EP0109019A2 - Surface heating element, especially for bandages or electric blankets - Google Patents

Abstract

A surface heating element is described. This has a positive temperature coefficient and a plurality of power supply lines (11) arranged at a distance from one another. The power supply lines (11) are formed by contact tracks (12) and are operatively connected to a flat support device. The contact tracks (12) and the carrying device are provided with an electrically conductive plastic with a positive temperature coefficient of electrical resistance. The surface heating elements are preferably used for bandages or electric blankets.

Description

The invention relates to a surface heating element, in particular for bandages or heated blankets, with a positive temperature coefficient and a plurality of power supply lines arranged at a distance from one another.

Surface heating elements are already known, in particular for an electrically heated bed cover - DE-OS 21 57 356 - the resistance of which increases with increasing temperature. The surface heating element is formed by individual heating conductors, which are worked independently of one another into a bedspread in the form of a mesh. In order to be able to generate the desired temperatures, these heating elements must have a relatively high temperature in order to cause heat to be emitted by heat radiation by radiation from the unheated surfaces lying between the individual heating elements. Although the use of materials in these heating elements, which increase their resistance as the temperature rises and accordingly enable fully automatic temperature control, complex circuit devices, such as bimetallic switches or the like, can be saved, but there is still a risk in the event of damage to the heating conductor that there may be a short circuit and, due to the high temperature of the heating elements, ignition of the bedspread. In addition, high voltages are required to generate the relatively high temperatures in the heating elements. The present invention has for its object to provide a surface heating element which can give off heat evenly distributed over a surface area and in which the risk of overheating of the surface heating element is eliminated by its construction.

This object of the invention is achieved in that the power supply lines are formed by contact tracks and are operatively connected to a flat support device, and that the contact tracks and the support device are provided with an electrically conductive plastic, in particular an elastomer with a positive temperature coefficient of electrical resistance. The advantages of this surprisingly simple solution lie in the fact that the use of a flat carrier device results in a uniform temperature release over practically the entire surface to be heated. Due to the large area over which the heat is emitted at the same time, it is therefore also possible to work with low voltages, so that the temperatures of the surface heating element are in a range in which ignition of surrounding materials hardly seems possible. In addition, the uniform coating with an electrically conductive plastic, e.g. an elastomer with a positive temperature coefficient of electrical resistance reaches a fully automatic limitation of the final temperature. This limitation relates not only to the power supplied, but also to the ambient temperature, which also influences the temperature of the surface heating element. This eliminates any security risk for users and achieves psychologically harmless heat application and adaptation. It is advantageous above all that there is a short wavelength of the surface heating element in the area of black radiation. This gives users a sense of well-being when using this surface heating element in duvets or bandages. Another advantage is that this black radiation has proven extremely useful for the treatment of certain symptoms of illness. In addition, with the surface heating element according to the invention, it is possible to find sufficiency without temperature control.

According to another very essential feature of the invention, it is possible for the carrying device, for example a network, to be made at least in part of an electrically conductive plastic, for example an elastomer with a positive temperature coefficient of electrical resistance is produced, and a thread-like carrier material, for example carbon fibers, metal threads or the like, are incorporated as contact tracks and / or reinforcing inserts in the carrying device or the network. This design enables cost-effective production of a surface heating element according to the invention, since subsequent coatings or the application of contact tracks can be saved if the thread-like carrier materials or the network of plastics, in particular elastomers, are designed accordingly. At the same time, when using filamentary carrier material consisting of carbon fibers, the network becomes resistant to mechanical stresses, so that damage to the contact tracks is avoided with certain stresses. The embedding of carbon fibers in the elastomer forming the network also has the advantage that when the network is used for heating purposes in the vicinity of aggressive air conditions or chemical reactions, no corrosion phenomena can occur on the contact tracks.

In the context of the invention it is further provided that the network consists of flexible thread-like carrier material and that the contact tracks are formed by flexible lanyards, and are preferably arranged parallel to a longitudinal fiber direction of the network, whereby a good adaptation, in particular with heating associations, is achieved and the mobility of the Patients who wear such bandages are not restricted too much.

According to a further embodiment of the invention, it is provided that the thread-like carrier material of the support device or of the network forms resistance tracks and consists, for example, of polyamide or polyester, and that the resistance tracks running perpendicular to the contact tracks have a higher conductance than the resistance tracks running parallel to these. This can also lead to thermal jams due to differences in conductance, no undesired local overheating of the surface heating element occurs.

According to the invention, it is also possible that the resistance tracks of the network running parallel to the contact tracks have a smaller cross-section than the resistance tracks running perpendicular to the contact tracks. The same basic material can thus be used to produce the threads of the network running parallel and perpendicular to the contact tracks.

It is advantageous according to a further embodiment of the invention that the resistance tracks of the network running perpendicular to the contact tracks and the elastomer applied to them are designed to be elastic in the longitudinal direction of the resistance tracks. As a result, changes in length that occur as a result of different temperatures can be taken into account in a simple manner without internal stresses occurring in the network or in the network coating.

Furthermore, it is also possible for the network to be coated with the elastomer in such a way that openings remain between the individual resistance tracks. This embodiment is distinguished in particular when the surface heating elements are used as bandages, since this ensures a consistently good vapor diffusion of the parts of the body being treated.

In the context of the invention, it is also possible for the network to be coated with an elastomer which has a non-linear temperature coefficient of electrical resistance with a characteristic curve, preferably in the region of the body temperature. This special design of the elastomer with which the net is coated ensures that the temperature of the surface heating element cannot rise significantly above the body temperature, because of the subsequent substantial increase in the electrical resistance in the elastomer automatically prevents further heating. This ensures a high level of security against undesired overheating of the surface heating element, especially in applications of surface heating elements for associations.

According to another embodiment variant of the invention it is provided that the elastomer, in particular its resistance, is designed to connect the contact tracks to a low-voltage current source and that the current source is preferably equipped with a step circuit for half-wave operation with a semiconductor diode. The use of a low-voltage power source for supplying the surface heating elements according to the invention enables the same to be used extremely universally, even in those areas where the use of these heating foils was not possible for safety reasons, especially as a result of the high voltage used.

According to the invention, it is also possible that at least on one side of the network, e.g. partially laminated foam coating is arranged, so that lying on surface heating elements according to the invention is not perceived as disturbing.

Finally, it is also possible within the scope of the invention that a heat reflection layer, e.g. a reflective tape or the like. is arranged. Unwanted heat radiation or energy emission in areas to be excluded from heating can thus be reduced or completely eliminated, as a result of which the energy consumption of such surface heating elements can be additionally reduced.

For a better understanding of the invention, it is explained in more detail below with reference to the exemplary embodiments shown in the drawings:

• Fig.l ein erfindungsgemäßes Flächenheizelement in schaubildlicher Darstellung;
• Fig.2 eine Stirnansicht des Flächenheizelementes im Schnitt, gemäß den Linien II - II in Figur 1;
• Fig.3 ein Flächenheizelement in einem Sicherheitsheizverband in schaubildlicher Darstellung;
• Fig.4 den Sicherheitsheizverband nach Figur 3 im Schnitt, gemäß den Linien IV - IV in Figur 3;
• Fig.5 ein erfindungsgemäßes Flächenheizelement in einer Heizkisseneinlage mit dem Flächenheizelement zugeordneter Reflexionsfolie, in Stirnansicht, teilweise geschnitten;
• Fig.6 ein Diagramm, in welchem die Leistungsaufnahme eines erfindungsgemäßen Flächenheizelementes sowie dessen Temperatur in Abhängigkeit von der Zeit dargestellt ist;
• Fig.7 einen Schnitt durch eine Ausführungsvariante eines erfindungsgemäßen Heizelementes;
• Fig.8 eine andere Ausführungsform eines Flächenheizelementes in schaubildlicher Darstellung.

Show it

• Fig.l a surface heating element according to the invention in a graphical representation;
• 2 shows an end view of the surface heating element in section, according to lines II - II in FIG. 1;
• 3 shows a surface heating element in a safety heating association in a diagram;
• 4 shows the safety heater bandage according to FIG. 3 in section, according to lines IV-IV in FIG. 3;
• 5 shows a surface heating element according to the invention in a heating pad insert with the reflecting film assigned to the surface heating element, in a front view, partially cut;
• 6 shows a diagram in which the power consumption of a surface heating element according to the invention and its temperature as a function of time is shown;
• 7 shows a section through an embodiment variant of a heating element according to the invention;
• 8 shows another embodiment of a surface heating element in a diagram.

In Figure 1, a surface heating element 1 is shown. This surface heating element comprises a flat support device, e.g. a network 2, the threads of which run perpendicular to one another and are networked with one another form resistance tracks 3 to 7 or 8.9. The resistance tracks 3 to 9 each consist of thread-like carrier materials 10 and 11, e.g. Polyamide or polyester fibers.

Some of the resistance tracks, namely the resistance tracks 3, 4 and 6, are assigned power supply lines 12, which are formed by contact tracks 13. These are e.g. formed by so-called Lahn tapes 14. These tapes 14 consist of a plurality of current-conducting threads 15 running parallel to one another, e.g. Metal and / or carbon threads or fibers that are woven or knitted together to form a band. The power supply lines 12 are connected to the resistance tracks 3, 4 and 6, 7.

For a better understanding of the representation in Figure 1, the network 2 is shown on a larger scale. The distance between the individual mutually perpendicular resistance tracks 3 to 9 is on average approximately 5 mm and the power supply lines 12 are usually arranged at a distance of between 25 and 50 mm from one another, depending on the performance of the surface heating element. In order to be able to better illustrate the connection of the individual resistance tracks and the power supply lines 12, the elastomer 16 with which these and the network 2 are coated is only shown over a small part of the length of the resistance tracks 4, 5 and 6, 7. The individual resistance tracks 3 to 9 are coated in such a way that openings 17 remain between the individual resistance tracks even after coating with the elastomer.

The elastomer 16 is formed with a positive temperature coefficient of electrical resistance and consists essentially of a conductive silicone rubber which e.g. with heating bandages, blankets or the like. about 0.5 - 3% increase in resistance per degree K.

• die Wärmeablage des Teilstückes L = k· Δt
• elektrische Leistungsaufnahme L = 1^2. R_o
• ergibt sich bei der Temperatur t_{: L =} I². R_oe^c·Δt

The value of the increase in resistance per degree K is to be determined as a function of the heat absorption of the medium to be heated. It should be taken into account that from a limit temperature up to which the heat distribution is uniform, it becomes unstable because the increase in power consumption is greater than the increase in heat output when the temperature is low. This effect occurs especially when the amount of heat given off is approximately proportional to the temperature difference between the surface of the resistor and the air, the resistor has a shape that does not allow internal heat exchange through heat conduction, for example wire, foil or the like a material with a positive temperature coefficient of electrical resistance. It is therefore important to ensure that the surface heating element 1 does not come into an unstable equilibrium, since this leads to an uneven heat distribution. The resistance then becomes much hotter in one place and remains almost cool in another. This can lead to overheating at the point that is too hot, so that the resistance burns or melts in this part. Now you bet for

• the heat deposit of the section L = k · Δt
• electrical power consumption L = 1 ^2. R _o
• at temperature t _{: L =} I ² . R _o e ^{c ·} .DELTA.t

By equating, Δ t can be calculated.

k.Δt = I ² .R _o · e ^c Δt

In a special case there is a solution (tangent) up to which the temperature is uniform.

k = e. I ² . R _o · c

An unstable heat distribution is achieved because, with little heating, the increase in power consumption is greater than the increase in heat output.

k · Δt ₌ I ² · R · e ^cΔt , ie limit temperature reached.

(k·Δ t) =

I². R_o·e^cΔt so folgt k = I² · R_o·c·e c.Δt = I². R_o.ecΔt --- c =

I Δt damit istΔt = U.c. D.h., daß bei einer angenommenen Meßspannung von 100 V und bei c = 0,05 → Δt = 20°C.If

(k · Δ t) =

I ² . R _o · ^cΔt then k = I ² · R _o · c · e c.Δt = I ² . R _o .ecΔt --- c =

I Δt is therefore Δt = UcDh that with an assumed measuring voltage of 100 V and at c = 0.05 → Δt = 20 ° C.

• L = Leistungsaufnahme bzw. -abgabe
• k = Konst. für Wärmeabgabe
• R = Grundwiderstand bei einer Meßtemperatur von t = 20°C
• t = Temperaturdifferenz zwischen Oberfläche des Widerstandes und der Luft
• c = Temperaturkoeffizient

In these equations:

• L = power input or output
• k = const. for heat emission
• R = basic resistance at a measuring temperature of t = 20 ° C
• t = temperature difference between the surface of the resistor and the air
• c = temperature coefficient

Even at an excess temperature of 20 ° C, if the temperature coefficient is 5%, you can theoretically get into the unstable area. However, since an internal heat exchange usually occurs in the film and I (current strength) is also not constant, the limit temperature is usually higher.

Due to the design according to the invention, in which the conductivity of the resistance tracks running transversely to the power supply lines have a higher conductance than the resistance tracks 3 to 7 running parallel to them, a relatively small distance can be maintained between the power supply lines 12, as a result of which an internal heat exchange takes place and the surface heating element is in the area of stable heat distribution.

A resistance mass according to AT-PS 274 965 or AT-PS 313 588 can preferably be used as the elastomer. If conductive plastics are produced according to the invention, surprisingly, plastic plants with a very strongly positive temperature coefficient are obtained. By adding the conductor particles known per se, for example graphite, it is possible to reduce the very large resistivities, the temperature coefficient of the plastic predominating as long as the conductor particles do not continuously touch to form a skeleton.

The process consists essentially in adding synthetic resin dispersions, synthetic resin solutions or synthetic resins with metal or semimetal compounds or their solutions in an amount such that there is approximately one metal or semimetal atom on a synthetic resin molecule, and reducing agents after mixing added in a slight excess or the metal or semimetal atoms are formed by known thermal decomposition, whereupon ions which are formed or are still present are washed out and the dispersions, solutions or granules are mixed with graphite or carbon black.

Some surprising observations and findings have led to this very general applicability of the method: If finely divided synthetic resins in the form of dispersions or solutions are brought into contact with silver nitrate in the form of a solution or in solid, finely comminuted form, individual silver ions adhere to the surface or in cavities of the large plastic molecules. This sticking occurs even with "dry" fabrics due to the surface moisture. As has been shown, the silver ions apparently migrate into the molecular cavities.

If the silver ion is subsequently reduced in accordance with the invention, the plastic surprisingly shows semiconductor properties: it now has an albeit high specific resistance with a strongly positive temperature coefficient. As the silver content increases, they become progressive coherent silver metal layers are formed, whereby the temperature coefficient of resistance shifts more and more to that of silver.

Additions of graphite and carbon black to achieve conductivity are known. In the plastics produced according to the invention, they should increase an existing conductivity and, when using the plastics as a resistance material or as a heating foil and the like, should achieve better heat dissipation. Accordingly, graphite addition is only an additional measure and is not part of the method according to the invention. The fact that embedded conductor particles, e.g. Graphite, not having to touch when a conductive plastic is used, such a composite is not only mechanically more resistant, but it is also the conductivity independent of mechanical or thermal stress. However, it is possible to make the electrical resistance direction-dependent by aligning the conductor particles.

In the process according to the invention, plastics are obtained which are free of ions. As has been shown, plastics that contain ions have only a low resistance to aging when exposed to electrical currents. An ion conduction will suddenly occur, apparently due to the effects of moisture, which can lead to momentary destruction of the plastic.

Surprisingly, it has been shown that not only customary polymers such as polystyrene, polyvinyl resins, polyacrylic acid derivatives and copolymers thereof, but also polyamides and their derivatives, polyfluorocarbons, epoxy resins and polyurethanes can be made electrically conductive by the process according to the invention. It shows that in addition to the electrical conductivity, other properties also change noticeably in some cases. The use of the process according to the invention is to be explained in more detail by the following formulation examples:

Example 1:

• 1470 parts by weight dispersion of fluorocarbon resins 55% in water
• 1 part by weight of wetting agent
• 28 parts by weight of silver nitrate solution 10%
• 6 parts by weight of chalk
• 8 parts by weight of ammonia
• 20 parts by weight of carbon black
• 214 parts by weight of graphite
• 11 parts by weight of hydrazine hydrate

Example 2:

• 1380 parts by weight acrylic resin dispersion 60% in water
• 1 part by weight of wetting agent
• 32 parts by weight of silver nitrate solution 10%
• 10 parts by weight of chalk
• 12 parts by weight of ammonia
• 6 parts by weight of carbon black
• 310 parts by weight of graphite
• 14 parts by weight of hydrazine hydrate

Example 3:

• 2200 parts by weight of distilled water
• 1000 parts by weight of styrene (monomeric)
• 600 parts by weight of ampholyte soap (15%)
• 2 parts by weight of sodium pyrophosphate
• 2 parts by weight of potassium persulfate
• 60 parts by weight of nickel sulfate
• 60 parts by weight of sodium hypophosphite
• 30 parts by weight of adipic acid
• 240 parts by weight of graphite

The graphite is only added after the hot polymerization, which results in a conductive, sprayable plastic after granulation and drying.

FIG. 2 shows how the individual resistance tracks 3 to 7 and 8 are coated with the elastomer 16. The power supply lines 12 are also coated with the elastomer 16.

In order to ensure uniform heat dissipation from the surface heating element 1, the cross section 18 of the resistance tracks 8, 9, which run perpendicular to the power supply lines 12, is smaller than the cross section 19, the resistance tracks 3 to 7 running parallel to the power supply lines 12 electrical voltage from one power supply line 12 to the other power supply line 12 avoided with high security.

FIG. 3 shows a surface heating element 1 incorporated in a heating unit 20. The heating bandage 20 is designed in the manner of a belt and has connecting surfaces 22 provided with a Velcro fastener in the end regions 21. By pressing the connecting surfaces 22 together, the two end regions 21 adhere to one another, so that the heating bandage 20 can be adapted to different body circumferences or circumferences of limbs or the like. A foam coating 23 has been removed over part of the heating bandage 20, so that the resistance tracks 24, 25 and the power supply lines 26 to 28 can be seen. The power supply lines 26 to 28 run in the longitudinal direction - arrow 29 - of the heating unit 20. A power supply source is connected to the power supply lines 26 to 28 via a feed line 30 which is provided with a plug contact 31. A low voltage in the range between 6 and 42 volts is preferably used to supply the power supply lines 26 to 28 in a heating unit 20.

In Figure 4 it can be seen that the surface heating element 1 is provided on both sides with a foam coating 23. The surface heating element 1 is accordingly between these Foam coatings 23 embedded. A textile coating 32 is applied to the side of the foam coating 23 facing away from the surface heating element 1.

The foam coatings 23 are connected to one another in their end regions by an adhesive or squeezing process, so that the surface heating element 1 is surrounded on all sides by this foam coating 23.

By using a mesh for the surface heating element 1 and a corresponding selection of the foam coating 23, an extremely good vapor diffusion behavior and a good breathing characteristic of the skin can be maintained even when the heating bandage 20 is applied. Sweat formation below such a heating association can thus be eliminated.

FIG. 5 shows the use of a surface heating element 1 according to the invention in a heating pad 33. The surface heating element 1 is surrounded on all sides by a textile-coated foam coating 34. To position the surface heating element 1 within the foam coating 34, the individual layers of the foam coating 34 are connected to one another in the region of the edge 35 and in the central region 36, for example by hot pressing. In order to enable increased heat application in the main radiation direction - arrow 37 - a reflective film 38 is arranged on the side opposite the surface heating element 1 of the main radiation direction - arrow 37. In order not to disturb the steam diffusion behavior of the heating pad too much, this reflection film is provided with small holes or through holes. It is essential that a radiation of the energy against the main direction of application - arrow 37 - is largely prevented.

FIG. 6 shows the course of the energy consumption as a function of time and the course of the temperature in comparison to the power consumption of a surface heating element 1 according to the invention.

As the characteristic curve 39 shows, the power consumption of the surface heating element 1 decreases with increasing time due to the rise in temperature and the consequent increase in resistance in the elastomer 16. This results in a self-stabilization of the surface heating element at a limit temperature that can be set by the elastomer. The temperature curve on the surface heating element with ideal thermal insulation can be seen from the characteristic curve 40 drawn in full lines. The characteristic curve 40 shown in broken lines shows the temperature profile of the surface heating element when heat is emitted, e.g. when used as a heating pad or bandage. The temperature stabilization is reached at approx. 50 °.

As shown with a characteristic line 41 shown in dash-dotted lines, the elastomer can also be adjusted in such a way that it has a non-linear temperature coefficient of the electrical resistance, the kink 42 in the characteristic line 41 indicating the sudden increase in resistance after this limit temperature has been reached. This causes a sudden increase in the resistance in the elastomer and a lowering of the power consumption, so that after a relatively short heating-up time the temperature in the surface heating element is stabilized quickly. This characteristic curve also represents the temperature profile when the surface heating element emits heat.

FIG. 7 shows a section through a surface heating element 43 which is formed by a network 44. The base material from which this mesh 44 is made consists at least in part of an electrically conductive elastomer 45. The mesh threads 46 and 47 form power supply lines 48, while the mesh threads 49, 50, 51 form resistance tracks 52. To produce the difference provided according to the invention Lichen conductivities are incorporated in the network threads 46, 47 a plurality of thread-like carrier materials 53, for example carbon fibers and / or glass or metal fibers. If carbon fibers are used, it is possible to use them both for reinforcement and for power conduction, while when using glass fibers, but possibly also in combination with the carbon fibers, metal fibers, for example made of copper or similar highly conductive material, are achieved to achieve better conductivity. can be embedded. Due to the different number of carrier materials 53 arranged in the mesh threads 46, 47 and 49, 50, 51, a different conductivity is achieved. In order to ensure the conductivity prescribed according to the invention in the mesh threads 49.50 running parallel to the mesh threads 46, 47, fewer thread-like carrier materials are embedded in these mesh threads 49.50 than in the transverse mesh threads 51.

Of course, it is also possible within the scope of the invention for the mesh to be knitted from the filamentary carrier materials, for example carbon fibers, optionally mixed with glass fibers or metal fibers, and for this mesh to be coated with an elastomer 45 with approximately the same coating thickness. However, it is also possible to achieve the different conductivities of the individual network threads by coating with an elastomer of different thicknesses.

As is also shown schematically in FIG. 7, the contact tracks 54 formed from carrier materials 53 can be connected to a low-voltage power source 55 via a connecting line. This current source 55 is preferably connected to a step circuit 56, which enables half-wave operation with a semiconductor diode.

In the context of the invention, the configuration of the foam coatings accommodating the surface heating element 1 or mesh 2 can be freely selected. So instead pure textile materials or other plastic materials can be used Assumption of the panel heating element 1 or network 2, are used. The type of control or the energy supply to the surface heating element or network can also be freely selected within the scope of the invention. In addition to the elastomer provided according to the invention, a thermal protection switch can also be provided for safety reasons in order to switch off overheating of the element in any case.

The use of a plastic net, which is coated with, or consists of, electrically conductive, deep black polymers, offers a number of advantages. This coating on the basis of an electrical semiconductor system provides automatic regulation of the current flow as a function of the temperature. The higher the temperature rises, the lower the current intensity becomes until it is immeasurably small at a certain thermal equilibrium. Burning through overheating is almost impossible due to this self-regulating effect. A medical advantage when using the surface heating element according to the invention as a heating bandage results from the fact that the emissivity of a body of temperature T for radiation of wavelength λ is equal to its absorption capacity for this radiation. Since the network is designed as a black body and thus has the greatest possible absorption capacity, a black body can also emit radiation of all wavelengths. Since, according to Wien's law of displacement, the wavelength of the most intense radiation λ is maximally proportional to the temperature, the intensity of the emitted wavelength increases with decreasing temperature. The radiation shifts more and more to the invisible infrared (heat radiation). Since the temperature of the heating bandage is around 300 ° K and the heat source can be seen as an almost black body, the radiation takes place in its essential spectral range in the infrared. Compared to visible light, this infrared radiation penetrates deeper into the body and even at low temperatures there is the same feeling of warmth as at higher temperatures. structures in a different radiation area. The deeper penetration of the infrared radiation and the stronger resonance of the biochemical macromolecules caused by the longer wavelength is the cause of the physiological effects, which means, for example, that there is no reddening of the skin even when the heating bandage is used for a long time, even when there is a great deal of heat. The normally occurring heat accumulation on the skin is therefore eliminated in a heating bandage using the surface heating element according to the invention.

A surface heating element 57 is shown in FIG. A carrier 58 is e.g. from a fabric 59 made of plastic. Contact sheets 60 are applied or woven into this fabric mat or into this fabric mat. These contact tracks 60 serve as power supply lines 61 and are formed, for example, by thread-like carrier materials, for example silver-coated carbon fibers, copper threads or lanyards made of such materials. An electrically conductive plastic with a positive temperature coefficient of electrical resistance is applied to the support device 58. The fabric parts between the contact tracks 60, together with the conductive plastic 62, serve as a resistance track that heats up when current is passed. Due to the positive temperature coefficient of the electrical resistance of the conductive plastic 62, the temperature rises rapidly with full power consumption, as a result of which the resistance in the plastic 62 increases, the power drops and a temperature stabilization is established. Only the amount of energy that is emitted to the outside by the surface heating element is supplied to the surface heating element 57 so that the temperature is kept constant in the desired range. Of course, it is possible to design the fabric 59 of the support device 58 in such a way that the fabric or the individual threads of the fabric which run perpendicular to the contact tracks 60 have a higher conductance than parallel to the contact tracks 60.

The shape of the support device 58 can be adapted to any application. So it is possible to use strip-shaped sheets, but also plates and molded parts with any outer circumference as a surface heating element.

Claims (12)

1. surface heating element, in particular for bandages or heated blankets, with a positive temperature coefficient and a plurality of power supply lines arranged at a distance from one another, characterized in that the power supply lines (11, 48) are formed by contact tracks (12, 54) and are operatively connected to a flat support device, and that the contact tracks (12, 54) and the carrying device (58) are provided with an electrically conductive plastic, in particular an elastomer (16, 45) with a positive temperature coefficient of the electrical resistance. 2. surface heating element according to claim 1, characterized in that the supporting device, e.g. a net (44), at least in part made of an electrically conductive plastic, e.g. an elastomer (45) with a positive temperature coefficient of electrical resistance, and a thread-like carrier material (53), e.g. Carbon fibers, metal threads or the like, are incorporated as contact tracks (54) and or or reinforcing inserts in the carrying device or the net (44). 3. surface heating element according to claim 1 or 2, characterized in that the network (2) consists of flexible thread-like carrier material (10,53) and the contact tracks (12,54) formed by flexible lanyards (13), and preferably parallel to a longitudinal fiber direction of the network (2,44) are arranged. 4. Surface heating element according to one of claims 1 to 3, characterized in that the thread-like carrier material (10, 53) of the carrying device or the network (2.44) forms resistance tracks (3 to 9, 52) and consists, for example, of polyamide or polyester and that the perpendicular to the Kon resistance paths (8,9,52) running in clock cycles (12, 54) have a higher conductance than the resistance paths (3 to 7, 52) running parallel to these. 5. surface heating element according to one of claims 1 to 4, characterized in that the thread-like carrier material (10,53) is formed by silver-coated carbon fibers. 6. Surface heating element according to one of claims 1 to 5, characterized in that the resistance tracks (5.52) of the network (2.44) running parallel to the contact tracks (12.54) have a smaller cross section (19) than that perpendicular to the contact tracks (12) running resistance tracks (9,8,52). 7. surface heating element according to one of claims 1 to 6, characterized in that the perpendicular to the contact tracks (12,54) resistance tracks (9,8,52) of the network (2,44) and the elastomer applied to this (16, 45) are made elastic in the longitudinal direction of the resistance tracks (3,9). 8. surface heating element according to one of claims 1 to 7, characterized in that the network (2,44) with the elastomer (16,45) is coated such that openings (17) remain between the individual resistance tracks (3 to 9). 9. surface heating element according to one of claims 1 to 8, characterized in that the network (2,44) is coated with an elastomer (16,45), which has a non-linear temperature coefficient of electrical resistance with a characteristic curve, preferably in the range of body temperature, having. 10. surface heating element according to one of claims 1 to 9, characterized in that the elastomer, in particular its resistance, is designed for connecting the contact tracks to a low-voltage current source (55) and that preferably the current source (55) with a step circuit (56) for half-wave operation is equipped with a semiconductor diode. 11. Surface heating element according to one of claims 1 to 10, characterized in that at least on one side of the network (2) a e.g. partially laminated foam coating (23, 34) is arranged. 12.Surface heating element according to one of claims 1 to 11, characterized in that on one side of the network (2) a heat reflection layer, e.g. a reflective film (38) or the like is arranged.

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