Abstract:
A non-resetable, bimetallic thermal switch. The bimetallic thermal switch includes a bimetallic element, first and second electrical contacts, and a component for electrically connecting and disconnecting the first and second electrical contacts based on movement of the bimetallic element. The switch also includes a non-resetable component configured to disallow electrical reconnection of the first and second electrical contacts after an electrical disconnection has occurred between the first and second electrical contacts.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Some commercially available thermal fuses have limited temperature capability. These thermal fuses use a solder that is alloyed to melt at a desired trip temperature. The solder is suspended between two points in a circuit (bridge). The solder “bridge” melts and falls away at the trip temperature, thereby opening the circuit (fuse). Other thermal fuses use the same solder, but contain a spring and contact bar. When the solder reaches its melting temperature, the spring pushes the bar away from the contacts thereby opening the circuit. Thus, solder fuses are not resetable, which is important for many applications. However, there is potential for the solder bridge to migrate back into place under vibration or changes in unit orientation, causing a re-closure of the switch to occur. Also, solder-type thermal fuses have a limited temperature range due to the melting point of the alloyed solder.  
           [0002]    Bimetallic thermal switches can be designed to trip over a range of temperature much greater than solder-type fuses. The setpoint for a bimetallic thermal switch is based on the type of bimetallic material used and the forming process of the bimetallic material. Although bimetallic switches can be produced to trip over a great range of temperatures, they are resetable. Bimetallic thermal switches toggle back to the “On” position (closed contacts) when the temperature drops below the trip value. However, many applications require that the thermal switch stays open even if the temperature returns to normal.  
           [0003]    Therefore, there is an unmet need for unresetable thermal switches that can be used over a wide temperatures range.  
         SUMMARY OF THE INVENTION  
         [0004]    A non-resetable, bimetallic thermal switch is provided. The bimetallic thermal switch includes a bimetallic element, first and second electrical contacts, and a component for electrically connecting and disconnecting the first and second electrical contacts based on movement of the bimetallic element. The switch also includes a non-resetable component configured to disallow electrical reconnection of the first and second electrical contacts after an electrical disconnection has occurred between the first and second electrical contacts.  
           [0005]    In one aspect of the invention, the non-resetable component is a spring-loaded stopper that disallows resetting motion of the bimetallic element.  
           [0006]    In a second aspect of the invention, the non-resetable component is a high-temperature non-conductive material that interrupts an electrical connection between the first and second electrical contacts after the first and second electrical contacts have been disconnected. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings.  
         [0008]    [0008]FIGS. 1A and B illustrate a first embodiment of a bimetallic thermal switch formed in accordance with the present invention; and  
         [0009]    [0009]FIGS. 2A and B illustrate a second embodiment of a bimetallic thermal switch formed in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0010]    The present invention is a nonresetable, bimetallic thermal switch. The trip temperature for a bimetallic thermal switch is based on the characteristics of a bimetallic disk that is included within the thermal switch. Bimetallic disks can be manufactured to trip at a temperature over a range of temperatures greater than solder-type thermal switches.  
         [0011]    [0011]FIGS. 1A and B illustrate an embodiment of a non-resetable, bimetallic thermal switch  20  formed in accordance with the present invention. The non-resetable, bimetallic thermal switch  20  includes a hermetically sealed housing  24  that includes electrical terminals  26  and  28  that extend from outside the housing  24  to inside the housing  24 . A flexible conducting beam  30  physically and electrically attaches to the first terminal  26  within the housing  24 . A non-conducting plunger  34  is attached to the conducting beam  30  at some predefined distance from the first terminal  26 . A bimetallic disk  36  is located at a base of the interior of the housing  24 . When the thermal switch  20  is experiencing temperatures below the temperature threshold of the bimetallic disk  36 , the bimetallic disk  36  is not in contact with the plunger  34 . Below the threshold temperature for the bimetallic disk  36 , the bimetallic disk  36  is concave relative to the plunger  34 . When the bimetallic disk  36  is not in contact with the plunger  34 , the conducting beam  30  maintains electrical contact with the second terminal  28 . This is the normal “ON” operation of the switch  20 . In this position, the bimetallic disk  36  maintains pressure on a spring-loaded stopper  40  due to the disk being in a convex configuration relative to the stopper  40 . The spring-loaded stopper  40  is attached to the base of the interior of the housing  24 . The spring-loaded stopper  40  provides a force that wants to push the stopper  40  into an upright position or a position predominately orthogonal to the bimetallic disk  36 . The force of the bimetallic disk  36  placed on the stopper  40  overcomes the force of the stopper  40 .  
         [0012]    [0012]FIG. 1B illustrates the switch  20  after the threshold temperature has been reached. Once the threshold temperature has been reached, the bimetallic disk  36  change shapes or snaps into contact with the plunger  34 , thereby disconnecting the conducting beam  30  from the second terminal  28  and opening the switch  20 . The bimetallic disk  36  is now in a concave position relative to the spring-loaded stopper  40 , thereby allowing the stopper  40  to spring into a position that is approximately orthogonal to the bimetallic disk  36  at approximately the center of the bimetallic disk  36 . The stopper  40  is made of a material, such as without limitation Inconel, that has enough strength to overcome any resetting force (i.e., if the temperature drops below the threshold temperature) of the bimetallic disk  36 . Therefore, the stopper  40  keeps the bimetallic disk  36  in contact with the plunger  34  thereby keeping the switch  20  open even if the temperature drops below the threshold temperature.  
         [0013]    [0013]FIGS. 2A and B illustrate another embodiment of a nonresetable, bimetallic thermal switch  90 . The non-resetable, bimetallic thermal switch  90  includes a hermetically sealed housing  94  that includes electrical terminals  96  and  98  that extend from outside the housing  94  to inside the housing  94 . A flexible conducting beam  100  attaches to the first terminal  96  within the housing  94 . A plunger  104  is attached to the conducting beam  100  at some predefined distance from the first terminal  96 . A bimetallic disk  106  is located at a base of the interior of the housing  94 . A high-temperature plastic piece  110  is suitably attached to the conducting beam  100 , an interior wall of the housing  94  or another component within the housing  94 . As shown in FIG. 2A, when the switch  90  is experiencing temperatures below the threshold temperature, the conducting beam  100  electrically connects the first terminal  96  to the second terminal  98 . Also, the high-temperature plastic piece  110  is spring-loaded to produce a force at the connection between the connecting beam  100  and the second terminal  98 . The force the piece  110  applies at the connection between the beam  100  and the second terminal  98  is not enough to overcome the force the beam  100  applies to the second terminal  98 .  
         [0014]    As shown in FIG. 2B, the temperature threshold has been reached and the bimetallic disk  106  toggles or snaps and places pressure on the plunger  104 , thereby forcing the conducting beam  100  to disconnect from the second terminal  98 . The piece  110  springs to a position between the conducting beam  100  and the second terminal  98 . Now, if the temperature drops back below the threshold temperature for the metallic disk  106 , the disk  106  stops putting pressure on the plunger  104 , and the piece  110  prevents the conducting beam  100  from electrically connecting with the second terminal  98 . A non-limiting example of the high-temperature plastic piece  110  is a Kapton strip.  
         [0015]    It will be appreciated that various other configurations of the electrically interrupting piece shown in FIGS. 2A and B or bimetallic disk preventers, such as that shown in FIGS. 1A and B, can be used for preventing reset of a bimetallic thermal switch.  
         [0016]    While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment.