CIRCUIT FUNDAMENTALS · BEGINNER

Choose a current-limiting resistor for an LED

An LED is not a resistor. Once it reaches its forward voltage, a small change in voltage can cause a large change in current. A series resistor sets a predictable and safer current.

Open the interactive LED example

The calculation

Subtract the LED forward voltage from the supply voltage. The remaining voltage appears across the resistor. Divide it by the desired current.

R = (V_supply − V_LED) ÷ I_LED

For a 5 V supply, a blue LED with an approximately 2 V forward voltage and a 10 mA target current, the result is 300 Ω. Choose the next preferred standard value above the result, such as 330 Ω, to avoid exceeding the desired current.

Try it in resistor.dev

Open the LED example and run the simulation.
Select the LED or resistor to read voltage, current and power.
Change the resistor from 330 Ω to 1 kΩ. The LED current decreases.
Try 100 Ω and observe the higher current and overload warnings.
Change the supply voltage, then recalculate a suitable resistor.

Check resistor power

A resistor also dissipates power: P = I²R. For small indicator LEDs the power is usually low, but calculate it whenever supply voltage or current increases. Select a resistor with a practical power margin and verify the LED's pulse and continuous-current ratings in its data sheet.

Polarity still matters

Current must flow from the LED anode to cathode. Reversing the LED blocks current until reverse breakdown, which can damage common indicator LEDs. A resistor limits forward current but does not replace correct polarity or a manufacturer data sheet.