LED Series Resistor
Resistor to safely drive an LED.
Formula
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Example
(5-2)/0.02 = 150 Ω
How it works
An LED does not behave like a simple resistor: once it reaches its forward voltage, the current can rise very quickly. This formula finds the series resistor needed to drop the extra supply voltage so the LED current stays at a safe, intended value. It comes from Ohm’s law, using the voltage left over after the LED’s forward drop: the resistor must handle (V_s - V_f) at current I.
When to use it
Use this when designing a basic LED indicator or any circuit where an LED is powered from a higher DC supply and needs a current-limiting resistor. It is common in breadboard projects, simple panels, Arduino outputs, and battery-powered LED setups.
Worked example, step by step
Suppose a red LED has a forward voltage of 1.8 V and you want 15 mA of current from a 9 V supply. R = (V_s - V_f) / I R = (9.0 - 1.8) / 0.015 R = 7.2 / 0.015 R = 480 Ω So the required series resistor is 480 ohms. In practice, you might choose the nearest standard value, such as 470 Ω.
Common mistakes to avoid
- Forgetting to convert milliamps to amps before calculating
- Using the supply voltage directly without subtracting the LED forward voltage
- Ignoring the resistor power rating, which can cause overheating
Did You Know?
Choosing the right series resistor keeps an LED bright without letting excess current destroy it.
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