Patent Publication Number: US-3876967-A

Title: Variable resistor

Description:
United States Patent 1191 [111 3,876,967 Hehl et a1. Apr. 8, 1975 [54] VARIABLE RESISTOR 2.218.204 10/1940 Marstcn 338/159 X 2. 5 ,3 5 2 1 Klaus Friedrich Hehk Bemd 2.398.833 7/135? iii/16? Schenk, both of Norderstedh 2.849.350 8/1958 338/16 x Germany 2.873.336 2/1959 Tassara 338/127 3.2 1,722 9 33 AS99139 Tool Works Chmgo- 3.431.133 1/1323 32312331, 8 a1. 333/133? 3.544.946 12/1970 Murakami 338/174 22 i Oct 31 1973 3699.493 10/1972 Oka et 211.  
  3,748,626 7/1973 Maurice 338/184 X 21 Appl. No.: 411,580  
 OTHER PUBL1CAT1ONS Dummea Variable Resistors Radio and Electronic 30 F A t P t I orelgn pphca Da a Components, V01. 11, Pitman &amp; Sons. pp. 79-81.  
 Nov. 4. 1972 Germany 2254085 52 us c1. 338/126; 338/127; 338/159; Reyno&#39;ds 338/161; 338/174; 338/202 [51] Int. Cl. H0lc 9/02 [57] ABSTRACT 1581 gg of 8 532 A variable resistor having a number of windings cong 1 g sisting of flat resistance strips which have been formed 9O by etching orgalvanic deposition is disclosed. The resister has a thermally conductive base and an electri- References Cited fliillyl/xilgznlating layer between the resistance strip and UNITED STATES PATENTS 2.213.078 8/1940 Stoekle 338/159 Clams 29 Draw Fgures 3,876,967 v SHEETBUF? Fig.26  
 IHIIIIIIIIII4 VARIABLE RESISTOR The invention relates to a variable resistor. especially a series resistor for a D.C. series motor, comprising a resistor unit possessing a number of windings, a slider having a contact which can be brought into contact with surfaces of the individual windings. and an actuating device for the slider.  
  Resistors of this type, as for example are used as series resistors for the fan motors of automobiles, are known in many forms of construction. The usual form consists of a wire resistance wound in or on a ceramic body and provided with special fixing elements and connections. The resistance wire is usually of iron or constantan. Resistance wires are also known which are fitted to relatively large ceramic plates. but which have a relatively low specific rating. The ceramic plates normally serve as heat removal plates. In other resistors of this type having a high specific rating, the resistors are often mounted outside the vehicle cabin of an automobile, to facilitate the dispersal of the thermal energy. If the above described resistors are used as series resistors, for example for fan motors, they are normally variable in steps or infinitely variable. This introduces certain problems, especially with regard to the form of the contact points, which are partly quite highly loaded. For this reason, the adjusting part and the load bearing part are frequently separated. It is also a requirement in such series resistors that they shall be as small as possible and as far as possible shall have very low surface temperatures. Their cost must also be as low as possible.  
  Resistors of so-called sandwich construction have also been disclosed, in which a light metal baseplate is laminated with glass fabric impregnated with epoxy resin. A sheet of relatively thin constantan or of other copper nickel alloys is applied onto the glass fabric. Individual resistance strips are then formed from the sheet by galvanic removal.  
  For many applications, the above described resistors can only be considered as partially satisfactory. The type initially described, which are constructed of ceramic bodies and resistance wire, are usually expensive to manufacture. The installation and repair of these resistors also present problems, since riveting or fixing of the ceramic body is difficult, due to its usually low duetility. To keep their size small, relatively thin wires must also be used. This in turn results in relatively high surface temperatures, for example 250C and more. lfiron resistance wires are used, the life of the resistors is adversely influenced by susceptibility to corrosion. Resistance wires made from copper nickel alloys, on the other hand. have a low specific resistance, so that their size is relatively large. Round resistance wire makes contacting difficult. Furthermore, numerous small components are necessary to make such resistors ready for installation. Due to the high surface temperatures, actuators of thermoplastics frequently cannot be used, or special thermal insulators must be provided, in order to reduce the temperature effect upon thermoplastics actuating elements.  
  Resistors made from the sandwich material referred to above possess the disadvantage that the method of manufacturing them is relatively expensive. Since the light metal baseplate serves as heat removal plate, it  
 must be of a certain size, with the consequence that quite large areas often have to be introduced into the etching machines. Since the cost of etching is a function of the surface etched such resistors. of a size to suit the necessary heat removal plate, are relatively expensive. Also, the temperature distribution in such builtup resistors is often unfavourable and local high temperature peaks occur. leading to a risk that the epoxy resins used as adhesive may reach the self-ignition tempera ture range. This is an extreme disadvantage especially when a resistor of this type is used in an automobile, where burning can result in the ignition of other components. A further disadvantage lies in the copper nickel alloy used, which because of its good conductivity is applied as a thin rolled sheet. A sheet of this type possesses firstly insufficient mechanical strength and is moreover extremely sensitive to cracking, tearing and other damage. Finally, the dielectric strength of glass fibre mats impregnated with epoxy resin is relatively low, so that there is a risk of short circuiting between individual points of contact strips.  
  It is the objective of the invention to provide a vari able resistor, especially a series resistor for a DC. series motor, which is of small size, inexpensive to manufacture and of high duty construction.  
  In a resistor of the type initially named, this objective is achieved in that the windings consist of flat strips of chrome nickel steel or another material possessing a high specific resistance. which are formed by etching or galvanic denudation of a suitable metal sheet or by galvanic deposition, and which are connected by an insu lating layer with a thermally conducting base unit. In the resistor according to the invention, the resistance material is first laminated onto insulating material and then etched. A plate made in this way is then applied onto a thermally conducting base unit. This unit may be constructed as a supporting unit, such as a housing, or the thermally conducting base unit may be connected by means of suitable attachment devices to a supporting unit. The individual resistance strips are preferably connected in series, but they may also be connected in parallel. A series circuit has the advantage that when set to maximum resistance value. all the conducting strips are loaded. This results in a uniform temperature loading and distribution. In a parallel circuit, the advantage achieved is that etching errors do not have an additive effect.  
  One form of embodiment of the invention provides that the metal sheet is laminated by means of an adhesive onto the insulating layer, before the resistance strips are etched out. For this purpose, a thermoplastics adhesive possessing high elongation at rupture is preferably used.  
  The chrome-nickel steel which is preferably used constitutes a suitable material for the resistance strips, since it possesses a relatively high specific resistance and good mechanical strength properties. According to one form of the invention, it is provided that the material of the resistance strips contains more than 17 percent chromium and 7 percent nickel. Such a steel is corrosion resistant. so that its resistance is not subject to variations during its operational life.  
  Another form of embodiment of the invention provides that the insulating layer is an insulating mat of fine glass fabric. Such a fine glass fabric mat is preferably impregnated or coated with an adhesive resistant to heat and to disruptive puncture. A fabric mat of this type possesses the advantage that the adhesive coating is uniformly distributed between the thermally conducting base unit and the resistance strips.  
  According to the invention, mica may also be chosen for the insulating layer. In the invention, a composite material of mica and polyamide fibres is to be preferred to mica alone for the insulating layer. An insulating layer constructed in this way possesses relatively good dielectric strength.  
  As initially mentioned, the surface temperature and the temperature distribution in the resistor is of extreme importance for the fixing and the location of fixing. It is for example important to protect adjacent parts, for example cables or components of thermoplastics, from excessive heat emission from the resistor. In this connection, the invention provides that the heat conducting basic unit is a plate and that the heat conducting plate possesses slits, through which the heat distribution in the plate is determined. The mechanically strongest part of the resistance plate, consisting of resistance strips, insulating layer and heat removal plate, constitutes the heat conducting plate, so that the heat removal plate either itself forms the supporting unit, or the resistance plate is attached by means of the heat removal plate to a supporting unit. In addition, the heat removal plate has good heat conducting properties, so that intensive heat conduction can take place, for example towards the fixing. If therefore special precautions are not taken, at least to reduce this thermal conductance, it will not be possible to use thermoplastics or other materials of limited temperature resistance for fixing. By means of slits in the heat removal plate. a particular temperature flow can therefore be achieved. In this connection, the invention also provides that transverse slits are incorporated between attachment points of the heat removal plate to a supporting unit and the resistance strips, these slits reducing heat conductance to the attachment points. On the side towards the winding strips, the attachment points are then sufficiently shielded against heat, even if the resistance strips lie relatively close.  
  For many applications, narrow winding strips are necessary. This can lead to relatively high local temperature loadings, which in turn has the result of softening the thermoplastics adhesive. To counteract this drawback. a further form of embodiment of the invention provides that the resistance strips comprise increases in width at predetermined points. In a further embodiment it is provided for this purpose that the increases in width lie in the regions of smallest radius of curvature of the resistance strips. The widened portions are made by non-etching and appreciably increase the peeling resistance of the adhesive, especially since they constitute a reduced local resistance due to the increase in area and therefore do not become so hot, as the remainder of the resistance strip.  
  In order that the resistor according to the invention can be operated as a variable resistor with a slider, contact strips connected to the winding strips are necessary, it being possible to bring the slider into engagement with these contact strips. In this connection, a fur ther embodiment of the invention provides that the contact surfaces are likewise flat strips of chromenickel steel, which are formed by etching or galvanic removal of the metal sheet or by galvanic deposition and which are connected by means of the insulating layer to the thermally conducting basic unit.  
  The contact strips are separated from one another by a so-called etch seam. The width of the seam determines the disruptive strength and the tendency to spark-drawing when the sliding contact of the slider moves over from one contact strip to another. One form of embodiment of the invention therefore provides that the seam between resistance strips and contact strips widens out upwards starting from the insulating layer. In the resistance strips, the smaller dimension of the seam at the base of the etch results in more effective adhesion of the resistance strips to the insulating layer. In the contact strips, there is the additional advantage that the wear of the sliding contact is appreciably reduced, because the hard edge of the resistance material is appreciably softened by the form of the etch seam.  
  In a further form of embodiment of the invention, it is provided that the thermally conducting basic unit possesses perforations in the region of the contact strips. Where the thermally conducting basic unit is of metal, an appropriate recess in the basic unit beneath these largely prevents short-circuiting to the heat conducting basic unit. Burning through of the insulating layer at this point therefore does not result in notable damage.  
  The contact strips must be so arranged and shaped that, with regard to their size and geometrical dimensions, the desired stepped adjustability of the resistor is ensured without excessive wear, without sparking or without excessive heating occurring. In this connection the invention provides that the slider possesses two sliding contacts spaced apart from each other and that the contact strips are arranged alongside each other and so shaped that, when the slider is moved, only the one sliding contact initially moves over onto the adjacent contact strip, while the other sliding contact still slides upon the previous contact strip. The staggered stepping of the contact strips permits a more favourable changeover from one resistance step to the next, since sparking is to a great extent eliminated.  
  Another form of embodiment for the contact strips provides that the slider possesses at least two sliding contacts spaced apart from one another, and that the contact strips are sub-divided into individual, mutually offset sections, so that in intermediate positions, the one sliding contact is in contact with the contact strip section of the next resistance strip, while the other sliding contact still slides upon the contact strip section of the preceding resistance strip.  
  In this form of embodiment, the resistance strip is connected to at least two contact strips, the offset or staggered arrangement of the contact strips once again appreciably reducing sparking.  
  Another form of embodiment for the construction of the contact strips provides that the individual contact strips are arranged concentrically in arcs radially one after the other and their ends are mutually staggered circumferentially, and that the slider possesses one sliding contact for each contact strip. The contact strips can be varied in any way with regard to their contact width and can be individually suited to the particular current intensities.  
 The arrangement and construction of&#39;the resistor according to the invention can be of any suitable type and the thermally conducting basic unit is a plate, which at the edge is at least partly raised and bent inwards, that the sliding contact of the slider is mounted upon a contact spring, by which the slider is pressed upwards with opposite ends against the inwardly bent edge of the section, which serves as a guide, and that the slider is pivotally mounted upon the resistance plate, constituted of the resistance strip, insulating layer and heat removal plate, Here, the resistance plate with the slider constitutes one unit, since the heat removal plate also acts as the guide element for the slider. With such a form of construction, relatively narrow tolerances are achieved.  
  In this connection the invention further provides that there is mounted upon the resistance plate a holder for a pivotal actuating lever, which can be brought into engagement with the slider arm by means ofa driver. The slider and actuating lever may be arranged and constructed in any suitable manner. In this connection the invention further provides that the pivot axes of the actuating lever and slider are mutually offset. Such offsetting of the two pivotal axes results in a transmission ratio not equal to unity, so that appropriate relationship can be achieved between the actuating range of the actuating lever and the sliding range of the slider.  
  As already mentioned above, the insulating layer supporting the resistance strips can be glued to the thermally conducting basic unit or heat removal plate, for example using a thermoplastics adhesive. For numerous applications, especially where the areas are large, glueing may be accompanied by disadvantages, so that mechanical connection between the insulating layer and the heat removing plate is more appropriate. The invention provides in this connection that projecting sheet noses or lugs are formed from the raised edge section of the heat removal plate, these lugs being bent in wards and pressing the plate consisting of the resistance strips and insulating layer against the heat removal plate.  
  The form and material of the slider can likewise be selected in any suitable manner. One form of embodiment of the invention provides that the slider is formed from a strip of spring material. The strip is first punched out and then plastically deformed, for example to construct the contact spring, so that it is ready for installation. For a slider suitable for incorporation into a resistor. in which it is guided by the inwardly bent edge section of the heat removal plate, the invention provides that at opposite ends facing points are provided, which bear against the underside of the inwardly bent edge section, This provides a conducting path from the slider through the facing points to the heat removal plate, from which the current can be conducted away by suitable means, for example a connecting lug.  
 In a further form of execution of the above-described slider, one form of embodiment provides that, at one end ofthe metal strip, a tongue is punched out, likewise bent downwards by means of plastic deformation and carries at its free end a bearing journal, which is seated in a corresponding recess of the resistance plate. The slider can be pivoted in one plane by means ofthisjournal, the sliding contact on the contact spring sliding over the contact strips, and the contact spring pressing the slider via the facing points against the inwardly bent edge section of the heat removal plate.  
 . When the contact strips are staggered, the slider must possess two or more sliding contacts. For this purpose the invention proposes that when there are two sliding contacts, a second contact arm is provided which is constituted of a punching out from the first contact arm, that the second contact arm is bent downwards by plastic deformation and that the first, longer Contact arm is downwardly in the end region, thus making it shorter. Such a form of construction permits free springing of the contact arms, without mutual interference.  
  As already explained above, a slider constructed in this way is actuated by the driver of an actuator. This must be capable of engaging in a suitable manner with the slider. For this purpose, one form of embodiment of the invention provides that the metal strip possesses a recess, with which the driver of the actuating lever can engage. The latter may also favourably be constructed of a strip, a portion of which is bent downwards to act as the driver.  
  As mentioned above, the thermally conducting base unit can also act as the supporting unit, by means of which the resistor is mounted at its fixing location. In this connection a particular form of embodiment provides that the thermally conducting basic unit is also constructed as supporting unit for installation purposes and that the slider is also actuating lever, out of which a contact spring carrying a sliding contact is formed. This results in one compact unit, distinguished by its especially small dimensions.  
  The supporting unit can be suited to the particular requirements and constructed in any appropriate manner. For this purpose one form of embodiment provides that the thermally conducting basic unit constructed as supporting unit is box-shaped and that the layer of resistance strip and insulating layer is secured to two or more sides of the basic unit. This enables an especially favourable heat distribution to be achieved for the resistor. In a further form of embodiment for this purpose it is provided that the highly loaded resistance strips are mounted upon the vertical sides of the basic unit. The vertical arrangement of the highly loaded resistance strips causes a certain convection effect, which promotes cooling of the resistance strips by air flowing past them.  
  In another form of embodiment, the slider may possess more than two sliding contacts, so that a wider possibility of variation for the switching point of the individual steps is afforded. It is therefore proposed according to the invention that theslider is formed from a metal strip, possessing facing points at opposite ends, which bear against the underside of the inwardly bent edge section, that a number of contact springs are provided extending transversely to the longitudinal axis of the slider and carrying at their free ends sliding contacts, these contact springs being formed as an integral part of a spring metal strip, which is held by the slider. Such a form of construction of the slider is also suitable for resistors, in which the slider constitutes, together with the resistance plate, one unit.  
  The attachment of the spring metal strip to the metal strip can also be carried out in any suitable manner. The contact springs of the slider should as far as possible exert an effective and uniform contact pressure. It is also desirable that the spring metal strip shall be simply mounted upon the strip. A further embodiment of the invention therefore provides that the spring metal strip is furnished with opposed projecting sections, which seat in corresponding recesses of the slider, and  
 that the distance between these recesses is so selected that the spring metal strip is held under a prestress.  
  The spring metal strip, from which the contact springs are integrally formed, may favourably be downwardly convex. It can then be removed out of the recesses, by bending it still further.  
  For numerous applications it is desirable for the supporting unit for the resistor to be of plastics, especially of thermoplastics. Care must therefore be taken that the heat evolution, which occurs when the resistor is in operation, cannot cause damage to the plastics. The invention provides for this purpose a form of construction which is characterised in that the resistance strip, insulating layer and heat conducting basic unit consitute a resistance plate, which is mounted at a distance from a supporting plate, that an actuating lever is pivotally mounted on the supporting plate, and that a contact spring carrying a sliding contact is held by the actuating lever, this contact spring being inserted selflockingly in a hole of the actuating lever. Here, the supporting plate and actuating lever can be made from plastics, while only the contact spring consists of an electrically conducting, spring material. The spring is preferably constructed according to the invention, in that it is joined integrally at its rear end to a contact lug, and that the contact spring is at least partially attenuated in cross-section. A certain thickness is necessary in the contact spring on account of the contact lug but this has an effect upon the resilience. Therefore, reduction in cross section is provided, for example by having one portion of smaller thickness. In order that the resistor slider can be made as inexpensively as possible. it is advantageous for it to be manufactured from simple spring steel. In such a case it is however necessary to construct a special sliding contact. For this purpose it is proposed by the invention that the contact spring shall possess at its free end a slit, which constitutes a web running transversely to the longitudinal axis of the contact spring, upon which a contact element is held as a rocker. The contact, rocker-mounted upon the contact spring. preferably consists of a material of high conductivity. such as bronze, brass or the like.  
  As already stated above. the holder for the actuating device of the resistor can be formed at the resistance plate constituted of the resistance strips, insulating layer and heat removal plate. A particularly preferred form of embodiment provides that the support for the holder is by means of domes, bearing against the resistance strips, that the domes have an internally threaded hole serving to receive a screw passing through a hole in the resistance plate and bearing with its head against the outside of the heat removal plate. Preferably, this internal hole is partially widened outwards leaving a gap between the screw and the wall of the hole. Especially when an insulating washer is additionally used together with the screw, a temperature drop is produced inside the screw towards the thread. ensuring a firm fit of the screw joint. If, as a result of some overload, the thermoplastics housing material flows, complete loosening of the resistance plate does not result therefrom, since the air gap thus produced means that the domes no longer bear and this gap itself acts as an insulator, so that no further temperature flow takes place.  
  The resistance strips must be furnished with a connecting lug, or they may be electrically connected to the heat removal plate. This can be carried out by various ways or means. One form of embodiment of the invention proposes for this purpose that the resistance strips are connected to the heat conducting plate and- /or to a connecting lug by means of a hollow rivet. The hollow rivet, when heated, causes an ascending air flow. resulting in a reduction of temperature at the connecting lug or heat removal plate.  
  Various examples of embodiment of the invention will be described in more detail below with reference to the drawings.  
  FIG. 1 shows a section through a resistance place according to the invention.  
  FIG. 2 shows a diagramatic plan of resistance strips of a resistance plate.  
 FIG. 3 shows a resistance plate with attachment lugs.  
  FIG. 4 shows a plan upon contact strips of a resistance plate.  
  FIG. 5 shows a plan of another form of embodiment of contact strips.  
 FIG. 6 shows a further form of construction of contact strips.  
  FIG. 7 shows in perspective a resistance plate with slider.  
  FIG. 8 shows a section through the arrangement according to FIG. 7, together with a slider actuator.  
  FIG. 9 shows a section through the fixing of the resistance plate according to FIG. 7.  
  FIG. 10 shows a plan view of a slider for a resistance unit according to FIG. 8.  
  FIG. II shows a section through the slider according to FIG. 10.  
  FIG. 12 shows a side view of the slider according to FIG. 10 in contact with a contact strip.  
  FIG. 13 shows a plan upon a slider for contact strips according to FIG. 5.  
  FIG. 14 shows a section through the slider according to FIG. 13.  
  FIG. 15 shows a side view of the slider according to FIG. 13.  
  FIG. 16 shows a plan upon a slider for contact strips according to FIG. 6.  
  FIG. 17 shows a section through a slider according to FIG. 16.  
  FIG. 18 shows a side view of the slider according to FIG. 16.  
  FIG. 19 shows a section through an attachment for a holder at a resistance plate.  
 FIG. 20 shows the attachment of a resistance lug.  
  FIG. 21 shows an electrically conducting connection between the resistance strip and the heat removal plate.  
  FIG. 22 show-s a section through another form of embodiment of a resistance unit.  
  FIG. 23 shows in enlarged section a resistance actuator of a resistor according to FIG. 22.  
  FIG. 24 shows in perspective a contact spring and a contact for an arrangement according to FIG. 23.  
 FIG. 25 shows a plan upon an installed contact.  
  FIG. 26 shows a partial plan upon an attachment lug for a resistance plate.  
  FIG. 27 shows an exploded view of an attachment for the attachment lug according to FIG. 26.  
  FIG. 28 shows a further form of embodiment of the resistor according to the invention.  
  FIG. 29 shows another form of embodiment of the resistor according to the invention.  
  FIG. 1 is a section through a resistor according to the invention, without slider. A resistance strip 8 of chrome-nickel steel is connected by means of an adhesive layer 9 to an insulating layer 10. A heat removal plate 11, consisting of heat conducting material, is attached by means of a further ashesive layer 9a. The resistance strip has been made by appropriate etching of a chromenickel steel sheet, which was previously glued to the insulating layer. The etching produces, between the individual resistance strips, etch seams, one of which is shown at 13 in FIG. 1. The etch seam l3 preferably is of smaller dimensions at the bottom than at the top edge. The construction of the etch seam facilitates maximum bearing surface for the resistance strip 8 upon the insulating layer and thus optimal adhesion and heat removal. In the slider region, the shape of the seam 13 also has the advantage that the sliding contact wear remains small. Furthermore, sparking is avoided. The insulating layer may consist for example of a composite mica material having high temperature resistance. This is also true for an insulating cover plate 12, which is applied onto the resistance strip. The adhesive layer 9a may be omitted, if the insulating layer is laid loosely upon the heat removal plate 11 and the parts are to be joined together by mechanical fixing means.  
  The sandwich construction according to FIG. 1 is so arranged that the dimensions X and X are as small as possible. A small dimension X, facilitates good heat removal via the heat removal plate 11. Also, the selection of the dimension X, is substantially dependent upon the dielectric strength of the insulating layer 10. Below the etch seam 13, a perforation a is indicated, which will be discussed later in more detail with reference to other figures.  
  FIG. 2 shows a diagramatic plan view upon a resistance plate according to FIG. 1. Three resistance strips are shown. 60, 61 and 62, the first ofwhich runs around the perimeter of the plate, while the innermost strip 62 is partially meandering. The resistance strips 60 and 62 are connected to contact strips 26 approximately in the middle of the resistance plate; the contact strips are not shown in more detail here. The resistance strips 60 and 62 can be connected in series or parallel. This also is not visible from FIG. 2. As can be seen, the strips are of different widths. By this construction, the temperature loading can be regulated in accordance with the power or rating. The innermost resistance strip 62 possesses circular widened portions 25, which are especially advantageous when the resistance strips are very narrow and which ensure improved adhesion between the resistance strip and the underlying insulating layer 10. Since the widened portions &#39;25 result in reduced local temperature loading, this means that the thermoplastics adhesive connecting the resistance strip to the insulating layer does not soften. Moreover, the widened portions 25, which are produced by non-etching,  
 appreciably increase the peeling strength of the adheslon.  
  FIG. 3 shows a special construction of a heat removal plate 63, such as shown in section in FIG. 1. The layer 16a of resistance strips (not shown individually) and insulating layer is mounted upon the plane plate 63. This may be done, for example, by lamination in an autoclave. In the vicinity of the slider region 26, which also is not shown in detail, a connecting lug 64 is indicated. This connecting lug 64 constitutes one terminal for the resistor, while the other is connected to the slider, not shown. The heat removal plate possesses, at three sides, fixing lugs 22 with holes 24 for attaching to a supporting unit or the like. Circular arc slits 23 are formed on the side towards the resistance strips around the attachment holes 24, screening the attachment holes against excessive heating.  
  FIG. 4 shows a form of embodiment of contact strips for a slider region 26 (compare FIGS. 2 and 3). A slider illustrated diagrammatically at 1 possesses two sliding contacts 3a and 3b at different radial distances. The sliding contacts 3a and 3b slide upon mutually staggered contact strips 65 and 66 respectively.  
  As can be seen, two contact strips, staggered relative to each other, belong to each resistance strip. This means that the jump of the slider from one resistance strip to the other is again sub-divided, thus reducing the sparking effect when transferring the contacts from one contact strip to the next. The actuation of the slider 1 is limited at both sides by two abutments 27a and 27b. Preferably, only the area of the slider range 26 bounded by the peripheral line 28 is open to engage ment by the sliding contacts, while the remaining portion (not shown) of the resistor as shown in FIG. 1, is covered by an insulating cover plate 12.  
  By suitable formation of the contact strips, they may be arranged to suit the travel-resistance curve. For example the contact width may be varied (g and f). The offset of contact strips 65 relative to contact strips 66 can also be modified (dimension e). The contact strip length can be varied (dimensions a and c) according to the current intensities. A measure of the spark-over resistance is the distance between adjacent and opposite contact strips (dimensions d and b). As already explained above, (FIG. 1), a special shape of the etch seam enables a high degree of adhesion strength to be achieved, with a low tendency to sparking. In order to inhibit disruption towards the heat removal plate in the slider region 26, thechangeover regions between the individual contact strips 65 and 66 may be punched out by appropriate perforations (see FIG. 1). This enables short-circuiting to be largely eliminated. Burning through of the insulating layer, caused by breakdown sparking, does not result in any notable damage.  
  FIG. 5 shows another form of construction of contact strips 66, over which a slider 67 moves. Five contact strips 66, at radial intervals, are provided, mutually offset circumferentially. The associated slider 67 accordingly possesses five sliding contacts, which can be brought into engagement with the individual contact strips 66 successively. Regarding the dimensions a to the same applies as that stated relative to FIG. 4. The dimensions d, to I1 represent the mutual offsetting of the individual contact strips, or the width of the etch seams.  
  In FIG. 6, a contact strip 68 is associated with each resistance strip. The contact strips 68 are of S-shape, so that a slider 69 possessing circumferentially and radially offset sliding contacts 70 and 71 engages an adjacent contact strip, when the slider 69 is pivoted. The contact strips 68 are otherwise ranged in a circular arc, concentrically to the pivot point of the slider 69.  
  In FIG. 7, a plate 16, consisting of an insulating layer with resistance strips glued thereon, is laid upon a metal plate 72, the edges 19 of which are bent through to constitute stiffening. The plate 16 is secured by means of noses or lugs 18 formed from and bent from the edge sections 19. The longitudinal edge sections also possess inwardly bent sections 73 and 74, beneath which the ends of a slider la are inserted, this slider bearing with contact springs on the slider region 75 of the plate 16.  
  FIG. 8 shows the arrangement of the slider 1a more clearly. It possesses two contact springs 76 and 77, which slide with sliding contacts upon the sliding region 75. It also comprises a tongue 32, fitted at its ends with a pivot pin 33, which is mounted in an opening 80 in the plate 16. The contact springs 76 and 77 press the slider la against the inwardly oriented edge sections 73 and 74, the slider Ia bearing with contact points 29 against the edge sections 73 and 74. The arrangement shown of slider 1a and resistance plate with bent edge sections constitutes one integral unit, which ensures extremely small tolerances.  
  Domes or studs 17 are secured by screws 82 to the plate 16 and bear against a holder 15. The holder 15 journals, by means of a journal 83, an actuating lever 14, from which a downwardly bent driver 84 engages in a hole 30 of the slider 1a, so that when the actuating lever 14 is pivoted, the slider is also pivoted. The holder 15 possesses an opening 86, through which the driver 84 extends downwards. The axes of the journals 33 and 83 are mutually offset by a distance y. This offsetting determines the transmission ratio between the actuating lever 14 and the slider In.  
  FIG. 9 shows in more enlarged detail the attachment of the resistance strips for the resistors according to FIGS. 7 and 8. The sandwich construction of the resistance plate 16 corresponds to that of FIG. 1. The same reference numerals have therefore been used. A nose or lug 18 of strip is bent inwardly from the raised edge section 19, as seen in FIG. 9; this nose presses the resistance strips glued to the insulating layer 10, which are laid loosely upon the lower heat removal plate, against that plate.  
  In FIGS. 10 to 12, a slider is shown, such as can be used for the resistor according to FIGS. 7 and 8. The contact springs 76 and 77 according to FIG. 8 are formed by arms 31a and 3112, which are constructed by punching out and plastic deformation in the manner illustrated in FIG. 11. The inner arm 31]) is cut out by suitable punching and bent downwards by plastic deformation. It carries a sliding contact 83. The outer, longer arm 31a surrounds the front region of the arm 31b and is bent downwards through 90 at 34 (FIG. 11). The result is, that without complete punching out, the arms 31a and 3111 can be moved independently of one another. The arm 31a carries a sliding contact 83a. As can be seen also from FIGSv 10 to 12, the slider is shaped complete with slide arms 31a and 31b and the tongue 32 from a single flat strip of spring metal. At the four corners of the approximately rectangular strip, are located contact points 29 u and 2912, by means of which the slider In (FIG. 8) slides on the under side of the edge sections 73 and 74.  
  FIG. 12 shows the slider according to FIGS. 10 and II in the installed position, the sliding contacts 83 and 83a sliding along the slide region 75 comprising the individual contact strips. The arrows P to P,-, explain the force conditions at the resistor. The slider is so constructed, and the contact points 29 are so arranged, that tilting ofthe slider when it is moved is as far as possible prevented. This depends, among other things, upon the selected values of the dimensions It, and 11 and 1,.  
  FIGS. 13 to 15 show a slider which is suitable for contact strips of the type shown in FIG. 5. This slider can also be incorporated in the same resistor as illustrated in FIGS. 7 and 8. The slider according to FIGS. 13 and 15 is once again made from one flat strip, which is provided with contact points 29 at its four corners. The contact points 29 bear, in this case also, against edge sections as described for example in FIGS. 7 and 8. By contrast to FIGS. 10 to 12, the contact springs are here provided as a separate component. This is made from a flat spring metal strip 85, from which contact arms 86 are integrally formed, holding contacts 87 at their ends. The contact springs 86 are plastically deformed out from the plane of the spring metal strip 85. The strip of the slider possesses, at each narrow end, downwardly bent edge sections 88, furnished with openings 89, through which the projections 90 of the strip penetrate. In this installed position, the strip 85 is downwardly curved and thus prestressed as a spring, so that the contact springs 86 are pressed downwards with a specific spring force. In other respects, the slider according to FIGS. 13 to 15 is constructed in the same manner as that according to FIGS. 10 to 12, and it bears the same references.  
  FIGS. 16 to 18 show a further form of embodiment of a slider, such as can be used for contact strips of the type shown in FIG. 6. In other respects, the slider according to FIGS. 16 to 18 is similarly constructed to the slider previously described, so that the same references are used for the same parts. It can also be used for a resistor of the type illustrated in FIGS. 7 and 8. In this form of construction, two spring arms and 91 point towards each other, the front end regions lying alongside each other. A rectangular hole 93 serves to receive the driver 84. By means of this slider, the radially and laterally offset arrangement of the contacts 70 and 71, for the contact strip layout of FIG. 6. is achieved.  
  It can be seen from FIG. 8 that the holder 15 for the resistor actuator is supported by means of domes or studs 17 against the resistance plate and secured there by screws 82. A more detailed representation of this attachment is given in FIG. 19. As can be seen, the dome 17 possesses an internal hole 94, provided with a thread 43, which engages with the thread of the screw 82. In the lower part, the hole 94 is widened out, giving an annular gap 45 between the dome 17 and the screw 82. The head of the screw 82 bears against the lower heat removal plate of the resistor plate 16 through an insulating washer 46. The dome l7 bears at its lower end against the resistance strips or insulating layer of the resistance plate 16. The form of attachment illustrated achieves in a simple manner a temperature drop from the resistance plate 16 to the holder 15.  
  FIGS. 20 and 21 show a section through a resistance plate, corresponding to the section of FIG. 1. A stepped opening is formed through the resistance plate, and a hollow rivet 48 passed through it. In the region of the heat removal plate 11, the opening is so broad that there is no contact between the rivet 48 and the heat removal plate 11. By-means of this rivet 48, a connection lug 64 is attached, which bears through an insulating washer 47 against the heat removal plate 11. With this form of the hollow rivet, heat generation produces an air flow which contributes to cooling the arrangement and improves the heat distribution.  
  In FIG. 21, the resistance strip 8 is directly connected by means ofa hollow rivet 96 to the heat removal plate 11.  
  FIG. 22 illustrates another form of embodiment for the construction of a resistor. A resistance plate 16, built up as shown in FIG. 1, is supported by insulating supports 97 from a supporting plate 99 of plastics. The supporting plate 99 journals, by means of a pivot pin 100, an actuating arm 14a, into which a sliding contact spring lb with a sliding contact 3a is inserted. The actuating lever 14a is also moulded from plastics. The construction of the actuating lever and the contact spring can be clearly seen from FIGS. 23 to 25. The contact spring 1b consists of a metal strip, at which a connecting lug 36 is formed at the rear end. The connecting lug 36, of smaller width, constitutes an abutment 38, which bears against an abutment of the actuating lever 140. An upwardly bent locking finger 39 engages in a recess 101 in the actuating lever 14a. By these means, the spring 1b is held self-lockingly in the actuating lever 140. Since a specific thickness is necessary for the connecting lug 36, and also the contact spring 1b must possess determined spring properties, a region 37 of reduced cross-section is formed by the cut-out constituted from the locking finger 39; this region of reduced cross-section may also be of smaller thickness.  
  At the front end, the contact spring 1b possesses a rectangular slot 102, which constitutes a web 40 perpendicular to the longitudinal axis. This web 40 and the slot 102 serve to receive the sliding contact 30. This contact is bent from highly conducting U-shaped metal strip, the two webs of which each contain a keyholeshaped recess 103. The width r at the narrowest part of the recess 103 is somewhat larger than the thickness s of the web 40. The width t of the web 40 is greater than the width r of the narrowest part and smaller than the width of the lower portion of the recess 103. When assembling. the contact piece 3a is first rotated through 90 with the recess 103 slid over the web 40 (as shown in dotted line in FIG. 25). The contact piece 3a is then rotated through 90. after which it becomes held as a rocker on the contact spring lb. The width of the contact piece 3a is so selected that it is guided as a neat fit in the slit 102. With this form of construction, accurate lateral guidance is ensured for the contact piece. This is of extreme importance, since when the contact strips are finally sub-divided, tilting would lead to jumping over a contact point.  
  FIG. 26 shows a portion in plan of the arrangement according to FIG. 3, the insulating layer a and resistance strips 8a being also shown here. FIG. 27 shows a method of fixing this arrangement in a housing or onto a support plate 104. For this purpose. the opening 24 possesses an axially, downwardly bent flange 49, which fits into the internal hole ofan insulating piece 50. The insulating piece 50 in turn fits with its external diameter into the axial flange a of the supporting plate 104,  
 which likewise is provided with an opening 105. A hollow rivet 52 is passed through the openings 105,&#34; the hole 50 and the opening 24 through an insulating washer 53, in order to join the individual components together. With this form of construction, the individual parts are accurately centered. Also, unfavourable temperature influences upon the supporting plate 104 are avoided, so that there is no need whatever for this plate to be made of a highly thermally stable material. The hollow rivet 52 can, however, be omitted also; the resistance plate being then directly held in position, without further fixing means, solely by the contact pressure of the slider.  
  FIG. 28 illustrates a form of embodiment, in which the element 106, constituted of resistance strips and insulating layer, is laminated upon a supporting plate 20, or glued thereto or attached by means of other suitable fixings, not shown. An actuating lever 107 is also provided with a contact spring 108, sliding upon the sliding region of the plate 106. The lever 107 is pivotally connected to the supporting plate 20 at 109. As can be seen, the lever 107 can adopt three stop positions, formed by a bent up and perforated edge portion of the supporting plate 20. A further lever 110, journalled in g the supporting plate 20, serves for actuating another device, for example an air valve in an automobile.  
  In FIG. 29, a layer 111 consisting of resistance strips and insulating layer is applied onto a box-shaped supporting unit 21. The resistance strips which can be highly loaded are provided on the vertical surfaces of the supporting unit 21. The resultant convection effect leads to better cooling of this arrangement. Here again, an actuating lever 112, provided with a contact spring 113, is pivotally mounted upon the supporting unit 21.  
 The invention claimed is as follows:  
  1. A variable resistor comprising a thermally conductive base, an electrically insulating layer, a resistance unit comprising a number of resistance strips which have change-over spaces therebetween and which are coupled by said electrically insulating layer to said thermally conductive base, a slider having a contact which can be brought into contact with contact surfaces of the individual resistance strips. and an actuating means for said slider wherein said base has open areas in alignment with said changeover spaces so that short circuiting through said base due to electrical breakdown of said insulating layer is substantially eliminated.  
  2. A variable resistor according to claim 1 in which said resistance strips comprise a metal sheet secured onto said insulating layer.  
  3. A variable resistor according to claim 1 in which the material of said resistance strips contains more than 17 percent chromium and 7 percent nickel.  
  4. A variable resistor according to claim 1 in which said insulating layer is an insulating mat of fine glass fabric.  
  5. A variable resistor according to claim 1 in which said insulating layer comprises mica.  
  6. A variable resistor according to claim 1 in which said insulating layer comprises a composite material of mica and polyamide fibres.  
  7. A variable resistor according to claim 1 in which said thermally conducting base is a plate and said thermally conducting plate comprises slots which determine the heat distribution in said plate.  
  8. A variable resistor according to claim 1 in which said resistance strips are covered with a nonconducting, non-metallic layer, the contact surfaces, however, being left free.  
  9. A variable resistor according to claim 1 in which said contact surfaces are flat strips of chrome nickel steel.  
 10. A variable resistor according to claim 1 comprising a contact spring wherein said thermally conducting base is a heat removal plate, which at an edge thereof is bent at least partially upwards and inwards to form a guide and said slider is pressed upwards by said contact spring against said inwardly bent edge section and is pivotally mounted on the combination of said really mounted on said supporting plate, and a contact spring comprising a sliding contact is held by said actuating lever, said contact spring being self-lockingly engaged in a recess of said actuating lever.  
  13. A variable resistor according to claim 1 comprising a separate heat conducting means wherein said resistance strips are thermally coupled to said heat conducting means by a hollow rivet.  
  14. A variable resistor according to claim 1 wherein said resistance strips are thermally coupled to said heat conducting means by a hollow rivet,