A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it in the forward direction. Like all diodes, it only allows current to flow one way.
How an LED works
When a sufficient forward voltage is applied across the LED (typically 1.5 to 3 V depending on colour), electrons recombine with holes at the junction of two semiconductor materials. This releases energy in the form of photons (light). The colour of the light depends on the semiconductor material used.
LED in a circuit
An LED must always be connected in the correct direction (anode to positive, cathode to negative). A protective resistor must be placed in series with the LED to limit the current, because even a small excess current will destroy the LED.
To calculate the required resistance: R = (supply voltage minus LED voltage) / desired current.
Advantages over filament lamps
| Property | LED | Filament lamp |
|---|---|---|
| Efficiency | High (little energy wasted as heat) | Low (most energy wasted as heat) |
| Lifetime | Very long (tens of thousands of hours) | Short (typically 1000 hours) |
| Speed | Switches on instantly | Slight warm-up delay |
| Size | Very small | Larger |
| Current needed | Very low (typically 10 to 20 mA) | Much higher |
Uses in physics
LEDs are used as indicator lights in circuits, for testing whether current flows in a particular direction, and in optical fibre communication systems. Their small size and fast switching make them suitable for digital signals.
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