Why LEDs Need a Current-Limiting Resistor

Unlike incandescent bulbs, LEDs are current-controlled devices. They have a forward voltage drop (typically 1.8–3.5V depending on colour) and will pass unlimited current if connected directly to a power supply — burning out in milliseconds. A series resistor limits current to the LED's rated value (typically 20mA for standard 5mm LEDs).

The Formula

R = (V_supply − V_forward) / I_forward

Where:

• V_supply = power supply voltage
• V_forward = LED forward voltage (from datasheet: ~1.8–2.2V red/yellow, ~3.0–3.5V blue/white/green)
• I_forward = desired current (typically 0.010–0.020A for standard LEDs)

How to Use This Calculator

Enter the supply voltage, LED forward voltage, and desired current. The calculator gives the exact resistor value, then shows the nearest standard E24 resistor values and the actual current each would produce.

Practical Examples

Example 1: Red LED (Vf = 2.0V, If = 20mA) on 5V supply. R = (5 − 2.0) / 0.02 = 150Ω. Standard value: 150Ω.

Example 2: White LED (Vf = 3.3V, If = 20mA) on 3.3V Arduino pin. R = (3.3 − 3.3) / 0.02 = 0Ω — DON'T connect directly! Use 3.3V and a 68Ω resistor if If = 5mA is acceptable.

Example 3: Multiple LEDs in series on 12V (3 red LEDs, Vf total = 6V). R = (12 − 6) / 0.02 = 300Ω.