LED Resistor Calculator
Find the theoretical series resistance and dissipation for one LED powered by DC.
Formula
R = (Vs − Vf)/I; Pr = (Vs − Vf)I; requires Vs > Vf and I > 0.
Supply voltage: 9 V · LED forward voltage: 2.4 V · Target current: 20 mA · Theoretical resistance ≈ ((9 V)-(2.4 V))÷(20 mA × 0.001 A/mA) · Resistor dissipation ≈ ((9 V)-(2.4 V))×(20 mA × 0.001 A/mA)
How to use
- Enter the DC supply voltage.
- Enter one LED’s forward voltage at your chosen positive current.
- Read theoretical resistance and resistor dissipation.
Examples
- 9 V, Vf = 2.4 V and 20 mA give 330 Ω and 0.132 W.
Frequently asked questions
Why must supply exceed forward voltage?
The series resistor needs positive voltage headroom. Equal or lower supply voltage is rejected.
Is the resistance a standard component value?
No. It is the theoretical value; choosing a nearby standard resistor changes the current.
Is the power result a resistor rating?
No. It is calculated dissipation. Select a rating with suitable margin and check tolerances and the LED datasheet.
Sources
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