Thermal protected varistor device

Abstract

In an embodiment a thermal varistor protection device includes a casing, a varistor embedded in the casing, wherein the varistor includes a first metallization electrode, which is only partly covered by an insulating material of the casing to allow an electrically conductive connection, a first terminal wire electrically conductively connected to the first metallization electrode of the varistor and a contact element electrically conductively connected to the first metallization electrode of the varistor in a region where the varistor is not covered by the insulating material, wherein the contact element is pre-stressed to ensure a fast separation of the contact element and the first metallization electrode when the electrically conductive connection between the contact element and the first metallization electrode becomes loose.

Claims

1. A thermal varistor protection device comprising: a casing; a varistor embedded in 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; a first terminal wire electrically conductively connected to the first metallization electrode of the varistor; and a contact element electrically conductively connected to the first metallization electrode of the varistor in a region where the varistor is not covered by the insulating material, wherein the contact element is pre-stressed to ensure a fast separation of the contact element and the first metallization electrode when the electrically conductive connection between the contact element and the first metallization electrode becomes loose, wherein a pre-stress is caused by a separate element which is not part of the contact element, and wherein the separate element is a spring.

2. The thermal varistor protection device according to claim 1, wherein the pre-stress is further caused by a part of the contact element itself, and wherein one part of the contact element is elastic.

3. The thermal varistor protection device according to claim 2, wherein the casing provides a feature configured to hold the contact element in place and to build up the pre-stress in an elastic part of the contact element.

4. The thermal varistor protection device according to claim 3, wherein the feature configured to hold the contact element in place and to build up the pre-stress in the contact element comprises rivets.

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

6. The thermal varistor protection device according to claim 5, wherein the characteristic temperature is a melting temperature of a solder, and wherein the melting temperature is in a range from 100° C. to 210° C.

7. The thermal varistor protection device according to claim 1, wherein the pre-stress of the contact element pushes the contact element against a wall of the casing away from the region where the varistor is not covered by the insulating material of the casing when the electrically conductive connection to the first metallization electrode of the varistor becomes loose.

8. The thermal varistor protection device according to claim 1, wherein the first terminal wire has an open loop at an end which is electrically conductively connected to the first metallization electrode of the varistor to increase a contact surface.

9. The thermal varistor protection device according to claim 1, wherein the contact element has an open loop at an end which is electrically conductively connected to the first metallization electrode of the varistor to increase a contact surface.

10. The thermal varistor protection device according to claim 1, wherein the casing comprises a cavity which is closed by a cap to protect inner parts against environmental influences.

11. The thermal varistor protection device according to claim 1, wherein the casing has generally a cuboid shape.

12. The thermal varistor protection device according to claim 1, further comprising a second terminal wire electrically conductively connected to a second metallization electrode of the varistor.

13. The thermal varistor protection device according to claim 1, wherein the casing comprises features, wherein one of the features holds the contact element in place and at least one other of the features is a hinge configured to build up the pre-stress in an elastic part of the contact element.

14. The thermal varistor protection device according to claim 1, wherein the pre-stress and a fixation of the contact element are configured to push the contact element away from the region of the first metallization electrode where the varistor is not covered by the insulating material in a movement parallel to a plane of the first metallization electrode.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the Figures:

(2) FIG. 1 shows a schematic perspective representation of a thermal varistor protection with a transparent casing;

(3) FIG. 2 shows a varistor which may be protected by the thermal varistor protection;

(4) FIG. 3 shows a schematic perspective representation of a thermal varistor protection with an embedded varistor and a connected spring contact element; and

(5) FIG. 4 shows a schematic perspective representation of a thermal varistor protection with an embedded varistor and a disconnected spring contact element.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

(6) The schematic representation of FIG. 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 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.

(7) FIG. 2 shows a possible embodiment of a varistor 2 that would be the object of protection in a thermal varistor protection 1. 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.

(8) A terminal wire that is connected to the varistor 2 can comprise an open loop at the connected end. In FIG. 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.

(9) FIG. 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 features 13 can be a hinge. 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.

(10) 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.

(11) FIG. 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.

(12) The invention described here is not restricted by the description provided in connection with the exemplary embodiments. Rather, the invention encompasses any novel feature and any combination of features, including in particular any combination of features in the claims, even if this feature or this combination is not itself explicitly indicated in the claims or exemplary embodiments.