Patent Publication Number: US-2023162934-A1

Title: Temperature-dependent switch

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 17/074,033 filed Oct. 19, 2020, which claims priority from German patent application DE 10 2019 128 367.1, filed on Oct. 21, 2019. The entire contents of both of these applications are incorporated herein by reference. 
    
    
     BACKGROUND 
     This disclosure relates to a temperature-dependent switch. 
     An exemplary temperature-dependent switch is disclosed in DE 10 2018 100 890 B3. 
     Such temperature-dependent switches are usually used for the purpose of protecting electrical devices from overheating. To this end, the switch is connected in series to the device to be protected and to the supply voltage thereof and is arranged mechanically on the device such that it is thermally connected to said device. 
     A temperature-dependent switching mechanism ensures that the two stationary contacts of the switch are electrically connected to each other below the response temperature of the switching mechanism. Hence, the circuit is closed below the response temperature and the load current of the device to be protected can flow through the switch. 
     If the temperature rises above an admissible value, the switching mechanism lifts off the movable contact member from the counter contact, opening the switch and disconnecting the load current of the device to be protected. The now current-less device can then cool down again. In this case, the switch, which is coupled thermally to the device, also cools down and would thereupon actually close again automatically. 
     However, in the case of the switch disclosed in DE 10 2018 100 890 B3, a closing lock ensures that this switching back does not occur in the cooled-down position, so that the device to be protected, once being switched off, cannot switch itself on again automatically. The closing lock mechanically locks the switching mechanism, so that the switching mechanism, once been opened, cannot close again, even if strong vibrations or temperature fluctuations occur. 
     This is a safety function that applies, for example, to electric motors that are used as drive units. This is intended in particular to prevent damage to the device or even injury to the person using the device. 
     Due to their switching behavior, such switches, which do not close again after being opened once, are also called one-time switches. 
     It goes without saying that “opening” the switch means the disconnection of the electrically conductive connection between the two contacts of the switch and not an opening of the switch housing in the mechanical sense. 
     A further switch of this type is disclosed in DE 10 2013 101 392 A1. This switch comprises a temperature-dependent switching mechanism having a temperature-dependent bimetal snap-action disc and a bistable spring disc which carries a movable contact or a current transfer member. When the bimetal snap-action disc is heated to a temperature above its response temperature, it lifts off the contact or the current transfer member from the counter contact or counter contacts against the force of the spring disc and thereby presses the spring disc into its second stable configuration in which the switching mechanism is situated in its high-temperature position. 
     When the switch and thus the bimetal snap-action disc cool down again, said bimetal snap-action disc returns to its low-temperature position. However, due its design, its edge cannot rest on a counter bearing, such that the spring disc remains in the stable second configuration in which the switch is open. 
     This means that the switch remains in its open position after opening once, even if it cools down again. However, tests carried out by the company of the applicant have shown that the switch disclosed in DE 10 2013 101 392 A1 does close again in the event of stronger mechanical vibrations such that—under safety aspects—it may not be the perfect solution in some applications. 
     There are also temperature-dependent switches with a so-called self-holding resistor which is connected in parallel with the two counter contacts so that it takes over part of the load current when the switch opens. Ohmic heat, which is sufficient to hold the snap-action disc above its response temperature, is generated in said self-holding resistor. 
     However, this so-called self-holding is only active for as long as the electric device is still switched on. As soon as the device is shut off from the supply circuit, no more current flows through the temperature-dependent switch either so that the self-holding function is cancelled. After the electric device has been switched on again, the switch would therefore be situated in the closed state again so that the device is able to heat up again, which could result in consequential damage. 
     This problem is avoided with the switches disclosed in DE 10 2007 042 188 B3 and DE 10 2013 101 392 A1, where the self-holding function is not realized electrically, but by means of a bistable spring part, which has two stable geometric configurations in a temperature-independent manner, as is described in the above-cited documents. 
     In contrast to this, the snap-action disc is a bistable snap-action disc that assumes either a high-temperature configuration or a low-temperature configuration in a temperature-dependent manner. 
     In the DE 10 2007 042 188 B3 mentioned at the outset, the spring disc is a circular snap-action spring disc on the middle of which the contact member is fastened. The contact member is, for example, a movable contact part which is pressed by the snap-action spring disc against the first stationary contact which is arranged on the inside of a cover of the housing of the switch. The snap-action spring disc presses by way of its edge against an inner bottom of a lower part of the housing which acts as a second contact. In this way, the snap-action spring disc, which is itself electrically conducting, produces an electrically conducting connection between the two counter contacts. 
     In its low-temperature position, the bimetal snap-action disc lies loosely against the contact part. If the temperature of the bimetal snap-action disc increases, it switches to its high-temperature position, in which it presses with its edge against the inside of the upper part of the housing and, concurrently with its center onto the snap-action spring disc such that said snap-action spring disc switches from its first to its second stable configuration, as a result of which the movable contact part is lifted off from the stationary contact and the switch is opened. 
     If the temperature of the switch cools down again, the bimetal snap-action disc switches back to its low-temperature position again. In this case, it moves with its edge into abutment with the edge of the snap-action spring disc and with its center into abutment with the upper part of the housing. However, the actuating force of the bimetal snap-action disc is not sufficient to let the snap-action spring disc switch back into its first configuration again. 
     The bimetal snap-action disc only bends further once the switch has cooled down a lot such that it is finally able to press the edge of the snap-action spring disc onto the inner bottom of the lower part by such a distance that the snap-action spring disc switches into its first configuration again and closes the switch again. 
     The switch disclosed in DE 10 2007 042 188 B3 therefore, after being opened once, remains open until it has cooled down to a temperature below room temperature, for which purpose a cooling spray can be used, for example. 
     Although said switch meets the corresponding safety requirements in many applications, it has nevertheless been shown that as a result of bracing the bi-metal snap-action disc between the upper part of the housing and the edge of the snap-action spring disc, in rare cases the snap-action spring disc nevertheless springs back in an unwanted manner. 
     DE 10 2013 101 392 A1 discloses a switch having a current transfer member as a movable contact member, for example in the form of a contact plate supported by the snap-action spring disc. Both stationary contacts are now arranged on the inner surface of the cover of the housing, wherein an electrically conductive connection between these two contacts is produced by placing the contact plate against these two contacts. 
     In the case of said switch, the snap-action spring disc is fixed with its edge on the lower part of the housing, while the bimetal snap-action disc is provided between the snap-action spring disc and the inner bottom of the lower part. 
     Below the response temperature of the bimetal snap-action disc, the snap-action spring disc presses the contact plate against the two stationary contacts. If the bimetal snap-action disc switches to its high-temperature position, it presses with its edge against the snap-action spring disc and pulls with its center the snap-action spring disc away from the upper part, so that the contact plate moves out of abutment with the two counter contacts. In order to make this geometrically possible, the contact plate, the snap-action spring disc and the bimetal snap-action disc are captively connected to each other by a centrally extending rivet. 
     When the temperature of the bimetal snap-action disc drops again, it switches back into its low-temperature position, but the spring disc remains in its assumed configuration as the bimetal snap-action disc lacks a counter bearing for its edge so that it is not able to press the current transfer member against the two stationary contacts again. 
     Said switch therefore comprises a self-holding function due to the design. In rare cases, in the event of strong mechanical vibrations, the snap-action spring disc can spring back unintentionally here too. 
     Further, DE 25 44 201 A1 discloses a temperature-dependent switch having a current transfer member realized as a contact bridge, where the contact bridge is pressed against two stationary counter contacts via a closing spring. The contact bridge is in contact via an actuating bolt with a temperature-dependent switching mechanism which consists of a bimetal snap-action disc and a spring disc, both of which are clamped at their edges. 
     As with the switch disclosed in DE 10 2007 042 188 B3, the spring disc and the bimetal snap-action disc are both bistable, the bimetal snap-action disc in a temperature-dependent manner and the spring disc in a temperature-independent manner. 
     If the temperature of the bimetal snap-action disc increases, it presses the spring disc into its second configuration, in which it presses the actuating bolt against the contact bridge, lifting it off the stationary counter contacts against the force of the closing spring. 
     Even when the bimetal snap-action disc cools down, the spring disc remains in said second configuration and keeps the switch open against the force of the closing spring. 
     Pressure can then be exerted onto the contact bridge from outside by means of a button such that, as a result, the spring disc is pressed back into its first stable configuration by means of the actuating bolt. 
     Along with the very complex design, said switch, on the one hand, comprises the disadvantage that in the open state, the spring disc lifts the contact bridge from the counter contacts against the force of the closing spring so that the spring disc, in its second configuration, has to overcome the force of the closing spring in a reliable manner. Because the closing spring, however, in the closed state ensures the secure abutment of the contact bridge against the counter contacts, a spring disc with a very high degree of stability is required here in the second configuration. 
     A further switch with three switching positions is disclosed in DE 86 25 999 U1. It comprises a flexible tongue, which is clamped-in at one end and carries a movable contact part at its free end, wherein the movable contact part interacts with a fixed counter contact. 
     A calotte is formed on said flexible tongue, which calotte is pressed into its second configuration, in which it distances the movable contact part from the stationary counter contact, by means of a bimetal plate which is also attached to the flexible tongue. 
     In the case of said switch, the calotte has to hold the movable contact part at a distance from the fixed counter contact against the closing force of the flexible tongue which is clamped-in at one end so that the calotte has to apply a high actuating force in its second configuration. 
     The switch consequently comprises the above-discussed disadvantages, namely that high actuating forces have to be overcome, which leads to high production costs and to a non-secure state in the cooled-down position. 
     The switch disclosed in DE 10 2018 100 890 B3, which was mentioned at the outset, has the mechanically most stable closing lock compared to the other mentioned switches. Due to the mechanical locking of the switching mechanism, which is produced by the closing lock, an accidental switch back after the switch has been open once is almost impossible. 
     It has been shown, however, that the closing lock disclosed in DE 10 2018 100 890 B3 is relatively complex to manufacture, so that the manufacturing costs of the switch are comparatively high. 
     SUMMARY 
     It is an object to provide a switch with an alternative closing lock which is simple and thus inexpensive to manufacture and yet still guarantees a safe disconnection of the electric circuit even in the cooled-down position of the switch and in the event of strong vibrations. 
     According to a first aspect, a temperature-dependent switch is provided, which comprises a first stationary contact, a second stationary contact, and a temperature-dependent switching mechanism having a movable contact member, wherein in a first switching position, the switching mechanism presses the movable contact member against the first stationary contact, thereby producing an electrically conductive connection between the first stationary contact and the second stationary contact via the movable contact member, and, in a second switching position, the switching mechanism keeps the movable contact member spaced at a distance from the first stationary contact, thereby disconnecting the electrically conductive connection, wherein the temperature-dependent switching mechanism comprises a temperature-dependent snap-action part, which is configured to switch from a geometric low-temperature configuration to a geometric high-temperature configuration upon reaching a switching temperature, and which is configured to switch back from the geometric high-temperature configuration to the geometric low-temperature configuration upon subsequently reaching a reset temperature that is lower than the switching temperature, wherein a switching of the temperature-dependent snap-action part from the geometric low-temperature configuration to the geometric high-temperature configuration moves the switching mechanism from the first switching position to the second switching position, thereby opening the switch, and wherein a closing lock is provided that prevents the switch once having opened from closing again by keeping the switching mechanism in its second switching position, wherein the closing lock comprises a locking element which comprises a shape-memory alloy and an opening through which the movable contact member protrudes, wherein the shape-memory alloy is configured to change a shape of the locking element upon reaching a locking element switching temperature from a first shape, in which the locking element does not activate the closing lock, to a second shape, in which the locking element activates the closing lock by exerting a force on a part of the switching mechanism, which force holds the switching mechanism in its second switching position. 
     The closing lock is activated upon reaching or exceeding a predefined temperature that in the present case is referred to as locking element switching temperature. As long as the locking element switching temperature is not reached or exceeded, the closing lock is not activated. 
     The closing lock particularly makes use of the temperature-dependent shape-memory effect (memory effect) of a shape-memory alloy. It comprises a locking element that is at least partly made of such a shape-memory alloy. This locking element comprises an opening through which a part of the switching mechanism protrudes. 
     In particular, the movable contact member of the switching mechanism protrudes through this opening and can move through the opening during the switching movement of the switch without colliding with the locking element. The shape of the opening can be varied, for example round or angular. 
     The temperature-dependent shape-memory effect of the shape-memory alloy of the locking element is preferably used as follows: As long as the locking element switching temperature is not exceeded, the locking element remains in its first shape. In this first shape, the locking element does not exert a force on the switching mechanism. Preferably, the locking element does not contact the switching mechanism at all as long as it is in its first shape. The switching function of the switching mechanism, which is particularly effected by the temperature-dependent snap-action part, is thus not impaired as long as the closing lock is not activated. The closing lock is only activated when the locking element assumes its second shape, which happens due to the shape-memory alloy upon exceeding the locking element switching temperature. In its second shape, the locking element exerts a force on a part of the switching mechanism. This force holds the switching mechanism in its second switching position and prevents it from returning to its first, closed switching position. 
     As soon as the locking element switching temperature is reached, the switch thus remains in its second, open switching position. A renewed closing of the switch is prevented by the closing lock. 
     Shape-memory alloys allow such temperature-dependent changes in the shape of components to be guaranteed very easily and reliably. The locking element can therefore be produced relatively inexpensively. Since otherwise the design of the switch and of the switching mechanism contained therein does not have to be changed, but only the locking element has to be added to the switch, the realization of the entire closing lock is very simple and inexpensive from a production point of view. The total costs of the switch are therefore hardly increased by the closing lock. 
     It should be noted that the terms “open switch” and “closed switch” do not refer to a housing position, but to the electrically conductive connection. Thus, the terms have nothing to do with whether a housing of the switch is open or closed. Rather, these terms refer to whether the electrically conductive connection between the two stationary contacts of the switch is open/established or closed/disconnected. 
     With regard to the terminology used in the present case, it should also be noted that the shape of the shape-memory alloy of the locking element changes upon “exceeding” the locking element switching temperature. In principle, the change in shape already occurs when the locking element switching temperature is reached. However, the word “exceeding” is intended to clarify that the shape of the locking element changes after a warm-up process, i.e. when the locking element switching temperature is reached, starting from a lower temperature and not during a cooling process when the locking element switching temperature is reached, starting from a higher temperature. 
     The locking element can, for example, be configured to change its shape from the first shape to the second shape upon reaching the locking element switching during a warm-up process, but to retain its second shape upon reaching the locking element switching temperature later again during a cooling process. 
     The change in shape that the locking element performs when the locking element switching temperature is exceeded can be manifold. For example, the locking element can change its shape from a cross-sectionally flat or straight shape to a cross-sectionally convex or concave shape. It is also conceivable that the locking element bends, tilts or expands in one direction when the locking element switching temperature is reached. 
     Preferably, the shape-memory alloy of the locking element is configured to move the locking element towards the switching mechanism upon reaching the locking element switching temperature, to touch it and to exert a compressive force on it which force holds the switching mechanism in its second switching position. This force exerted by the locking element on the switching mechanism is preferably higher than the force exerted by the temperature-dependent snap-action part, by means of which the temperature-dependent snap-action part in its low-temperature configuration attempts to close the switch, i.e. to bring the switching mechanism into its first switching position. 
     If the switch is open after the switching temperature has been reached and the closing lock is activated after the locking element switching temperature has been reached, the closing lock prevents the switch from closing again, even if its temperature drops below the reset temperature again and the temperature-dependent snap-action part tries to snap back into its geometric low-temperature configuration. 
     According to a refinement, the locking element is substantially plate-shaped or disc-shaped. 
     This has the advantage that the locking element and thus the entire closing lock hardly increases the overall height of the switch. The dimensions of the switch as well as the design of the switching mechanism do not or hardly need to be adjusted compared to regular switches without closing lock. 
     Herein, “plate-shaped” and “disc-shaped” are understood to mean that the length and width extension of the locking element is considerably greater than its thickness. While “plate-shaped” can be almost any shape when viewed from above, “disc-shaped” preferably refers to a circular, oval or elliptical shape of the locking element. 
     According to another refinement, the opening in the locking element is configured as a through hole. 
     This has the advantage that such a hole can be produced relatively easily and inexpensively. The locking element can be produced as a kind of perforated plate, i.e. a plate with a through hole. Such a locking element can be mounted very easily in the housing of the switch and put over the movable contact member of the switching mechanism. Preferably, the opening or through-hole is centrally arranged in the locking element. 
     The locking element as well as the remaining design of the switch can, for example, be rotationally symmetrical. 
     In a further refinement, the locking element comprises at least one slit that penetrates the locking element and adjoins the opening. 
     The advantage of such a slit is that the shape-memory effect can be increased thereby. In other words, by means of the shape-memory alloy, the locking element can achieve a greater change in shape with the same amount of force. The at least one slit in the locking element also avoids internal stresses that could otherwise occur due to the deformation of the locking element caused by the shape-memory alloy. 
     The at least one slit may be rectilinear and may extend radially outward from the opening. 
     This has the advantage that the locking element can arch more. Parts of the locking element can unfold along the slit without producing any greater shear forces in the area of the opening. 
     The locking element may comprise two, three, four or more slits in the locking element, each of which adjoins the opening, is rectilinear and extends radially outward from the opening. 
     Hence, according to this refinement, the slits are inserted into the locking element in a star shape manner starting from the hole. Each of these slits preferably penetrates the entire thickness of the locking element. This has the advantage that the slits create individual, separate areas in the locking element which can bend separately upon reaching the locking element switching temperature in order to exert the force that is required for the closing lock on the switching mechanism separately from each other. 
     The locking element can be a kind of slotted spring disc or slotted disc spring, which is flat in its first shape, i.e. purely disc-shaped, and in its second shape is convex or concave curved. 
     In a further refinement, the switch comprises a housing and that the locking element is attached to the housing with its edge. 
     Since the opening is preferably centrally arranged in the locking element, such a peripheral attachment of the locking element has the advantage that the shape change caused by the shape-memory alloy is hardly affected. In addition, the locking element can be fixed at its edge to the housing in a very stable manner. 
     The locking element is preferably attached to the housing along its entire circumferential edge. The attachment can be in a non-positive locking manner, a positive locking manner and/or in a firmly bonded manner. The locking element is particularly preferably at its peripheral edge clamped in the housing. Such a fastening can be realized most cost-efficiently in terms of production. 
     The edge of the locking element may be made of an electrically insulating material or may be coated with an electrically insulating material. 
     For example, a central part of the locking element can be made of shape-memory alloy, which is connected to an electrically insulating material on its outer circumference. Similarly, the entire locking element can be made of the shape-memory alloy and coated on its peripheral edge with an electrically insulating material, e.g. plastic. Furthermore, an adhesive foil can be applied to the shape-memory alloy at the edge or along the circumference to electrically insulate the edge of the locking element. The coating or the adhesive foil can be applied to the locking element on one or both sides (top and bottom). 
     The electrical insulation of the edge of the locking element has the advantage that the locking element can be used to achieve an electrical insulation of two housing parts of the switch. Since the edge of the locking element is preferably attached to the housing and parts of the housing are current-carrying, such insulation also has the advantage that the locking element itself does not carry any current. This in turn has a positive effect on the function and lifetime of the shape-memory alloy. 
     According to another refinement, the housing comprises a lower part that is closed by an upper part, wherein the locking element rests on a circumferential shoulder that is arranged in the lower part and is arranged clamped between the lower part and the upper part. 
     Such an arrangement of the locking element has the advantage that it only has to be placed on the shoulder in the lower part during production and is automatically clamped between the upper part and the lower part during the closing of the switch housing and is thus fixed. Typically, the lower part has a raised edge which is at least partially bent or flanged to the upper part when the switch housing is closed in order to hold the upper part to the lower part. 
     Further, the first stationary contact or each of the two stationary contacts may be arranged on an inner side of the upper part. 
     This measure ensures that when the upper part is mounted on the lower part, the geometrically correct assignment between the first contact or the first and second contact to the movable contact member is also established at the same time. 
     In a further refinement, the locking element is arranged on a first side of the temperature-dependent snap-action part facing the first contact and is configured to exert in its second shape the force that holds the switching mechanism in its second switching position directly or indirectly on the temperature-dependent snap-action part. 
     Viewed from the temperature-dependent snap-action part, the locking element according to this refinement is thus arranged on the same side of the snap-action part as the first contact. As soon as the locking element assumes its second shape upon reaching the locking element switching temperature, it presses on the switching mechanism from this first side of the temperature-dependent snap-action part. 
     Depending on the design of the switching mechanism, the locking element can either contact the temperature-dependent snap-action part directly and apply the force directly to it or contact another component of the switching mechanism so that it applies the force only indirectly to the temperature-dependent snap-action part. Both cases have the advantage that both a direct application of force to the temperature-dependent snap-action part and a direct application of force to the temperature-independent spring part are possible without any problems, since both components are typically designed with a relatively large surface area and thus offer large-area possibilities for the application of force. 
     In an alternative refinement, the locking element is arranged on a second side of the temperature-dependent snap-action part facing away from the first contact and is configured to exert in its second shape the force that holds the switching mechanism in its second switching position directly or indirectly on the contact member. 
     Viewed from the temperature-dependent snap-action part, the locking element in this refinement is therefore not arranged on the side of the first contact (first side), but on the opposite second side of the temperature-dependent snap-action part. Upon reaching the locking element switching temperature it exerts the force that holds the switching mechanism in its second switching position preferably directly on the movable contact member. This has the advantage that the force exerted by the locking element is directly applied to the part that is to be kept apart from the first contact when the closing lock is activated. Since the movable contact member is usually a solid component, there is also hardly any risk of damage to the switching mechanism by the closing lock. 
     According to another refinement, the shape-memory alloy of the locking element is a shape-memory alloy with a one-way memory effect. 
     By using a shape-memory alloy with a one-way memory effect, the locking element and thus also the closing lock can be designed irreversibly. In this case, the switch according to the invention is a so-called one-time switch. The shape-memory alloy allows only one single change of shape of the locking element. After it has changed its shape from the first shape to the second shape upon exceeding the locking element switching temperature, cooling again in the case of such a shape-memory alloy with a one-way effect does not cause a renewed change in shape. 
     Alternatively, the shape-memory alloy can be a shape-memory alloy with two-way memory effect, wherein the locking element is configured to change its shape from the second shape to the first shape when falling below a locking element reset temperature, and wherein the locking element reset temperature is lower than the locking element switching temperature. 
     In this case, the switch is a switch with a closing lock that is reversibly designed, i.e. it can be released again. Shape-memory alloys with a two-way effect can, so to speak, remember two shapes, one at high and one at low temperature. With such a two-way shape-memory alloy, the locking element can change its shape from the first shape to the second shape upon reaching the locking element switching temperature, and then return to its first shape upon reaching the locking element return temperature during a subsequent cooling. 
     According to a further refinement, the locking element switching temperature is equal to or higher than the switching temperature of the temperature-dependent snap-action part. 
     If the two switching temperatures are selected to be the same, the closing lock is activated at the same time at which also the switch opens. If, on the other hand, the locking element switching temperature is selected to be higher than the switching temperature of the temperature-dependent snap-action part, the closing lock is activated after the switch has opened. In fact, the electric circuit is disconnected when the switch is opened. In practice, however, the switch usually heats up a little before the cooling process begins due to the residual heat typically remaining in the device to be protected. After opening the switch, the temperature slightly overshoots, which is why it is referred to as the overshoot temperature range. It is therefore possible to set the locking element switching temperature to be in this overshoot temperature range. 
     According to a further refinement, the locking element reset temperature is lower than the reset temperature of the temperature-dependent snap-action part. 
     This has the advantage that, if the switch cools down regularly after opening, the closing lock remains activated even if the temperature-dependent snap-action part reaches or falls below the reset temperature. A deactivation of the closing lock (if it is reversible) can then be carried out, for example, by means of a corresponding cold treatment. For example, the switch can be treated manually with a cooling spray, which deactivates the closing lock and closes the switch again. 
     According to a further refinement, the switching mechanism comprises a temperature-independent spring part which is connected to the movable contact member, wherein the temperature-dependent snap-action part acts on the temperature-independent spring part when the switching temperature is exceeded and thereby lifts off the movable contact member from the first contact. The spring part may be a bistable spring part with two temperature-independent, stable geometric configurations. 
     If the spring part is configured as a bistable spring disc, it is preferred that the spring disc in its first stable configuration presses the movable contact member against the first contact and in its second stable configuration keeps the movable contact member spaced apart from the first contact. This has the advantage that in the closed state of the switch (in the first switching position of the switching mechanism) the spring disc causes the closing force and thus the contact pressure between the movable contact member and the first contact. This mechanically relieves the temperature-dependent snap-action part, which has a positive effect on its service life and the long-term stability of its response temperature (switching temperature). 
     If the spring part is configured as a bistable spring disc having two temperature-independent stable geometric configurations, this has the additional advantage that the bistable spring disc keeps the switch in its open state after it has been opened. 
     The temperature-dependent snap-action part is preferably a bimetal or trimetal snap-action disc. 
     According to a further refinement, the movable contact member comprises a movable contact part interacting with the first contact, and that the spring part interacts with the second contact, wherein it is preferred that the spring part, at least in its first geometric configuration, is electrically connected to the second contact via its edge. 
     A similar refinement is disclosed in DE 10 2018 100 890 B3, DE 10 2007 042 188 B3 or DE 10 2013 101 392 A1. The result is that the temperature-dependent snap-action part is not current-loaded in any position of the switch, but that the load current of the electrical device to be protected flows through the spring part. 
     In an alternative refinement, the movable contact member comprises a current transfer member that interacts with both stationary contacts. 
     Here it is advantageous that the switch can carry considerably higher currents than the switch disclosed in DE 10 2007 042 188 B3. The current transfer member arranged on the contact member ensures the electrical short circuit between the two contacts when the switch is closed, so that not only the temperature-dependent snap-action part but also the temperature-independent spring part is no longer traversed by the load current. A switch having such a current transfer member is disclosed in DE 10 2013 101 392 A1. 
     It goes without saying that the features referred to above and yet to be explained below can be used not only in the respective given combinations, but also in other combinations or alone without leaving the spirit and scope of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    shows a schematic sectional view of a first embodiment of the switch in its low-temperature position; 
         FIG.  2    shows a schematic sectional view of the first embodiment of the switch shown in  FIG.  1    in its high-temperature position; 
         FIG.  3    shows a schematic sectional view of the first embodiment of the switch shown in  FIG.  1    in its high-temperature position with activated closing lock; 
         FIG.  4    shows a schematic sectional view of a second embodiment of the switch in its low-temperature position; 
         FIG.  5    shows a schematic sectional view of the second embodiment of the switch shown in  FIG.  4    in its high-temperature position; 
         FIG.  6    shows a schematic sectional view of the second embodiment of the switch shown in  FIG.  4    in its high-temperature position with activated closing lock; 
         FIG.  7    shows a schematic sectional view of a third embodiment of the switch in its low-temperature position; and 
         FIG.  8    shows a schematic top view of a locking element according to an embodiment. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG.  1    shows a schematic sectional view of a switch  10 , which is rotationally symmetrical in top view and preferably has a circular shape. 
     The switch  10  comprises a housing  12  in which a temperature-dependent switching mechanism  14  is arranged. The housing  12  comprises a pot-shaped lower part  16  and an upper part  18 , which is held to the lower part  16  by a bent or flanged upper edge  20 . 
     In the embodiment shown in  FIG.  1   , both the lower part  16  and the upper part  18  are made of an electrically conductive material, preferably metal. The upper part  18  rests on a shoulder  22  of the lower part with an interposed insulating foil  24 . The shoulder  22  is designed as a circumferential shoulder and comprises a substantially annular bearing surface on which the upper part  18  rests with the interposed insulating foil  24 . 
     The insulating foil  24  provides electrical insulation of the upper part  18  against the lower part  16 . The insulating foil  24  also provides a mechanical seal that prevents liquids or impurities from entering the interior of the housing from outside. 
     Since the lower part  16  and the upper part  18  are in this embodiment each made of electrically conductive material, thermal contact to an electrical device to be protected can be produced via their outer surfaces. The outer surfaces are also used for the external electrical connection of the switch  10 . 
     Another insulating foil  26  can be applied to the outside of the upper part  18 , as shown in  FIG.  1   . 
     The switching mechanism  14  comprises a temperature-independent spring part  28  and a temperature-dependent snap-action part  30 . The spring part  28  is preferably designed as a bistable spring disc. Accordingly, it has two temperature-independent stable geometric configurations. The first configuration is shown in  FIG.  1   . The temperature-dependent snap-action part  30  is preferably designed as a bimetal snap-action disc. It has two temperature-dependent configurations, a geometrical high-temperature configuration and a geometrical low-temperature configuration. In the first switching position of the switching mechanism  14  shown in  FIG.  1   , the temperature-dependent bimetal snap-action disc  30  is in its geometrical low-temperature configuration. 
     The temperature-independent spring disc  28  rests with its edge  32  on a further circumferential shoulder  34  formed in the lower part  16 . In its low-temperature configuration, the temperature-dependent bimetal snap-action disc  30  can be freely suspended in the housing  12  in such a way that its edge  36  does not contact the housing  12 . Among other things, this has the advantage that the closing pressure in the closed state of the switch  10  is generated by the spring disc  28  alone. Also, when the switch  10  is closed, the current then flows only through the spring disc  28 , but not through the bimetal snap-action disc  30 . 
     The edge  36  of the bimetal snap-action disc  30  in its low-temperature configuration can alternatively rest on the inner bottom surface  38  of the lower part  16 . For this purpose, the inner bottom surface  38  may be laterally raised, as indicated by the dotted line  39  in  FIG.  1   . In such a case, the closing pressure of the switch  10  in its closed state would be generated not only by the spring disc  28  but also by the bimetal snap-action disc  30 . 
     The temperature-independent spring disc  28  is with its center  40  fixed to a movable contact member  42  of the switching mechanism  14 . The temperature-dependent bimetal snap-action disc  30  is with its center  44  also fixed to this movable contact member  42 . 
     The movable contact member  42  comprises a contact part  46  and a ring  45  which is pressed onto the contact part  46 . The ring  45  comprises a circumferential shoulder  47  on which the bimetal snap-action disc  30  rests with its center  44 . The spring disc  28  is clamped between the ring  45  and the upper, widened section of the contact part  46 . In this way, the temperature-dependent switching mechanism  14  is a captive unit consisting of contact member  42 , spring disc  28  and bimetal snap-action disc  30 . When mounting the switch  10 , the switching mechanism  14  can thus be inserted as a unit directly into the lower part  16 . 
     The contact part  46  of the movable contact member  42  interacts with a fixed counter contact  48 , which is arranged inside the upper part  18 . This counter contact  48  is herein also referred to as the first stationary contact. The outside of the lower part  16  serves as the second stationary contact  50 . 
     In the position shown in  FIG.  1   , the switch  10  is in its low-temperature position, in which the spring disc  28  is in its initial configuration and the bimetal snap-action disc  30  is in its low-temperature configuration. The spring disc  28  presses the movable contact member  42  against the first stationary contact  48 . 
     In the closed low-temperature position of the switch  10  as shown in  FIG.  1   , an electrically conductive connection is thus established between the first stationary contact  48  and the second stationary contact  50  via the movable contact member  42  and the spring disc  28 . 
     If the temperature of the device to be protected, and thus the temperature of the switch  10  and the temperature-dependent bimetal snap-action disc  30  arranged therein now increases, the snap-action disc will switch from the low-temperature configuration shown in  FIG.  1    to its concave high-temperature configuration shown in  FIG.  2   . When this snap-action occurs, the edge  36  of the bimetal snap-action disc  30  is supported by a part of the switch  10 , in this case by the edge  32  of the spring disc  28 . Thereby, the snap-action disc  30  pulls with its center  44  the movable contact member  42  downwards and lifts off the movable contact member  46  from the first stationary contact  48 . At the same time, it bends the temperature-independent spring disc  28  downwards at its center  40  so that the spring disc  28  switches from its first stable geometric configuration shown in  FIG.  1    to its second stable geometric configuration shown in  FIG.  2   .  FIG.  2    thus shows the high-temperature position of the switch  10  in which it is open. The electric circuit is thus disconnected. 
     If the device to be protected and thus the switch  10  together with the temperature-dependent bimetal snap-action disc  30  then cool down again, the bimetal snap-action disc  30 , upon reaching its reset temperature, would actually snap back to its low-temperature configuration as shown in  FIG.  1   . Then the bimetal snap-action disc  30  would actually move the spring disc  28  back to its first configuration as shown in  FIG.  1    and thus close the switch again. However, with the switch  10  as shown in  FIG.  1   , this reset process can be prevented by a closing lock  52 . 
     The closing lock  52  comprises a locking element  54 , which is substantially plate-shaped or disc-shaped. In the first embodiment shown in  FIGS.  1 - 3   , this locking element  54  is clamped between the lower part  16  and the upper part  18 . More precisely, the locking element  54  is clamped between the circumferential shoulder  22  and the insulating foil  24 . In addition to this clamping arrangement, the locking element  54  can also be firmly bonded to the lower part  16  (e.g. glued, welded or soldered). 
     An embodiment of the locking element  54  is shown in  FIG.  8    in a schematic top view. At least a major part of the locking element  54  is made of a shape-memory alloy. This shape-memory alloy is configured to change the shape of the locking element  54  from a first shape to a second shape upon exceeding a predefined temperature, which is herein referred to as the locking element switching temperature.  FIG.  8    shows the first shape of the locking element  54 . This also corresponds to the shape of the locking element  54  indicated in  FIGS.  1  and  2    in the schematic section, in which the closing lock  52  is not yet activated. 
     In its first shape, the locking element  54  has substantially the shape of a circular disc. It comprises an opening  56 , which in the embodiment shown here is designed as a centrally arranged hole. The movable contact member  42  of the switching mechanism  14  protrudes through the opening  56  (see  FIGS.  1 - 3   ). The opening  56  is therefore preferably dimensioned in such a way that the contact member  42  neither collides with the switching mechanism  14  in the first switching position of the switching mechanism  14  nor during its switching movement. It goes without saying that the opening  56  does not necessarily have to be a round hole, but can also have a different shape, e.g. oval, elliptical or angular. 
     The edge  58  of the locking element  54 , with which it is attached to the housing  12 , is preferably made of an electrically insulating material or coated with an electrically insulating material. This additionally improves the electrical insulation between the lower part  16  and the upper part  18 . In addition, the stability of the clamping of the locking element  54  in the housing  12  can be increased. 
     For example, the base body of the locking element  54  can be made entirely of the shape-memory alloy, which is provided with an adhesive foil or plastic coating  60  at the edge  58 . This adhesive foil or plastic coating  60  is preferably applied to both sides of the shape-memory alloy base body. 
     The locking element  54  shown in  FIG.  8    further comprises four slits  62 , which extend radially outward from the opening  56  in a star-shaped manner. The slits  62  extend through the entire thickness of the locking element  54 . Hence, they are not only superficially inserted into the locking element  54 , but cut completely through it. Starting from the central opening  56 , they run radially outward, but end before the outer edge  58  of the locking element  54 . 
     The slits  62  allow a kind of unfolding of the locking element  54  when the shape-memory alloy brings the locking element  54  into its second shape upon reaching the locking element switching temperature. The four sectors of the locking element  54  that are separated by the slits  62  then fold down as shown in  FIG.  3   . The individual sectors of the locking element  54  bend or bulge downwards. 
     In  FIG.  3    the curvature of the locking element  54  in its second shape is such that it may be convex on its upper side and concave on its lower side. Depending on the design of the shape-memory alloy, the curvature of the locking element  54  in its second shape can also be reversed so that its upper side is concave and its lower side is convex (similar to the two discs  28 ,  30  in  FIG.  3   ). 
     In principle, such a temperature-related change in shape can also be achieved with a shape-memory alloy locking element without slits  62  or with fewer slits  62 . Slit  62 , however, helps to reduce internal stresses caused by the deformation of the locking element  54 . In addition, the shape change of the locking element  54  can be increased. 
     In the first embodiment shown in  FIG.  1 - 3   , the following interaction between the switching mechanism  14  and the closing lock  52  or the associated locking element  54  results: as long as the switching temperature of the bimetal snap-action disc  30  is not exceeded, the switch remains in its closed position as shown in  FIG.  1   . When the switching temperature is reached, the bimetal snap-action disc  30  snaps into its high temperature configuration as shown in  FIG.  2    and lifts off the movable contact member  42  from the first stationary contact  48 , thus opening the switch  10  and disconnecting the current flowing through the switch  10 . The locking element switching temperature, i.e., the temperature at which the shape-memory alloy brings the locking element  54  to its second shape, is preferably selected to be slightly higher than the switching temperature of the bimetal snap-action disc  30 . For example, the shape-memory alloy of the locking element  54  can be configured such that the locking element switching temperature is 5-40 K above the switching temperature of the bimetal snap-action disc  30 . Hence, upon reaching the switching temperature, the locking element  54  initially remains in its first shape as shown in  FIG.  2   . The closing lock  52  is therefore not yet activated in the situation shown in  FIG.  2   . 
     If the temperature of the switch  10  and thus also the temperature of the locking element  54  now increases further, the shape-memory alloy causes the already mentioned shape change of the locking element  54  when the locking element switching temperature is reached, so that it assumes its second shape as shown in  FIG.  3   . In this second shape or configuration, the locking element  54  presses on the top of the spring disc  28  as shown in  FIG.  3   , causing the locking element  54  to exert a force on the switching mechanism  14  which acts directly on the spring disc  28  and indirectly on the movable contact member  42 . This force keeps the switching mechanism  14  in its second switching position. The closing lock  52  is activated. 
     Even if the switch  10  now cools down again starting from the situation shown in  FIG.  3   , the switching mechanism  14  cannot be moved to its first switching position as long as the closing lock  52  is activated. If the switch  10  cools down below the reset temperature, the bimetal snap-action disc  30  would snap back into its low-temperature configuration as shown in  FIG.  1   . However, the switching mechanism  14  would still remain in its second switching position because the edge  36  of the bimetal snap-action disc  30  would snap into the void, so to speak, without being able to support itself on the housing  12 . 
     Even if the inner bottom surface  38  is laterally raised, as indicated in  FIG.  1 - 3    by the dotted line  39 , the bimetal snap-action disc  30  in its low-temperature configuration could be supported with its edge on the housing  12 . However, as long as the closing lock  52  is activated, the spring disc  28  would still be pressed down by the locking element  54 , so that the movable contact member  42  would remain spaced apart from the first stationary contact  48  and the switch  10  would remain open. 
     In order to be able to effectively prevent an inadvertent closing of the switch  10  with the closing lock  52  activated, even if the bimetal snap-action disc  30  in its low-temperature configuration can rest on the housing  12 , the spring constant of the locking element  54  is in such case preferably higher than the spring constant of the bimetal snap-action disc  30 . 
     Depending on the design of the locking element  54 , deactivation of the closing lock  52  is either not possible at all or is possible by a cold treatment. 
     In the first case of an irreversible closing lock  52 , the switch  10  is a one-time switch. For this purpose, a shape-memory alloy with a one-way memory effect is selected for the locking element  54 . 
     By using a shape-memory alloy with a two-way memory effect, the closing lock  52  can alternatively be configured to be reversible. In this case, the shape-memory alloy of the locking element  54  is configured to return the shape of the locking element  54  from the second shape shown in  FIG.  3    to the first shape shown in  FIGS.  1  and  2    when the temperature of the locking element falls below a locking element reset temperature. In this case the locking element  54  can, so to speak, remember both forms. 
     The shape-memory alloy of the locking element  54  is preferably configured such that the locking element reset temperature is lower than the reset temperature of the bimetal snap-action disc  30 . For example, the shape-memory alloy of the locking element  54  can be configured such that the locking element reset temperature is lower than room temperature and, for example, set to be in a temperature range of 0-15° C. By means of a cold treatment, the closing lock  52  can thus be released again so that the switch  10  would return from the switching position shown schematically in  FIG.  3    to the switching position shown schematically in  FIG.  1   . 
       FIGS.  4 - 6    show a second embodiment of the switch  10 . 
       FIG.  4    shows, similar to  FIG.  1    above, the switch  10  in its closed position, in which the switching mechanism  14  is in its first switching position, the bimetal snap-action disc  30  is in its low-temperature configuration and the closing lock  52  is not activated.  FIG.  5    shows, similar to  FIG.  2    above, the switch  10  in its open position, in which the switching mechanism  14  is in its second switching position, the bimetal snap-action disc  30  is in its high-temperature configuration and the closing lock  52  is not activated.  FIG.  6    shows, similar to  FIG.  3   , the switch  10  in its open position, in which the switching mechanism  14  is still in its second switching position, but the closing lock  52  is activated. 
     The switching function as well as the interaction between the switching mechanism  14  and the closing lock  52  in the second embodiment shown in  FIGS.  4 - 6    is the same as mentioned above with regard to the first embodiment shown in  FIGS.  1 - 3   . 
     In contrast to the first embodiment, the locking element  54  of the closing lock  52  in the second embodiment shown in  FIGS.  4 - 6    is arranged on the opposite side of the switching mechanism  14 . While the locking element  54  of the first embodiment of the switch  10  shown in  FIGS.  1 - 3    is arranged on the upper side of the switching mechanism  14  facing the first contact  48 , the locking element  54  of the second embodiment of the switch  10  shown in  FIGS.  4 - 6    is arranged on the lower side of the switching mechanism  14  facing away from the first contact  48 . 
     The locking element  54  is clamped between two spacer rings  64 ,  66 . The first spacer ring  64  is arranged on the inner bottom surface  38  of the lower part  16 . The locking element  54  rests on this first spacer ring  64 . The second spacer ring  66  is arranged on the locking element  54 . The bimetal snap-action disc  30  rests with its edge  36  on the upper side of the second spacer ring  66 . 
     A further spacer ring  68  is arranged at the position where the locking element  54  was arranged between the lower part  16  and the upper part  18  according to the first embodiment. This spacer ring  68  serves as a spacer between the lower part  16  and the upper part  18 . Furthermore, the spring disc  28  can be supported from below by this spacer ring  68  when the switching mechanism  14  is in its second switching position (see  FIGS.  5  and  6   ). 
     In the second embodiment of the switch  10  shown in  FIGS.  4 - 6   , the movable contact member  42  is further designed slightly differently. In the area of its lower end, it comprises a laterally protruding socket  70  whose diameter is slightly larger than the diameter of the opening  56  provided in the locking element  54 . The movable contact member  42  protrudes through the opening  56  provided in the locking element  54 . The widened socket  70  is arranged below the locking element  54 . 
     The locking element  54  is designed in the same way as mentioned above with regard to the first embodiment shown in  FIGS.  1 - 3    (see  FIG.  8   ). However, in its second shape, which it assumes after reaching the locking element switching temperature, the locking element  54  now directly engages the movable contact member  42 . As shown in  FIG.  6   , the locking element  54  presses the widened base  70  from above, thus keeping the movable contact member  42  spaced apart from the first stationary contact  48 . Thus, also in this embodiment, it is not possible to close the switch  10  again as long as the closing lock  52  is activated. 
     Also in this embodiment, the closing lock  52  can be designed reversibly or irreversibly, depending on whether a shape-memory alloy with a one-way memory effect or a shape-memory alloy with a two-way memory effect is used for the shape-memory alloy of the locking element  54 . 
       FIG.  7    shows a third embodiment of the switch  10 ′. The closing lock  52  is designed in the same way as switch  10  shown in  FIGS.  4 - 6   . 
     Since the interaction between the switching mechanism  14 ′ and the closing lock  52  is realized in the same way as mentioned before with the switch  10 ′ shown in  FIG.  7   , this will not be discussed again explicitly at this point. Likewise, the switch  10 ′ is only shown in its closed position, in which the switching mechanism  14 ′ is in its first switching position. 
     The design of the switch  10 ′ shown in  FIG.  7    is slightly different from the design of the switch  10  according to the first two embodiments shown in  FIG.  1 - 6   . 
     The lower part  16 ′ is again made of electrically conductive material. The flat upper part  18 ′ is made of electrically insulating material. It is held to the lower part  16 ′ by the bent edge  20 ′. 
     A spacer ring  68 ′ is provided between the upper part  18 ′ and the lower part  16 ′ to keep the upper part  18 ′ at a distance from the lower part  16 ′. On its inside, the upper part  18 ′ comprises a first stationary contact  48 ′ and a second stationary contact  50 ′. The contacts  48 ′ and  50 ′ are designed as rivets which extend through the upper part  18 ′ and end outside in the heads  72 ,  74  which serve for the external connection of the switch  10 ′. 
     The movable contact member  42 ′ here comprises a current transfer member which is designed as a contact plate, the upper side of which is coated with an electrically conductive coating so that the current transfer member  76 , in the closed position of the switch  10  shown in  FIG.  7   , rests on the contacts  48 ′,  50 ′ and provides an electrically conductive connection between the contacts  48 ′ and  50 ′. The current transfer member  76  is connected to the spring disc  28  and the bimetal snap-action disc  30  via a rivet  78 , which is also to be regarded as part of the contact member  42 ′. Upon exceeding the switching temperature, the bimetal snap-action disc  30  of the switching mechanism  14 ′ ensures, similar to the previous one, that the switching mechanism  14 ′ is moved to its second switching position, in which the current transfer member  76  is kept spaced apart from the two contacts  48 ′,  50 ′ and the circuit is thus disconnected. 
     A difference of the switch design shown in  FIG.  7    is that, in contrast to the embodiment of the switch  10  shown in  FIGS.  1 - 6   , no current flows through either the spring disc  28  or the bimetal snap-action disc  30  when switch  10  is closed. When the switch  10 ′ is closed, current flows only from the first external connection  72  via the first contact  48 ′, the current transfer member  76  and the second contact  50 ′ to the second external connection  74 . 
     The locking element  54  of the closing lock  52  engages the rivet  78  as soon as the closing lock  52  is activated, i.e. as soon as the temperature of the switch  10 ′ and thus the temperature of the locking element  54  exceeds the locking element switching temperature. Similar to the second embodiment shown in  FIGS.  4 - 6   , the rivet  78  is provided with a widened base  70  at its lower end. At this base  70 , the locking element  54  engages to press down the rivet  78  and thus the entire movable contact member  42 ′ and to hold the switching mechanism  14 ′ in its second switching position as soon as the closing lock  52  is activated. 
     In principle, the closing lock  52  can also be designed for the switch  10 ′, as shown schematically in  FIG.  7   , in the same way as the first embodiment of the switch  10  shown in  FIGS.  1 - 3   . 
     A reversible design of the closing lock  52  is also possible with the third embodiment of the switch  10 ′ shown in  FIG.  7   . 
     It is to be understood that the foregoing is a description of one or more preferred exemplary embodiments of the invention. The invention is not limited to the particular embodiment(s) disclosed herein, but rather is defined solely by the claims below. Furthermore, the statements contained in the foregoing description relate to particular embodiments and are not to be construed as limitations on the scope of the invention or on the definition of terms used in the claims, except where a term or phrase is expressly defined above. Various other embodiments and various changes and modifications to the disclosed embodiment(s) will become apparent to those skilled in the art. All such other embodiments, changes, and modifications are intended to come within the scope of the appended claims. 
     As used in this specification and claims, the terms “for example,” “e.g.,” “for instance,” “such as,” and “like,” and the verbs “comprising,” “having,” “including,” and their other verb forms, when used in conjunction with a listing of one or more components or other items, are each to be construed as open-ended, meaning that the listing is not to be considered as excluding other, additional components or items. Other terms are to be construed using their broadest reasonable meaning unless they are used in a context that requires a different interpretation.