Calculate the current limiting resistor for an LED circuit
Supply voltage Vs (V)
LED forward voltage Vf (V)
LED current (mA)
Result
Answer
Step-by-Step Derivation
LED Resistor Formula
R = (Vs - Vf) / I
P = I^2 R
The LED resistor drops the extra supply voltage that the LED does not use. Ohm law converts that voltage difference and target current into a safe series resistance.
⚠Choose the next higher standard resistor value and a resistor wattage above the calculated power. LED forward voltage varies by color and device.
How LED Current Limiting Works
An LED behaves differently from a simple resistor and can draw excessive current if connected directly to a voltage source. A series resistor stabilizes the current.
Voltage Headroom
The resistor drops Vs minus Vf.
Current Target
LED current is entered in milliamps and converted to amps.
Power Rating
The resistor must safely dissipate I squared R.
Standard Values
Use the nearest higher standard resistor for safety.
Example: For Vs = 5 V, Vf = 2 V, and I = 20 mA, R = (5-2)/0.02 = 150 ohm.
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