Introduction
A resistor is a two-terminal component that limits current, develops a voltage drop, divides voltage, sets operating conditions, and converts some electrical energy into heat. Resistors are among the most frequently used parts in electronic circuits.
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The colored bands on many axial resistors identify resistance value and tolerance. The physical size often gives a rough indication of power rating, but the manufacturer's markings and datasheet are the reliable guides. Surface-mount resistors usually use printed codes or may be too small to carry a readable value.
What resistance does
Resistance opposes electric current. For a given applied voltage, increasing resistance reduces current. For a given current, increasing resistance produces a larger voltage drop.
- V is voltage in volts.
- I is current in amperes.
- R is resistance in ohms.
Common jobs performed by resistors
- Limit current through LEDs and other components.
- Set transistor and amplifier bias conditions.
- Create voltage dividers and reference levels.
- Provide pull-up and pull-down paths for digital signals.
- Set gain and timing in analog circuits.
- Terminate transmission lines and reduce reflections.
- Measure current by producing a small, known voltage drop.
- Discharge capacitors after power is removed.
Important specifications
| Specification | Meaning |
|---|---|
| Resistance | The nominal opposition to current, stated in ohms. |
| Tolerance | The allowed difference between actual and nominal resistance. |
| Power rating | The heat the resistor can safely dissipate under stated conditions. |
| Temperature coefficient | How much resistance changes as temperature changes. |
| Maximum working voltage | The greatest continuous voltage permitted across the part. |
| Pulse rating | The short-duration energy or power the resistor can withstand. |
Types you may encounter
Common fixed-resistor constructions include carbon composition, carbon film, metal film, metal oxide, wirewound, thick film, thin film, and metal-strip current-sense resistors. Variable resistors include potentiometers, rheostats, and preset trimmers. Other resistive devices deliberately change resistance in response to their environment: thermistors respond to temperature, photoresistors or LDRs respond to light, and varistors change strongly with applied voltage. Each construction offers a different balance of cost, accuracy, noise, stability, pulse capability, power handling, and high-frequency behavior.
Continue through the resistor pages
The Basics page explains Ohm's law, color bands, preferred values, power, series and parallel connections, and simple applications. The History page follows the development of resistance measurement and practical resistor construction. The Advanced page covers resistivity, temperature effects, noise, parasitics, precision measurement, pulse loading, and engineering selection. The Variable Resistors page introduces potentiometers, rheostats, and trimmers. The Using Variable Resistors page shows how the same three-terminal part can be wired as a voltage divider or as a two-terminal variable resistance. The next pages cover thermistors, photoresistors, and varistors—resistive parts whose values change because of temperature, light, or voltage rather than by turning a knob.