Patent GB-1046939-A (GB)
Title: Adjustable resistor
IPC: H01C10/24
Filed: 1963-09-27
Abstract: 1,046,939. Variable resistors. DAYSTROM Inc. Sept. 27, 1963 [Sept. 28, 1962], No. 38104/63. Heading H1S. A variable electrical resistor comprises an electrically non-conductive tubular base member 10 on the inside wall of which is a thin resistive film 26 in the form of a helix, and a contactor 60 movably mounted within the member 10 and having an electrically conductive protuberance 62 in electrical contact with the film 26, means so constraining the contactor 60 for movement along and inside the member 10 whereby the protuberance 62 moves along the helix, being elongated in the direction of its movement. The member 10 may be of ceramic, the inner surface being glazed prior to having a thin film such as carbon or a nickel alloy deposited on it by sublimation or other known method. The film 26 is adjusted to the desired resistance value by " spiralling " using an electric probe, the resultant helix having typically 15 turns each of a width of 0.03 inch. A lead screw 12, gold-plated for improved conductivity and corrosion resistance, is rotatably mounted at one end by a bushing 38 and fixedly supported at the other end in a fixed bushing 42 mounted within an insulating bushing 54. A carriage 44 in the shape of a half cylinder but with a portion 46 at either end extending more than 180 degrees around surrounds the lead screw 12 and has its right-hand end force fitted on to the bushing 38 so that it can be driven by rotation of an adjustment head 36, the bushing 38 and the head 36 being made into an integral unit by a plastic moulding 40. The left-hand end portion 46 of the carriage 44 is loosely fitted over the fixed bushing 42 to permit its free rotation about that bushing. A follower 14 is threaded over the lead screw 12, being cylindrical in shape with a portion 48 of enlarged diameter having an axial groove 50 and a radial groove 52. The pitch angles of the lead screw 12 and the helical film 26 are selected to be substantially identical. Upon rotation of the head 36, the carriage 44 engages the portion 48 of the follower 14 and causes it to rotate about the lead screw 12. The follower 14 may be of brass and is preferably rhodium-plated for improved conductivity and wear and corrosion resistance. The resilient metal contactor 60 carried by the follower 14 is cross-shaped, having one pair of short arms 66 and one pair of long arms 68, the former being located in the axial groove 50 and the latter in the radial groove 52. The arms 68 rest upon the bottom of the groove 52 so that upon depression of the contactor 60 the arms 68 spread somewhat. An elongated protuberance 62 on the face of the contactor 60 has its major axis canted to be in substantial alignment with the helical film 26. The arms 68 ensure that the protuberance 62 is maintained in contact with the film 26. A spring 70, e.g. of beryllium copper and a pair of washers 72, e.g. of gold-plated brass, are placed on either end of the lead screw 12 to provide a clutch action. The ends of the lead screw 12, which are not threaded, are of reduced diameter and the follower 14 disengages from the threads as it reaches either end of the film 26. The washers 72 are located upon shoulders formed at either end of the lead screw 12 and as the follower 14 disengages, it compresses one of the springs 70 so that upon reversal of rotation of the head 36, the compressed spring 70 forces engagement of the follower 14 with the threads of the lead screw 12. Terminals 24 of a metallic frit or gold-plating are provided on the ends of the base member 10 and these contact ends caps 28, 29 which are connected to pins 30, 32, respectively, on a terminal board. A terminal 35 soldered to the fixed bushing 42 which in turn is soldered to the lead screw 12 is connected to the centre pin 31 on the terminal board. The entire unit is encapsulated with a casing 20 of a plastic such as an epoxy resin.