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LED Resistor Calculator

Calculate the correct series resistor value for an LED circuit using Ohm's law: R = (Vs - Vf) / If. Enter the supply voltage, LED forward voltage, and desired current to find the resistance in ohms and power dissipation in the resistor. Essential for Arduino projects, hobby electronics, breadboard prototyping, and LED strip driver design.

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Formula

R = (Vs - Vf) / If; P = If x (Vs - Vf).

The voltage across the resistor equals the supply voltage (Vs) minus the LED forward voltage drop (Vf). Dividing this by the desired current (If, in amps) gives the resistance in ohms via Ohm's law. The power dissipated in the resistor equals the current times the voltage drop across it, which determines the minimum wattage rating needed.

Worked Example

Supply voltage 12V, LED forward voltage 3.2V, desired current 20 mA: Step 1: Voltage across resistor = 12 - 3.2 = 8.8V Step 2: Current in amps = 20 mA = 0.02A Step 3: R = 8.8 / 0.02 = 440 ohms Step 4: Nearest standard value = 470 ohms (slightly reduces current to 18.7 mA) Step 5: Power = 0.02 x 8.8 = 0.176W (a 1/4W resistor is sufficient)

Frequently Asked Questions

What if the supply voltage is less than the LED forward voltage?

The LED will not light properly. The supply must exceed the forward voltage for current to flow through the circuit. If Vs < Vf, the formula gives a negative resistance, which is physically meaningless. Check your power supply and LED datasheet.

Should I use the exact calculated value or the nearest standard resistor?

Use the nearest higher standard resistor value (from the E12 or E24 series). Going slightly higher reduces current marginally but protects the LED from overcurrent. Common standard values include 220, 330, 470, 680, 1K, 1.5K, and 2.2K ohms.

What is the typical forward voltage for different LED colors?

Red LEDs: 1.8 to 2.2V. Yellow/Green: 2.0 to 2.4V. Blue/White: 3.0 to 3.4V. UV LEDs: 3.2 to 3.8V. Always check the specific LED datasheet for exact forward voltage at your target current.

Can I use this for multiple LEDs in series?

Yes. Add the forward voltages of all LEDs in the series chain, then use that total as Vf. For example, 3 red LEDs at 2V each: total Vf = 6V. Make sure the supply voltage exceeds the total forward voltage.

What wattage resistor do I need?

The calculator shows power dissipation. Choose a resistor rated for at least double the calculated power for reliability. For example, if the calculation shows 0.18W, use a 1/2W resistor. Most hobby projects use 1/4W resistors, which handle up to 0.25W.

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