Thermal protection device

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.

This patent application is a national phase filing under section 371 of PCT/EP2019/058413, filed Apr. 3, 2019, which claims the priority of Chinese patent application 201810299514.X, filed Apr. 4, 2018, each of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The invention concerns a thermal protection device to protect a varistor against overheating.

BACKGROUND

A varistor has a special current-voltage characteristic. The varistor changes from an electrically insulating state to an electrically conductive state if an applied voltage exceeds a certain value. This effect is used to protect voltage-sensitive electrical elements. However, if the varistor becomes conductive, a high current can flow through it. The current can heat up the varistor to a point of explosion or other damage.

To protect the varistor and other electrical elements against such overheating one may use thermal protection as described in U.S. Pat. No. 6,430,019 B1. One type of such protection is a thermal fuse.

SUMMARY

Embodiments provide a fast and reliable thermal protection device. Further embodiments provide a thermal protection device to protect a varistor in cases of overheating due to a persistently high voltage applied to the varistor for a certain time.

Embodiments relate to a thermal protection device comprising a housing defining at least one cavity. In the housing a varistor is partly embedded. If a voltage applied to the varistor reaches a characteristic value, the varistor changes from an electrically insulating state to an electrically conductive state and allows a flow of a high current. The varistor comprises a contact surface on one side. Furthermore the housing comprises an inner wall of insulating material, which is placed adjacent to the varistor and to the contact surface of the varistor. There is a window provided in the inner wall, which allows a connection to the contact surface of the varistor during an operational state of the thermal protection device. In a fault state of the thermal protection device, the window in the inner wall is covered by a moveable insulation block. Thereby, the contact surface of the varistor would be completely electrically insulated by the covered window and the inner wall.

The operational state is the state of the thermal protection device as long as a system, where the thermal protection device is installed, is in an ordinary state. If a high voltage is applied to the varistor due to an error in the system, the varistor becomes electrically conductive. As a result a high current flows through the thermal protection device. This current can heat up the varistor and the electrical connection to it. The thermal protection device is designed to protect the varistor against such a heating up. Therefore the electrically conductive connection between the varistor and the system gets interrupted by the thermal protection device in the case of a heating up. The case of the interruption of the connection is the fault state of the thermal protection device.

The moveable insulation block is arranged inside the housing. The function of the moveable insulation block is to insulate the varistor in a region of the window during a fault state of the thermal protection device.

Such a housing provides optimal protection for any inner parts against environmental influences, e.g., humidity or dust. The inner wall on the contact surface of the varistor with the window in it defines a small contact area on the contact surface, which is easier to control by the thermal protection device compared to an open contact surface. In the operational state the thermal protection device allows a connection to the contact surface of the varistor. Due to the reduction of useable contact area of the varistor, it is easier to reduce the risk of electric arcs. The covering of the window in the inner wall of the housing is a safe disconnection, since the whole contact surface would be electrically insulated. Furthermore, the cover can be designed to prevent an electric arc, which would elude the provisions of the thermal protection device.

In an embodiment the thermal protection device is designed with a varistor comprising a second contact surface with a terminal. This contact surface with the terminal can be arranged on an opposite side to the first contact surface of the varistor mentioned above. The terminal on the second contact surface can be larger than the dimensions of the varistor. In a special modification the terminal on the second contact surface can protrude from the housing. The terminal protruding from the housing can be formed so as to plug the varistor on a circuit board. Thereby the housing and also the thermal protection device can be plugged on a circuit board via the terminal. The varistor can be a metal oxide varistor. In a certain modification it would be a zinc oxide varistor. The varistor can have a rectangular shape but is not limited to it. The varistor may also have a round shape, for example. The possible terminal protruding from the housing with the possibility to be plugged can improve the handling and exchanging of the thermal protection device. This would allow an easy and quick replacement.

Another embodiment of the thermal protection device comprises a metal contact formed to a terminal. This metal contact can define a spring terminal. The spring terminal would be fixed to the housing. It can be partly arranged in the cavity of the housing. During the operational state of the thermal protection device one end of the spring terminal can be connected to the first contact surface of the varistor through the window in the inner wall of the housing inside the cavity. Such a connection can cause a pre-stress in the fixated spring terminal which would cause a spring force directed away from the connection perpendicular to the first contact surface of the varistor. The possible pre-stress in the spring terminal during the operational state of the thermal protection device can lead to a shorter response time in the case of triggering. Additionally, the spring terminal would not revert to its initial position of connection to the first contact surface of the varistor without an external force after triggering of the thermal protection device. A part of the spring terminal can protrude from the housing. This part can be designed as a pluggable terminal. Therefore, it can provide a stably plugged thermal protection device.

In an embodiment of the thermal protection device, the connection between the spring terminal and the first contact surface of the varistor is implemented as a low temperature solder joint during the operational state of the thermal protection device. Therein “low temperature” can be a characteristic temperature at which the solder joint would reach a state where it would allow a separation of the before-connected metal contact and the contact surface due to a force or pre-stress as it could exist in the metal contact. It is possible that the “low temperature” is the melting temperature of the low temperature solder. This characteristic temperature would be called “low” because of its possibly lower value compared to a usual characteristic temperature of solder. The characteristic temperature of the low temperature solder can be in a range from 100° C. to 210° C., e.g., 138° C. If the low temperature solder joint is heated up to the characteristic temperature, the connection can become loose and the solder can become liquid. If the connection between the spring terminal and the first contact surface becomes loose, a separation of the two becomes easy. Such a separation would lead to a fault state of the thermal protection device. Therein the term “fault” is chosen due to the origin of the state, which is a fault in the system in which the thermal protection device is installed.

In this embodiment heating of the low temperature solder joint can be caused either by a high current which flows through the connection as a consequence of the fault in the system or by the heat radiated from the varistor itself since the solder joint is in direct contact with the varistor. Additionally, the heating of the low temperature solder joint can be caused by any other source from outside the thermal protection device. A high current or a heated varistor are the cases against which the thermal protection device should protect the varistor. The arrangement of the low temperature solder joint can ensure a short response time of the thermal protection device.

A possible combination of an embodiment with a low temperature solder joint with the spring terminal from the embodiment above can lead to a safe and fast disconnection of the first contact surface of the varistor, due to the arrangement of the connection close to the varistor and the stress in the connected spring terminal.

In a further embodiment of the invention, the housing is closed by a removable plastic cap. The removable plastic cap would allow access to the parts in the cavity of the housing. Furthermore, with an accurate application of heat it can be possible to rebuild the low temperature solder joint between the spring terminal and the first contact surface after triggering of the thermal protection device. This would allow a skilled person to reset the thermal protection device to an operational state.

Another embodiment of the thermal protection device comprises a spring in the cavity of the housing. The spring can be a torsion spring. The spring can be arranged adjacent to the inner wall of the housing opposite to the varistor. Furthermore the thermal protection device of this embodiment can comprise a moveable insulation block inside the cavity of the housing adjacent to the inner wall. Specifically, the moveable insulation block can be partly located between the spring terminal and the inner wall. Both the spring and the moveable insulation block or only one of them can have a common centre of rotation. At this centre of rotation a rivet would be arranged on the inner wall that reaches in the cavity of the housing. On the rivet both the spring and the moveable insulation block can be fixated.

In the operational state of the thermal protection device the spring can push the moveable insulation block against the spring terminal, which is connected to the first contact surface of the varistor. The moveable insulation block pushed by the spring can represent a source of force acting on the connection between the spring terminal and the first contact surface of the varistor, wherein the force performed by the spring and the moveable insulation block would be directed parallel to the inner wall. The force can be directed to separate the spring terminal from the first contact surface of the varistor in a fault state of the thermal protection device.

Since the spring terminal can be already pre-stressed in an operational state of the thermal protection device, the moveable insulation block in combination with the described spring force represents a further separation mechanism. Such a further separation mechanism in the form of the moveable insulation block pushed by a spring would increase the safety of the thermal protection device since it ensures a separation of the connection to the first contact surface if the connection gets loose.

Since the moveable insulation block can be designed to move in a rotation around a rivet in the centre of rotation, no guide rails are needed. Therefore it is unlikely for the moveable insulation block to jam. This would improve the safety of the thermal protection device.

In a preferred embodiment of the thermal protection device the moveable insulation block as described above can be pushed in an end position by the spring in the case of a fault state of the thermal protection device. Such a fault state can be reached if the connection to the first contact surface of the varistor becomes loose due to a heating up of the varistor. The moveable insulation block can be designed to separate a terminal from the first contact surface of the varistor. Furthermore it can be designed to cover the window in the inner wall of the housing, as a result of which the first contact surface of the varistor would be completely electrically insulated and therefore defy electric arcs. Such a design would ensure the disconnection of the varistor and prevent an emergency of a flashover in a fault state of the thermal protection device.

A possible embodiment of the thermal protection device comprises an indicator to signal the current state of the thermal protection device. The indicator would be partly arranged in the housing and can comprise signal contacts which protrude from the housing and an indicator trigger which is arranged in the housing. The indicator may be a micro-switch. It is also possible that the moveable insulation block reaches the trigger of the indicator in a fault state of the thermal protection device. The moveable insulation block can be designed to trigger the indicator if it is pushed in the end position of the moveable insulation block. It is possible that any further electronic be connected to the signal contacts of the indicator to display the state of the thermal protection device.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1shows an exemplary embodiment of a thermal protection device100in an operational state. In the figure a housing1is represented as transparent to bring a varistor2into view, which is partly embedded in the housing1. The housing1comprises an inner wall16, which is arranged parallel to a contact surface21of the varistor2. The housing defines at least one cavity. Here, the inner wall16can also be a wall of the cavity mentioned before. The inner wall16comprises a window12, which offers the possibility for forming a connection from a part in the cavity to the contact surface21of the varistor2. The varistor2comprises a terminal22. Said terminal is arranged on an opposite side of the contact surface21and protrudes from the housing1.

The thermal protection device100comprises a spring terminal13, which is partly arranged in the cavity of the housing1. The spring terminal is held in its position by a fixation feature14in the housing1. The protruding parts of the spring terminal13and of the terminal22of the varistor2are designed to make the thermal protection device pluggable. During the operational state of the thermal protection device100an end of the spring terminal13in the cavity of the housing1is connected to the contact surface21of the varistor2through the window12in the inner wall16of the housing1. The connection between the spring terminal13and the first contact surface causes a stress in the spring terminal, which is directed away from the contact surface21of the varistor2. To hold the connection between the spring terminal13and the contact surface21, an electrical connection11is used. The electrical connection11is 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.

The shown embodiment of the thermal protection device100in an operational state comprises a moveable insulation block43, which is arranged in the cavity of the housing1and abuts on the inner wall16. A spring42is placed in the cavity and pushes the moveable insulation block43against the spring terminal13near the electrical connection11. The spring42is held in place by a feature to fix the spring41, which is arranged on the inner wall16and reaches into the cavity of the housing1. The path of movement for the moveable insulation block43and the spring42is a path of gyration with its centre at the feature41to fix the spring42, in a plane parallel to the inner wall16and therefore parallel to the contact surface21of the varistor2.

The embodiment of the thermal protection device comprises an indicator31, as shown inFIG. 1. The indicator31is partly arranged in the housing1and can be reached by the moveable insulation block43. The indicator comprises signal contacts32, which are arranged outside the housing1.

FIG. 2shows an embodiment of the housing1for a thermal protection device in a transparent view to show the partly embedded varistor2. The shown embodiment of the housing comprises the inner wall16with the window12, wherein the inner wall16is arranged parallel to the contact surface21of the partly embedded varistor2and delimits a cavity in the housing1. The window12in the inner wall16allows a connection to be formed to the contact surface21of the varistor2. The shape or arrangement of the window12within the inner wall16is not limited to the example shown in the figures. The terminal22of the varistor2is located on an opposite side of the contact surface21and protrudes from the housing1.

In the housing1there is a feature3to fix the indicator31. This feature3is a notch in the outer wall of the housing, which reaches from an upper boundary of the housing to the level of the inner wall16. The housing1comprises the feature14to fixate a spring terminal. The feature14also is a notch in the outer wall of the housing1.

In the cavity of the housing there is the feature41to fix the spring42. The feature41to fix the spring42is designed like a rivet which is arranged on the inner wall16and reaches into the cavity of the housing1.

FIG. 3represents a schematic overview of a possible embodiment of the thermal protection device100in an operational state. In this operational state the spring terminal13is connected to the contact surface21of the varistor2, which is partly embedded in a cavity of the housing1. The connection is realised through the window12in the inner wall16of the housing1and is held by means of the electrical connection11. The solder joint is stable at low temperatures and the low temperature solder stays solid until its temperature reaches a critical temperature. The spring terminal13is fixed at the housing1via the feature14to fix the spring terminal partly in the cavity of the housing1. Due to the fixation of the spring terminal13and the connection to the contact surface21, a stress is built up in the spring terminal near the end which is connected to the contact surface21of the varistor2. This stress is directed away from the contact surface and causes a fast separation of the spring terminal13and the contact surface21of the varistor2, if the low temperature solder becomes liquid.

In this embodiment the thermal protection device100comprises the moveable insulation block43. During the operational state the moveable insulation block43is arranged between the inner wall16and the spring terminal13inside the cavity of the housing1. The spring42pushes the moveable insulation block43against the spring terminal13near the electrical connection11as long as the thermal protection device100is in its operational state. The spring42and the moveable insulation block43have a common rotation axis. This rotation axis is defined by the feature41to fix the spring42, which is arranged on the inner wall16an reaches into the cavity of the housing1. The torsion spring has two arms, wherein one arm is to push the moveable insulation block43and the other arm is pushed against a wall of the housing1in order to provide a torque in the spring42.

Inside the cavity of the housing a trigger33of the indicator31is arranged. During the operational state of the thermal protection device100, the trigger33of the indicator31is not activated. A corresponding electrical signal can be tapped at the signal contacts32of the indicator31, which protrude from the housing1.

FIG. 4shows a schematic overview of a possible embodiment of the thermal protection device100in a fault state. The fault state is a result of a chain reaction, initiated by a critically high voltage between the spring terminal13and the terminal22of the varistor over a certain time. If the voltage between these terminals reaches a characteristic value, the varistor2changes 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 connection11between the spring terminal13and the contact surface21of the varistor2. That would liquefy the used low temperature solder. Thereby the electrical connection11becomes loose either due to the stress in the spring terminal, which bends the spring terminal13away from the contact surface21of the varistor2, or due to the torque of the spring42, which pushes the moveable insulation block43between the spring terminal13and the inner wall16, over the window12. As a result of this chain reaction, the spring terminal13is separated from the varistor2and the moveable insulation block closes the window12in the inner wall and activates the trigger33of the indicator31.

FIG. 5shows an exemplary embodiment of the varistor2. The varistor comprises the contact surface21on one side of the varistor2. The contact surface21is arranged on a metallization layer23of the varistor2. This metallization layer23can partly or completely extend over one side of the varistor2. The varistor2comprises a further contact surface with the terminal22on the opposite side with respect to the contact surface21. The terminal22of the varistor2protrudes over the dimensions of the varistor2. In a possible embodiment, the terminal22is designed in such a way that it reaches out of the housing1. 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.