Nonresettable Thermally Actuated Switch

This invention relates to a thermally actuated switch, and more particularly to a one-time or nonresettable thermally actuated switch.

Generally, such one-time switches are used to protect a motor, transformer, or other electrical apparatus from operating in ambient temperatures which exceed a predetermined maximum safe level. Such one-shot switches are typically used as back-up devices for protecting electrical apparatus which may also have current sensing devices or other thermostatic switches or the like adapted to provide normal overload protection for the apparatus, the switch of this invention functioning where the normally relied upon devices might fail. For example, such a switch may be serially connected to the windings of a motor so as to protect the motor by interrupting the power supplied thereto in the event that, because of overheating of the motor winding or for any other reason, the ambient temperature to which the switch is exposed exceeds a level which could cause damage to the motor. These switches are often relatively small (e.g., 0.50 inches or 1.27 cm. long and 0.135 inches or 0.34 cm. in diameter) so they may be readily incorporated in the apparatus they are to protect so as to be responsive to the ambient temperatures to which the apparatus is exposed and to be responsive to any abnormal rise in temperature the apparatus might undergo due to overload or short circuit conditions in the apparatus.

More particularly, such switches utilize a fusible material which rapidly undergoes a change of state when heated to a predetermined temperature (i.e., the fusible material melts) to effect breaking of the circuit. Reference may be made to such U.S. Pat. Nos. as 3,180,958, 3,291,945, 3,309,481, 3,505,630 and 3,519,972 for such one time thermally actuated switches.

Among the several objects of this invention may be noted the provision of a one-time thermally actuatable switch which effects a more positive interruption of an electrical circuit upon the switch being heated above a predetermined temperature; the provision of such a switch which has a minimum number of parts, which is convenient and inexpensive to manufacture, and which is reliable in operation. Other objects and features of this invention will be in part apparent and in part pointed out hereinafter.

Briefly, a nonresettable thermally actuated switch of this invention comprises a case of electrically conductive material, a first power lead electrically connected to the case, a second power lead electrically insulated from the case, and a deformable connector of conductive material in electrical contact with an inside surface of the case and with the second lead thereby to complete a circuit. Means is provided actuable in response to being heated above a predetermined temperature for deforming the connector means away from the inside surface of the case and for electrically insulating the connector from the case thereby to break the circuit.

FIG. 1 is an enlarged longitudinal section of the switch of this invention showing it in its operating position in which the current is carried from one power lead through the case to a connector and to another power lead;

FIG. 2 is a view similar to FIG. 1 showing the connector deformed away from the case thereby breaking the circuit; and

FIG. 3 is an enlarged end elevational view illustrating the deformable connector.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

Referring now to the drawings and more particularly to FIG. 1, a nonresettable (or one-time) thermally actuated switch of this invention, generally indicated at 1, is shown to comprise a cylindric case or housing 3 of electrically conductive material, such as formed from silver-plated red brass or the like. One end of the case is closed, as indicated at 5, and a first power lead or terminal 7 is secured in electrical contact with the closed end 5 of the case. A second power lead 9 is electrically insulated from the case and secured thereto by insulator and securement means 11. This second lead extends into an open end of the housing opposite closed end 5. A deformable connector 13 of electrically conductive material, such as silver sheet material or the like, is normally in electrical contact with an inside surface 15 of case 3 and in electrical contact with the inner end of lead 9. As generally indicated at 17, means are provided which are actuable in response to being heated above a predetermined ambient temperature for deforming (i.e., bending) connector 13 away from inside surface 15 of case 3 and for electrically insulating the connector from the case thereby to break the circuit.

More particularly, lead 7 is shown to have a flange 19 adjacent its end and a short portion 21 extending beyond the flange insertable through an opening 23 in end 5 of case 3. Once inserted through opening 23, portion 21 is upset, as indicated at 25, with flange 19 abutting end 5 of the case so as to positively secure the lead to the case in electrical contact therewith.

Connector 13, as best shown in FIG. 3, is a star-shaped contact member having 8 bendable legs or contact portions 27 and a center portion 29. The legs are each bent, as indicated at 31, from the plane of the center portion so that only the outer peripheral edges 33 of the legs contact the inner surface 15 of case 3 thereby to provide electrical contact between the case and connector 13. The center portion 29 of connector 13 is adapted to bear against the inner end of power lead 9 thereby to complete the circuit between case 5 and lead 9. As the span of connector 13 between opposed outer edges 33 is somewhat greater than the inside dimensions of case 3 and as its legs 27 are resilient, its legs bend inwardly upon being inserted in the case and thus are resiliently biased into contact with inside wall 15 of case 3 to provide good electrical contact between the case and the connector. While connector 13 is shown as star-shaped to provide readily deformable legs, it will be understood that connectors of other configurations, such as a cup-shaped, radially slotted connector may be used.

Means 17 for deforming connector 13 is shown to comprise a solid body 35 of fusible material engageable with the center portion 29 of the connector, an insulator 37, and a spring 39. The fusible material is preferably an organic compound such as a petroleum wax compound or the like which upon being heated to a predetermined temperature rapidly changes state from a solid to a liquid. Insulator 37 is spaced from connetor 13 by the fusible member while the fusible member is in its solid preformed shape (i.e., before it melts). Insulator 37 has an edge 41 engageable with connector 13 for deforming the connector (i.e., for bending legs 27 of the connector) so as to break electrical contact between the legs and the inside surface of the case. Spring 39 is a compression coil spring interposed between end 5 of case 3 and the inner end of insulator 37 to bias the insulator, the fusible member, and connector 13 into positive current-carrying engagement with the inner end of lead 9, this inner end of lead constituting an abutment for connector 13. Insulator 37 is a generally cylindric cup-shaped member having outer dimensions somewhat less than those of the inside of case 3 so that it is freely movable longitudinally within the case from a retracted position (see FIG. 1) in which the fusible material maintains it spaced from connector 13 and an advanced circuit-breaking position (see FIG. 2) in which its edge 41 engages connecctor 13 and deforms the latter to break the circuit. Insulator 37 is movable from its retracted position (see FIG. 1) to its circuit-breaking position by spring 39 upon fusible member 35 melting. It will be understood that with the insulator in its retracted position, coil spring 39 is compressed in a preloaded position and contains sufficient stored energy to move the insulator forward and to deform legs 27 of connector 13 and to break the circuit.

Insulator 37 has a central bore or recess 43 having a base wall 45 at its inner end facing fusible member 35, cylindric side walls 46, and a rounded transition surface 47 between edge 41 and the side walls. The insulator also has a shoulder 49 against which one end of spring 39 bears and a central hub portion 51 extending into the coil spring.

Fusible wax body 35, like insulator 37 is cylindric and has a generally flat face 53 engageable with portion 29 of connector 13 and a central hub portion 55 extending into recess 43 of the insulator and bearing against base wall 45 thereof.

Insulator and securement means 11 comprises an integral collar 57 formed on lead 9 and spaced from the inner end of the lead. Collar 57 is of substantially larger diameter than lead 9 but is somewhat smaller than the inner diameter of the open end of case 3 and has a circumferential groove 59 therearound. Means 11 further comprises a sleeve 61 of a flexible electrical insulator material, such as may be formed from a polyimide resin available commercially from E. I. DuPont de Nemours and Company under the trademark KAPTON. The outer diameter of sleeve 61 is only slightly less than the inner diameter of the open end of case 3 so as to be conveniently insertable into the open end of case 3 and be disposed between the case and the outer surface of the collar. An inwardly projecting shoulder 63 within case 3 is engageable with the inner end of the sleeve upon inserting the sleeve in the case and thus serves as a locating stop for the sleeve. Lead 9 with collar 57 thereon is inserted axially endwise into sleeve 61 until the inner end of terminal 9 is positioned within the case, such as is shown in FIG. 1. The outer end of the case and sleeve 61 are crimped, as indiated at 65, into groove 59 of collar 57 thereby to positively secure and to electrically insulate lead 9 relative to the case. It will be understood that with terminal 9 secured within case 3 as above described, the inner end of lead 9 serves as an abutment for connector 13 and the preload compression force of spring 39 is reacted against the case via insulator 37, wax member 35, connector 13, lead 9 and collar 57.

Upon switch 1 being heated above a predetermined ambient temperature level, thermal sensing member 35 melts and thus permits spring 39 to rapidly move insulator member 37 from its retracted to its circuit-breaking position. As insulator 37 moves forward, its leading edge 41 engages legs 27 of connector 13 and forces the connector to bend at a point 67, as indicated in FIG. 2, thereby to move the outer edges 33 of the connector away from the inner face 15 of case 3. Upon continued further movement of insulator member 37 by spring 39, the transition portion 47 of the insulator member cammingly engages the connector and causes edges 33 to further move away from the inner surface 15 of the case (i.e., to move radially inwardly toward lead 9). Base wall 45 of the insulator is engageable with portion 29 of the connector and thus provides a hard stop for the insulator. With the insulator in its circuit-breaking position, a portion of the insulator is interposed between the connector and the inside surface 15 of the case thereby to positively insulate the connector and lead 9 from the case. It will be understood that when the thermal sensing member 35 melts, the melted wax will merely pool (not shown in FIG. 2) in the lower portion of case 3 and does not prohibit insulator 37 from moving from its retracted to its circuit-breaking position. It will further be understood that the fusible material is not electrically conductive.

In assembling a nonresettable thermally actuated switch 1 of this invention, spring 39, insulator 37, wax body 35 and connector 13 are inserted in the above specified order into the open end of case 3, such as shown in FIG. 1. Sleeve 61 is inserted into the open end of the case until its forward end engages shoulder 63. Lead 9 with collar 57 thereon is inserted into the open end of the case through sleeve 61 until the forward end of the lead engages the back face of connector 13. Lead 9 is then further pushed into the case thereby to compress spring 39 to its preloaded condition (as shown in FIG. 1) so as to positively maintain connector 13 in current-carrying relation with the inner end of lead 9. Case 3 is then crimped at 65 thereby to positively lock lead 9 in position within the open end of the case may be sealed with a conventional molded in place potting compound, as indicated at 69.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.