LED Resistor Calculator

Calculate the proper current-limiting resistor value for your LED circuit. Supports single or multiple LEDs in series.

LED Resistor Formula: R = (Vs - Vf) / If For multiple LEDs in series: R = (Vs - n×Vf) / If Where: • R = Resistance in ohms (Ω) • Vs = Source voltage (V) • Vf = LED forward voltage (V) • If = LED forward current (A) • n = Number of LEDs in series Power Dissipation: P = I² × R = (Vs - Vf) × I Typical LED Forward Voltages: • Red: 1.8-2.2V • Green: 2.0-3.5V • Blue/White: 3.0-3.6V • Yellow: 2.0-2.2V • Infrared: 1.2-1.8V
Example 1 (Red LED from Arduino): Source: 5V LED Vf: 2V (red) Current: 20mA R = (5 - 2) / 0.02 = 150Ω Use 180Ω or 220Ω standard resistor P = 0.02² × 150 = 0.06W (use 1/4W resistor) Example 2 (White LED from 12V): Source: 12V LED Vf: 3.2V (white) Current: 20mA R = (12 - 3.2) / 0.02 = 440Ω Use 470Ω standard resistor P = 0.02² × 440 = 0.176W (use 1/4W resistor) Example 3 (Three LEDs in Series): Source: 12V LED Vf: 2V each (3 red LEDs) Current: 20mA Total Vf = 3 × 2 = 6V R = (12 - 6) / 0.02 = 300Ω Use 330Ω standard resistor P = 0.02² × 300 = 0.12W Example 4 (Low Power Indicator): Source: 3.3V LED Vf: 2V Current: 5mA (dimmer but saves power) R = (3.3 - 2) / 0.005 = 260Ω Use 270Ω standard resistor

Why do LEDs need a current-limiting resistor?

LEDs have very low internal resistance and will draw excessive current if connected directly to a voltage source, causing immediate burnout. A current-limiting resistor controls the current to safe levels (typically 5-30mA for standard LEDs).

How do I calculate LED resistor value?

Use the formula R = (Vs - Vf) / If, where Vs is source voltage, Vf is LED forward voltage, and If is desired forward current. For example, a red LED (Vf=2V) at 20mA from 5V needs R = (5-2)/0.02 = 150Ω.

What are typical LED forward voltages?

Red: 1.8-2.2V, Green: 2.0-3.5V, Blue/White: 3.0-3.6V, Yellow: 2.0-2.2V, Infrared: 1.2-1.8V, UV: 3.1-4.4V. Always check your specific LED datasheet for exact values.

What current should I use for my LED?

Standard 5mm LEDs typically use 10-20mA. Maximum is often 30mA, but running at 20mA gives good brightness with longer life. For low power applications, 5-10mA still provides visible light. High-power LEDs can handle 350mA to 3A+.

What wattage resistor do I need?

Calculate power with P = I² × R or P = (Vs-Vf) × I. Use a resistor rated at least 2× the calculated power for safety. For most standard LED circuits (20mA or less), 1/4W (0.25W) resistors are sufficient.

Can I use one resistor for multiple LEDs?

Only if LEDs are in series (current flows through all). For parallel LEDs, each needs its own resistor to ensure equal current distribution. If they share one resistor, one LED failing short will destroy the others.

What happens if I use the wrong resistor value?

Too low resistance = excessive current, overheating, shortened life or immediate burnout. Too high resistance = dim LED or no light. Always round up to the next standard resistor value for safety (e.g., 150Ω calculated → use 180Ω or 220Ω).

How do I connect multiple LEDs in series?

Series LEDs share the same current. Total forward voltage = sum of individual Vfs. Formula becomes R = (Vs - Vf1 - Vf2 - Vf3...) / If. Ensure Vs > total Vf, otherwise LEDs won't light.

What are standard resistor values?

E12 series (common): 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82, and multiples (100, 120, 150, 180, 220, 270, 330, 390, 470, 560, 680, 820). Always choose the next higher standard value from your calculation.

Why does my LED flicker or dim?

Possible causes: insufficient supply voltage, wrong resistor value (too high), poor connections, failing power supply, PWM control, or LED operating near threshold voltage. Check all connections and verify resistor value.

Can I use LEDs without resistors?

Only with a proper constant-current driver or current-regulated power supply. Some specialized circuits use active current limiting instead of resistors. Never connect LEDs directly to voltage sources without current limiting.

How does temperature affect LED performance?

High temperatures increase LED current and can cause thermal runaway without proper current limiting. Vf typically decreases ~2mV/°C as temperature rises. Heat sinking is important for high-power LEDs to maintain performance and longevity.

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