Resistors

Resistors are passive two-terminal electrical components that provide a defined opposition to current flow. They are used to set current levels, divide voltages, bias active devices, terminate transmission lines, and protect circuits. Resistance determines power dissipation according to P = I^2R or P = V^2/R, producing heat in proportion to current and resistance. In circuit diagrams, resistors are represented by a zigzag line in the United States or a rectangular box under IEC standards. The resistance is expressed in ohms (Ω).

Most resistors are fixed in value. Common categories include carbon composition, carbon film, metal film, metal oxide, wirewound, and thin-film types. They differ in noise, stability, temperature coefficient, power rating, and voltage rating. Resistors come in axial through-hole, radial, and surface-mount (SMD) packages, with SMD sizes such as 0402, 0603, and 0805. Precision parts may have tight tolerances (as low as 0.1%) and defined temperature coefficients; general-purpose parts typically range from about 1% to 5% tolerance.

Variable resistors, including potentiometers and trimmers, provide adjustable resistance for calibration and user control. They are used in bias networks, gain control, and tuning or trimming circuits.

Specifications and standards: resistance values are drawn from standard series (E12, E24, E96, etc.). Color codes have traditionally been used on carbon resistors to indicate value, tolerance, and temperature coefficient; many modern components use printed markings or coded SMD markings.

Reliability and handling: resistors are designed for various power ratings, temperature ranges, and voltage limits. They can drift with temperature, moisture, or overvoltage and may fail due to mechanical stress or aging.