Thermal fuse containing bimetallic sensing element

[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.

[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.

[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] 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] 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] 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.