LED circuits

A light-emitting diode, or LED, is a semiconductor device that produces light when an electric current passes through it in the forward direction. Like any diode it conducts in one direction only, and as current flows the energy released by charge carriers crossing its junction is emitted as light of a color set by the semiconductor material. LEDs are efficient, compact and long-lived, which has made them the standard choice for indicators, displays, signaling and, increasingly, general lighting.

The key fact in driving an LED is that it is a current-operated device with a roughly fixed forward voltage drop. If an LED were connected directly across a supply, the current would rise sharply and could destroy it, so almost every LED circuit includes a means of limiting current. The simplest method is a series resistor chosen so that, given the supply voltage and the LED's forward voltage, the current settles at a safe value. For multiple LEDs or for efficiency, more elaborate circuits use constant-current drivers that hold the current steady regardless of small voltage changes.

LED circuits range from a single indicator lamp to arrays, displays and decorative lighting. Designs may flash or dim LEDs, sequence several of them, or sense ambient light to switch them on automatically. Important considerations include selecting the right current-limiting component, observing the LED's polarity, and managing heat in high-power devices. Collections of LED circuits show practical examples of driving, dimming and arranging LEDs, and serve as an accessible introduction to working with diodes and current limiting.

Frequently asked questions

Why does an LED need a current-limiting resistor?

An LED has a nearly fixed forward voltage, so without limiting, the current would rise sharply and could destroy it.

What determines an LED's color?

The semiconductor material of the junction, which sets the energy and therefore the wavelength of the emitted light.

Does an LED's orientation matter?

Yes. Like any diode it conducts and emits light only when connected in the correct forward direction.