LED Series Resistor
The resistor between your supply and the LED, plus its dissipation.
Your recent runs (stored only in your browser)
No calculations yet — results land here so you can compare runs.
The engineering
The resistor absorbs the difference between a stiff supply and a diode that wants a fixed voltage. Typical Vf: red ≈ 1.8–2.0 V, green/yellow ≈ 2.0–2.2 V, blue/white ≈ 3.0–3.4 V.
Round up to the next standard value — LEDs are brighter than you think and last longer under-driven.
Where this math comes from
Henry Round saw silicon carbide glow in 1907 and wrote two paragraphs about it; Oleg Losev mapped the phenomenon carefully in the 1920s and was ignored. Nick Holonyak's 1962 red LED at GE finally made electroluminescence a product, and Shuji Nakamura's 1993 blue LED completed the palette — earning the 2014 Nobel and putting a Vf of 3.2 V on this card.
The series resistor is the humble half of the story: diodes have exponential I–V curves, so something linear has to take responsibility for the current.
- 1907H. J. RoundFirst observation of electroluminescence (SiC).
- 1962Nick HolonyakFirst practical visible LED (red, GaAsP).
- 1993Shuji NakamuraBlue GaN LED — white light, Nobel 2014.
See the full timeline of the math behind every calculator →
Runs entirely in your browser — nothing you enter leaves this page. Your recent runs are stored only on your device.