Patent Description:
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.

<CIT> 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.

<CIT> refers to a varistor component comprising a first external contact and a second external contact.

<CIT> relates to a SPD (Surge Protection Device) with a separation mechanism.

<CIT> relates to a SPD comprising a zinc oxide type varistor.

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.

The invention as disclosed in the independent claim <NUM> offers a solution to this problem. The dependent claims can lead to a preferable solution.

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.

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.

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.

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.

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.

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.

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.

A value of the characteristic temperature of the low-temperature solder can be in a range from <NUM> to <NUM>. In a special embodiment the value of the characteristic temperature is <NUM>.

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.

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.

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.

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.

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.

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.

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.

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.

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

<FIG> shows a possible embodiment of a varistor <NUM> that would be the object of protection in a thermal varistor protection <NUM> of this invention. The varistor <NUM> comprises a first metallization electrode <NUM>, on which a first terminal wire <NUM> is electrically conductively connected. Furthermore, the varistor <NUM> of the shown embodiment has a second metallization electrode <NUM> (not visible) on the opposite side of the first metallization electrode <NUM>. There is a second terminal wire <NUM>, which is electrically conductively connected to the second metallization electrode <NUM> of the varistor <NUM>.

A terminal wire that is connected to the varistor <NUM> can comprise an open loop at the connected end. In <FIG> the first terminal wire <NUM> shows an open loop <NUM> 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 <NUM>.

<FIG> shows a schematic perspective of an embodiment of the thermal varistor protection <NUM> without a cap <NUM>. In a casing <NUM> of insulating material <NUM> a varistor <NUM> is embedded. A first terminal wire <NUM> and a second terminal wire <NUM> are electrically conductively connected to two metallization electrodes on opposite sides of the varistor <NUM> and protrude out of the casing <NUM>. The contact element <NUM> is electrically conductively connected to a metallization electrode <NUM> of the varistor adjacent to the point of connection of the first terminal wire <NUM>, which is in a region <NUM> where the varistor <NUM> is free from insulating material <NUM>. The casing <NUM> comprises features <NUM> to hold the contact element <NUM> and build up a pre-stress in the contact element <NUM>. The contact element <NUM> is elastic to build up the pre-stress. The connection between the metallization electrode <NUM> of the varistor <NUM> and the contact element <NUM> can be realized with a low-temperature solder.

In cases of high voltage between the contact element <NUM> and the second terminal wire <NUM> the varistor <NUM> changes from an electrically insulating state to an electrically conductive state, and a high current flows through the varistor <NUM> and the connections at the varistor <NUM>. 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 <NUM> of the varistor <NUM> and the contact element <NUM> becomes liquid, the contact element <NUM> gets pushed away from the region <NUM> without insulating material <NUM> due to its inner pre-stress caused by the features <NUM> of the casing <NUM>.

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