The materials used for resistor construction are generally metal alloys, pure metal or metal-oxide films, or carbon (solid or in thin-film form). Wire wound resistors use metal alloy wire wound onto ceramic formers. The windings must have a low self-inductance value, so that the wire is wound using the method shown in Figure 1 with each half of the winding wound in the opposite direction.
Figure 1 (Non-inductive winding of a wire-wound resistor. The two halves of the total length of wire are wound in opposite directions so that their magnetic fields oppose each other.)
Wire-wound resistors are used when very low values of resistance are needed or when very precise values must be specified (for meter shunts, for example).
Large resistance values in the region of 20 kW upwards need such fine-gauge wire that failure can occur due to corrosion, especially in tropical conditions of high temperature and high humidity, so high-value, wire wound resistors should not be used for marine or tropical applications unless the wire can be protected satisfactorily.
Carbon composition resistors, once the main type of resistor used for electronics, are now rarely used. They consist of a mixture of graphite and clay whose resistivity depends on the proportion of graphite in the mixture. Because the resistivity value of such a mixture can be very high, greater resistance values can be obtained without the need for physically large components.
Resistance value tolerances are high, however, because of the greater difficulty in controlling the resistivity of the mixture and the final dimensions of the carbon composition rod after heat treatment. You should not specify carbon composition resistors for any new design unless cost is an overriding factor.Figure 2 Typical resistors: carbon film, wirewound.
Metal film, carbon film and metal-oxide film resistors are more recent types that form the vast majority of resistors used today. They are made by evaporating metals (in a vacuum or an inert atmosphere), or metal oxides (in an oxidizing atmosphere) onto ceramic rods.
The resistance value is controlled by controlling the thickness of the film and by cutting a spiral path on the film after it has been deposited.
These resistors are considerably cheaper to make than wire-wound types and can be made to much closer tolerances than carbon-composition types. The costs of such resistors are now almost the same as those of composition types. Figure 2 shows typical fixed resistor shapes.
Variable Resistors and Potentiometers
Variable Resistors and Potentiometers can be made using all the methods that are employed for fixed resistors.
The component is termed a potentiometer when connections are made to both ends of the resistive track and also to a sliding connection.Figure 3 Typical potentiometer and trimmer shapes.
Variable resistor uses only one connection to one end of the track and one to the sliding connector. By convention, both are wired so that the quantity that is being controlled will be increased by clockwise rotation of the shaft as viewed by the operator.
A trimmer is a form of potentiometer, often miniature, that is preset on test and not normally alterable by a user of equipment. Figure 3 illustrates the variety of potentiometer and trimmer shapes.