GB819724A - Precision wire wound resistors - Google Patents

Abstract

819,724. Wire wound resistors and potentio-meters. UNITED AIRCRAFT CORPORATION. Nov. 3, 1955, No. 31405/55. Class 37. A resistor is given a predetermined value by placing a fusible electrically conductive member in contact with a number of adjacent turns of the resistance wire and then fusing the fusible member progressively so that the value of resistance is incrementally increased to the predetermined value. Figs. 1 and 2 show a fixed resistor in which a resistance wire 10 is helically wound on an insulating core 11, a strip of electrically conductive fusible material 12 is in contact with a number of adjacent turns of wire 10 thus effectively shorting these turns. The fusible strip 12 which may be of gold or gold alloy has a lower melting point than that of the wire 10 and the resistor is initially wound to a value below that finally required. A voltage source is applied across the resistor so that the fusible material progressively fuses as the current increases until the resistor has reached the prescribed value of resistance. In a modification, Fig. 3 (not shown), a greater precision of adjustment is obtained by connecting in parallel with a fixed resistor a vernier resistor having a fusible metal strip. The resistance wire may be a nickel chrome alloy such as that sold under the Registered Trade Mark of " Nichrome " and the fusible strip a gold copper alloy. By the use of suitable contact members contact may be made with adjacent turns of the resistance wire to fuse the strip between them without passing current longitudinally along the resistance wire. In a further modification, Fig. 4 (not shown), high precision of adjustment is obtained by connecting the adjustable resistance having a fusible metal strip between one end and a tapping point of a fixed resistor. Figs. 5 and 6 show a precision potentiometer which is accurately adjusted by means of a fusible metal strip to attain a predetermined angle resistance characteristic. A contact member 44, the pressure of which may be adjusted by a screw 46 makes sliding contact with a circular wire wound " contact " resistor 48 wound on core 49. The resistance has substantially uniform resistance per unit length. Wire wound " function " resistor 54 and " vernier " resistor 55 have spaced points along them connected by bridging wires to the " contact " resistor 48 so that the three resistors have these points connected in parallel. The cores of the three resistors are formed of metal which is covered with a suitable insulating material the metal having the same thermal expansion coefficient as the resistance wire used. Both the resistors 54 and 55 are provided with fusible metal strips 12 which enable their values to be adjusted. The " function" " resistor 54 and the " vernier " resistance 55 have their values sequentially adjusted so as to obtain a given resistance value variation with respect to a given angular position of the shaft.