second portion  45   b  of the second movable piece  4   b  are the same as those in the breaker  1 . 
     In the movable piece  4 B, too, the first elastic portion  43   a  of the movable piece  4   a  may be provided with a slit, and the second elastic portion  43   b  of the movable piece  4   b  may be provided with a slit, similarly to the movable piece  4 A shown in  FIG.  6   . 
     Further, the movable piece  4 B may include two or more pairs of movable pieces. 
     While detailed description has been made of the breaker  1 , etc. according to the present invention, the present invention can be embodied in various forms without being limited to the above-described specific embodiments. 
     That is, it is sufficient for the breaker  1 , etc. to include at least: 
     the first terminal piece  2  on which the fixed contact  21  is formed;
 
the movable piece  4  with the movable contact  41  for pressing the movable contact  41  against the fixed contact  21 ;
 
the thermally-actuated element  5  deforming with a change in temperature and moving the movable piece  4  so that the movable contact  41  is separated from the fixed contact  21 ; and
 
the second terminal piece  3  electrically connected to the movable piece  4 , wherein
         the movable contact  41  includes
 
the first movable contact  41   a  arranged at the first end portion  40   a  of the movable piece  4  in its longitudinal direction, and
 
the second movable contact  41   b  arranged at the second end portion  40   b  of the movable piece  4  in the longitudinal direction, and
   the fixed contact  21  includes
 
the first fixed contact  21   a  arranged at a position with which the first movable contact  41   a  can contact, and
 
the second fixed contact  21   b  arranged at a position with which the second movable contact  41   b  can contact.
       

     For example, by forming the movable piece  4  with a laminated metal such as a bimetal or a trimetal, the movable piece  4  may be integrally formed with the thermally-actuated element  5 . In this case, the structure of the breaker  1 , etc. is simplified. 
     Further, although the present embodiment has the self-holding circuit by the PTC thermistor  6 , such configuration may be omitted, which can reduce the resistance between the terminals. 
     Further, in the breaker  1 A, the movable piece  4  may be provided with three or more pairs of movable contacts  41  and the like. 
     Further, the breaker  1 . etc. of the present invention can be widely applied to a secondary battery pack, a safety circuit for an electric device, and the like.
           FIG.  8    shows the secondary battery pack  500 .       

     The secondary battery pack  500  comprises a secondary battery  501 , and the breaker  1 , etc. provided in the output circuit of the secondary battery  501 .
           FIG.  9    shows a safety circuit  502  for electric device.       

     The safety circuit  502  comprises the breaker  1 , etc. provided in series in the output circuit of the secondary battery  501 . 
     By a cable including a connector in which the breaker  1 , etc. is incorporated, a part of the safety circuit  502  may be constituted. 
     DESCRIPTION OF THE SIGNS 
     
         
           1  breaker 
           1 A breaker 
           2  first terminal piece 
           3  second terminal piece 
           4  movable piece 
           4 A movable piece 
           5  thermally-actuated element 
           21  fixed contact 
           21   a  first fixed contact 
           21   b  second fixed contact 
           22  first terminal 
           32  second terminal 
           33   a  convex portion 
           33   b  convex portion 
           40   a  first end portion 
           40   b  second end portion 
           41  movable contact 
           41   a  first movable contact 
           41   b  second movable contact 
           42  first contact region 
           44   a  protrusion 
           44   b  protrusion 
           45   a  first portion 
           45   b  second portion 
           46   a  bent portion 
           46   b  bent portion 
           47   a  bent portion 
           47   b  bent portion 
           51  top portion 
           500  secondary battery pack 
           501  secondary battery 
           502  safety circuit