Abstract:
A variable resistor of modular design having a hub-flange and at least one housing frame having a central opening. A spring carrier provided with a contact spring is arranged in the central opening of the housing and is rotatable by means of an actuation shaft. A carrier plate, on which the resistor course and the collector course are mounted, is positioned adjacent the spring carrier such that the contact spring provides an electrical connection between the resistance and the collector courses. The housing frame has a pot-shaped front side for receiving a rotor. Means is provided for arresting rotation of the rotor to within predetermined angular limits, and the rotor has a contoured rim for engaging a click-stop device.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an arrangement for plural variable resistors on a common actuation shaft. 
     2. The Prior Art 
     In order to enable reasonable and low-cost production of the numerous variations demanded by the user of one type of variable resistor, variable resistors are assembled in the so-called modular fashion. In this way, it is more readily possible to fulfil the requirements of the user for single-, tandem-, duplotandem-, duotandem-, doubletandem-, etc., variable resistors, with specified requirements as to shaft configurations and resistance values, with a resistor whose major components are the same for all variations. 
     A variable resistor is already known which is modularly constructed such that several modules of the same kind are couplable with one another. Each module consists of a pot-shaped housing having a front and a back surface. A spring carrier is rotatably mounted in the housing with the aid of a shaft which extends through a central opening in the housing. A contact spring is mounted on the spring carrier, the contacts of the spring resiliently resting against a resistance course as well as against a collection course. The resistance course is mounted on a carrier plate which forms the end of the pot-shaped housing. The collector is developed as a planar collector ring with a central opening, the ring being arranged in the bottom of the pot-shaped housing and having a central opening through which the shaft extends. Thus, the contacts of the contact spring are arranged on different sides of the spring carrier. The housing itself is connected, by a locating plate, with an externally threaded hub-flange. 
     To obtain better protection against unintentional adjustment of the variable resistor and to obtain precisely defined resistance values, for example, for use with volume control systems, conventional variable resistors are additionally provided with a click-stop device. Ordinarily, an odd number of stops is chosen in order to make possible a defined center position, for example, when used for audio adjustment. If precision adjustment is required, as many as 41 stop-positions may be provided. With simpler apparatus, a single center-position stop may suffice. 
     A variable resistor having a click-stop device, consisting of a hub-flange on which a carrier plate is mounted for carrying the resistance course and the collector course, is already known. An actuation shaft extends through the hub-flange; on one end of the actuation shaft a spring carrier is mounted. On the side of the spring carrier turned towards the carrier plate, a contact spring is mounted with its contacts lying resiliently against the resistance and collector courses in order to produce an electrical connection between the two courses. A row of dome-like elevations are provided along a circumference at a predetermined radius from the axis of said spring carrier, and said elevations cooperate with two diametrically-opposed projecting noses of a stop-spring. This stop-spring is arranged on a pot-shaped housing which surrounds the entire variable resistor and which is mounted on the carrier plate. It is a disadvantage that this click-stop device is not readily transferable to a variable resistor of modular design. 
     With known variable resistors, each spring carrier is ordinarily turned independently by the actuation shaft. Because the coupling between the spring carrier and the shaft is a loose one and the construction parts participating in the transmission of the turning movement are subject to the usual functional tolerances, a relatively low level of synchronous precision results for tandem resistors of this type. By the term synchronous is meant the electrical uniformity of the individual resistance values realized within a predetermined actuation zone. The measurement is done by a voltage comparison in which the synchronous tolerance is calculated in dB from the common logarithm of the ratio between the two voltages tapped, multiplied by a factor of 20. With known variable resistor arrangements, a synchronism of only 3 dB can be attained. For high-quality apparatus, however, synchronism conditions of 1 dB and less are required. 
     A tandem resistor is already known in which the actuation shaft between the mounting positions of the two spring carriers has a reduced cross-section. This cross-section reduction is obtained by an indentation of the shaft, or by using two surface millings symmetrically displaced 180° apart, or by means of a transverse bore through the shaft. Torsional stress on the actuation shaft causes a deformation at the portion with reduced cross-section, such that the spring carriers mounted on the actuation shaft can be turned relative to one another. In this manner, a synchronisation correction is made possible. To facilitate the twisting, the end of the actuation shaft is provided with a screw driver slot. With this arrangement it is disadvantageous that no possibility of having a detent position exists. 
     SUMMARY OF THE INVENTION 
     Therefore, it is the purpose of the invention to provide a variable resistor of modular design, the variable resistor providing in a very small space a simple, easily exchangeable click-stop device with as many stop positions as possible, and at the same time to limit the angular rotation zone. 
     A further purpose of the present invention is to improve the synchronous precision of such a variable tandem resistor. 
     These purposes are satisfied by the invention in that one of the housing frames has a pot-shaped front side, between the walls of which a rotor provided with an arresting segment is mounted for rotation. The rotor has a stop contour on the side turned away from the housing frame in which a click-stop device, biased by a resilient member, engages. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Two preferred embodiments of the invention will now be described in detail with reference to FIGS. 1 to 9, in which: 
     FIG. 1 shows an exploded, perspective view of a first embodiment of the variable tandem resistor according to the invention; 
     FIG. 2 shows a cross-sectional view through the variable resistor of FIG. 1; 
     FIG. 3 shows a top view of the rotor of the variable resistor of FIGS. 1 and 2; 
     FIG. 4 shows a cross-sectional view taken along line A-B through the rotor of FIG. 3; 
     FIG. 5 shows a bottom view of the rotor of FIGS. 1 to 4; 
     FIG. 6 shows a cross-sectional view through a further embodiment of a variable tandem resistor according to the invention; 
     FIG. 7 shows a top view of the rotor of the variable resistor of FIG. 6; 
     FIG. 8 shows a cross-sectional view taken along line C-D through the rotor of FIGS. 6 and 7; and 
     FIG. 9 shows a bottom view of a rotor of FIGS. 6 to 8. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a first embodiment of the tandem variable resistor according to the invention in exploded, perspective view. A tandem resistor comprises two resistors which are variable by movement of a single actuation shaft. Such resistors are particularly needed for stereophonic purposes. It is to be noted, however, that because the resistor is constructed in modular fashion, many further combinations are realizable beyond simply the tandem arrangements illustrated here. 
     The tandem resistor shown in FIG. 1 comprises a hub-flange 1, two modular construction units, an insulating plate 2, and a shield plate 3. The entire tandem resistor assembly is held together by rivets 4. Each modular construction unit comprises essentially a pot-shaped housing frame 5, a spring carrier 6, an intermediate frame 7 of synthetic material, and a carrier plate 8. The spring carrier 6 is rotatably mounted in a central opening 9 of the housing frame. A contact spring 10 is mounted on the spring carrier, the slide contacts of the contact spring producing an electrical connection between the resistor course 11 and the collector course 12. The resistor course and the collector course are, according to conventional assembly procedures, mounted on the carrier plate. Connecting lugs 13, which are fastened by means of the clamp technique on the carrier plate, reach outwardly from the bottom of the housing. An insulating plate 14 is attached to the carrier plate 8 and is identical to the insulating plate 2. A shield plate 15 completes the first construction unit and has, in contrast to the shield plate 3, a central opening through which the actuation shaft 16 from the first unit extends into the second unit. Each of the shield plates has a solder connection lug 17 for soldering of the shield plate to a ground terminal of a printed circuit. Spring arms 18 molded integrally on opposite sides of the intermediate frame cooperate with and engage corresponding recesses in the carrier plate and the housing frame. The carrier plate recesses are designated as 19 in FIG. 1, while the recesses of the housing frame are not indicated in that Figure. 
     A generally O-shaped leaf spring 20 is mounted by one of its narrow sides on the shield plate 15, while on the opposite narrow side a recess 21 is provided in which a ball detent 22 is placed. The second construction unit also comprises essentially a housing frame 24, a spring carrier 6, an intermediate frame 7 of synthetic material, and a carrier plate 8, the spring carrier, with its contact spring 10, being mounted for rotation in the opening 9 in the back side of the housing frame 24. The front side of the housing frame is pot-shaped, the walls thereof being taller than those of the housing frame 5 of the first construction unit. A rotor 23 is mounted for rotation within the space defined by these walls and is provided with a circumferential click-stop contour on the face of the rotor turned away from the housing frame. The click-stop contour consists of radial slots 25 which are located around and extend inwardly from the rim of the rotor. An odd number of slots is provided in order to make possible a defined center position for audio adjustments. If precision adjustment capability of the resistor is desired, up to 41 slots and, hence, stop positions, can be provided. With much simpler apparatus a center stop suffices. Ball detent 22, resiliently biased by the leaf spring 20, engages the slots 25. The special configuration of the leaf spring and the click stop counter on the rotor makes possible 41 click-stop positions even when the variable resistor is constructed to relatively small dimensions. 
     The rotor 23 is shown in enlarged scale in FIGS. 3, 4 and 5 and has a circumferentially-running ring-shaped groove 26 on the face turned towards the housing frame, the ends of this groove being defined by an arresting segment 27. An arresting pin 35 extends into the groove, the arresting pin being integrally formed as a part of the housing frame. The arresting segment and the arresting pin cooperate to limit the rotation zone of the variable resistor. The rotor&#39;s click-stop contour preferably extends over the same angular region as the ring-shaped groove. 
     Rotor 23 is coupled with the spring carrier 6 by two diametrically-opposed, elevated ring segments 28. The ring segments mate with corresponding openings 29 of the spring carrier 6. An entrainment member 30 on the actuation shaft 16 cooperates with projections 31, 32 extending radially inwardly of the central opening of the rotor to cause the rotor and the spring carrier to rotate with the actuation shaft. 
     The actuation shaft 16 is secured in position in the opening of hub-flange 1 by means of a snap ring 33. Between the hub-flange and the first housing frame 5 is an intermediate spacer 34 which serves to assure that the distance from the housing wall, on which the hub-flange is fastended, to the connecting lugs 13 is in accordance with the grid of the printed circuit on which the resistor is to be mounted. 
     A further embodiment is shown in FIGS. 6 to 9. The tandem resistor shown in these Figures has synchronous precision which is somewhat better than in the first embodiment. In so far as the same components as in the first embodiment are used in the second embodiment, the same numbering is used. 
     The tandem resistor shown in cross-section in FIG. 6 consists of a hub-flange 1, two modular construction units, and an insulating plate 2 and a shield plate 3. The entire tandem resistor assembly is held together by a rivet or screw connection. Each modular unit essentially comprises a pot-shaped housing frame 5, a spring carrier 46, an intermediate frame 7 of synthetic material, and a carrier plate 8. The spring carrier is mounted for rotation in a central opening of the housing frame. A contact spring 10 is mounted on the spring carrier, the sliding contacts of the contact spring providing an electrical connection between the resistor course and the collector course. Both courses are mounted on the carrier plate according to conventional construction techniques. Connecting lugs 13, which are fastened by means of a clamp technique or by rivets on the carrier plate, extend outwardly from the bottom of the case. An insulating plate 14 is adjacent to the carrier plate 8, plate 14 being substantially identical with the insulating plate 2. The completion of the first construction unit is a shield plate 15 which has, in contrast to the shield plate 3, a central opening through which an actuation shaft 36 from the first unit extends into the second unit. Each of the shield plates has a solder-connection lug 17 for soldering of the shield plate to a ground terminal of a printed circuit. 
     Spring arms 18 (not illustrated in FIG. 6) formed integrally on opposite sides of the intermediate frame cooperate with and engage corresponding recesses in the carrier plate and the housing frame. 
     The second construction unit essentially also comprises a housing frame 24, a spring carrier 47, an intermediate frame 7 of synthetic material, and a carrier plate 8, the spring carrier with its contact spring 10 being mounted for rotation in an opening in the back side of housing frame 24. The front side of housing frame 24 is pot-shaped, the walls thereof being taller than those of housing frame 5. A rotor 37 is mounted for rotation in the space defined by these walls and is provided with a click-stop contour on the face of the rotor turned away from the housing frame. The click-stop contour consists of radial slots 38 which are located around and extend inwardly from the rim of the rotor. An odd number of slots is provided in order to make possible a defined center position for audio adjustments, as mentioned with regard to the first embodiment. Ball detent 22, resiliently biased by the leaf spring 20, engages the slots. The generally O-shaped leaf spring is fastened at a narrow side to the shield plate 15, while the opposite narrow side is provided with a recess in which the ball detent 22 is placed. 
     Rotor 37 is shown in enlarged scale in FIGS. 7, 8, and 9, and has a circumferentially-running ring-shaped groove 26 on the rotor face turned towards the housing frame, the ends of this groove being defined by an arresting segment 27. An arresting pin 35 extends into the groove, the arresting pin being integrally formed as a part of the housing frame. The arresting segment and the arresting pin cooperate to limit the rotation zone of the variable resistor. The click-stop contour therefore preferably extends over the same angular region of the rotor as the ring-shaped groove. 
     Extending outwardly in the axial direction from the respective faces of the body of rotor 37 are integrally-formed ring segments 39, 40; each face of the rotor body has two diametrically opposed such ring segments. Spring carrier 47 is press-fit on the two shorter ring segments 40, while the other spring carrier 46 is press-fit on the longer ring segments 39. To guarantee a play-free coupling between the spring carriers and the rotor, the ring segments 39, 40 are tapered conically from a larger to a smaller diameter in the direction away from the rotor body. Thus, fully synchronous rotation of the two spring carriers is ensured. Entrainment members 41 located on the actuation shaft cooperate with slots 42 in the rotor to cause the rotor to rotate with the actuation shaft. The axial length of the entrainment members is less than the total axial length of the rotor body and ring segments, as can be seen from FIG. 6. The actuation shaft 36 is secured in position in the opening of the hub-flange by means of a snap ring 43 and a U-plate 44 of metal, the latter serving to improve the torsional moment. To prevent an axial displacement of the actuation shaft 36, a spacer disc 45 is provided, the spacer disc being preferably made of hard paper and rests against the housing frame 5.