LED Series Resistor Calculator
Calculate the required resistance and power rating for your LED circuit.
Calculated Specifications
Required Resistance:
0 Ω
Power Dissipated by Resistor:
0 W
Recommended Resistor Wattage:
0 W
Warning: Source voltage must be higher than the total LED forward voltage.
Understanding LED Resistance Calculations
When building circuits with Light Emitting Diodes (LEDs), you cannot connect them directly to a power source like a battery or power supply without a resistor. Because LEDs are non-linear devices, they will attempt to draw as much current as possible, which leads to immediate "thermal runaway" and destroys the component.
To prevent this, we use a Current Limiting Resistor. This calculator uses Ohm's Law to determine the exact resistance needed to protect your LEDs while ensuring they shine at their optimal brightness.
The LED Resistor Formula
The math behind this calculator follows a simple variation of Ohm's Law (V = I × R):
R = (Vsource – (Vforward × N)) / Iforward
- Vsource: The voltage provided by your power supply.
- Vforward: The voltage drop across a single LED (see table below).
- N: The number of LEDs connected in series.
- Iforward: The desired current in Amperes (Note: 1000mA = 1A).
Typical LED Forward Voltages
| LED Color | Typical Forward Voltage (V) | Typical Current (mA) |
|---|---|---|
| Red | 1.8V – 2.2V | 20mA |
| Yellow / Orange | 2.0V – 2.2V | 20mA |
| Green | 2.1V – 3.3V | 20mA |
| Blue / White | 3.0V – 3.6V | 20mA |
| Infrared | 1.2V – 1.5V | 20mA |
Real-World Example Calculation
Suppose you have a 12V battery and you want to light up 3 Red LEDs in series. Each LED has a forward voltage of 2.0V and requires 20mA (0.020A).
- Calculate total LED voltage: 3 LEDs × 2.0V = 6.0V.
- Calculate voltage the resistor must drop: 12V – 6.0V = 6.0V.
- Apply Ohm's Law: 6.0V / 0.020A = 300 Ohms.
- Calculate Power: 6.0V × 0.020A = 0.12 Watts. A standard 1/4 watt (0.25W) resistor would work perfectly.