Table of Contents
LED Resistor Basics
LEDs (Light Emitting Diodes) require a current-limiting resistor to prevent excessive current that would destroy them. Unlike incandescent bulbs, LEDs have a very steep current-voltage curve; even small voltage increases above the forward voltage threshold cause large current increases. The series resistor drops the excess voltage and limits current to a safe operating level.
The forward voltage varies by LED color and material. Red LEDs typically have 1.8-2.2V forward voltage, while blue and white LEDs need 3.0-3.5V. Power dissipation in the resistor must be considered to select an appropriately rated component, especially with high-power LEDs or series strings.
LED Resistor Formula
LED Color Voltage Table
| LED Color | Forward Voltage | Typical Current |
|---|---|---|
| Infrared | 1.1 - 1.5 V | 20 mA |
| Red | 1.8 - 2.2 V | 20 mA |
| Orange | 2.0 - 2.2 V | 20 mA |
| Yellow | 2.0 - 2.2 V | 20 mA |
| Green | 2.0 - 3.5 V | 20 mA |
| Blue | 3.0 - 3.5 V | 20 mA |
| White | 3.0 - 3.5 V | 20 mA |
Frequently Asked Questions
Can I run LEDs without a resistor?
Running LEDs without current limiting is risky. If the supply voltage is exactly the forward voltage, the LED might work briefly but will likely fail from thermal runaway. Always use a resistor or constant-current driver. The exception is when using a regulated constant-current power supply designed for LEDs.
How do I calculate for LEDs in parallel?
Each parallel LED branch should have its own resistor. Do not share a single resistor among parallel LEDs because manufacturing variations in forward voltage cause unequal current sharing, leading to potential failure of the LED carrying the most current.
What resistor wattage do I need?
Calculate P = I^2 x R and choose a resistor rated at least 2x this value for reliable operation. For a 20mA LED with a 500 Ohm resistor, power is 0.2W, so a 1/2W resistor provides adequate margin.