An electronic device that possesses resistance and is selected for use because of that property. There are several different types of resistor in common use; the type of resistor chosen depends on the particular application for which it is designed. The three main types of resistance element are carbon, wire-wound, and film resistors. These may be produced as fixed-value resistors or adjustable resistors.
Carbon resistors consist of finely ground particles of carbon mixed with a ceramic material and encapsulated in an insulating material. The encapsulation has a set of coloured stripes or dots – the colour code – denoting the value of the resistance (see Table 2 in the back matter). Carbon resistors are compact, robust, and relatively cheap to manufacture and are widely used in electronic circuits in which the resistance value is not critical. The value of the resistance however is a function of the operating voltage and of temperature and close tolerances cannot be maintained over a wide range of load and ambient conditions. When used at power levels above one megohm the level of thermal noise becomes high, precluding their use at high levels of power. The relatively short length of these resistors causes them to have a noticeable shunted capacitance and therefore at high (VHF and higher) operating frequencies the effective resistance is reduced as a result of the dielectric losses involved.
Adjustable carbon composition resistors may be formed on an insulating base or moulded at a high temperature onto a moulded plastic base. The composition is formed with a linear rotating contact or it may be tapered to produce a nonlinear characteristic. The resistance change is continuous. These resistors, particularly the thinner ones, tend to be noisy and are subject to mechanical wear in frequent use.
Wire-wound resistors are formed from a wire of uniform cross section wound on a suitable former. The value of resistance may be determined very accurately and for uses where the resistance value is critical, wire-wound resistors are usually preferred. Wire-wound resistors however are unsuited to use above 50 kilohertz as they have marked inductive and capacitive effects even when specially wound. At high frequencies the skin effect causes an increase in the effective resistance.
Adjustable wire-wound resistors are produced in a wide range of values and types. Linear or circular types are made with single-turn or multiturn contacts. By tapering or producing other shapes of the former the contact rotation versus resistance curve may be altered to generate various functions: logarithmic, sine wave, or other characteristics are available. Wire-wound resistors almost invariably change resistance values in steps. This can sometimes provide an unwanted pulse in the output. The motion of the contact across the turns also tends to generate noise.
Resistive film resistors are the most suitable type of resistor for high-frequency applications since the inductance, even in high-value spiral films, is very much less than with wire-wound resistors. Special high-frequency film resistors with very low values of inductance are available.
Metal film resistors are the most widely used type for precision requirements. They are manufactured by depositing nichrome alloys on a substrate. Extremely good characteristics may be obtained with normal tolerances of ±1% achievable; ±0.5%, ±0.25%, and ±0.1% are also available.