Electrical resistance heating element

Electrical resistance heating element comprising at least one heating conductor in the form of an element which is provided at opposite surfaces with an electrical contacting means and is made of a material having a positive temperature coefficient of electrical resistance (PTC element), and further comprising two substantially plane contact plates, the contact plates being held together with the PTC element interposed between them to form a single unit acting as a contact arrangement, and a sleeve of electrically insulating, heat conductive material having an inner space whose internal cross-section is adapted to the cross-section of the contact arrangement, the contact arrangement being inserted into the inner space in the sleeve under a pressure perpendicular to the plane of the contact plates. The sleeve consists of elastomeric material. With a view to achieving good and uniform heat dissipation the contact plates project a considerable distance beyond the PTC element, a spacer web portion of the sleeve being provided between them to form guide ducts into which the contact plates are inserted. These ducts may be formed to diverge in the direction of insertion of the contact arrangement. At the outer end of the sleeve, connecting leads are bonded to the ends of the contact plates and these are embedded in a holding element of insulating material. This element may be formed as a plug which closes and seals the sleeve when the contact arrangement is inserted; alternatively it may be cast into the outer end of the sleeve after insertion of the contact arrangement.

The invention relates to an electrical resistance heating element, 
comprising at least one heating conductor in the form of an element which 
is provided with electrical contacting means at opposite surfaces and 
which is made of a material having a positive temperature coefficient of 
electrical resistance (PTC element), said heating element also having two 
substantially plane contact plates, the contact plates being held together 
with the interposed PTC elements as a single structure or assembly, to 
constitute a contact arrangement. 
In electrical heating appliances, more particularly electrically heated 
domestic appliances such as beverage-preparation machines (e.g. coffee 
making machines), fan heaters, hair dryers and so on, resistance heating 
elements whose heating conductors are one or more PTC elements are being 
used to an increasing extent instead of conventional resistance heating 
elements whose heating conductors consist of a metal wire. Such PTC 
elements are usually of prismatic form, with two opposite plane parallel 
surfaces and round or polygonal in plan, usually consist of a ceramic 
material, more particularly with a barium titanate base, and have the 
property that they are more-or-less self-stabilising in their electrical 
power consumption, since the electrical resistance increases sharply in a 
specific temperature range. Electrical connection to these PTC elements is 
usually achieved by providing the two opposite surfaces with electrical 
contacts in the form of a suitably applied metallised layer and connecting 
these to a current supply line by means of connecting elements. 
In the case of PTC elements the electrical power consumption and thus the 
attainable heat emission depends to a substantial extent on heat 
dissipation and therefore on the existance of a low thermal resistance 
level towards the components or media which are to be heated. If there is 
high thermal resistance the PTC element passes into the transition 
temperature range in which the considerable increase in electrical 
resistance occurs at a low power consumption level. Thus the detrimental 
results of overheating are in fact avoided, but the heating power remains 
unsatisfactory. It follows that low thermal resistance is a desirable aim. 
It is known (cf. German laid-open specification No. 26 14 433) 
corresponding to U.S. Pat. No. 4,147,927 to insert PTC elements into a 
tubular sleeve and to make the connection with elastic bars which are used 
for heat dissipation at the same time. It is also known (cf. German 
published specification No. 26 41 894 corresponding to U.S. Pat. No. 
4,104,509) to connect the PTC elements with connection wires, for example 
by soldering, and to embed them in a housing with a heat-conducting, 
electrically insulating compound. These known measures can be carried out 
only with relatively considerable outlay and with considerable use of 
manual work, are suitable only for a very limited range of use in each 
case, and leave much to be desired more especially as regards discharging 
heat from the PTC elements. 
Starting from the state of the art described above, it has already been 
proposed (cf. German Patent application No. P 28 45 965.6-34 and British 
Patent Application No. 2033709 corresponding to commonly assigned U.S. 
application Ser. No. 086,175, filed Oct. 18, 1979), as initially 
indicated, to provide as connection elements two substantially flat 
contact plates and to form a sandwich type of contact arrangement from 
these contact plates with the PTC element (or several PTC elements) 
interposed between the plates, said contact arrangement being held 
together elastically. The contact plates are adapted to the plan shape of 
the PTC elements, and are held together by holding elements placed on the 
edges. The unified structure thus obtained ensures that such a contact 
arrangement can be manipulated without difficulty, but it does not ensure 
the application pressure between contact plates and PTC elements which is 
necessary for heat transfer. This pressure, which is necessary for 
operational reasons, is applied only when the arrangement is inserted in 
an appliance, by arranging for suitable appliance parts to clamp the 
contact arrangement. 
An object of the present invention is to develop an electrical resistance 
heating element of the type initially indicated and discussed 
hereinbefore, to form a self-contained unit which can be used in a very 
wide range of applications, which guarantees optimum heat shedding from 
the PTC element independently of further measures when fitted, and can be 
manufactured in a simple and inexpensive manner. 
According to the present invention there is provided an electrical 
resistance heating element, comprising at least one heating conductor in 
the form of an element which is provided at opposite surfaces with an 
electrical contacting means and is made of a material having a positive 
temperature coefficient of electrical resistance (PTC element), and 
further comprising two substantially plane contact plates, the contact 
plates being held together with the PTC element interposed between them to 
form a single unit acting as a contact arrangement, and a sleeve of 
electrically insulating heat conductive material having an inner space 
whose internal cross-section is adapted to the cross-section of the 
contact arrangement, the contact arrangement being inserted into the inner 
space in the sleeve under a pressure perpendicular to the plane of the 
contact plates. 
The sleeve proposed according to the invention is, as the term implies, 
closed circumferentially and comprises an interior space which is open at 
at least one end for the insertion of the contact arrangement. The outer 
surface of the sleeve can be substantially optionally shaped in accordance 
with mounting or fitting conditions in an appliance, more particularly 
being given a cylindrical shape in one constructional form as a "heating 
cartridge" for example, in which case the inner space extends parallel to 
the axis of the cylinder. Since the widths of the contact plates and the 
PTC element are normally substantially identical, the cross-section of the 
contact arrangement is substantially rectangular and correspondingly the 
inner space usually has an internal cross-section of a rectangular shape, 
which is substantially constant over the length of the inner space. In the 
width direction the inner space and contact arrangement are so dimensioned 
that the said arrangement can be pushed-in without difficulty. On the 
other hand, as will be clear from the foregoing, as regards the height of 
the inner space and contact arrangement the adaption is such as to bring 
about pressure perpendicularly to the plane of the contact plates, this 
pressure ensuring that the contact plates abut tightly on the PTC element 
with low transfer or contact resistance for current and heat. 
The pressure application force is taken up by the sleeve, and on the other 
hand the contact plates are pressed with their full surface area against 
the wall of the inner space, so that good heat transfer to the outside of 
the sleeve results. The pressure is produced by giving the inner space 
slightly smaller dimensions in the sense of height relatively to the 
contact arrangement, the amount of undersize depending substantially on 
the compressive strength of the PTC elements and the elastic properties of 
the sleeve material, and the necessary amount can be determined by simple 
experiments. After the insertion of the contact arrangement the inner 
space can be filled up additionally by casting into it an electrically 
insulating, heat conductive material such as, for example, heat conductive 
silicone rubber, to obtain optimum heat dissipation from the side surfaces 
of the PTC element also. 
Of course it is also possible to provide the PTC element in a composite 
form by arranging a plurality of PTC elements in series, behind one 
another and/or adjacent to one another (side by side) between the contact 
plates. 
Current is supplied with the use of connecting leads which can be secured 
to the rear ends of the contact plates (considered in the insertion 
section) by soldering, spot welding, terminal clips or the like. 
As will be clear from the foregoing, according to the invention the 
pressure between contact plates and PTC element is produced by the 
elasticity of the sleeve. Particularly advantageous adaption can be 
achieved is the sleeve consists of elastomeric material, for example heat 
conductive silicone rubber. In every case the sleeve can consist of a 
length of tubular extruded material of suitable cross-section, and used 
open at both ends or subsequently closed at the end. But preferably the 
sleeve is cast or moulded as a part closed at one end. 
In order to facilitate insertion of the contact plates can be given a 
slightly wedge-shaped form. But in this and other respects it is 
particularly advantageous to use a constructional form wherein the contact 
plates are constructed as heat-emitting surfaces, project beyond the PTC 
element in the longitudinal direction at at least one side, and are 
completely enclosed by the sleeve. With the contact plates (which in every 
case should consist of material which is a good conductor of heat such as 
copper or aluminium) are constructed in this way, the heat transfer from 
the PTC element to the sleeve is substantially improved, and thus also the 
heat transfer from the outer surface of the sleeve to the heat-using 
device or medium. Moreover if the contact plates project beyond the PTC 
element at least at the front side considered in the insertion direction, 
they can be introduced into the inner space in a wedge-shaped formation 
inclined relatively to one another. As a result, insertion requires the 
application of only slight force, and yet a high pressure is achieved in 
the inserted state, in that the contact plates abut over the whole surface 
on the PTC element because of the elasticity of the sleeve material. 
Preferably the contact plates project beyond the PTC element at least to 
an extent corresponding to the length of the said element. 
A further advantageous form is characterised in that the sleeve comprises a 
spacer web extending axially within the inner space, and a guide duct for 
one of the contact plates is formed in each case between the respective 
neighbouring inner wall of the sleeve and the opposite side of the spacer 
web, one duct for each plate. Spacer web and guide ducts are so 
dimensioned that the portions of contact plates projecting beyond the PTC 
element abut on the spacer web and on the sleeve wall with elastic 
application pressure, with further improvement of the heat transfer. The 
spacer web can be formed in the sleeve in a simple manner, when the sleeve 
is being cast or moulded, integrally with the sleeve and starting from the 
closed sleeve end, and preferably merging into the sleeve wall at the 
sides. In other words, in this constructional form the sleeve comprises, 
after the inner space, a solid end portion in which the guide ducts are 
formed. 
It is also advantageous in the case of this constructional form to make the 
guide ducts diverge in the direction of insertion of the contact 
arrangement. This results in the front ends of the contact plates being 
opened out from one another on entry into the guide ducts, so that as a 
result of the rear ends of the contact plates, between which the PTC 
element lies, are pressed together because of the lever effect. This 
measure results in an overall better heat transfer between the contact 
plates and the sleeve and uniformity of heat transfer over the length of 
the contact plates. 
The manipulation of the contact arrangement, more particularly in the 
insertion operation, can be simplified by embedding the rear ends of the 
contact plates, considered in the insertion direction, in a holding 
element made of electrically insulating material, preferably likewise heat 
conductive silicone rubber. At the same time the holding element provides 
tension relief for the connection leads connected to the contact plates. 
Preferably the holding element is so constructed that it closes and seals 
the inner space of the sleeve when the contact arrangement is inserted.

A "cartridge" type of electrical resistance heating element is shown in the 
Figures. It comprises substantially a sleeve 1 of heat conductive silicone 
rubber and a contact arrangement 2 which is inserted into the interior of 
the sleeve 1. The contact arrangement 2 is assembled from two flat contact 
plates 3 made of copper and a PTC element 4 which is interposed between 
the contact plates 3 and is provided at its surfaces facing towards the 
contact plates 3 with an electrical contacting layer or other contacting 
means (not shown). The contact plates 3 are of the same width as the PTC 
element substantially--apart from a light amount of oversize (see FIG. 2). 
The PTC element 4 is supplied with current by way of the contact plates 3 
by means of connection leads 5 soldered or otherwise bonded to the ends of 
the contact plates 3. 
The sleeve 1, cast or moulded from heat conductive silicone rubber, is 
closed at one end and comprises an inner space 6 of rectangular 
cross-section (see FIG. 2) which is open towards the other end and into 
which the contact arrangement 2 is inserted. In the width direction the 
inner space 6 allows the contact arrangement 2 a certain amount of 
clearance. In the vertical direction at right angles to the plane of the 
contact plates, on the contrary, the inner space 6 is so dimensioned that 
in the fitted state the contact arrangement 2 is pressed together because 
of the elasticity of the sleeve 1. Externally the sleeve 1 is of circular 
cylindrical shape. 
As FIG. 1 shows, the contact plates 3 project beyond the PTC element at the 
front side in the insertion direction (arrow 7) by more than twice the 
length of the said element. The length of the contact plates 3 thus 
corresponds approximately to the length of the sleeve 1, so that a good, 
uniform heat transfer over a considerable surface area is achieved. 
Between the portions of the contact plates 3 which project beyond the PTC 
element 4 there engages a spacer web 8 which is integrally formed on to 
the closed end of the sleeve 1 when the said sleeve is cast or moulded, 
and the said web projects into the inner space 6, merges into the wall of 
the sleeve 1 laterally, and leaves free between itself and the wall of the 
sleeve 1, in the height direction, guide ducts 9 into which the contact 
plates 3 engage. The guide ducts 9 are given narrower dimensions than the 
contact plates 3, so that in the inserted state the spacer web 8 and the 
wall of the sleeve 1 abut with elastic pressure on the contact plates 3, 
so that good heat transfer is ensured. The guide ducts 9 may be formed to 
diverge slightly in the insertion direction 7, as previously mentioned 
herein; this feature is not illustrated in the drawings as it can be 
readily visualized having regard to the advantageous effect thereof 
previously mentioned herein. 
At the outer end (the rear end in the insertion direction 7) the contact 
plates 3 are embedded in a holding element 10 of heat conductive silicone 
rubber which at the same time acts as a strain relief means for the 
connection leads 5. The holding element 10 is so dimensioned that it 
closes and seals the inner space 6 in the fitted position (shown in FIG. 
1) of the contact arrangement 2. 
If a holding element 10 closing the inner space 6 is not provided, after 
the contact arrangement 2 has been inserted the remaining space inside the 
sleeve (shown in FIG. 2.) can be filled with an electrically insulating, 
readily heat conductive, casting material 11--again, for example, heat 
conductive silicone rubber.