Patent Application
20100277268
The present invention relates to a thermal safety device for a heating element deposited on a substrate, forming a heating subassembly particularly included in a household electrical appliance.
There is a temperature-sensitive cutout known from the document EP 0654170, intended to be fixed to a substrate, comprising a leaf spring provided at each of its ends with a contact soldered to a corresponding contact pad on the substrate. Between the two soldered contacts, the leaf spring comprises an arm composed of a roof-shaped part followed by a flat section. The cutout is activated by applying pressure on the roof, which generates a bending of the flat section and a lifting moment of the contact disposed next to the section. When the substrate overheats, the solder of the contact melts and the lifting moment produces a lateral displacement of the contact, which opens the circuit. However, the opening of the contact is generated by a slight deformation of the roof, thus requiring very precise control of the dimensions of the spring and of the various parts exerting pressure on the roof. Moreover, this type of cutout requires a considerable amount of space between the contacts in order to accommodate the roof-shaped part and the flat section.
There is another thermal safety device known from the document WO 98/20510, comprising a metal strip soldered to a substrate between two contact pads. The strip has a cantilevered end which is mechanically stressed by a wall of a housing. When there is overheating in the vicinity of the contact pads, the solder joints melt and the stress exerted on the end of the strip causes the circuit to open by separating the contact located on the end opposite the cantilevered end. However, this thermal safety device only works if the two solders of the pads melt simultaneously, which requires the same thermal environment for each pad. Moreover, the metal strip provided with a cantilevered end is long and thus difficult to install in a small space.
The object of the present invention is to eliminate the aforementioned drawbacks and to propose a thermal safety device that offers high reliability and safe operation.
Another object of the invention is to propose a thermal safety device that is simple and economical to use.
Another object of the invention is to propose a thermal safety device that is compact so that it can be installed in a small space.
These objects are achieved with a thermal safety device intended to be fixed to a substrate, comprising a strip fuse comprising a metal strip provided with a first contact zone electrically connected to a first pad on the substrate and also provided with at least one other contact zone capable of being soldered to another pad on the substrate, the strip fuse also comprising a lever having a free end not connected to the metal strip, characterized in that the lever generates a tilting moment of the soldered contact zone about at least one pivot point disposed between the two contact zones when the end of the lever is pressed.
The thermal safety device thus produced comprises a strip fuse whose shape is stable when it is soldered to the substrate, and the device is subsequently activated by pressing on the end of the lever. According to the invention, the pivot point around which the tilting moment is generated by the lever is disposed between the two contact zones, thus enabling the device to have a compact structure.
Advantageously, the pressure on the end of the lever occurs outside the two contact zones.
This feature makes it possible to obtain a lever arm long enough to guarantee, when the device overheats, a displacement of the contact zone relative to the pad with an even greater margin of safety in terms of electrical isolation.
Preferably, the lever has two arms disposed alongside the metal strip.
This feature makes it possible to balance the forces generating the tilting moment of the soldered contact zone.
Advantageously, each arm of the lever has stiffening means.
This feature makes it possible to keep the arms of the lever from bending in order to transmit the tilting moment to the soldered contact zone.
Preferably the stiffening means of each arm of the lever are formed by a longitudinal bend.
This feature makes it possible to stiffen the lever arms simply and economically without adding any parts.
Advantageously, the metal strip has at least two bends located between the pivot point and the first contact zone.
This feature makes it possible to create additional pivot points which increase the opening of the contact zone.
Preferably, the metal strip and the lever are formed in the same strip of metal.
This feature makes it possible to produce the strip fuse of the thermal safety device simply and economically.
Advantageously, at least the soldered contact zone is reflow soldered.
This feature makes it possible to attach all of the components that must be soldered to the substrate of the type electronic components, connection elements . . . in a single production phase.
The invention will be more clearly understood by studying the embodiment taken as a nonlimiting example and illustrated in the attached figures, in which:
As seen in
The heating resistor 4 has an end provided with a hot point 7 which, as the appliance heats up, stays relatively hot due to the pattern of the track of the heating resistor 4. The power pin 12 is connected to a track having an end provided with a cold point 8, which stays relatively cold as the appliance heats up.
As seen in
The strip fuse 20 also includes a lever 30 comprising two arms 3435, disposed alongside the metal strip 21 and integral with the latter at the level of the contact zone 22 soldered to the hot point 7. The two arms 34, 35 of the lever 30 each include a first part 32, 33 parallel to the metal strip 21 and a second, inclined part 38, 39. The inclined parts 38, 39 join to form an end 36, which serves as a pressing zone. The junction between the first part 32, 33 and the second, inclined part 38, 39 defines a pivot point 37 about a transverse axis 42.
The metal strip 21 comprises, between the pivot point 37 and the contact zone 23 soldered to the cold point 8, three successive bends 24, 25, 26 forming an inverted V.
Each arm 34, 35 of the lever 30 comprises, in its inclined part 38, 39, an outer edge provided with a reinforcing longitudinal bend 40, 41.
As seen in
All the solders of the power pins 11, 12 and the contact zones 22, 23 of the strip fuse 20 are reflow soldered in the same production operation.
After soldering, the plastic cover 15 of the connector (
In operation, in the event of a failure of the means for controlling the heating of the heating resistor 4, the temperature around the hot point 7 reaches the melting temperature of the solder (
It is understood that the invention is not in any way limited to the embodiment described and illustrated, which has been given only as an example. Modifications are possible, particularly in terms of the composition of the various elements or the substitution of technical equivalents, without going outside the scope of protection of the invention.
Thus, the metal strip of the strip fuse may not include bends and may not be rectilinear.
In a variant of embodiment, the lever arm may be connected to the metal strip by a connecting means of the soldering or crimping type.
In another variant of embodiment, the lever may be cut out of the center of the metal strip. In that case, the free end of the lever is located between the pivot point and the contact zone soldered to the cold point.
| Number | Date | Country | Kind |
|---|---|---|---|
| 08 00136 | Jan 2008 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR09/00004 | 1/6/2009 | WO | 00 | 7/9/2010 |
