LED Series Resistor Calculator (Formula + Examples)
LED Series Resistor Calculator (Formula + Examples)
Formula
[ R = (V_\text{supply} - V_\text{LED}) / I_\text{LED} ]
Example 1: 5V + red LED (2.0V) at 15 mA
R ≈ 200Ω → 220Ω
Example 2: 12V + white LED (3.1V) at 20 mA
R ≈ 445Ω → 470Ω
Common mistakes
mA vs A, parallel LEDs without a resistor per branch, using random Vf tables.
Frequently Asked Questions
Why do LEDs need resistors?
LEDs have very low internal resistance once conducting. Without a current-limiting resistor, they draw excessive current and burn out immediately. The resistor drops excess voltage and limits current to safe levels.
Can I use a higher resistor value than calculated?
Yes! Using a slightly higher resistor is safer and extends LED lifespan. The brightness difference between 15mA and 20mA is usually imperceptible. When in doubt, choose the next higher standard value.
What happens if I don't use a resistor?
Without a resistor, the LED will draw excessive current (limited only by power supply capacity), overheat, and fail immediately. Always use a current-limiting resistor.
How do I connect multiple LEDs?
- Series: Connect LEDs end-to-end with one resistor. Voltages add up (3 LEDs × 2V = 6V total).
- Parallel: Each LED branch needs its own resistor. Don't share one resistor across parallel LEDs.
What's the difference between series and parallel?
- Series: LEDs share current, forward voltages add up. One resistor for the entire chain.
- Parallel: Each LED gets full supply voltage, but each branch needs its own resistor for proper current limiting.
What if my calculated resistor value doesn't exist?
Use the nearest standard E24 resistor value. Standard values: 100, 120, 150, 180, 220, 270, 330, 390, 470, 560, 680, 820, 1000Ω, etc. Slightly higher is safer than lower.
