Excessive power dissipation may raise the temperature of the resistor to a point where it can burn the circuit board or adjacent components, or even cause a fire. If the average power dissipated by a resistor is more than its power rating, damage to the resistor may occur, permanently altering its resistance this is distinct from the reversible change in resistance due to its temperature coefficient when it warms. Power resistors are physically larger and may not use the preferred values, color codes, and external packages described below. Resistors required to dissipate substantial amounts of power, particularly used in power supplies, power conversion circuits, and power amplifiers, are generally referred to as power resistors this designation is loosely applied to resistors with power ratings of 1 watt or greater. They usually absorb much less than a watt of electrical power and require little attention to their power rating.Īn aluminium-encased power resistor rated for dissipation of 50 W when mounted on a heat-sink Discrete resistors in solid-state electronic systems are typically rated as 1/10, 1/8, or 1/4 watt. Resistors are rated according to their maximum power dissipation. This power is converted into heat which must be dissipated by the resistor's package before its temperature rises excessively. Using Ohm's law, the two other forms can be derived. Where V (volts) is the voltage across the resistor and I (amps) is the current flowing through it. The behaviour of an ideal resistor is described by Ohm's law: The nominal value of the resistance falls within the manufacturing tolerance, indicated on the component. The electrical function of a resistor is specified by its resistance: common commercial resistors are manufactured over a range of more than nine orders of magnitude. Resistors are also implemented within integrated circuits. Practical resistors as discrete components can be composed of various compounds and forms.
Resistors are common elements of electrical networks and electronic circuits and are ubiquitous in electronic equipment. Variable resistors can be used to adjust circuit elements (such as a volume control or a lamp dimmer), or as sensing devices for heat, light, humidity, force, or chemical activity. High-power resistors that can dissipate many watts of electrical power as heat, may be used as part of motor controls, in power distribution systems, or as test loads for generators.įixed resistors have resistances that only change slightly with temperature, time or operating voltage. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active elements, and terminate transmission lines, among other uses.
A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element.
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