PTC-Heizzelle

Abstract

A PTC heating cell with a cuboid PTC element (2) having main side surfaces that each form the largest surface area of ​​the PTC element (2), wherein the main side surfaces are opposite each other and extend parallel to each other, and two electrically separated metallizations (6) provided for current introduction into the PTC element (2), characterized in that the two metallizations (6) are provided on each of the main side surfaces (4) of the PTC element (2) as strips (6) extending over the entire longitudinal extent (L) of the PTC element (2) on diagonally opposite surface sections of the PTC element (2), that each of the strips (6) has a width (b) of less than one quarter of the width of the PTC element (2), and that two U-shaped contact rails (10) extending in the longitudinal direction (L) of the PTC element (2) and each associated with a metallization (6) are provided.which have spring projections (16) formed in one part, which bear against the metallization (6) under elastic preload and clamp the PTC element (2) and the associated metallization (6) between them.

Description

The present invention relates to a PTC heating cell having a cuboidal PTC element and metallizations which are electrically separated from one another. Two metallizations are provided on the surface of the ceramic PTC element for current introduction into the PTC element.Such a PTC heating cell is generally known. In the conventional PTC heating cells, which are used in the field of air conditioning of motor vehicles in question here as elements of a motor vehicle heating device for heating air or a liquid medium, the metallization is usually located on opposite main side faces of the PTC element. This main side surface is the largest surface of the PTC element. In the case of a cuboidal PTC element, the main side faces are situated opposite one another and extend parallel to one another. The current path through the PTC element is accordingly effected at right angles to the main side faces and in the thickness direction of the PTC element.If a voltage is applied to a PTC element, this heats up. With increasing voltage, the ohmic resistance of the ceramic PTC element must be increased in the aforementioned metallization of the ceramic PTC element which is generally customary in the prior art. In addition to the PTC range of the characteristic curve, it also exhibits an NTC range. This has adverse effects on low temperature operation. The shape of the characteristic curve is voltage-dependent. The NTC behavior increases with a higher specific resistance of the PTC element. The voltage dependence (varistor effect) increases with a smaller number of grain boundaries between the individual electrodes. The grain boundaries are defined by the individual ceramic grains which compactly form the PTC element.Further relevant prior art is described in the following documents: DE 695 32 131 T2, DE 10 2017 208 086 A1, DE 101 18 599 B4, EP 3 334 242 A1, CN 1 06 060 977 A and DE 10 2011 081 830 A1.The present invention seeks to provide a PTC heating cell which is compact and has as good heating and heat emission behaviour as possible.To solve this problem, the present invention proposes a PTC heating cell having the features of claim 1. This has two metallizations which are formed over the entire longitudinal extent of the PTC element, namely as relatively thin strips. The strips are located on diagonally opposite surface sections of the PTC element. The strips are usually designed as strictly rectangular strips. The strips are usually located on opposite major side surfaces but are diagonally offset. Each of the individual strips is associated with a specific polarity. In addition to the aforementioned strips, no metallization is usually provided for the current introduction on the surface of the PTC element. The electrodes, which are formed as strips, cause diagonal flow through the PTC element. The current path through the PTC element is accordingly substantially longer than in a conventional construction in which the current passes through the PTC element in the direction of thickness. Thus, it is possible to provide a PTC heating cell with a relatively thin construction, in which the current passes through a maximum number of grain boundaries. The current path is also longer than a current path when contacting the PTC element on opposite end side surfaces which connect the main side surfaces. This is because, in the case of diagonally opposite electrodes, the current path is composed of the width and additionally the thickness of the PTC element.The thickness of the PTC element can be reduced to up to 0.9 mm at voltages of up to 800 V. With this thickness, the risk of breakage due to thermal stress is ruled out. The thickness of the PTC element (without metallization) is preferably in a range from 0.9 mm to 2 mm.Without impairing the thermal performance of the PTC element, PTC heating cells and thus electrical heating devices for motor vehicles comprising such heating cells can thus be formed which are of very compact construction. In this case, the voltage dependence of the characteristic curve can be reduced on account of the relatively long current path. At a higher operating voltage, lower specific resistances result, whereby the NTC effect is reduced.The PTC element according to the invention is accordingly suitable in particular for high-voltage applications, especially for the formation of electric heating devices for electromobility. Thus, the vehicle electrical system voltage available for the drive can also be used directly for air conditioning the vehicle and in particular the interior or for heating technical components of the electric vehicle. Voltages with up to 1,000 VDC, generally between 600 and 1000 VDC, are considered to be a high voltage range. The control of the PTC element can be effected by PWM. This pulse width modulation is usually set in a high kHz up to the MHZ range.It is understood that the aforementioned advantages can only be utilized if the strip has a width of less than half the width of the PTC element. The same applies to the ratio of the thickness-wise extension of the metallizations to the thickness of the PTC element. In order to achieve an at least 50% percent varistor effect, i.e. a switching-on current which is less by 10%, the ratio B / b or D / d must be at least two, with B=width or D=thickness of the cuboidal PTC element and b=width or d=thickness of the electrode strip, and the ratio should particularly preferably be between 3 and 6.The electrode strips preferably extend in the width direction as far as the adjacent edge of the cuboidal PTC element. Thus, the electrode strips are at the greatest possible distance from one another.The strips should also be attached to an edge of the cuboidal PTC element in the longitudinal direction. The strips preferably extend from edge to edge and accordingly over the entire length of the PTC element.In order to contact the electrode strips, according to a preferred development of the present invention, a respective contact rail is provided for each of the metallizations. This contact rail is usually formed from a metal, particularly preferably from a sheet metal material. The contact rail usually forms, at its free end projecting beyond the PTC element, a connection lug for the plug-in contact of the PTC element. The terminal tab is the male element of an electrical plug connection. The contact rail can be glued or soldered to the metallization.According to the invention, the contact rail engages around the PTC element and the associated metallization. The contact rail is for this purpose U-shaped in cross section. This U-shaped contact rail has two opposing legs which receive the PTC element between them, and a web which connects the two legs. The two legs clamp the PTC element between them, wherein the metallization of the electrode is provided on one side between the ceramic material of the PTC element and the contact rail. There, the power current is introduced alone into the PTC element. The uncoated surface region of the ceramic PTC element has a resistance in the M ohm range. This part thus remains stress free, even if no further insulating material is provided between the corresponding limb of the U-shaped rail and the opposite ceramic surface. Thus, the PTC element can be clamped in the U-shaped contact rail in a relatively simple manner.With a view to a secure, permanent clamping which also gives rise to certain manufacturing tolerances, the spring rail according to the invention has spring projections which are formed integrally thereon and which bear against the metallization under elastic prestress. Usually, corresponding spring projections are provided on both opposing legs, so that the spring projections abut on both sides against the PTC element or the metallization. It is understood that usually a plurality of spring projections are provided one behind the other in the longitudinal direction in order to distribute the clamping force with respect to the PTC element uniformly in the longitudinal direction.According to a further preferred embodiment of the present invention, an overmoulding made of an insulating plastic is provided. This injection molding is produced by placing the PTC element with the metallization and the contact rails provided thereon in an injection mold in which the contact rails are injection molded. The overmolding surrounds the contact rails, but otherwise leaves heat-emitting surfaces of the PTC element free. The insulating plastic forming the encapsulation accordingly completely seals the contact rails. Only the connection lug can protrude through the encapsulation and be exposed on the outer side of the PTC heating cell for the electrical connection thereof. The main side surfaces directly adjacent to the contact rails and the metallization are not encapsulated by the plastic material by injection molding and remain free. This is because these main side surfaces serve for dissipating the heat generated by the PTC heating cell. The end side surfaces of the cuboidal PTC element which extend at right angles thereto and are not contacted or engaged around by the contact rails can form the outer surface of the PTC heating cell or else be provided with insulation which is formed by the injection-molded plastic.For the overmolding, it is preferable to round the edges of the PTC element surrounded by the overmolding. There, during the injection molding, a radius and no sharp edge is regularly provided.According to a further preferred embodiment of the present invention, surfaces of the PTC element which are not provided with the metallization, in particular the main side surfaces of the PTC element which are opposite one another and on the edge of which the metallization is provided, are covered with an insulating layer. Against this insulating layer, usually a heat-emitting element in the form of a corrugated fin is directly applied if the aforementioned heating cell is the heat-emitting element of an air heater with a layered structure, which is formed by the heating cell and corrugated fin layers bearing against it on both sides. The insulating layer can also enclose all free surfaces of the ceramic PTC element. The insulating layer usually encloses the PTC element in such a way that an external medium cannot reach the current-carrying parts of the heating cell. Formed in this manner, the PTC heating cell can also be inserted as a heating fin into a heating chamber of a liquid heater, which is separated from a connection chamber, in which the connection lugs of the heating cell can be exposed, via a partition wall, in order to connect said connection lugs to a connection for the power current. The injection molding can form a sealing collar which is traversed by the regions of the contact rail extended beyond the edge of the PTC element, which regions form the connection lug on the side opposite the PTC element. This sealing collar can be accommodated in a sealing manner in a plug contact receptacle of a partition wall which separates the heating chamber from the connection chamber, as is known from EP 3 334 242 A1. Thus, the heating cell can be used per se, with appropriate insulation, for heating a heating fin of the previously known electric heating device in a motor vehicle.With regard to a better fastening of the insulating layer, it is proposed according to a preferred development of the present invention that a longitudinal edge of the insulating layer, which is provided on a main side surface of the PTC element, is overlapped by the contact rail and / or the encapsulation and is thus positively incorporated into the PTC heating cell. In the case of an injection molding, it is possible to seal the insulating layer via the injection plastic.The present invention is explained schematically below with reference to an exemplary embodiment in conjunction with the drawing. In these show: FIG. 1 shows a perspective end view of a PTC heating cell; FIGS. 2 aand 2 b show two variants for contact rails for electrical contacting of the exemplary embodiment; and FIG. 3 shows a sectional view of the edge region of the exemplary embodiment with an injection molding.FIG. 1 shows a cuboidal PTC element 2. the longitudinal direction is denoted by L, the width direction by B and the thickness direction by D. Mutually opposite main side faces 4 span the width direction B and the longitudinal direction L. Strip-shaped metallizations 6 are provided at the edge of each main side surface 4. These extend in the longitudinal direction L from edge to edge. In width direction B, metallizations 6 each abut the edge that laterally delimits main side surface 4. The metallization 6 has a rectangular cross section and is applied to the ceramic main side surface 4 by sputtering. The two metallizations 6 are of identical design and are associated with opposite longitudinal edges. The polarities of the metallizations 6 are marked with + and -, respectively. Arrows within the PTC element 2 illustrate the current path through the PTC element 2. in the exemplary embodiment shown, the width b of the strip 6 corresponds to approximately 20% of the width B of the PTC element 2.The contacting with the power current takes place via two respective contact rails 10, which are shown by way of example in FIGS. 2 aand 2 b. The respective contact rails 10 have a U-shaped cross section with mutually opposite legs 12 which are connected to one another via a web 14. Spring projections 16 formed by punching and bending protrude from the inner surfaces of the legs 12. In the exemplary embodiment according to FIG. 2, these form ramp surfaces which rise in the longitudinal direction of the contact rail 10 and thus in the longitudinal direction and thus make it possible to introduce the PTC element in the direction of the arrow P. In the exemplary embodiment according to FIG. 2 b, the ramp surfaces are provided at right angles thereto, so that the PTC element 2 can be inserted between the legs 12 in the width direction. The legs 12 abut with their spring projections 16 against the PTC element 2. The spring projections resting against the metallization 6 serve for the introduction of the power current via the contact rail 10, whereas the opposite leg 12 serves only for the mechanical fastening. Since there is no metallization and thus an electrode on the surface of the ceramic PTC element 2, there is no introduction of the power current.FIG. 3 shows a sectional view for a variant in which an insulating layer 18 in the form of a ceramic plate or plastic film is placed on the region of the main side surface 6 not provided with the metallization 6. The edge region of the insulating layer 18 extending in the longitudinal direction L is overlapped by the legs 12 of the contact rail 10 and is accordingly mechanically secured. FIG. 3 also illustrates an overmolding, denoted by reference numeral 20, made of an insulating plastic material, which overlaps the contact rail 10 and an extended edge region of the insulating layer 18 and seals the contact rail 10. Thus, the outer surface of the insulating layer 18 forms the heat-emitting outer surface of the PTC heating cell. This can be used directly as a heating element in an electric heating device of a motor vehicle for heating a liquid or gaseous medium.List of reference characters2 PTC element 4 main side surface 6 metallization / strip 10 contact rail 12 leg 14 web 16 spring projection 18 insulating layer 20 overmolding L longitudinal direction b width B width direction D thickness direction

Claims

A PTC heating cell having a cuboidal PTC element (2) which has main side faces which each form the largest surface of the PTC element (2), the main side faces being situated opposite one another and extending parallel to one another, and two metallizations (6) which are electrically separated from one another and are provided for the introduction of current into the PTC element (2), characterized in that the two metallizations (6) are provided on each of the main side faces (4) of the PTC element (2) on diagonally opposite surface portions of the PTC element (2) as strips (6) formed over the entire longitudinal extent (L) of the PTC element (2), each of the strips (6) has a width (b) of less than a quarter of the width of the PTC element (2), and two U-shaped contact rails (10) are provided which extend in the longitudinal direction (L) of the PTC element (2) and are each assigned to a metallization (6) and have spring projections (16) formed integrally thereon, which rest against the metallization (6) under elastic prestress and clamp the PTC element (2) and the assigned metallization (6) between them.PTC heating cell according to claim 1, characterised in that the strips (6) extend in the width direction (B) as far as the adjacent edge of the cuboidal PTC element (2).PTC heating cell according to claim 1 or 2, characterised in that the strips (6) extend in the longitudinal direction (L) from edge to edge of the cuboidal PTC element (2).PTC heating cell according to one of the preceding claims, characterized byan encapsulation (20) made of an insulating plastic, which surrounds the contact rails (10) and leaves heat-emitting surfaces (4) of the PTC element (2) free.PTC heating cell according to one of the preceding claims, characterized in that surfaces (4) of the PTC element (2) which are not provided with the metallization (6), in particular main side surfaces (4) which are opposite one another and on the edge of which the metallization (6) is provided, are covered with an insulating layer (18).PTC heating cell according to Claim 5, characterized in that the contact rail (10) and / or the encapsulation (20) engages over a longitudinal edge of the insulating layer (18).

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