Thermal protection device

Abstract

In an embodiment a thermal protection device includes a housing, a varistor partly embedded in the housing, wherein the housing electrically insulates the varistor, and wherein the varistor includes a partly uninsulated contact surface, an inner wall of insulating material arranged adjacent to the contact surface of the varistor, a window in the inner wall configured to allow an electrical connection of the contact surface of the varistor in an operational state of the thermal protection device and a moveable insulation block configured to cover the window in the inner wall to insulate the varistor in a region of the window of the inner wall in a fault state of the thermal protection device.

Claims

1. A thermal protection device comprising: a housing; a varistor partly embedded in the housing, wherein the housing electrically insulates the varistor, and wherein the varistor comprises a partly uninsulated contact surface; an inner wall of insulating material arranged adjacent to the contact surface of the varistor; a window in the inner wall configured to allow an electrical connection of the contact surface of the varistor in an operational state of the thermal protection device; a moveable insulation block configured to cover the window in the inner wall to insulate the varistor in a region of the window of the inner wall in a fault state of the thermal protection device, wherein the moveable insulation block is configured to move in a rotation around a rivet in a center of rotation; a metal contact defining a spring terminal such that the spring terminal is electrically connected through the window to the contact surface of the varistor during the operational state of the thermal protection device; and an indicator configured to signal a current state of the thermal protection device, wherein the indicator comprises an indicator trigger, and wherein the moveable insulation block is configured to press the indicator trigger when the moveable insulation block is moved in its end position in the fault state of the thermal protection device thereby operating the indicator trigger of the indicator.

2. The thermal protection device of claim 1, wherein the metal contact is subject to a pre-stress acting in a direction away from the contact surface during the operational state of the thermal protection device.

3. The thermal protection device of claim 1, wherein the electrical connection between the contact surface of the varistor and the spring terminal comprises a low temperature solder joint during the operational state of the thermal protection device, and wherein a low temperature is a characteristic temperature at which the solder joint reaches a state where it allows a pre-stress to interrupt a contact with the contact surface.

4. The thermal protection device of claim 3, wherein the characteristic temperature is a melting temperature of a solder, and wherein the melting temperature is in a range from 180° C. to 210° C.

5. The thermal protection device of claim 1, wherein the moveable insulation block is pressed against the spring terminal by a spring force from a spring during the operational state of the thermal protection device.

6. The thermal protection device according to claim 1, wherein the moveable insulation block is configured to be pushed in an end position by a spring and to thereby cover the window of the inner wall in the fault state of the thermal protection device.

7. The thermal protection device according to claim 1, wherein the housing comprises the indicator on a front side such that signal contacts of the indicator reach out of the housing and the indicator trigger reaches into the housing.

8. The thermal protection device according to claim 7, wherein the indicator is a micro-switch.

9. The thermal protection device according to claim 7, wherein the moveable insulation block is configured to be pushed in its end position by a spring thereby operating the indicator trigger of the indicator in the fault state of the thermal protection device.

10. The thermal protection device according to claim 1, wherein the varistor has a rectangular shape or a round shape.

11. The thermal protection device according to claim 1, wherein the varistor comprises a second contact surface provided with a terminal.

12. A thermal protection device comprising: a housing; a varistor partly embedded in the housing, wherein the housing electrically insulates the varistor, and wherein the varistor comprises a partly uninsulated contact surface; an inner wall of insulating material arranged adjacent to the contact surface of the varistor; a window in the inner wall configured to allow an electrical connection of the contact surface of the varistor in an operational state of the thermal protection device; a moveable insulation block configured to cover the window in the inner wall to insulate the varistor in a region of the window of the inner wall in a fault state of the thermal protection device; and an indicator configured to signal a current state of the thermal protection device, wherein the indicator comprises an indicator trigger, wherein the moveable insulation block is configured to press the indicator trigger when the moveable insulation block is moved in its end position in the fault state of the thermal protection device thereby operating the indicator trigger of the indicator, and wherein the thermal protection device is configured such that, with an accurate application of heat, a low temperature solder joint between a spring terminal and the contact surface is rebuildable after triggering of the thermal protection device so that the thermal protection device is resettable to the operational stats.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the figures:

(2) FIG. 1 shows a schematic perspective representation of a thermal protection device. Here the housing is transparent;

(3) FIG. 2 shows a transparent housing to bring the embedded varistor into view;

(4) FIG. 3 represents a thermal protection device in an operational state;

(5) FIG. 4 represents a thermal protection device in a fault state; and

(6) FIG. 5 represents a schematic view of a metal oxide varistor disc.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

(7) FIG. 1 shows an exemplary embodiment of a thermal protection device 100 in an operational state. In the figure a housing 1 is represented as transparent to bring a varistor 2 into view, which is partly embedded in the housing 1. The housing 1 comprises an inner wall 16, which is arranged parallel to a contact surface 21 of the varistor 2. The housing defines at least one cavity. Here, the inner wall 16 can also be a wall of the cavity mentioned before. The inner wall 16 comprises a window 12, which offers the possibility for forming a connection from a part in the cavity to the contact surface 21 of the varistor 2. The varistor 2 comprises a terminal 22. Said terminal is arranged on an opposite side of the contact surface 21 and protrudes from the housing 1.

(8) The thermal protection device 100 comprises a spring terminal 13, which is partly arranged in the cavity of the housing 1. The spring terminal is held in its position by a fixation feature 14 in the housing 1. The protruding parts of the spring terminal 13 and of the terminal 22 of the varistor 2 are designed to make the thermal protection device pluggable. During the operational state of the thermal protection device 100 an end of the spring terminal 13 in the cavity of the housing 1 is connected to the contact surface 21 of the varistor 2 through the window 12 in the inner wall 16 of the housing 1. The connection between the spring terminal 13 and the first contact surface causes a stress in the spring terminal, which is directed away from the contact surface 21 of the varistor 2. To hold the connection between the spring terminal 13 and the contact surface 21, an electrical connection 11 is used. The electrical connection 11 is realised by a low temperature solder joint. Such a low temperature solder joint has a critical temperature at which the solder becomes liquid. The critical temperature at which the solder becomes liquid can be in a range between 100° C. and 210° C., e.g., 138° C.

(9) The shown embodiment of the thermal protection device 100 in an operational state comprises a moveable insulation block 43, which is arranged in the cavity of the housing 1 and abuts on the inner wall 16. A spring 42 is placed in the cavity and pushes the moveable insulation block 43 against the spring terminal 13 near the electrical connection 11. The spring 42 is held in place by a feature to fix the spring 41, which is arranged on the inner wall 16 and reaches into the cavity of the housing 1. The path of movement for the moveable insulation block 43 and the spring 42 is a path of gyration with its centre at the feature 41 to fix the spring 42, in a plane parallel to the inner wall 16 and therefore parallel to the contact surface 21 of the varistor 2.

(10) The embodiment of the thermal protection device comprises an indicator 31, as shown in FIG. 1. The indicator 31 is partly arranged in the housing 1 and can be reached by the moveable insulation block 43. The indicator comprises signal contacts 32, which are arranged outside the housing 1.

(11) FIG. 2 shows an embodiment of the housing 1 for a thermal protection device in a transparent view to show the partly embedded varistor 2. The shown embodiment of the housing comprises the inner wall 16 with the window 12, wherein the inner wall 16 is arranged parallel to the contact surface 21 of the partly embedded varistor 2 and delimits a cavity in the housing 1. The window 12 in the inner wall 16 allows a connection to be formed to the contact surface 21 of the varistor 2. The shape or arrangement of the window 12 within the inner wall 16 is not limited to the example shown in the figures. The terminal 22 of the varistor 2 is located on an opposite side of the contact surface 21 and protrudes from the housing 1.

(12) In the housing 1 there is a feature 3 to fix the indicator 31. This feature 3 is a notch in the outer wall of the housing, which reaches from an upper boundary of the housing to the level of the inner wall 16. The housing 1 comprises the feature 14 to fixate a spring terminal. The feature 14 also is a notch in the outer wall of the housing 1.

(13) In the cavity of the housing there is the feature 41 to fix the spring 42. The feature 41 to fix the spring 42 is designed like a rivet which is arranged on the inner wall 16 and reaches into the cavity of the housing 1.

(14) FIG. 3 represents a schematic overview of a possible embodiment of the thermal protection device 100 in an operational state. In this operational state the spring terminal 13 is connected to the contact surface 21 of the varistor 2, which is partly embedded in a cavity of the housing 1. The connection is realised through the window 12 in the inner wall 16 of the housing 1 and is held by means of the electrical connection 11. The solder joint is stable at low temperatures and the low temperature solder stays solid until its temperature reaches a critical temperature. The spring terminal 13 is fixed at the housing 1 via the feature 14 to fix the spring terminal partly in the cavity of the housing 1. Due to the fixation of the spring terminal 13 and the connection to the contact surface 21, a stress is built up in the spring terminal near the end which is connected to the contact surface 21 of the varistor 2. This stress is directed away from the contact surface and causes a fast separation of the spring terminal 13 and the contact surface 21 of the varistor 2, if the low temperature solder becomes liquid.

(15) In this embodiment the thermal protection device 100 comprises the moveable insulation block 43. During the operational state the moveable insulation block 43 is arranged between the inner wall 16 and the spring terminal 13 inside the cavity of the housing 1. The spring 42 pushes the moveable insulation block 43 against the spring terminal 13 near the electrical connection 11 as long as the thermal protection device 100 is in its operational state. The spring 42 and the moveable insulation block 43 have a common rotation axis. This rotation axis is defined by the feature 41 to fix the spring 42, which is arranged on the inner wall 16 an reaches into the cavity of the housing 1. The torsion spring has two arms, wherein one arm is to push the moveable insulation block 43 and the other arm is pushed against a wall of the housing 1 in order to provide a torque in the spring 42.

(16) Inside the cavity of the housing a trigger 33 of the indicator 31 is arranged. During the operational state of the thermal protection device 100, the trigger 33 of the indicator 31 is not activated. A corresponding electrical signal can be tapped at the signal contacts 32 of the indicator 31, which protrude from the housing 1.

(17) FIG. 4 shows a schematic overview of a possible embodiment of the thermal protection device 100 in a fault state. The fault state is a result of a chain reaction, initiated by a critically high voltage between the spring terminal 13 and the terminal 22 of the varistor over a certain time. If the voltage between these terminals reaches a characteristic value, the varistor 2 changes from an electrically insulating state to an electrically conductive state and allows a high current as a first result. The high current can heat up the electrical connection 11 between the spring terminal 13 and the contact surface 21 of the varistor 2. That would liquefy the used low temperature solder. Thereby the electrical connection 11 becomes loose either due to the stress in the spring terminal, which bends the spring terminal 13 away from the contact surface 21 of the varistor 2, or due to the torque of the spring 42, which pushes the moveable insulation block 43 between the spring terminal 13 and the inner wall 16, over the window 12. As a result of this chain reaction, the spring terminal 13 is separated from the varistor 2 and the moveable insulation block closes the window 12 in the inner wall and activates the trigger 33 of the indicator 31.

(18) FIG. 5 shows an exemplary embodiment of the varistor 2. The varistor comprises the contact surface 21 on one side of the varistor 2. The contact surface 21 is arranged on a metallization layer 23 of the varistor 2. This metallization layer 23 can partly or completely extend over one side of the varistor 2. The varistor 2 comprises a further contact surface with the terminal 22 on the opposite side with respect to the contact surface 21. The terminal 22 of the varistor 2 protrudes over the dimensions of the varistor 2. In a possible embodiment, the terminal 22 is designed in such a way that it reaches out of the housing 1. The varistor can be designed as a metal-oxide varistor. Such a metal-oxide varistor has a characteristic electrical behavior if a voltage is applied between the two contact surfaces. If the applied voltage reaches a certain value, the varistor resistance changes rapidly from an insulating state to a conductive state.