GB1575531A

GB1575531A - Thermal switch

Info

Publication number: GB1575531A
Application number: GB2214377A
Authority: GB
Original assignee: MCCAUGHNA J
Current assignee: MCCAUGHNA J
Priority date: 1978-05-09
Filing date: 1978-05-09
Publication date: 1980-09-24

Description

(54) THERMAL SWITCH (71) I, JAMES RUSSELL MCCAUGHNA, of 5521 E. Exeter Blvd., Phoenix, Arizona 85018, U.S.A., a Citizen of the United States of America, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement This invention relates to a thermal switch, which as used herein means an electrical device having a pair of terminals and a movable conductive part normally biassed into an operative position in which it establishes a conductive path between the terminals and being movable out of such position to break said conductive path upon collapse of a normally solid fusible pellet.

Thermal switches are used for a wide variety of purposes, primarily to give protection against faults or hazards giving rise to an undue rise in the enviromental temperature, but in addition to give overcurrent protection, at least to a limited extent, at the same time.

One known construction of thermal switch comprises a conductive casing having a first terminal connected to one closed end of the casing and a second terminal projecting through an insulating mass closing the other end of the casing. Within the casing the insulated terminal is in electrical contact with a thin resilient disc which slidably contacts the interior wall of the casing. This disc establishes an electrically conductive path from said first terminal to the second terminal. The disc is maintained in its operative position in electrical contact with the insulated terminal by means of a compression spring acting against an abutment formed by a pellet of fusible waxes. A weaker trip spring is operative in the opposite direction between the disc and insulating mass. When due to overheating the fusible pellet collapses, the compression spring is released to allow the weaker trip spring to drive the disc slidably along the interior of the casing into an inoperative position spaced from the insulated terminal.

The conductive path between the first and second terminals is thereby broken.

In the known arrangement of thermal switch, the construction of the slidable disc is of critical importance. It can be made by stamping thin conductive foil in a star shape, slightly oversized relative to the casing, so as to be resiliently deformed when the switch is assembled and thereby ensure good electrical contact with the interior wall of the casing.

However, while the use of such a thin foil construction is in some ways disadvantageous, the use of a more robust disc, which would assist reliable operation, would entail materially increased manufacturing costs.

It is an object of the present invention to provide a thermal switch having an improved contact means which can be easily and inexpensively manufactured and assembled in the switch to ensure reliable operation.

According to the present invention, there is provided a thermal switch comprising a conductive casing, a terminal connected to the casing and a terminal entering the casing in insulated relationship thereto, and within the casing a resilient contact means normally in electrically conductive relationship with the insulated terminal and which is compressed in a first coordinate direction against an abutment in the form of a normally solid fusible pellet, whereby said contact means is normally expanded outwards in a second coordinate direction to contact the casing and thereby establish an electrical connection between the casing and the insulated terminal, said contact means consisting of a hollow element having spaced opposed end parts compressed towards one another in said first coordinate direction and spaced opposed side parts which are thereby expanded apart in the second coordinate direction, said element being open-sided in the third coordinate direction, and said pellet having a preselected temperature of disinte gration at which will collapse to release the compressive force acting on the contact ele ment in the first coordinate direction and thereby permit its contraction in the second coordinate direction.

In one construction the contact means takes the form of a hollow double diaphragm having opposed dished walls, preferably of elongated elliptical shape. Normally the diaphragm is compressed directly between the fusible pellet and an abutment fixed to the insulated terminal. When the pellet collapses, the electrical contact between the contact means and the casing is broken. In a second construction, the contact means takes the form of an open-ended box having two side walls which are normally resiliently deformed in the radially outwards direction to contact the casing. The box is compressed between the pellet and the insulated terminal; however, a spring is preferably provided to urge the box axially away from the insulated terminal when the pellet collapses, whereby the normal electrically conductive relationship between the contact means and the insulated terminal is broken. Only a relatively weak spring is necessary to urge the box axially due to radial contraction of the box which occurs when the axial pressure on the box is released due to collapse of the pellet.

In the accompanying drawings: Figure 1 illustrates one thermal switch construction in its normal condition; Figure 2 shows the construction of Figure 1 in the operated condition; Figure 3 illustrates a second thermal switch construction, in the normal condition ready for operation; Figure 4 shows the construction of Figure 3 in the operated condition; Figure 5a shows a contact means for the switch of Figure 1 in perspective view, while Figure 5b shows a strip for use in producing the contact means of Figure 5a; Figure 6 shows in perspective view a contact means for the switch of Figure 3.

The construction shown in Figure 1 comprises a tubular conductive casing 10 electrically connected to a terminal conductor 11 fixed in position to seal one endof the casing.

A terminal conductor 12 projects through an insulator 17, which closes the other end of the casing. The insulator 17 is preferably a ceramic bead or alternatively a high-temperatureresistant plastics material which does not outgas readily, such as that known by the Trade Mark Ryton R-4. The periphery of the casing is crimped over the insulator 17 and sealing is completed by an insulating mass 13, for example of an epoxy resin. Within the casing is a contact mass means in the form of a rectangularly-shaped open-ended conductive tube or box 500 positioned with its axis perpendicular to the axis of the casing 10. This contact box 500 is located and compressed (during assembly of the switch) between a fusible pellet 16 and the head of the insu lated terminal 12. The face of the head of the insulated terminal may be either rounded (as shown) or flat. In the normal operative con dition of the switch, the contact box 500 serves to complete the circuit connection between the casing 10 and the terminal 12.

Thus, due to the compression of the box 500 in the axial direction of the casing, its lateral walls 500a are deformed radially outwards of the casing axis to contact the wall of the casing 10. When the pellet 16 collapses on reaching a preselected temperature, the axial pressure on the box 500 is released, allowing it to contract radially out of contact with the casing, The electrical connection between the casing 10 and the terminal 12 is thus broken. Preferably, as shown in the drawing, a weak spring 501 is provided, normally compressed between the box 500 and a tapered shoulder 17a formed on the ceramic bead 17. The spring 501 is weak enough not to jeopardise the electrical contact between the contact box 500 and the end of the terminal 12, but is strong enough to displace the box 500 axially away from the terminal 12 when the pellet 16 collapses and the box 500 contracts radially. It is thereby doubly ensured that the electrical connection between the casing 10 and the insulated terminal 12 is broken. The switch is shown in its operated condition in Figure 2; the pellet 16 has collapsed, and the contact box 500 has contracted radially and has been pushed by the spring 501 out of contact with the terminal 12.

The contact box 500 for the switch of Figure 1 is shown in Figure 5a in its relaxed condition. The top and bottom walls of the box 500 which, in the assembled switch, abut the terminal 12 and the pellet 16, either directly or through load-distributing spacers, are designated 500b. The contact tube or box 500 may be easily formed from a metal strip (Figure 5b) by bending along the dotted lines indicated.

After forming by bending, the contact box is heat treated to effect spring tempering.

In the event that the spring 501 is not provided in the above-described construction, the top wall 500b of the contact box 500 is preferably fixed to the terminal 12 centrally on the casing axis, whereby to avoid the small risk that, when the pellet 16 collapses, the box is displaced into a tilted position which does not properly break the electrical connection between the casing 10 and the insulated terminal 12.

In the alternative construction shown in Figure 3, the contact box is replaced by a hollow double diaphragm 600 of conductive material having opposed elongate ellipticallyshaped dished walls 600a, 600b. The spring is omitted, the contact diaphragm 600 being fixed to the insulated terminal 12 by means of a crimped ring 600c. The contact diaphragm 600 is axially compressed, during assembly of the switch, between the fusible pellet 16 and the terminal 12, thereby to be radially expanded so as to contact the casing 10 at the opposite ends of its major axis. When the pelllet collapses, the diaphragm 600 expands axially and contracts radially, out of contact with the casing 10, thus breaking the electrical connection between the casing 10 and the terminal 12.

The switch is shown in its operated condition in Figure 4.

The contact diaphragm 600 is also shown in Figure 6, in its relaxed condition, and the elongate elliptical shape of this diaphragm is apparent from this Figure.

In a modification of the construction of Figure 3, the contact diaphragm 600 is freely located between the terminal 12 and the pellet 16 during assembly of the switch, and a spring analogous to the spring 501 of Figure 1 is provided to urge the radially contracted contact diaphragm 600 out of contact with the terminal 12, when the pellet collapses.

WHAT I CLAIM IS,- 1. A thermal switch comprising a conductive casing, a terminal connected to the casing and a terminal entering the casing in insulated relationship thereto, and within the casing a resilient contact means normally in electrically conductive relationship with the insulated terminal and which is compressed in a first coordinate direction against an abutment in the form of a normally solid fusible pellet, whereby said contact means is normally expanded outwards in a second coordinate direction to contact the casing and thereby establish an electrical connection between the casing and the insulated terminal, said contact means consisting of a hollow element having spaced opposed end parts compressed towards one another in said first coordinate direction and spaced opposed side parts which are thereby expnaded apart in the second coordinate direction, said element being open-sided in the third coordinate direction, and said pellet having a preselected temperature of disintegration at which it will collapse to release the compressive force acting on the contact element in the first coordinate direction and thereby permit it contraction in the second coordinate direction.

2. A thermal switch according to claim 1, wherein the conductive casing defines a switch axis corresponding to the first coordinate direction, the contact means being expanded radially outwards in the second coordinate direction to contact the casing.

3. A thermal switch according to claim 1 or claim 2, wherein the contact element consists of a tubular box which is open-sided in the third coordinate direction.

4. A thermal switch according to claim 3 when appendant to claim 2, including a spring acting on the contact box to urge said box in the axial direction of the switch when the pellet collapses.

5. A thermal switch according to claim 1 or claim 2, wherein the contact element consists of a hollow double diaphragm having opposed walls of dished cross-section in planes normal te the third coordinate direction.

6. A thermal switch according to claim 5 wherein the conatact diaphragm has one of its opposed walls fixed to the insulated terminal.

7. A thermal switch according to claim 5 when appendant to claim 2, including a spring acting on the contact diaphragm to urge said diaphragm in the axial direction when the pellet collapses.

8. A thermal switch according to claim 3 or claim 4, wherein the contact box is formed by bending a metal strip which is subsequently heat treated.

9. A thermal switch according to claim 5 or claim 6 or claim 7, wherein the walls of the double diaphragm, viewed along the first coordinate direction, are of elongate elliptical shape.

10. A thernal switch substantially as herein.

before described with reference to Figures 1,2 and 5 or to Figures 3, 4 and 6 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

600 is axially compressed, during assembly of the switch, between the fusible pellet 16 and the terminal 12, thereby to be radially expanded so as to contact the casing 10 at the opposite ends of its major axis. When the pelllet collapses, the diaphragm 600 expands axially and contracts radially, out of contact with the casing 10, thus breaking the electrical connection between the casing 10 and the terminal 12.

The switch is shown in its operated condition in Figure 4.

The contact diaphragm 600 is also shown in Figure 6, in its relaxed condition, and the elongate elliptical shape of this diaphragm is apparent from this Figure.

In a modification of the construction of Figure 3, the contact diaphragm 600 is freely located between the terminal 12 and the pellet 16 during assembly of the switch, and a spring analogous to the spring 501 of Figure 1 is provided to urge the radially contracted contact diaphragm 600 out of contact with the terminal 12, when the pellet collapses.

WHAT I CLAIM IS,- 1. A thermal switch comprising a conductive casing, a terminal connected to the casing and a terminal entering the casing in insulated relationship thereto, and within the casing a resilient contact means normally in electrically conductive relationship with the insulated terminal and which is compressed in a first coordinate direction against an abutment in the form of a normally solid fusible pellet, whereby said contact means is normally expanded outwards in a second coordinate direction to contact the casing and thereby establish an electrical connection between the casing and the insulated terminal, said contact means consisting of a hollow element having spaced opposed end parts compressed towards one another in said first coordinate direction and spaced opposed side parts which are thereby expnaded apart in the second coordinate direction, said element being open-sided in the third coordinate direction, and said pellet having a preselected temperature of disintegration at which it will collapse to release the compressive force acting on the contact element in the first coordinate direction and thereby permit it contraction in the second coordinate direction.
2. A thermal switch according to claim 1, wherein the conductive casing defines a switch axis corresponding to the first coordinate direction, the contact means being expanded radially outwards in the second coordinate direction to contact the casing.
3. A thermal switch according to claim 1 or claim 2, wherein the contact element consists of a tubular box which is open-sided in the third coordinate direction.
4. A thermal switch according to claim 3 when appendant to claim 2, including a spring acting on the contact box to urge said box in the axial direction of the switch when the pellet collapses.
5. A thermal switch according to claim 1 or claim 2, wherein the contact element consists of a hollow double diaphragm having opposed walls of dished cross-section in planes normal te the third coordinate direction.
6. A thermal switch according to claim 5 wherein the conatact diaphragm has one of its opposed walls fixed to the insulated terminal.
7. A thermal switch according to claim 5 when appendant to claim 2, including a spring acting on the contact diaphragm to urge said diaphragm in the axial direction when the pellet collapses.
8. A thermal switch according to claim 3 or claim 4, wherein the contact box is formed by bending a metal strip which is subsequently heat treated.
9. A thermal switch according to claim 5 or claim 6 or claim 7, wherein the walls of the double diaphragm, viewed along the first coordinate direction, are of elongate elliptical shape.
10. A thernal switch substantially as herein.

before described with reference to Figures 1,2 and 5 or to Figures 3, 4 and 6 of the accompanying drawings.