Resistor: Resistance, Ohms and Power Rating

A resistor is a passive electrical component designed to provide resistance in a circuit. Resistance is the electrical quantity describing the relationship between voltage across an ohmic element and the current through it. The component and the quantity are related, but they are different meanings. Bourns resistor definition · ROHM resistance explanation

Resistance and the ohm

Resistance is commonly represented by R and measured in ohms (Ω). For an ideal ohmic resistor with a constant resistance, current increases in proportion to applied voltage. At the same voltage, a higher resistance means a lower current. This is the basic relationship described by Ohm’s law; it is not a claim that every electrical device has a constant resistance. ROHM Ohm’s-law explanation

A resistor’s stated resistance is a nominal value. Its resistance tolerance describes the allowed difference from that value. Actual resistance can also change with temperature, so “fixed resistor” does not mean a value that remains identical under every condition. Bourns tolerance and temperature coefficient

What is inside a resistor?

A resistor contains a resistive element and electrical connections. The element’s construction varies: ROHM describes carbon-film and metal-film elements on an insulating core, as well as wire wound around a ceramic support. These are different ways of providing resistance, rather than different names for the quantity itself. ROHM resistor constructions

Fixed, variable, leaded and surface-mounted

Fixed resistors have a specified nominal resistance. Variable resistors provide an adjustment mechanism; a moving contact changes the portion of the resistive element involved in the electrical path. This distinction concerns adjustment. ROHM fixed and variable resistors

Leaded and surface-mounted describe connection and mounting forms. They are a separate classification from fixed versus variable, and do not by themselves state a resistance value or power rating. ROHM mounting forms

Resistance value is not power rating

Current through a resistor causes electrical energy to be dissipated as heat. Its power rating concerns the power it can dissipate under specified conditions, while its resistance value concerns the voltage–current relationship. Temperature and the manufacturer’s stated rating conditions matter when interpreting a power rating. Bourns power dissipation · ROHM rated power

These properties answer different questions. A resistance value alone does not establish how much heat a component may safely dissipate. This page gives no universal derating factor or suitable part for a particular circuit.

For a component that stores separated charge, see capacitor basics. Continue through the electronic components section for related component definitions.