LED Resistor Calculator
Required Resistance
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Resistor Power Rating
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Nearest Standard Value
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How to Calculate an LED Resistor
LEDs need a current-limiting resistor: R = (V_supply - V_forward) / I_LED. Without it, the LED draws too much current and fails.
Common forward voltages: Red=1.8-2.2V, Green=2.0-3.5V, Blue/White=3.0-3.5V. Standard current: 20mA for through-hole LEDs.
The resistor must handle power dissipation: P = (V_supply - V_forward) x I_LED. Choose a rating above the calculated value.
Frequently Asked Questions
What happens without a resistor?
The LED draws excessive current, overheats, and burns out almost instantly.
One resistor for series LEDs?
Yes. R = (V_supply - V_LED1 - V_LED2...) / I. Ensure supply exceeds sum of forward voltages.
What forward voltage to use?
Red: 1.8-2.2V, Yellow: 2.0-2.2V, Green: 2.0-3.5V, Blue/White: 3.0-3.5V. Check datasheet.
Use exact or standard resistance?
Use nearest standard value equal to or higher than calculated. Slightly higher is safer for the LED.