LED Series Resistor Calculator
Calculate the exact and nearest standard E24 resistor value and power rating to safely drive any LED from 3.3V, 5V, 12V, or custom DC supply.
How it works
An LED (Light Emitting Diode) is a current-driven device with a relatively constant voltage drop (Vf). If connected directly to a voltage source higher than Vf, it will draw excessive current and burn out instantly.
A series resistor limits the current by dropping the remaining voltage. The formula is derived from Ohm's Law:
R = (Vs - Vf) / I
Where Vs is the supply voltage, Vf is the LED's forward voltage drop, and I is the desired current.
Worked example
You want to power a standard Red LED (typically Vf = 2.0V) from a 5V USB supply. A safe current for most standard 5mm LEDs is 20mA (0.02A).
Using R = (Vs - Vf) / I,
R = (5V - 2.0V) / 0.02A = 150 Ω.
The exact value is 150 Ω, which happens to be a standard E24 resistor value.
Frequently Asked Questions
What happens if I use a larger or smaller resistor?
A larger resistor will reduce the current, making the LED dimmer but extending its lifespan. A smaller resistor will allow more current, making the LED brighter but risking a reduced lifespan or instant burnout if the maximum current is exceeded.
Can I use this for high-power (e.g. 1W, 3W, 10W) LEDs?
No. High-power LEDs heat up, which lowers their forward voltage (Vf), causing them to draw even more current if driven by a simple resistor (thermal runaway). High-power LEDs require active constant-current driver circuits.
What are typical Forward Voltage (Vf) values?
Typically: Red/Yellow/Orange: ~1.8V to 2.2V. Green/Blue/White/UV: ~3.0V to 3.4V. Infrared (IR): ~1.2V to 1.5V.
Sources
- Basic LED driving circuits (The Art of Electronics).
- Typical LED datasheets (Vishay, Cree, Kingbright).
Results are for engineering assistance. Verify important calculations and check official datasheets. See the Engineering Disclaimer.