How to use
- Enter the DC supply voltage, the LED forward voltage at the intended operating point and the target current in milliamperes. Use the LED data sheet for its forward voltage; color alone is not enough.
- Press Calculate to see resistance, voltage drop, current and resistor dissipation. The diagram shows the supply, resistor and LED in series. Copy result copies the confirmed values. Edits and Clear remove the old diagram too.
Formula and interpretation
R = (V_supply − V_LED) / I, with I in amperes. Resistor power is P_R = (V_supply − V_LED) × I = I² × R. The displayed resistance is the ideal calculated value; this tool does not select a commercial resistor or a power rating.
Worked example
With a 5 V supply, a 2 V forward voltage and 10 mA target current, the resistor drops 3 V. The calculation gives 300 Ω and 0.03 W of resistor dissipation. The LED current is 0.01 A.
Limits and input errors
Supply: 0–1,000 V; LED forward voltage: greater than 0 up to 100 V; current: greater than 0 up to 1,000 mA. These are numerical input limits, not approved component operating ranges. Supply must exceed forward voltage. The ideal model excludes temperature changes, tolerances, transients and multiple LEDs.
Positive numeric inputs below 10⁻¹⁰⁰ are rejected to avoid numerical underflow. Use a decimal point or comma without grouping; scientific notation such as 1e-6 is accepted.
Frequently asked questions
Why is equal or lower supply voltage rejected?
There is no positive voltage headroom for a series resistor at the specified forward voltage and target current. A zero-ohm result would not provide current limiting.
Can I use the power result as the part rating?
No. Calculated dissipation is not a rating recommendation or thermal guarantee. Check the actual resistor and LED data sheets and their operating conditions before choosing components.