Abstract:
The present invention relates to a PTC heating element with at least one PTC resistance element which is arranged between two electrically conductive plates in a housing opening of a housing and, with at least one of the electrically conductive plates as an intermediate layer, which is pressed with an initial tension against a heat-emitting element which is held at the housing. With the present invention, a PTC heating element of the type mentioned at the beginning that can be manufactured easily and economically is to be specified. To solve this problem, the present invention further develops the PTC heating element mentioned at the beginning in such a way that attachment tabs arranged on the edge of the housing opening protrude beyond the housing opening and in such a way that the heat-emitting element has tab cuts, behind which the tabs engage.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a PTC heating element with at least one PTC resistance element that is arranged between two electrically conductive plates in a housing opening of a housing and that, with at least one of the electrically conductive plates as an intermediate layer, is pressed with an initial tension against a heat-emitting element that is held by the housing.  
         [0003]     2. Description of the Related Art  
         [0004]     Such a PTC heating element is known, for example, from EP 0 350 528, which is traced back to the applicant. In this state of the art, a number of PTC heating elements are arranged in a frame in a number of planes with heat-emitting elements, in the form of metal strips curved in a meandering manner, layered between. Located within the frame is a spring, which presses the two electrically conductive plates lying on the exterior of the PTC heating element against the PTC element for good thermal and electrical contacting and the heat-emitting elements against the adjacent electrically conductive plate for good heat transfer of the heat generated by the PTC resistance element. In the mentioned state of the art, the PTC heating element is used for heating air. There are, however, other embodiments also known in which the PTC resistance element heats a heating plate. Such embodiments are particularly known for heating or keeping warm foods, for example, baby food.  
         [0005]     The problem forming the basis of the present invention is to specify a heating element of this category that can be manufactured simply and therefore economically.  
         [0006]     To solve this problem, the present invention specifies a PTC heating element with the characteristics of Claim  1 . This differs from the category-defining state of the art by a housing opening, whereby attachment tabs that are arranged around the edge of the housing opening protrude from it and whereby the heat-emitting element has tab cuts behind which the tabs engage when a heat-emitting element is mounted on the housing.  
         [0007]     As a result of this engaging, the heat-emitting element is connected to the housing in a simple manner, for example, by being locked in place. Alternatively, the tabs can also have a thickening on their free end that engages behind the tab opening, whereby this thickening is formed by surface-fusing the respective tab. Such thickened tabs are achieved, for example, by hot stamping.  
         [0008]     The tabs, however, serve more than just to attach the heat-generating element to the housing. Rather, the tabs encircle the housing opening and protrude from the exterior of the housing opening. In this way, the tabs form a guide that forms a guide of the housing in the insertion direction of the electrically conductive plates and the at least one PTC resistance element, as a result of which the assembly is simplified. In addition to this, the tabs can be formed with a funnel-shape on the upper free end, in order to facilitate simple insertion of the two strip conductors with the at least one PTC resistance element arranged between them.  
       OBJECT OF THE INVENTION  
       [0009]     The tabs of the PTC heating element according to the invention accordingly take on a double function. On the one hand, because of their arrangement relative to the housing opening, they allow a simple, guided insertion of the electrically conductive plates together with the at least one PTC resistance element. Furthermore, they securely hold in position the heat-emitting element after the final assembly.  
         [0010]     According to a preferred further development of the present invention, the tabs project beyond an upper contact surface for the heat-emitting element, whereby this contact surface is formed by the housing and is at a height that is dimensioned in such a way that the electrically conductive plate that is raised above the contact surface by the initial tension before the placement of the heat-emitting element does not extend higher than the upper end of the tabs. In this way, it becomes possible to fix in place during the assembly the layer composition that emits the heat and that is held in the housing opening. Preferably the tabs are dimensioned in such a way that the upper electrically conductive plate that is raised above the contact surface is roughly at the same height as the upper end of the tabs. With a view to the most sparing use possible of material for forming the housing and tabs, these should project beyond the upper electrically conductive plate only slightly in the raised position.  
         [0011]     For further simplification of the PTC heating element, according to a preferred development of the present invention it is proposed that the electrically conductive plate lying opposite the heat-emitting element, i.e., that electrically conductive plate that is provided on the side other than the heat-emitting element with respect to the PTC resistance element, be provided with at least one spring element that is formed by means of stamping processing and by bending the sheet metal material that forms the electrically conductive plate as a single piece on this sheet metal material and that is supported on the housing. The spring element is preferably cut out from the sheet metal material that forms the electrically conductive plate in such a way that it initially projects beyond the outside of the contact surface of the at least one PTC resistance element at the corresponding electrically conductive plate. The projecting section is then preferably bent under the electrically conductive plate at the back of the contact surface, and it can be supported on the housing with this spring leg, whereby this housing is preferably closed on its underside lying opposite the housing opening. In the context of the present invention, a cut that opens towards the contact surface for the heat-emitting element is particularly understood as a housing opening, whereby the size of this cut roughly corresponds to the dimensions of the PTC resistance element(s) held in the housing. In other words, the housing opening, when seen from the top, has roughly the contour of the PTC resistance element(s) of the PTC heating element.  
         [0012]     With a view to secure and simple localisation of plug sockets connected with cables to an electrical current source for the electrical connection of the PTC heating element, according to a further preferred development of the present invention it is proposed that the housing be given two plug holders, each of which has a locking opening for engaging with a snap-in pin formed on the plug socket, whereby a contact stud formed by stamping processing and bending the electrically conductive plate is arranged in the plug holder and consequently electrically connected in a permanent and simple manner to the plug socket undetachably secured in the housing by means of the locking connection.  
         [0013]     For simple securing of the two electrically conductive plates in the housing, it has proven advantageous to guide each contact stud to the respective electrically conductive plate out of the housing through a contact stud cut that is cut outside the housing opening on the underside in a direction at right angles to the plane of the electrically conductive plate, and by deflection into the plug holder, to bring it into an alignment in which the contact stud runs essentially parallel to the electrically conductive plate and is inserted into the plug holder. In this further development, only that cut in the housing that serves to hold the PTC resistance element(s) is understood as the housing opening. Consequently, a housing opening formed with the area of a single PTC resistance element can, for example, be cut into the housing. The electrically conductive plates have a contact section lying on the PTC resistance element, whereby a connection section that has been punched free protrudes on its exterior circumferential surface. This connection section initially lies in the plane of the sheet metal strip that forms the electrically conductive plate. A sub-area of the connection section, i.e., the front end of the connection section forming the contact stud, is then bent by 90°, so that the contact stud essentially extends from the plane of the sheet metal material at a right angle. In this state, the electrically conductive plate, with its contact section, is introduced into the opening of the housing. The contact stud here penetrates the contact stud cut that is cut into the underside of the housing and protrudes beyond the housing from the exterior. Subsequently, the protruding section is reshaped, for example, by a slide, and introduced into the contact stud holder that runs essentially parallel to the contact surface of the contact section, i.e., parallel to the bearing surface for the heat-emitting element. After this reshaping step, the respective strip conductor is held securely in the housing. The connection section of the electrically conductive plate is preferably surrounded relatively tightly by walls of the housing for this, and consequently held in a fixed position in the housing.  
         [0014]     Further advantages and details of the present invention result from the following description, in conjunction with the drawing. This drawing shows: 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1 a  top view of the upper side of an embodiment of a PTC heating element with the heating plate removed;  
         [0016]      FIG. 2 a  perspective top view of the underside of the embodiment shown in  FIG. 1 ;  
         [0017]      FIG. 3 a  longitudinal sectional view along the line III-III according to the depiction in  FIG. 1  with the heating plate positioned; and  
         [0018]      FIG. 4 a  longitudinal sectional view along the line IV-IV according to the depiction in  FIG. 1  with the heating plate positioned. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]     The embodiment depicted in the drawing is a PTC heating element for heating baby food or keeping it warm, and this PTC heating element is held inside an injection moulded housing (not shown) which forms the holder for the container holding the baby food (bottle, jar). The PTC heating element shown in the drawing is located in the base of this injection moulded housing. This, with its heating plate  2 , forms the support for the container that holds the baby food and that stands directly on the heating plate  2 .  
         [0020]     The embodiment shown in the drawing has a housing  4 , which essentially consists of three separated segments. Consequently, the housing  4  has a holder section  6  for holding a heat-emitting layer composition comprising a lower electrically conductive plate  8 , a PTC heating element  10  arranged in between with an essentially circular shape and an upper electrically conductive plate  12  (cf.  FIG. 3 ). For holding this layer composition, the holder section  6  has an essentially round opening  14  that is only slightly larger than the layer composition. In this layer composition, the lower electrically conductive plate  8  and the upper electrically conductive plate  12  each have a round contact section  16  formed according to the contour of the round PTC element.  
         [0021]     On opposite sides, electrically conductive plate receiving chambers  18 ,  20  are provided that fork off through the housing  4  from the opening  14  and that open towards the opening  14 , whereby the bases of these can be raised slightly with respect to the base of the opening  14 .  
         [0022]     The housing  4  forms two plug holders  22 ,  24 , in addition to the holder section  6 . These plug holders  22 ,  24  are covered with respect to the upper side of the PTC heating element shown in  FIG. 1  by a covering  26  that is located below a contact surface  28  for the heating plate  2 , whereby this contact surface  28  is formed by the upper side of the housing  4 . This contact surface  28  runs essentially across the entire contour of the housing  4  along the edge.  
         [0023]     Each of the plug holders  22 ,  24  is formed for holding one plug socket  30 . The plug socket  30  essentially has a rectangular cross-section with flat upper and undersides  32 ,  33 , as well as a snap-in pin  34  that projects from the upper side  32  (cf.  FIG. 3 ). Corresponding to this, each of the plug holders  22  and  24  has a locking opening  36  that is cut into the interior side of the covering  24  or  26  in the shown embodiment, without penetrating the respective covering  24 ,  26 .  
         [0024]     As the section view according to  FIG. 3  shows, the exterior base  40  of the plate receiving chambers  18 ,  20  is at the same height as the underside  42  of the housing  4 . In extension of the edges of the plug holder  22 ,  24 , contact bars  43  form the underside  42  of the housing  2 . Between the base  40  and the underside of the covering  24 ,  26 , the housing  4  forms a limit stop  44  for the plug socket  30 . Below this limit stop  44 , the cut that is formed by the plug holder  22 ,  24  and that is on the underside of the housing  2  is penetrated up to the back end of the same. In other words, the plug holder  22  or  24  continues beyond the cut  46  and until the rear end of the housing  4 . A contact stud holder  48  that, as a slot, connects the base  40 , exposed at the bottom, of the housing to the interior of the holder section  40 , opens towards the cut  46 , and is provided outside the opening  14  and in extension of the plug holders  22 ,  24 .  
         [0025]     The plug holders  22 ,  24  are bordered on the bottom by a partition wall  50  that is provided in the front area of the plug holder  22  or  24  in the insertion direction of the plug socket  30  and that connects the two contact bars  43  of the respective plug holders  22 ,  24 . Behind this, in the insertion direction, each of the plug holders  22 ,  24  has a contact stud insertion opening  52  that, for reasons of injection moulding manufacture of the housing  4 , is formed with the width of the plug holder  22  or  24  (cf.  FIG. 2 ).  
         [0026]     As can be seen in  FIG. 1 , three tabs  54  project beyond the contact surface  28 , whereby these three tabs  54  are provided on the edge of the round interior circumferential area of the opening  14 . The tabs  54  can be provided as a single piece with the housing  4  by means of injection moulding.  
         [0027]     Alternatively, when the housing  4  is manufactured by injection moulding, tab cuts can be cut out and the tabs  54  can be inserted into these. In this development, it is, for example, possible to form the tabs with protruding springs on their underside ends, whereby these protruding springs are supported in the interior on the base of the housing opening  14  and press against the lower electrically conductive plate  8 , in order to press it, together with the PTC resistance element  10  and the upper electrically conductive plate  12 , against the heating plate  2  after assembly.  
         [0028]     In an alternative development, which is realised in the shown embodiment, the lower electrically conductive plate  8  has protruding springs  56  formed in a single piece with it (cf.  FIG. 4 ). These protruding springs  56  are initially formed by cutting free the sheet metal material forming the lower electrically conductive plate  8  and, in a following manufacturing step, bending them under the underside of the lower electrically conductive plate  8 , so that protruding springs  56  provided on the interior of the base  40  in a single piece with the lower electrically conductive plate  8  are formed. Preferably, a number of protruding springs  56  are provided on the circumference of the round contact section  16  and from there guided under the lower electrically conductive plate  8 .  
         [0029]     The tabs  54  project beyond the contact surface  28  with a height that depends on two factors: firstly, the tabs serve to hold the pre-mounted layer composition. Due to the force of the protruding springs  56 , the layer composition is pressed against the interior side of the heating plate  2  after the assembly, for good heat transfer to the heating plate  2 . This means that before the final assembly, i.e., before the attachment of the heating plate  2  to the housing  4 , the layer composition protrudes beyond the contact surface  28 . The tabs  54  provided around the opening  14  and distributed around the circumference here ensure that the upper electrically conductive plate  12  remains within the circumferential surface of the opening  14 , even in this raised position.  
         [0030]     Furthermore, the heating plate  2  has tab cuts  58  corresponding to the tabs  54 , whereby these tabs  54  engage behind these tab cuts  58  after the heating plate  2  is positioned on the contact surface  28 . For this purpose, the tabs  54  can have snap-in projections that engage behind the tab cuts  58 . In the embodiment shown, the tabs  54  are formed with a thickening  60  by means of hot stamping after the assembly of the heating plate  2 , whereby this thickening  60  protrudes beyond the tab cut  58 . Accordingly, the housing  4  and the heating plate  2  are undetachably connected to each other by means of the thickenings  60  of the tabs  54 .  
         [0031]     For the electrical connection of the lower and upper electrically conductive plates  8 ,  12 , these have connection sections  62  that go off from the contact sections  16  at opposite sides. Each connection section  62  can be subdivided into a contact stud base  64 , which is cut out by means of stamping processing and which runs in a radial direction with respect to the contact section  16 , and a contact stud  66 , which runs perpendicular thereto. The contact stud base  64  lies within the electrically conductive plate chambers  18  or  20 ; the contact stud  66  is electrically connected to the plug socket  30 .  
         [0032]     For assembly, first the contact stud  66  is bent out of the plane of the sheet metal material at the transition to the contact stud base  64  by 90°. A fold  68  formed in this way is dimensioned in such a way that it lies in an extension of the slot-shaped contact stud holder  48  when the electrically conductive plate  8 ,  12  is introduced into the opening  14 . During the assembly, i.e., during the insertion of the layer composition into the opening  14 , the contact studs  66  are accordingly inserted through the contact stud holders  48 . Subsequently, the linear section of the contact studs  66  that protrudes beyond the base  40  is turned by 90° on to the respective plug holder  22  or  24  by a slide, so that the front end of the contact stud  66  is deviated through the contact stud insertion opening  52  and into the plug holder  20 ,  24 . This reshaping step normally takes place after the heating plate  2  has been attached to the housing  4  and consequently the layer composition is held in the opening  14  with respect to the height due to the initial tension of the protruding springs  56 . In this case, the contact stud  66  can be positioned between the contact stud holder  48  and the limit stop  44 , essentially against the base  40 , without there being any grounds for fearing the loss of the desired good heat transfer due to a flat fit of the upper electrically conductive plate  12  and the underside of the heating plate  2 . After the bending of the contact stud  66 , this then runs essentially parallel to the upper or underside  42  of the housing  4 , is essentially held in place with respect to the height by the base  40  and, due to the guide within the contact stud holder  48 , is fixed in place across the width within certain limits and secured in such a way that the plug socket  20  can be slid on to the contact stud  66  and consequently electrically contacted, without it being possible for the contact stud  66  to avoid the frictional forces acting here.