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Size a Current-Limiting Resistor for an LED

Work out the resistor value an LED needs from supply voltage and current.

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V
A
Ω
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Enter any two values and the other two are solved automatically

💡 Ohm's law: V = I × R. Power adds P = V × I. Knowing any two lets you derive the rest — handy for sizing resistors, checking LED current, or working out heat dissipation.

LEDs tips

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Resistor value R = (supply V − LED forward V) ÷ LED current. Enter the voltage drop and target current to get R.

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A typical LED wants ~20mA (0.02A). With a 5V supply and a 2V LED, that needs ~150Ω.

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Check the power too — the calculator shows watts so you can pick a resistor that won't overheat.

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