LED Series Resistor Calculator

Calculate the series resistor for one or several LEDs in series. Enter Supply Voltage, LED forward voltage, desired LED current and number of LEDs. Leave all fields filled and press Calculate.

.led-calc {
max-width: 520px;
margin: 28px auto;
padding: 18px;
border-radius: 10px;
background:#fff;
box-shadow: 0 6px 18px rgba(0,0,0,0.06);
font-family: Arial, sans-serif;
}
.led-calc h2 { margin:0 0 6px; text-align:center; }
.led-calc p.caption { text-align:center; color:#555; margin:0 0 12px; font-size:14px; }
.grid {
display:grid;
grid-template-columns: 1fr 190px;
gap:10px 12px;
align-items:center;
}
.grid label { font-size:14px; color:#222; }
.grid input[type=”number”], .grid select {
width:120px;
padding:8px;
box-sizing:border-box;
border:1px solid #ccc;
border-radius:6px;
font-size:14px;
}
.actions { text-align:center; margin-top:12px; }
.btn {
display:inline-block;
padding:9px 18px;
border-radius:7px;
border:none;
cursor:pointer;
font-weight:600;
}
.btn.calc { background:#007bff; color:#fff; margin-right:8px; }
.btn.reset { background:#fff; color:#007bff; border:1px solid #007bff; }
.result {
margin-top:14px;
padding:12px;
border-radius:8px;
background:#fafafa;
border:1px solid #eee;
font-weight:600;
text-align:center;
}
.note { margin-top:12px; font-size:13px; color:#444; }
table.vf-table { width:100%; margin-top:10px; border-collapse:collapse; font-size:13px; }
table.vf-table th, table.vf-table td { border:1px solid #eee; padding:6px 8px; text-align:center; }
@media (max-width:520px){
.grid { grid-template-columns: 1fr; }
.grid select { width: 100%; }
}

function formatOhms(R) {
if (!isFinite(R) || R === null) return ‘-‘;
if (Math.abs(R) >= 1e6) return (R/1e6).toFixed(2) + ‘ MΩ’;
if (Math.abs(R) >= 1e3) return (R/1e3).toFixed(2) + ‘ kΩ’;
return R.toFixed(2) + ‘ Ω’;
}
function formatPower(P) {
if (!isFinite(P) || P === null) return ‘-‘;
if (Math.abs(P) >= 1) return P.toFixed(2) + ‘ W’;
return (P*1000).toFixed(2) + ‘ mW’;
}
function parseInputValue(id, unitId) {
const v = parseFloat(document.getElementById(id).value);
if (isNaN(v)) return NaN;
const u = parseFloat(document.getElementById(unitId).value);
return v * u;
}
function calcLedResistor() {
const vs = parseInputValue(‘vs’,’vsUnit’); // V
const vf_single = parseInputValue(‘vf’,’vfUnit’); // V
const n = parseInt(document.getElementById(‘nLed’).value) || 1;
const i = parseFloat(document.getElementById(‘iLed’).value) * parseFloat(document.getElementById(‘iUnit’).value); // A

const out = document.getElementById(‘ledResult’);
out.style.display = ‘none’;
out.innerHTML = ”;

// validation
if (!isFinite(vs) || !isFinite(vf_single) || !isFinite(i) || n <= 0) {
alert('Please enter valid numeric values for supply, forward voltage, current and number of LEDs.');
return;
}
if (i <= 0) {
alert('Current must be greater than zero.');
return;
}

const vf_total = vf_single * n;
const vdrop = vs – vf_total;

if (vdrop <= 0) {
out.style.display = 'block';
out.innerHTML = `

`;
return;
}

const R = vdrop / i; // ohms
const P_R = vdrop * i; // watts (power dissipated in resistor)

// Suggest standard resistor wattage: if P>0.25W recommend 1/2W etc. (simple hint)
let wattHint = ”;
if (P_R <= 0.125) wattHint = 'Suggested resistor rating: 1/8 W or higher';
else if (P_R <= 0.25) wattHint = 'Suggested resistor rating: 1/4 W or higher';
else if (P_R <= 0.5) wattHint = 'Suggested resistor rating: 1/2 W or higher';
else if (P_R <= 1) wattHint = 'Suggested resistor rating: 1 W or higher';
else wattHint = 'Suggested resistor rating: ≥2 W (use proper heat dissipation)';

// Show results
out.style.display = 'block';
out.innerHTML = `

`;
}

function resetLedCalc(){
[‘vs’,’vf’,’nLed’,’iLed’].forEach(id=>document.getElementById(id).value = ”);
document.getElementById(‘vsUnit’).value = ‘1’;
document.getElementById(‘vfUnit’).value = ‘1’;
document.getElementById(‘iUnit’).value = ‘0.001’;
const out = document.getElementById(‘ledResult’);
out.style.display = ‘none’;
out.innerHTML = ”;
}

(adsbygoogle = window.adsbygoogle || []).push({});