Calculate Resistor for Led

LED Resistor Calculator

function calculateLedResistor() { var supplyVoltage = parseFloat(document.getElementById('supplyVoltage').value); var ledForwardVoltage = parseFloat(document.getElementById('ledForwardVoltage').value); var ledForwardCurrent_mA = parseFloat(document.getElementById('ledForwardCurrent').value); var resistorValueResult = document.getElementById('resistorValueResult'); var powerDissipationResult = document.getElementById('powerDissipationResult'); // Input validation if (isNaN(supplyVoltage) || isNaN(ledForwardVoltage) || isNaN(ledForwardCurrent_mA) || supplyVoltage <= 0 || ledForwardVoltage <= 0 || ledForwardCurrent_mA <= 0) { resistorValueResult.innerHTML = 'Required Resistor Value: Please enter valid positive numbers for all fields.'; powerDissipationResult.innerHTML = 'Resistor Power Dissipation: –'; return; } if (supplyVoltage <= ledForwardVoltage) { resistorValueResult.innerHTML = 'Required Resistor Value: Supply Voltage must be greater than LED Forward Voltage.'; powerDissipationResult.innerHTML = 'Resistor Power Dissipation: –'; return; } // Convert mA to Amps for calculation var ledForwardCurrent_A = ledForwardCurrent_mA / 1000; // Calculate voltage drop across the resistor var voltageDropAcrossResistor = supplyVoltage – ledForwardVoltage; // Calculate resistor value using Ohm's Law: R = V / I var resistorValue = voltageDropAcrossResistor / ledForwardCurrent_A; // Calculate power dissipation of the resistor: P = V * I var powerDissipation = voltageDropAcrossResistor * ledForwardCurrent_A; resistorValueResult.innerHTML = 'Required Resistor Value: ' + resistorValue.toFixed(2) + ' Ω'; // Ω is Ohm symbol powerDissipationResult.innerHTML = 'Resistor Power Dissipation: ' + powerDissipation.toFixed(3) + ' W'; }

Understanding the LED Resistor Calculator

Light Emitting Diodes (LEDs) are incredibly useful components in electronics, but they require careful handling. Unlike traditional incandescent bulbs, LEDs are current-driven devices, meaning their brightness and lifespan are primarily determined by the amount of current flowing through them, not just the voltage applied.

Why Do LEDs Need a Resistor?

If you connect an LED directly to a power source without a current-limiting resistor, it will draw excessive current. This "overcurrent" can quickly damage or destroy the LED, often causing it to burn out instantly. The resistor acts as a current limiter, dropping a portion of the supply voltage and thus restricting the current to a safe level for the LED.

Every LED has a specific "forward voltage" (Vf) and a "forward current" (If) rating. The forward voltage is the voltage drop across the LED when it's operating correctly, and the forward current is the optimal current for its brightness and longevity. These values are typically found in the LED's datasheet.

How the Calculation Works

The calculator uses Ohm's Law and basic circuit principles to determine the appropriate resistor value and its power dissipation. Here's a breakdown:

1. Voltage Drop Across Resistor (V_resistor): The resistor needs to drop the difference between your supply voltage and the LED's forward voltage.
   V_resistor = Supply Voltage (V) - LED Forward Voltage (Vf)
2. Resistor Value (R): Once you know the voltage drop across the resistor and the desired current through the LED (which is also the current through the resistor), you can calculate the resistance using Ohm's Law (R = V / I).
   R = V_resistor / LED Forward Current (A)
3. Resistor Power Dissipation (P_resistor): Resistors convert electrical energy into heat. It's crucial to select a resistor with a power rating higher than its calculated dissipation to prevent it from overheating and failing. The power dissipated by the resistor is calculated as:
   P_resistor = V_resistor * LED Forward Current (A)

Example Calculation:

Let's say you have a 5V supply voltage, an LED with a 2.0V forward voltage (Vf), and you want to drive it with 20mA (0.02A) forward current (If).

• Voltage Drop Across Resistor: 5V – 2.0V = 3.0V
• Required Resistor Value: 3.0V / 0.02A = 150 Ω
• Resistor Power Dissipation: 3.0V * 0.02A = 0.06 W

Based on this, you would need a 150 Ohm resistor. For power dissipation, a standard 1/4 Watt (0.25W) resistor would be more than sufficient, as 0.06W is well below its rating.

Choosing the Right Resistor

After calculating the ideal resistor value, you'll likely need to choose a standard resistor value that is close to, but usually slightly higher than, your calculated value. This ensures the current remains at or below the desired level. Always ensure the resistor's power rating is significantly higher than the calculated power dissipation to ensure reliability and prevent overheating.