The resistance element is used as a wire, made from special alloys, which have small low temperature coefficient and good stability against environmental conditions. Ceramic substrate is used because the mismatch between linear expansion of ceramic substrate and wire wound is minimized and thus stable characteristics could be obtained. The wire is chosen according to its resistivity. The actual resistance is adjusted during winding. The resistance required is driven from the length of the wire used, gauge and their resistivity. During winding, one end of the wire is welded on a butt cap. Known length of wire is tightly wound around the ceramic substrate and the other end of the wire is soldered to cap on other end. Since wire is susceptible to mechanical strain, they are annealed at high temperatures. Wire wound resistors suffer from high frequency effects due to winding. Also it is difficult to get high ohm values. Thus a back to back or bifilar winding is used. These resistors suffer from mechanical vibrations and shocks. Thus it is necessary to given adequate protections.
(a) OPEN WOUND: Wire of nickel is wound on a ceramic rod. These rods are grooved or plain. The wire is thus unprotected. The wire used normally thick and thus limits maximum resistance value. Values available are from 25 kilo ohm to 150 kilo ohm. Wattage rating 10 to 60W.
(b) VITREOUS ENAMELLED: THESE resistors are coated with vitreous enamel. It enamels excellent protection against moisture penetration.
Range of value 0.1 ohm to 106 ohms
Temperature coefficient 5 ppm/0C
Working voltage < 300 V
Working temperature 35 to 3750C
Applications: These resistors are suitable for low resistance and low noise applications. Also these are used when high power handling capacity is required in small size. They find immense used in regulated power supplies, MultiMate's, Wheatstone bridge etc.