HEAT-PROTECTED VARISTOR ELEMENT

Description

[0001] The invention concerns a thermal protection device to protect an electrical element against overheating, for example a varistor.

[0002] In electrical circuits it is important to protect threatened electrical elements against overheating. A varistor is such an electrical element. The varistor can change from an electrically insulating state to an electrically conductive state with a characteristic current-voltage behaviour. On the one hand, if an overvoltage is applied to an electrical circuit, the varistor can protect the electrical circuit. On the other hand, the varistor has to be protected in turn when the overvoltage persists and a high current flows through the varistor.

[0003] US 6 323 750 B1 relates to an electrical component, in particular to a varistor, which is installed in a plastic cup and has at least two electrical connecting leads.

[0004] WO 2017 / 140463 A1 refers to a varistor component comprising a first external contact and a second external contact.

[0005] JP 2009 218508 A relates to a SPD (Surge Protection Device) with a separation mechanism.

[0006] WO2007 / 142152 A1 relates to a SPD comprising a zinc oxide type varistor.

[0007] It is the purpose of this invention to create a fast and reliable thermal protection device. To be more specific, the task is a thermal protection device to protect a varistor in cases of overheating due to a persistently high voltage applied to the varistor over a certain time.

[0008] The invention as disclosed in the independent claim 1 offers a solution to this problem. The dependent claims can lead to a preferable solution.

[0009] The invention relates to a thermal varistor protection device with a casing comprising an insulating material and a varistor which is embedded in the insulating material of the casing, wherein the varistor comprises a first metallization electrode, which is only partly covered by an insulating material of the casing to allow an electrically conductive connection to the first metallization electrode of the varistor. Furthermore the thermal varistor protection device comprises a first terminal wire that is electrically conductively connected to the first metallization electrode of the varistor. The thermal varistor protection device also comprises a contact element which is electrically conductively connected to the first metallization electrode of the varistor in a region where the varistor is not covered by the insulating material of the casing and wherein the contact element is pre-stressed to provide a fast separation of the contact element and the first metallization electrode if the electrical connection between the contact element and the first metallization electrode gets loose.

[0010] The varistor is protected against environmental influences and is largely electrically insulated as a result of being embedded by the insulating material of the casing. Therefore the varistor is protected against unwanted contact. Since the first metallization electrode is only partly embedded in the insulating material of the casing, an electrically conductive connection is possible. The pre-stressed contact element ensures a fast and secure separation of contact element and first metallization electrode. Therefore an improvement in the protectional function is provided.

[0011] The pre-stress of the contact element is caused by the contact element itself. The contact element comprises an elastic part, which causes the pre-stress during an existent connection between the contact element and the first metallization electrode of the varistor.

[0012] If the pre-stress is caused by the contact element itself or by an elastic part of it, the thermal varistor protection device can be built in smaller dimensions, since no additional feature is needed to generate the pre-stress.

[0013] The casing provides a feature to hold the contact element in place. If the pre-stress to the connection element is caused by a part of the connection element, it is possible to use the feature to build up the pre-stress. The feature can be designed in the form of a rivet. The feature can comprise more than one rivet.

[0014] Such a rivet can be part of the casing. In this case it would be possible to produce the rivet in one production step together with the casing itself. That would save production time and costs.

[0015] The electrically conductive connection between the first metallization electrode of the embedded varistor and the contact element can be realized as a low-temperature solder joint. Therein the low temperature would be a characteristic temperature at which the solder reaches a state where it would allow the pre-stress to interrupt the connection. The low temperature can be a characteristic temperature at which the solder becomes liquid.

[0016] A value of the characteristic temperature of the low-temperature solder can be in a range from 100°C to 210°C. In a special embodiment the value of the characteristic temperature is 138°C.

[0017] By using such a low-temperature solder as described above, a thermally triggered interruption of a pre-stressed connection can be ensured. The triggering may be caused by a temperature increase of the varistor as well as by a high current which flows through the electrically conductive connection and heats it up. Both triggering mechanisms can be realized in the electrically conductive connection between the contact element and the first metallization electrode of the varistor, since the connection is close to the varistor and therefore shows a similar temperature behaviour, and the contact element and the connection are connected in series to the varistor and thus have the same current which flows through the varistor and which would heat up all the elements on the current path.

[0018] In one embodiment of the invention, if the electrically conductive connection between the first metallization electrode of the varistor and the contact element becomes loose, the pre-stress of the contact element pushes the contact element away from the region where the metallization electrode of the varistor is free from insulating material of the casing. The contact element can be pushed in a region where the metallization electrode of the varistor is covered by the insulating material of the casing. Thereby the contact element can get pushed against a wall of the casing by the pre-stress.

[0019] A local separation of contact element and metallization electrode can improve a save disconnection of those parts if the connection becomes loose. The separation by the pre-stress can lead to a fast separation, in addition. Here it is not important if the pre-stress is caused by a part of the contact element or by something else.

[0020] The first terminal wire can comprise a loop-like-shaped end which is electrically conductively connected to the first metallization electrode of the varistor. More specifically, the end can be shaped as an open loop or an open lug. This modification of the first terminal wire can increase a contact area between the first terminal wire and the metallization electrode of the varistor. As a result, the loop-like shape of the connected end of the first terminal wire can lead to an improved electrically conductive contact with higher stability and conductivity.

[0021] In one embodiment of the invention the contact element is a wire. Here the contact element can comprise an end which is electrically conductively connected to the metallization electrode of the varistor. For the same reasons as outlined above in view of an improved electrically conductive contact with higher stability and conductivity, it is possible to modify the connected end of the contact element, too.

[0022] The thermal varistor protection device can comprise a cap. The cap can be designed to be removably placed on the casing. Here the casing can define a cavity which is closed by the cap. Such a cavity would protect inner parts against environmental influences. The set of parts in the cavity can comprise the region on the metallization electrode of the varistor which is free from insulating material of the casing, a part of the contact element, the feature to hold the contact element, and the electrically conductive connection between the contact element and the metallization electrode of the varistor.

[0023] A general shape of the casing can be adjusted to the shape of the varistor. Therefore the casing can have a generally cuboid shape. An alteration of the casing can reduce the needed material to embed the varistor and therefore reduce costs.

[0024] The thermal varistor protection device can comprise a second terminal wire. The second terminal wire would be electrically conductively connected to a second metallization electrode of the varistor. Furthermore an arrangement of the second metallization electrode on the varistor at an opposite side to the first metallization electrode is possible.

[0025] In the figures: Figure 1shows a schematic perspective representation of a thermal varistor protection with a transparent casing. Figure 2shows a varistor which may be protected by the thermal varistor protection. Figure 3shows a schematic perspective representation of a thermal varistor protection with an embedded varistor and a connected spring contact element. Figure 4shows a schematic perspective representation of a thermal varistor protection with an embedded varistor and a disconnected spring contact element.

[0026] The schematic representation of figure 1 gives a perspective view on an embodiment of a thermal varistor protection 1. A casing 10 is made from insulating material 11 and is represented transparent. In this casing a varistor 2 is embedded and is partly covered by the insulating material 11. In a region 12 which is free of the insulating material 11, the varistor 2 can be accessed for establishing an electrically conductive connection. The thermal varistor protection 1 comprises a cap 19 to cover a cavity in the casing 10 and to protect parts from environmental influences. A first terminal wire 31 and a second terminal wire 32 are electrically conductively connected to opposite sides of the varistor 2 and protrude from the casing 10. A contact element is electrically conductively connected to the varistor 2 in a region 12 which is free of the insulating material 11, and protrudes from the casing 10, too. In the shown embodiment of the invention the first terminal wire 31 and the contact element 33 are adjacent to one another and connected to the same side of the varistor 2 in the region 12 which is free of insulating material 11. Both the first terminal wire 31 and the contact element 33 have an open loop 311,331 at their respective ends connected to the varistor.

[0027] Figure 2 shows a possible embodiment of a varistor 2 that would be the object of protection in a thermal varistor protection 1 of this invention. The varistor 2 comprises a first metallization electrode 21, on which a first terminal wire 31 is electrically conductively connected. Furthermore, the varistor 2 of the shown embodiment has a second metallization electrode 22 (not visible) on the opposite side of the first metallization electrode 21. There is a second terminal wire 32, which is electrically conductively connected to the second metallization electrode 22 of the varistor 2.

[0028] A terminal wire that is connected to the varistor 2 can comprise an open loop at the connected end. In figure 2 the first terminal wire 31 shows an open loop 311 at its connected end. It should be mentioned that the cuboid-like shape of the varistor is an example only. A cylinder-like shape or other shapes are also possible for an embodiment of the protected varistor 2.

[0029] Figure 3 shows a schematic perspective of an embodiment of the thermal varistor protection 1 without a cap 19. In a casing 10 of insulating material 11 a varistor 2 is embedded. A first terminal wire 31 and a second terminal wire 32 are electrically conductively connected to two metallization electrodes on opposite sides of the varistor 2 and protrude out of the casing 10. The contact element 33 is electrically conductively connected to a metallization electrode 21 of the varistor adjacent to the point of connection of the first terminal wire 31, which is in a region 12 where the varistor 2 is free from insulating material 11. The casing 10 comprises features 13 to hold the contact element 33 and build up a pre-stress in the contact element 33. The contact element 33 is elastic to build up the pre-stress. The connection between the metallization electrode 21 of the varistor 2 and the contact element 33 can be realized with a low-temperature solder.

[0030] In cases of high voltage between the contact element 33 and the second terminal wire 32 the varistor 2 changes from an electrically insulating state to an electrically conductive state, and a high current flows through the varistor 2 and the connections at the varistor 2. If a high electrical current flows through a solder joint of a low-temperature solder, the solder gets heated up and becomes liquid. If the low-temperature solder in the connection between the metallization electrode 21 of the varistor 2 and the contact element 33 becomes liquid, the contact element 33 gets pushed away from the region 12 without insulating material 11 due to its inner pre-stress caused by the features 13 of the casing 10.

[0031] Figure 4 shows a case where the connection between a contact element 33 and the metallization electrode 21 of a varistor 2 embedded in the casing 10 has become loose. Due to the inner pre-stress of the contact element 33 and the loosened connection, the contact element 33 is pushed to a wall of a cavity in the casing 10 and away from a region 12 where the varistor 2 is free from electrically insulating material 11. The inner pre-stress of the contact element 33 is caused by a feature 13 of the casing 10 that has the additional function to hold the contact element 33 in its position, even if the connection to the metallization electrode 21 of the varistor 2 is undone. Both the terminal wire 31 and the contact element 33 have an open loop 311,331 at their respective ends which are supposed to be electrically conductively connected to the metallization electrode 21 of the varistor 2. The electrically conductive connection between the contact element 33 and the metallization electrode 21 of the varistor 2 can be realized with a low-temperature solder. If the low-temperature solder becomes liquid due to a high current that is caused by a high voltage which makes the varistor 2 switch from an electrically insulating state to an electrically conductive state, the inner pre-stress of the terminal 33 pushes the end with the open loop 331 against a wall of the cavity of the casing 10. As a result, the electrical connections between the contact element 33 and the varistor 2 and between the contact element 33 and the first terminal wire 31 become loose. This reaction protects the varistor against too much current and a resulting heating of the varistor, and it is possible to recognize a drop in voltage at the first terminal wire 31 by means of external signal processing. The present invention is not limited to the aforementioned specific examples but a variety of modifications can be contemplated within the technological scope of the present invention which is defined by the appended claimsList of Reference Signs

[0032] 1thermal varistor protection 10casing 11insulating material 12region free of insulating material 13feature to hold an elastic contact element 19cap 2varistor 21first metallization electrode 22second metallization electrode 31first terminal wire 311open loop of first terminal wire 32second terminal wire 33contact element 331open loop of contact element

Claims

1. A thermal varistor protection device (1), comprising - a casing (10) comprising an insulating material (11), - a varistor (2) embedded in the insulating material (11) of the casing (10), wherein the varistor (2) comprises a first metallization electrode (21), which is only partly covered by the insulating material (11) of the casing (10) to allow an electrically conductive connection, - a first terminal wire (31), which is electrically conductively connected to the first metallization electrode (21) of the varistor (2), - a contact element (33)which is electrically conductively connected to the first metallization electrode (21) of the varistor (2) in a region (12) where the varistor is not covered by the insulating material (11) of the casing (10), and - wherein the contact element (33) is pre-stressed to ensure a fast separation of the contact element (33) and the first metallization electrode (21) if the electrically conductive connection between the contact element (33) and the first metallization electrode (21) gets loose, - wherein the pre-stress to the contact element (33) is caused by a part of the contact element (33) itself, and therefore one part of the contact element (33) is elastic and - wherein the casing (10) provides a feature (13) to hold the contact element (33) in place and to build up the pre-stress in the elastic part of the contact element (33).

2. The thermal varistor protection device (1) according to claim 1, wherein the electrically conductive connection between the first metallization electrode (21) of the varistor (2) and the contact element (33) is realized as a low-temperature solder joint, wherein the low temperature is a characteristic temperature at which the solder joint reaches a state where it allows the pre-stress to interrupt the contact.

3. The thermal varistor protection device (1) according to claim 1, wherein the characteristic temperature is the melting temperature of the solder, which is in a range from 100°C to 210°C, e.g. 138°C.

4. The thermal varistor protection device (1) according to one of the preceding claims, wherein the feature (13) to hold the contact element (33) in place and to build up the pre-stress in the contact element (33) is designed in the form of rivets.

5. The thermal varistor protection device (1) according to one of the preceding claims, wherein the pre-stress of the contact element (33) pushes the contact element (33) against a wall of the casing (10) away from the region (12) where the varistor (2) is not covered by the insulating material (11) of the casing (10), if the connection to the first metallization electrode (21) of the varistor (2) becomes loose.

6. The thermal varistor protection device (1) according to one of the preceding claims, wherein the first terminal wire (31) has an open loop (311) at the end which is electrically conductively connected to the first metallization electrode (21) of the varistor (2) to increase the contact surface.

7. The thermal varistor protection device (1) according to one of the preceding claims, wherein the contact element (33) has an open loop (331) at an end which is designed to be electrically conductively connected to the first metallization electrode (21) of the varistor (2) to increase the contact surface.

8. The thermal varistor protection device (1) according to one of the preceding claims, wherein the casing (10) defines a cavity which is closed by a cap (19) to protect the inner parts against environmental influences.

9. The thermal varistor protection device (1) according to one of the preceding claims, wherein the casing (10) has a generally cuboid shape.

10. The thermal varistor protection device (1) according to one of the preceding claims, comprising a second terminal wire (32), which is electrically conductively connected to a second metallization electrode (22) of the varistor (2).