EP1246208A1 - PTC element with mounting system for PTC polymeric element - Google Patents

Abstract

Within a preset expansion stretch in a main expansion direction a mounting (2) is designed to oppose expansion of a PTC polymer element (1) with a small counter force. The mounting has a limiting device (3) designed to oppose any further expansion in the PTC polymer element with a large counter force if the preset expansion stretch is exceeded, in order to limit the expansion of the PTC polymer element.

Description

Technical field

This invention relates to a PTC device, which is a PTC polymer element and contains a holder for it.

State of the art

PTC polymer elements are known per se. They consist of a matrix made of a polymer material, often a thermoplastic such as polyethylene, with at least one highly conductive filler added to the matrix, for example a metal powder, another conductive powder such as TiB ₂ , graphite or carbon black. The amount of electrically conductive filler should be above a material-dependent so-called percolation limit. Other fillers may also be present, such as those with varistor properties. The material aspect of the PTC polymer element plays no decisive role for the present invention and is not further problematized here because PTC polymer materials are extensively documented in the prior art.

The PTC transition, which is in one certain relatively narrow transition temperature range takes place. With this The PTC transition increases the specific electrical resistance by many Orders of magnitude. The reason for this is due to the temperature increase induced mechanical expansion of the PTC polymer material that the previously for the conductivity essential intermediate bridges and contacts between the Filler particles and agglomerations weaken or even interrupt. The reason this is because the thermal expansion coefficient of the Matrix material are much larger than those of the filler. The mechanical expansion of the PTC polymer material in the PTC transition can in Percentage range.

PTC polymer elements are exploited using this specific behavior used technically in different ways. A main application is in Limitation and interruption of electrical currents. The can by the PTC polymer element flowing and interrupting current through the Joule heat loss either responsible for the PTC transition itself be or be induced by external heat supply such a transition.

However, the invention is not based on applications of the PTC device Resistor for limiting current or as a switch for interrupting current limited. Other possible applications are e.g. self-regulating Heating elements and sensors.

Presentation of the invention

The underlying technical problem of the invention is a With regard to use even with very sudden PTC transitions specify improved structure of a PTC component.

According to the invention, a PTC component with a PTC polymer element containing a polymer matrix and at least one filler has that is good electrical conductivity, and all in one Response temperature range shows a PTC transition that with a strong Increased conductivity and a mechanical expansion is connected, and with a holder for the PTC polymer element, characterized in that the bracket is designed so that it extends at a PTC transition of the PTC polymer element within a predetermined expansion distance in a main direction of expansion at most a small counterforce opposed, and that the bracket has a limiting device which is designed so that when the specified expansion distance is exceeded a possible further expansion of the PTC polymer element by the Limiting device is opposed to a large counterforce Limit the expansion of the PTC polymer element, the predetermined Expansion distance is dimensioned so that the conductivity of the PTC polymer element has risen sharply when the PTC polymer element up has extended to the outer end of the specified expansion distance, however, mechanical overloading of the PTC polymer element is avoided.

Preferred embodiments are the subject of the dependent claims.

The invention is based on the knowledge that very sudden PTC transitions to mechanical loads or damage to the PTC polymer element can lead, especially in the area of Response zone, i.e. where the actual PTC transition took place and the material is therefore the warmest. Because the mechanical expansions in the case of sudden PTC transitions, especially when limiting or Interruption of short-circuit currents, in the millisecond range or even Parts of the PTC polymer element may appear more quickly accelerated significant speeds. It can be done through the Mass inertia of the accelerated parts at the end of the PTC transition, i.e. at Completion of the actual mechanical expansion, through the sudden Delay of the briefly rapidly moving parts to considerable tensions come within the PTC polymer material. Especially in the warm Areas that are usually areas with a reduced material cross section are, mechanical damage can occur, the resistance in the undesirably increase the normal conducting state or the PTC polymer element to make something useless.

According to the invention, a holder for the PTC polymer element is therefore provided to be trained in such a way that for a given given Expansion distance, which is the desired expansion during the PTC transition corresponds, allows an expansion of the PTC polymer element and this opposes at most small forces, when this is exceeded Limiting distance, however, the expansion due to significantly larger counterforces limited. The inertial forces should therefore be limited to that limit provided limiting device are caught and therefore not lead to excessive tensile stresses in the PTC polymer material itself can. It is important that the force applied to the PTC polymer element done in two stages. Within the given Mechanical expansion, the mechanical expansion is necessary and desirable, where their handicap due to excessive opposing forces the PTC transition could hinder, weaken or slow down. However, the extent is significantly larger than this specified distance, it should be as effective as possible limited, so the PTC polymer element are braked.

Since the PTC transition takes place over a certain area in which the desired increase in resistance occurs to such a large extent that usually part of this increase in resistance for technical applications is sufficient, it is not absolutely necessary that the specified Expansion distance exactly on the maximum possible movement stroke in the PTC transition is coordinated. Rather, it can be somewhat shorter, and depending on the requirements placed on the PTC transition become. Conversely, it is not necessarily harmful if the default Movement distance slightly larger than the maximum possible stroke in the PTC transition is. The main advantages of the invention can already be achieved if only a significant part of the "overshoot" in violent PTC transitions is intercepted.

Preferred ranges for the quantitative measurement of the given Expansion distance in relation to the maximum expansion of the PTC polymer element a PTC transition is between 35%, better 50%, as the lower limit and 110%, better 100%, as the upper limit. It is mind the maximum static expansion of the PTC polymer element to go out; when measuring this expansion must therefore Overshoot effects are omitted. It is based on the explanations in Reference within the description of the exemplary embodiments.

In very many applications, the PTC polymer elements are elongated Rods or similar shaped. In these cases, it essentially plays Expansion of the PTC polymer element in the longitudinal direction of this shape Role, then the invention can also be limited to the extent of the PTC polymer element in this longitudinal direction. With the Extensions in the other directions perpendicular thereto are in such Geometries then correspondingly lower speeds and thus Inertia and possibly also smaller masses are connected. Naturally However, the invention can also be applied in more than one direction. This is of no further importance for the basic principle of the invention, which is why this description applies to cases with a limitation in only one direction limited.

As a rule, it will be the cheapest to unilaterally close the PTC polymer element hold, about on and with the help of an electrical contact, and the movement of the limit opposite end of the PTC polymer element. At this opposite end can then be a flexibly connected contact be provided. Incidentally, this electrical contact can also be an essential one Cause for the mechanical damage to be prevented by the invention be the inertia of the accelerated part of the PTC polymer element elevated. Of course, the PTC polymer element can also For example, be held in the middle and extend from the center to two sides stretch out. Then two (or more) limiting devices can also be used Find application.

It should preferably be avoided that the limiting effect of the Limiting device is used discontinuously, for example the PTC polymer element with one end against a limiting device in the form of a simple attack without having been slowed down beforehand. A such an approach would be within the scope of the invention, but is due to of the shock-like vibrations as less favorable. Rather, the Limiting effect can be built up continuously by inserting the Sense of the choice of the concept of the main claim large counterforce already one of the Expansion of the PTC polymer element counteracting counterforce rises continuously.

For example, this continuously increasing counterforce can be caused by a be built elastic medium, which is designed so that it in its Effect of the limiting device is connected upstream. For this purpose, the Reference examples, in which various preferred Embodiments for the elastic medium are shown, namely a Spring, an enclosed gas volume - also in combination with a Liquid - and an elastomer element (or a plurality of each Elements). Of course, these variants could also be mixed together become.

It can be particularly advantageous to use the elastic medium in such a way that the elastic limit of the elastic medium is reached before the large counterforce of the limiting device is applied, and not in the sense of destruction of the elastic medium (something like a tension spring breaking), but in Form of a very strong increase in the counterforce with further expansion of the PTC polymer element. For example, this can be done by the end of the spring travel of a softer first spring, whereupon only a second, harder spring is compressed, which can also be so hard that it can itself build up the large counterforce of the limiting device, that is, the limiting device itself. Furthermore, in the case of an enclosed gas volume, it is possible to have the volume compressed by the PTC polymer element to such an extent that the increase in pressure becomes very strong with further expansion of the path or even the so-called covolume of the gas becomes essential, i.e. the behavior of an ideal gas with the volume reversed proportional pressure in the sense of a disproportionate pressure increase with volume reduction is left. Of course, gases with relatively large molecules are particularly suitable, for example CO ₂ or gases with even larger covolumes.

Finally, it is also conceivable for elastomer elements to be designed in this way be that a near the end of the specified distance The elasticity constant increases. This can be due to over-strong Compression of the elastomer material by installing a harder core suitable shape can be achieved.

Overall, the limiting device should at best be designed in such a way that overshoot of the PTC polymer element in the event of a very sudden expansion as a result of a PTC transition is completely prevented. Which quantitative values are necessary for this is a question of the individual case. The counterforce applied to the PTC polymer element within the predetermined expansion distance should preferably not be greater than about 25 N / cm ² (based on the minimum cross section of the PTC polymer element). With an elastic counterforce, at the end of the specified expansion distance, this value should be at most approximately, i.e. the elastic constant should be dimensioned accordingly, taking into account the length of the permissible expansion distance (for an exemplary movement distance of 200 µm, for example, with a 20 mm long PTC polymer element to a surface-specific elastic constant of ≤ 1250 N / cm ³ .

Brief description of the drawings

In the following the invention is based on various embodiments in individual illustrated. Features disclosed here can also be found in others than the combinations shown or individually essential to the invention.

FIG. 1 shows a typical time course diagram of one by an inventive one PTC component limited short-circuit current and the mechanical Expansion of the PTC polymer element.

FIG. 2 shows a first exemplary embodiment for a PTC component according to the invention with a double spring as an elastic medium.

Figure 3 shows a second embodiment with a closed Volume of gas as an elastic medium and a liquid.

Figure 4 shows a third exemplary embodiment with a gas as elastic Medium.

Figure 5 shows a last embodiment with elastomer elements as elastic medium.

Ways of Carrying Out the Invention

In Figure 1 is a typical timing diagram for a PTC transition shown. The abscissa shows the time axis t in ms, the left coordinate the Size of a short-circuit current I to be limited in amperes and the right one Coordinate, the expansion d of the PTC polymer element that occurs due to the PTC transition in micrometers, in the longitudinal direction of a rod-shaped PTC element, as shown schematically in Figures 2-5. you recognizes that the short-circuit current jumps from the time of 1.5 ms rises to its maximum value within half a ms and that in the process approximately 0.2 - 0.3 ms after the current rise the characteristic expansion of the PTC polymer material starts. At the time of 2.1 ms, this already has one current limiting effect. The current is almost up at 2.2 ms zero dropped, although the expansion is just a fifth of the static Has reached maximum. This is due to the extreme increase in electrical Resistance in the PTC transition. The inertia must also be taken into account the heat conduction from the place of their generation at the contact points of the Filler material in the expanding polymer matrix material.

The mechanical expansion of the PTC polymer element continues to increase and shows between an amount of 50 microns and an amount of 70 Micrometer, i.e. between the times 2.5 ms and 3.3 ms, one pronounced overshoot. After this overshoot the expansion stabilizes at the static maximum value of about 50 Micrometers. In the present case, the extension of the response zone was in the Longitudinal direction about 5 mm, so that one has an expansion coefficient of can run out about 1%. The maximum static expansion was however in The overshoot is exceeded by about 30%. This Overextension and the subsequent swinging back are with a massive mechanical stress on the response zone of the PTC polymer element connected, which is to be reduced or avoided according to the invention. To do this the expansion of the PTC polymer element to an amplitude of approximately 25-50 Micrometers (in the present case), because obviously already at relatively low expansion values the desired current limiting function to a considerable extent.

Figure 2 shows a schematically illustrated example. Here is the PTC polymer element 1 installed in a holder 2 in the form of a rigid frame, the left end of the PTC polymer element 1 in the figure being firmly attached to the Bracket 2 is anchored. Between one of the PTC polymer elements opposite end of the holder 2 and the free end of the PTC polymer element 1 is a double spring 3,4 used, the PTC polymer element 1 facing part 3 a relatively large spring constant has, while the other part 4 has a very small spring constant. The Order could also be reversed).

If the PTC polymer element 1 is in the course of the PTC transition with its right end extends to the right, as in the second illustration of FIG. 2 indicated by dashed lines, the soft spring 4 is first compressed until it is completely compressed. The length of the soft spring 4 in The “cold” state of the PTC polymer element 1 in the left representation in FIG. 2 adapted to the desired maximum expansion of the PTC polymer element 1, so set to about 40 microns. It goes without saying that this one schematically drawn coil springs do not form a realistic representation. In practice, very flat elastic metal elements, such as disc springs, use an elastic deformation over a distance of 40 Allow micrometers (or another desired distance). portable Electrical contacts could be caused, for example, by weak wires be contacted.

The PTC polymer 1 can thus extend over the predetermined expansion distance of about 40 microns against the relatively small spring counterforce of the soft Extend spring 4 and only then will the much greater counterforce the hard spring 3 prevented from further expansion. The difference between the two spring constants should be significant, for example correspond to a factor of 10 - 1000 or greater. This will make it Prevents overshoot of the accelerated parts of the PTC polymer element 1 by slowing them down effectively. The hard spring 3 and the frame of the Bracket 2 thus absorb the forces that without the invention of the Response zone should be worn.

In this and the following representations, the PTC polymer element 1 is as simple bar drawn. Of course, it actually has a more complicated one Shape, in particular it has a central constriction to define a Response zone. However, these details are for the principle of the invention irrelevant.

The second exemplary embodiment in FIG. 3 differs from the first Embodiment of Figure 2 in that the double spring 3, 4 is replaced by a gas volume 9 and a liquid volume 8 in one closed vessel 7. The PTC polymer element 1 is at its in Figure 3 upper end attached to a part 6 of the bracket and protrudes with its lower End in a sealed manner into the vessel 7. In the vessel 7 is a liquid 8 in contact with the lower side of the PTC polymer element 1 and also a volume of gas 9. If the PTC polymer element 1 as a result of a PTC transition with its underside downward, it pushes the Liquid level below itself, so that the liquid level in the remaining area of the vessel 7 rises and the gas volume 9 compressed. Due to the increasing intrinsic pressure of the gas volume 9 on the one hand and by the hydrodynamic resistances within the liquid 8 on the other hand, the PTC movement becomes a certain counterforce opposed. The hydrodynamic damping should not be too great, just to have a smoother transition between the spring constants generate, but not to hinder the PTC transition.

When the gas volume has almost disappeared, its internal pressure increases dramatic because it reversed from the covolume of the gas increases in proportion to the decreasing volume. The The elastic constant of the gas volume 9 thus has a linear one Expansion of the PTC polymer element 1 a pole. This will make the Expansion of the PTC polymer element 1 towards the end of the intended Expansion distance with continuously but strongly increasing counterforce limited.

The next exemplary embodiment from FIG. 4 largely corresponds to the second Exemplary embodiment from FIG. 3, but in this case the liquid 8 is omitted. The PTC polymer element 1 is here through a seal 10 sealed that the gas volume 9 can not escape from the vessel 7. Expansion of the PTC polymer element 1 causes compression of the Gases 9 within the vessel 7, the pressure again increasing sharply. Due to the inverse proportionality between pressure and volume is also here a strong disproportionate increase in the elasticity constant of the Given gas volume 9. This can be done by choosing a gas with a large Kovolumen be increased.

The last exemplary embodiment in FIG. 5 uses elastomer elements 11 which between the top of the PTC element 1 and a part 13 of the holder lie. The underside of the PTC element 1 is on another part 12 of the Bracket attached. When the PTC polymer element 1 now expands the elastomeric elements 11 are compressed, here also by corresponding Material choice or shape or through a harder core within the Elastomer elements 11 achieved a disproportionate increase in the counterforce can be. Finally, the expansion of the PTC element 1 by Stop the top against the bracket part 13 are limited.

LIST OF REFERENCE NUMBERS

1

PTC polymer element

2

bracket

3

hard feather

4

soft feather

6

supporting member

7

closed vessel

8th

liquid volume

9

gas volume

10

poetry

11

elastomer element

12

supporting member

13

supporting member

Claims (15)

PTC device
with a PTC polymer element (1) which has a polymer matrix and at least one filler which is highly electrically conductive, and which shows a PTC transition in a response temperature range, which is associated with a strong increase in conductivity and mechanical expansion, and
with a holder (2, 6, 7, 12, 13) for the PTC polymer element,
characterized in that the holder (2, 6, 7, 12, 13) is designed in such a way that, in the case of a PTC transition, it opposes at most a small counterforce to the expansion of the PTC polymer element (1) within a predetermined expansion distance in a main expansion direction, and
that the holder (2, 6, 7, 12, 13) has a limiting device (3, 7, 8, 9, 11, 13) which is designed such that if the predetermined expansion distance is exceeded, any further expansion of the PTC polymer element (1) a large counterforce is opposed by the limiting device in order to limit the expansion of the PTC polymer element (1),
the predetermined expansion distance being dimensioned such that the conductivity of the PTC polymer element (1) has risen sharply when the PTC polymer element (1) has extended to the outer end of the predetermined expansion distance, mechanical overloading of the PTC polymer element ( 1) is avoided. PTC component according to Claim 1, in which the PTC polymer element (1) has an elongated shape and the limiting device (3, 7, 8, 9, 11, 13) the expansion of the PTC polymer element (1) only in the The longitudinal direction of this shape is limited. PTC component according to claim 1 or 2, wherein the PTC polymer element (1) is held on one side and on an opposite Interacts with the limiting device (3, 7, 8, 9, 11, 13). PTC component according to one of the preceding claims, wherein the predetermined expansion distance in the range of 35%, preferably of 50%, up to 110%, preferably up to 100%, of the maximum static Expansion of the PTC polymer element (1). PTC component according to one of the preceding claims, wherein the Bracket (2, 6, 7, 12, 13) is designed so that the limiting device (3, 7, 8, 9, 11, 13) the great counterforce after continuous Rise of a counterforce against the expansion of the PTC polymer element (1) generated. PTC component according to Claim 5, in which the holder (2, 6, 7, 12, 13) an elastic medium (3, 4, 9, 11) which the continuous Increase in the counterforce generated. PTC component according to claim 6, wherein the elastic medium first spring (4). PTC component according to claim 6, wherein the elastic medium enclosed gas volume (9). PTC component according to claim 6, wherein the elastic medium Elastomer element (11). PTC component according to one of claims 6-9, wherein the holder is designed so that the continuously increasing counterforce of the elastic medium (3, 9, 11) at a given Expansion distance extending expansion of the PTC polymer element (1) continuously into the great drag of the Limiting device passes. PTC component according to claim 7 and claim 10, wherein the Limiting device a second spring (3) with much larger Has spring constant than that of the first spring (4). PTC component according to Claim 8 and Claim 10, in which the gas (9) has a high covolume. PTC component according to claim 9 and claim 10, wherein the Bracket (3, 7, 8, 9, 11, 13) is designed so that the elastomer element (3, 9, 11) at the end of the specified stretch to the limit reaches its elastic range. PTC component according to one of the preceding claims, wherein the large counterforce is so large that it is at the maximum permissible No stresses on the PTC polymer element (1) during a PTC transition to an overshoot of the expansion movement of the PTC polymer element (1) is coming. PTC component according to one of the preceding claims, in which the low counterforce within the predetermined expansion distance, based on the minimum cross-sectional area of the PTC polymer element (1), is at most 25N / cm ² .

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