DE20006599U1 - Heating device with electric heating elements for water beds - Google Patents

Description

Patent Attorney Dipl.-Ing. Lambert Eichelbaum

D-45659 Recklinghausen Krüppeleichen 6

Telephone: (02361) 21091-2 Fax: (02361) 22949

30.03.2000 of 2607a/99

Applicant:

VONTANA Waterbeds GmbH

Karlstr. 31 - 35, D - 45739 Oer-Erkenschwick

"Heating device with electric heating elements for waterbeds"

•♦

Page 2 II &iacgr; . &iacgr;I'..'...&diams; I ^3<>-03.2000

.-ii "&iacgr;-&idigr;* *&idiagr;&idigr;* 2 *&idigr;· ^; * "vtT 2607a/99

Description:

The invention relates to a heating device with electric heating elements for waterbeds, which is arranged between a bed frame and a safety foil and regulates the temperature of a metal plate which is attached under the safety foil of a waterbed core lying thereon, wherein the heating elements are thermally connected to the underside of the metal plate in a flat housing via a highly thermally conductive layer.

In a heating device of this type according to DE 195 08 315 Cl, the electrical heating elements consist of electrical conductors made of a pasty mixture of particles of precious metals, such as gold, silver or ruthenium, as well as ceramic components, such as glass and aluminum oxides, which are baked in the form of a hybrid conductor loop on a ceramic plate made of an aluminum oxide substrate. These electrical conductors generate heat when current flows through them, which is transferred to the metal plate of the housing of this heating device via the ceramic plates as heat exchangers and via the highly heat-conducting layer. The ceramic plates do not function as heat generators, but only as pure heat exchangers of the heat generated by the electrically baked resistance conductor loops when current flows through them.

i»:.. &iacgr;::3&ogr;&iacgr;.03.2000 I *&udigr;' *νγ* 2607a/99

Heat. For this purpose, the heat-conducting layer with which the ceramic plates are glued to the underside of the metal plate is made of an adhesive with high thermal conductivity. An NTC sensor is arranged on the underside of the rigid metal plate, while the current conductors leading to the burned-in resistance wire conductors are connected to a TRIAC, which is also attached to the underside of the metal plate. Both the NTC sensor and the TRIAC are connected to a &iacgr;&ogr; control device.

According to DIN-EN 60335-2-66 from February 1996, the temperature on the surface of a waterbed heater must not exceed 60 ⁰ C and the temperature of the surface of the waterbed mattress must not exceed 37 ⁰ C. Furthermore, the temperature increase on the surface of the waterbed heater must not exceed 125 ⁰ C.

The waterbed heater described above in accordance with DE 195 08 915 Cl cannot meet these requirements because, on the one hand, the NTC sensor is attached to the metal plate and therefore only measures the temperature of the metal plate and can therefore only regulate the temperature of the metal plate. An NTC sensor of this type operates on the principle that its electrical resistance is smaller the higher the temperature is and vice versa. If the cable breaks, the resistance increases to infinity and simulates a low temperature, which gives the controller the command to heat up.

Page 4 *· * J _t \ JJ ₄ Jj * \ * '3OJ. 03. 2000

.L ^: .V *w* V '-'W 2607a/99

Furthermore, this heating device cannot accommodate different waterbed cores, i.e. waterbed cores with different water volumes, with different frame properties made of metal, foam or wood, as well as different covers, for example a covered and uncovered waterbed core, because the different heat radiation losses for the different

&iacgr;&ogr; the aforementioned conditions cannot be detected by the NTC sensor. Experience has shown that the temperature of the waterbed core deviates considerably from the desired control temperature. Furthermore, since the metal plate is fitted with relatively small ceramic plates on its underside as heat exchangers, which only take up a small area of the metal plate, there are zones of very different temperatures on the metal plate during the heating phase. As a result, it has a considerable temperature ripple on its radiating surface. This temperature ripple refers to deviations in the temperature on the surface of the metal plate in the form of temperature peaks directly above the ceramic conductors and temperature drops between the individual ceramic conductors, which is noticeable during the heating phase.

This in turn means that the NTC sensor transmits a temperature to the controller that is

·

· ■

*U -Vi V.-* I '«■'*&ngr;&ogr;* 2607a/99

actual average temperature of the heating phase, but at best in the stationary state.

In WO 98/36664, a waterbed heater has been disclosed in which a layer with a high electrical resistance is placed underneath the metal plate and an electrical resistance wire heater is arranged on top of it, which in turn is shielded from the housing by a layer with a high electrical resistance.

&iacgr;&ogr; is. This heater has similar disadvantages to the one mentioned at the beginning, namely that, for example, if the metal plate is subjected to an unintentionally high load and bends, the conductor loops can break completely or partially, interrupting the current flow. This is all the more true as there is often a layer of air or another insulating layer made of soft material between the electrical conductor loops and the base of the housing underneath for thermal insulation, which encourages the metal plate to bend. This waterbed heater does not have a protective device that automatically prevents the metal plate from overheating under any circumstances if a control line leading to it fails. It is nothing more and nothing less than a conventional resistance wire heater, known in many variations, with all the known disadvantages.

Page 6 'S!!.! It.:* * I :·3(&03.2000

.'- "it* « * *i."'from 2607a/99

Based on this closest prior art, the invention is based on the object of creating a heating device of the type mentioned at the outset which, while avoiding the aforementioned disadvantages, on the one hand reliably excludes overheating of the metal plate and on the other hand ensures reliable control of the temperatures of the waterbed core with different water volumes and different properties of the frame, be it made of metal, foam or wood, as well as with different room temperatures and covers and thus with different heat transfer conditions.

This object is achieved according to the invention in connection with the generic term mentioned at the outset in that the heating elements consist of several current and heat-conducting metal elements held together in a force-fitting and/or form-fitting manner by current-conducting coupling elements and PTC heating elements clamped between them which generate heat when current flows through them, and that the highly heat-conducting layer between the underside of the metal plate and the heating elements is designed as a double-sided adhesive film layer with good current-insulating properties.

This design creates a waterbed heater with real ceramic heating elements for the first time, i.e. with PTC heating elements that generate the desired heat directly when current flows through it and

Page 7 i : *.&iacgr; ::.:««! ::3&ogr;|.&ogr;3.2&ogr;&ogr;&ogr;

.*.-*.-·.-* *is* * **·'Vo" 2607a/99

not just function as a heat exchanger of a burned-in hybrid resistance-conductor loop, as in the prior art. Since the heating elements now consist of several current- and heat-conducting metal elements held together by force- and/or form-fitting current-conducting coupling elements, the entire underside surface of the metal plate can be heated completely evenly and without any notable waviness.

In addition, the PTC heating elements offer an automatic and therefore self-acting safety function, because their electrical resistance increases steeply at a temperature of, for example, 90 ⁰ C and approaches infinity at a temperature of 100 ⁰ C, so that the flow of current through the metal elements is interrupted and overheating is therefore excluded.

In an advantageous development of the invention, the metal elements consist of aluminum or copper profiles with a large mass and thus also a large storage capacity. The PTC heating element is advantageously made of barium carbonate, titanium oxide and other additives, while the film layer consists of a permanently elastic heat-conducting film filled with an acrylate adhesive.

The coupling element is advantageously formed by a spring-loaded contact clamp, which connects two metal elements in phase with the PTC heating elements arranged between them in a force-, current- and form-fitting manner.

Page 8 JI * « &iacgr; II.I. * I * I 3(3.03. 2000

2607a/99

coupled with each other. In this way, an extremely compact overall heating element is created using simple means, which can dispense with any adhesive layers arranged in between. To simplify the coupling of the contact clamps, the metal elements are provided with dovetail-shaped recesses on their side facing away from the current-insulating and heat-conducting foil for the positive engagement of the contact clamps.

&iacgr;&ogr; According to an advantageous development of the invention

The heating device has a total of four metal elements running parallel to each other with a total of six PTC heating elements arranged between them below the metal plate and the foil layer as an overall heating system.

According to a particularly advantageous development of the invention, the heating device is controlled by two NTC sensors, the first of which is arranged outside the area of the metal plate in an outer area of the housing in such a way that it is in direct measuring contact with the waterbed core and the second of which is arranged in a known manner below the metal plate. As a result, the second NTC sensor always measures the temperature of the metal plate and the first NTC sensor always measures the temperature of the waterbed core. Both NTC sensors are connected to a microcomputer, which constantly evaluates both temperatures and controls the temperature of the metal plate in such a way that the water temperature of the waterbed core is at the desired

Page 9 I . &iacgr;&iacgr;&iacgr;*&iacgr;* « &iacgr; *. 13CJ. 03. 2000

' '*.* Ve* 2607a/99

This ensures that the desired water bed core temperature is not only reached but also kept constant, regardless of the size of the water volume, the completely different nature of the water bed frame, whether it is made of metal, foam or wood, and regardless of the room temperature and the covering conditions of the water bed core. The micro-computer is designed in such a way that

&iacgr;&ogr; set so that the surface temperature of the metal plate does not exceed 60 ⁰ C. Furthermore, if one or both NTC sensors or a line leading to them breaks, the microcomputer ensures that a malfunction is excluded by means of a plausibility detection and thus both the temperature of the surface of the metal plate and the temperature of the waterbed core are limited to the values permitted by the DIN standard.

An embodiment of the invention is shown in the drawings.

Fig. 1 is a perspective exploded view of a waterbed with a heating device arranged between a padding of the bed frame and the safety foil,

Fig. 2 shows a cross-sectional view of a waterbed with a gap between the upholstery of the bed frame and the

Page 10 "· · · &diams; p.*- _&Lgr; \ *«3&THgr;.03.2000

^< /

..· ·&diams;■ "·.-.* J ¹ Ji''.&ngr;,&iacgr; 2607a/99

Safety film arranged heating device according to Fig. 1,

Fig. 3 shows the enlarged detail III of Fig. 2 of a side area of the waterbed heater.

Fig. 4 is an exploded view of the heating device consisting of the metal plate, with metal elements with PTC heating elements and coupling elements arranged therebetween,

Fig. 5 the bottom view of the heating device with the housing removed,

Fig. 6 is a sectional view along line VI - VI of Fig. 5,

Fig. 7 shows the enlarged detail VII of Fig. 6,

Fig. 8 is an equivalent circuit diagram of the heating device of Fig. 4 and 5,

Fig. 9 a diagram of the electrical resistance of a PTC heating element as a function of its temperature,

Fig. 10 the view in the direction of arrow X of Fig. on a controller housing with the micro-computer and

&ngr;» -

Page 11 ; &iacgr;&idigr; * &iacgr;1'.Ai * * &iacgr;&iacgr; 3(p. 03. 2000

,'.'*/ Vi"''α*'νogr; 2607a/99

Fig. 11 is a side view in the direction of arrow XI of Fig. 10.

The new heating device 1 for a water bed 2 is arranged according to Figures 1 and 2 between a padding 3 of a bed frame 4, which can be made of metal, plastic or wood, and a safety foil 5. The controller of the water bed heating is connected to 6, the flexible cables are

Î with 7 and 7a and the plug is designated with 8. The waterbed core 9 is placed on the tub-shaped safety film 5. The tub-shaped safety film is designed to catch the entire water content of the waterbed core 9 if it leaks. The waterbed core 9, the safety film 5 and the padding 3 are wrapped in an upper cover part 10, which is connected to a lower cover part 10a of the cover 10, 10a by means of a not shown and circumferential zipper at 10b.

(see Fig. 2).

In Fig. 2, parts that correspond to Fig. 1 are designated with the same reference numbers. The waterbed heater 1 is intended to heat the water 9a in the waterbed core 9 to the desired temperature. This water 9a is subject to completely different heat losses through radiation and through heat conduction, depending on whether, for example, there is an upper bed on the cover top 10 or not, or depending on whether the

Page 12 *t *. j &iacgr; j &iacgr; · · * &diams; 3<f. 03.2000

-'■··■.' * ^: *&idigr;**"&Ggr; * *ie*'XÖ 2607a/99

Frame 4 is made of wood, metal or foam. All previously known waterbed heaters only measure the temperature of this metal plate 11 with an NTC sensor 13, which is attached to a metal plate 11. Consequently, such an NTC sensor 13 can only regulate the temperature of this metal plate 11. This NTC sensor 13 cannot influence the different radiation and heat dissipation losses of the water 9a within the waterbed core 9.

&iacgr;&ogr; take.

According to Fig. 4, the heating device 1 according to the invention consists, from bottom to top, of a plastic housing 14 that provides good heat insulation and, if possible, reflects the downward heat upwards by means of a coating 14b, e.g. made of aluminum or a vapor-deposited chrome layer, of four metal elements 15-18 made of extruded aluminum or copper profiles, of a total of six PTC heating elements 19-24 that generate heat when current flows through them and are held together in a force-fitting and form-fitting manner and in a current-conducting manner by current-conducting coupling elements 25, 26. The heating device 1 also includes a layer 27 that conducts heat well and is current-insulating and is arranged between the underside 11a of the metal plate 11 and the metal elements 15-18 and the PTC heating elements 19-24. The coupling elements 25, 26 are either partially or completely surrounded in their central region by a current-insulating layer 28, 28a, so that

Page 13 &Idigr; J J.i * J.!. , J iI30.O3.2OOO

2607a/99

both coupling elements 25, 26 in order to avoid a short circuit, the metal element 16 or 17 located between the ends 25a, 25b of the coupling element 25 and the ends 26a, 26b of the coupling element 26 has no current contact with the coupling elements 25, 26 in this area, but only the in-phase metal elements 15 and 17 on the one hand and 16 and 18 on the other.

According to Figures 3 and 4, the heating device 1 is controlled by two NTC sensors 12, 13, of which a first 12 is arranged outside the area of the metal plate 11 on an outer area 14a of the housing 14 in such a way that it is in direct measuring contact with the waterbed core 9 via the safety film 5, whereas a second NTC sensor 13 is in measuring contact with the underside 11a of the metal plate 11 via the film 27 in a manner known per se. The coupling elements 25, 26, which are designed like leaf springs at least at their ends 25a, 25b and 26a, 26b, hold both the metal elements 15-18 and the six PTC heating elements 19-24 together in a force-fitting and form-fitting manner and in a current-conducting manner. For this purpose, the ends 25a, 26b and 26a, 26b engage in dovetail-shaped grooves 29 of the metal elements 15-18 in a form-fitting and force-fitting manner. This means that adhesives or other connecting means can be dispensed with in order to ensure excellent heat transfer from the heat-generating elements when the current surge flows through.

Page 14 "&iacgr;&idiagr; S · ! 1 !.** . S * &diams; 3CJ. 03 . 2000

* * ' **** &iacgr; '.i'V 2607a/99

PTC heating elements 19-24 to the metal elements 15-18.

Figures 5 and 6 show the heating device 1 according to Fig. 4 in the assembled state with the housing 14 removed. Parts that correspond to Fig. 4 are designated with the same reference numbers. Figures 5 and 6 show a strain relief area 14c and Î power cable 7a below the metal plate 11 as well as the earthing connection 11b for the metal plate 11. Parts that correspond to Fig. 4 are designated with the same reference numbers. This also applies to Fig.

From this Fig. 7, the leaf spring-like character of the ends 25a, 25b and correspondingly also of the ends 26a, 26b of the coupling elements 25, 26 is evident, as well as the fact that after a single clipping of these ends 25a, 25b, 26a, 26b into the dovetail-shaped grooves 29 of the metal elements 15-18, automatic release is no longer possible. With the spring preload of the ends 25a, 25b of the coupling element 25 and the corresponding preload of the ends 26a, 26b of the coupling element 26, both the metal elements 15-18 and the PTC heating elements 19-24 are connected with appropriate force and thus with good heat and current conductivity.

Fig. 8 shows an equivalent circuit diagram of the heating device 1 of Figures 4 and 5. The

Parts that correspond to these figures are designated with the same reference numerals. A current path 30 is electrically connected to the coupling element 26 and a further current path 31 is electrically connected to the coupling element 25.

Via the current path 30, the alternating current flows in both directions via the end 26a of the

coupling element 26 into the metal element 16 through the two PTC heating elements 19, 20 into the metal element 15 and from there via the end 25a of the coupling element 25 into the current path 31.

Once again, the alternating current flows via the current path 30 and the coupling element 26 via its end 26b into the metal element 18, from there through the two PTC heating elements 23, 24 into the metal element 17 and from there via the end 25b of the coupling element 25 into the current path 31.

And finally, a current flows via the current path 30, the end 26a of the coupling element 26 into the metal element 16, from there through the two PTC heating elements 21, 22 into the metal element 17 and from there via the end 25b of the coupling element 25 into the current path 31.

As a result, when an alternating current is applied to the ends 32 of the current paths 30, 31, all PTC heating elements 19-24 are switched on via the coupling elements 25, 26

Page 16 '; *· J. J ··.:.«: j ·3(|. 03.2000

" :\ ^#&bgr; ΢ 2607a/99

and the metal elements 15-18 in both directions of the described current passages through which the alternating current flows, thereby generating heat. The degree of heat generation and the temperature that can be achieved depends on the composition of the PTC heating elements 19-24, which consist of barium carbonate, titanium oxide and other additives. Depending on the composition, the PTC heating element 19-24 can increase its electrical resistance to the flow of current above a certain temperature.

For a specific PTC heating element 19-24, which increases its electrical resistance R from a temperature of approximately 90 ⁰ C, this is shown in Fig. 9. As this diagram shows, from a temperature of approximately 90 ⁰ C, the electrical resistance R of the PTC heating element increases steadily from 10 ³ Ω and reaches a resistance of λ ⁸ Ω at 150 ⁰ C. As the resistance increases, the current flow and thus also the heat generation of the PTC heating elements 19-24 automatically decreases.

Since according to the DIN-EN 60335-2-66 of February 1996 mentioned above, the temperature of the surface - in this case the metal plate 11 - 150 ⁰ C must not exceed, due to the automation of the PTC heating elements 19-24 an automatic and always functioning temperature limitation is created with guaranteed safety, which no longer depends on any control elements.

Page 17 ·· f**· ·**· ·

Furthermore, since the metal elements 15-18 consist of either aluminum or copper profiles of large mass and thus of large heat storage capacity, on the one hand the stationary state of heat transfer of the metal plate is not only achieved very quickly and evenly, but also a current flow with low electrical resistance of the metal elements 15-18 is ensured, whereby the heat generation is left exclusively to the PTC heating elements 19-24.

The good heat-conducting layer 27 between the underside Ha of the metal plate 11 and the PTC heating elements 19-24 and the metal elements 15-18 is provided with good current-insulating properties and advantageously consists of a permanently elastic heat-conducting foil filled with an acrylate adhesive. The permanent elasticity is particularly important in this case so that an air gap that impairs heat transfer cannot arise between the PTC heating elements 19-24 or metal elements 15-18 on the one hand and the underside Ha of the metal plate 11 on the other hand due to embrittlement.

The coupling elements 25, 26, designed as spring-loaded contact clips, couple two metal elements 15-18 in phase with the PTC heating elements 19-24 arranged between them in a force-locking, current-locking and form-locking manner. This eliminates the need for adhesive layers that would otherwise be required, with the associated problems of poorer current conduction due to the

Page 18 *··!··*·! J 5 3(J. 03.2000

&bull;; ·.. ······: ·; _{ν <} 5 2607a/99

electrical resistance of such adhesive layers is unnecessary. In addition, since the springy
Since the contact clamps 25a, 25b; 26a, 26b of the coupling elements 25, 26 are not exposed to dynamic loads, long-term durability is ensured.

The first NTC sensor 12 (see Fig. 3) used to measure the temperature of the waterbed core 9 and the second to measure the temperature of the metal plate 11

The NTC sensors 13 used are connected to a microcomputer 33 in accordance with Figures 10 and 11, which is located in the controller housing 6. This microcomputer 33 continuously evaluates both temperatures and regulates the temperature of the metal plate 11 in such a way that the water temperature of the waterbed core 9 rises to the desired temperature set on the controller housing 6. The microcomputer 33 also ensures that the surface temperature of the metal plate 11 is limited to 60 ⁰ C. The desired temperature can be set using a plus-minus switch 34 on the controller housing 6 on the corresponding temperature scale 35 using a diode light strip 36 and thus without the previously usual slider.

In the event of failure of one or both of the NTC sensors 12, 13 or in the event of a break in a line leading to them, this micro-computer 33 ensures the exclusion of a malfunction by means of a plausibility detection and monitors both the temperature of the surface of the

Page 19. . I i I : . *. I I March 20, 2000

of 2607a/99

·

Metal plate 11 as well as the temperature of the waterbed core 9 are limited.

The PTC elements 19-24 are designed in such a way that in the event of a fault, e.g. in the event of a failure of the electronics or of one of the two NTC sensors 12, 13, a temperature limitation below the maximum permissible temperature of 125 ^° C is automatically given due to the increasing electrical resistance from a certain temperature.

Since these PTC heating elements 19-24 consist of certain ceramic elements, a real ceramic waterbed heater is created for the first time.

Page 20 ". IIII '. . ; ; ;*' J ³ O. 03.2000

List of reference symbols

Heating device 1

Waterbed 2

Lower part 3

Bed frame 4

Security film 5

Control device 6

flexible cables 7, 7a

Plug 8

Waterbed core 9

Water 9a

Cover top

Cover bottom part 10a

Metal plate

Page 21 '''» IIII . &idigr;"&Igr;&idigr;'?0 .03.2000

&iacgr;&igr;&igr; *: t ·*:* * : :: ^ ₀ &igr; β?7a/99

Bottom of the metal plate 11 11a

NTC sensors 12,

Plastic housing 14

Exterior of the housing 14 14a

heat-reflecting layer 14b

Metal elements 15, 16, 17,

PTC heating elements 19, 20, 21,

22, 23,

Coupling elements 25,

Ends of coupling elements 25, 26

current insulating layer

dovetail grooves of the metal elements 15-18

Current paths

Ends of the current paths 30, 31 Micro-computer 33

25a, 25b, 26a, 26b 28, 28a 29 30, 31 32

Page 22 *&iacgr;I'. * I j . ; * ; -03/30/2000

i i 2607a/99

Plus-minus switch 34

Temperature scale 35

Diode light strip 36

electrical resistance R

Claims (14)

1. Heating device with electrical heating elements for waterbeds, which is arranged between a bed frame and a safety foil and regulates the temperature of a metal plate which is attached under the safety foil of a waterbed core lying on top of it, the heating elements being connected in a heat-conducting manner to the underside of the metal plate in a flat housing via a layer which conducts heat well, characterized in that the heating elements consist of several metal elements ( 15-18 ) which conduct electricity and heat and are held together by means of force-fitting and/or positive locking by current-conducting coupling elements ( 25 , 26 ) and PTC heating elements ( 19-24 ) which are clamped between them and generate heat when current flows through them, and that the layer which conducts heat well between the underside ( 11a ) of the metal plate ( 11 ) and the heating elements ( 15-24 ) is designed as a double-sided adhesive film layer ( 27 ) with good current-insulating properties. 2. Heating device according to claim 1, characterized in that the metal elements ( 15-18 ) consist of aluminum or copper profiles of large mass and thus large heat storage capacity. 3. Heating device according to claim 1 or 2, characterized in that the PTC heating elements ( 19-24 ) consist of barium carbonate, titanium oxide and other additives. 4. Heating device according to one of claims 1 to 3, characterized in that the film layer ( 27 ) is made of a permanently elastic heat-conducting film filled with an acrylate adhesive. 5. Heating device according to one of claims 1 to 4, characterized in that the coupling element ( 25 , 26 ) is formed by a resilient contact clamp which couples two in-phase metal elements ( 15 , 17 ; 16 , 18 ) with the PTC heating elements ( 19-24 ) arranged between them in a force-fitting, current-fitting and form-fitting manner. 6. Heating device according to one or more of claims 1 to 5, characterized in that the metal elements ( 15-18 ) are provided on their side facing away from the current-insulating and heat-conducting foil layer ( 27 ) with dovetail-shaped recesses ( 29 ) for the positive engagement of the contact clamps ( 25 , 26 ). 7. Heating device according to one or more of claims 1 to 6, characterized in that a plurality of metal elements ( 15-18 ) with PTC heating elements ( 19-24 ) located between them are arranged almost over the entire surface of the metal plate ( 11 ). 8. Heating device according to one of claims 1 to 7, characterized in that it has a total of four metal elements ( 15-18 ) running parallel to one another with a total of six PTC heating elements ( 19-24 ) arranged between them below the metal plate ( 11 ) and the foil layer ( 27 ). 9. Heating device according to one of claims 1 to 8, characterized in that the heating device ( 1 ) is controlled by two NTC sensors ( 12 , 13 ), of which a first ( 12 ) is arranged outside the area of the metal plate ( 11 ) in an outer region ( 14a ) of the housing ( 14 ) in such a way that it is in direct measuring contact with the waterbed core ( 9 ) and of which a second ( 13 ) is arranged in a manner known per se below the metal plate ( 11 ). 10. Heating device according to claim 9, characterized in that the first NTC sensor ( 12 ) used to measure the temperature of the waterbed core ( 9 ) and the second NTC sensor ( 13 ) used to measure the temperature of the metal plate ( 11 ) are connected to a microcomputer ( 33 ) which continuously evaluates both temperatures and regulates the temperature of the metal plate ( 11 ) such that the water temperature of the waterbed core ( 9 ) rises to the desired temperature which can be set on a controller ( 6 ). 11. Heating device according to claims 9 and 10, characterized in that the microcomputer ( 33 ) limits the surface temperature of the metal plate ( 11 ) to 60°C. 12. Heating device according to one of claims 1 to 11, characterized in that the PTC heating elements ( 19-24 ) are designed in such a way that in the event of a fault, e.g. in the event of failure of the electronics and/or one of the NTC sensors ( 12 , 13 ), a temperature limitation below the maximum permissible temperature of 125°C is automatically provided due to the increasing electrical resistance. 13. Heating device according to one of claims 8 to 11, characterized in that in the event of failure of an NTC sensor ( 12 , 13 ) or in the event of a break in a line leading thereto from the microcomputer ( 33 ), the exclusion of a malfunction is ensured by a plausibility detection and both the temperature of the surface of the metal plate ( 11 ) and the temperature (9) of the waterbed core are limited. 14. Heating device according to one of claims 10 to 13, characterized in that the microcomputer ( 33 ) and the controller ( 6 ) are accommodated in a common housing.