The Difference Between Fixed and Variable Resistors

A resistor is an electronics component that opposes the flow of electrons in an electrical circuit. There are several varieties of resistors available, and in a variety of resistance ratings, but generally each resistor will fall into one of two categories: fixed resistors and variable resistors. As the names indicate, fixed resistors have a fixed and unchangeable resistance value, while variable resistors have variable resistance values. What follows is a brief discussion of the various common types of fixed and variable resistors in the electronics world.

Fixed Resistors

Fixed resistors are discrete electronics components that are available in a variety of resistance values and rated in Ohms. Common types of fixed resistors include carbon composition resistors, carbon film resistors, metal film resistors, wire wound resistors, and foil resistors. Each of the above types of fixed resistors has particular characteristics that suit that resistor type for particular purposes. For example, wire wound resistors generally are able to handle larger amounts of power than, say, carbon composition resistors, but their wire coils often result in unwanted inductive reactance in AC or RF circuits. Fixed resistors are often marked according to the conventional resistor color code.

Variable Resistors

Variable resistors, as the name implies, are resistors whose resistance is able to be varied, whether manually or automatically. Common types of variable resistors include rheostats and potentiometers and are often differentiated from one another by means of the linearity of their variable resistance. For example, potentiometers are available in linear-taper varieties and logarithmic-taper varieties that differ in the linearity of their variability as the potentiometer is turned. Potentiometers are often low-Wattage devices, while rheostats are often used for high-power applications in industrial electronics.

The choice between a fixed and variable resistor in circuit-design will be based upon whether one's circuit calls for resistance values or circuit characteristics that require variability or stability. For example, if one is constructing a radio receiver that will be fixed on a single radio frequency, then a circuit that is composed entirely of fixed-value resistors and other components will probably suffice. If however, one is building a motor control circuit that calls for variation in motor speed, a variable resistor would be a more appropriate choice since a variable resistor could be used to vary the speed of the motor. Thus there is a need for both fixed and variable resistors in electronics circuits, and a familiarity with both types of resistors is essential if one is to have comprehensive knowledge of the various types of circuits that will call for resistors.