LED String Series Resistor Calculator

Want to know the exact current limiter resistor value for a specific LED series string? You can now use this calculator toll to quickly get the precise LED series string resistor value.

So first we are doing a very basic LED series resistor, just the normal LED protection idea that we all already know but still we calculate because guessing is bad, right, so we use this calculator to get the accurate results...

LED Supply Voltage

We start with supply voltage. This is simply the voltage that you already have in hand, like 5V, 9V, 12V, 24V, anything. We put this value because the LED does not see full supply directly, but still it is the source, so we need to know it.

LED Series Forward Voltage Drop

Then we talk about LED forward voltage. Each LED eats some voltage when it glows. Red LED eats less, blue and white pull more, but anyway we already know the datasheet value or approximate value, so we put that number here.

LED Current

Then comes LED current. This is the current at which the LED is comfortable. Normally 10mA, 20mA, this may be sometimes lower or higher. Calculator asks it in mA, but internally it quietly converts it into Ampere because formulas do not like mA.

Then we ask how many LEDs are connected in series. This is important because one LED is not same as three LEDs. Series means voltages add up, current stays same so now we multiply LED forward voltage with number of LEDs which gives total LED voltage.

How the Formula Works

Now this is the key thinking moment. Supply voltage minus total LED voltage gives us leftover voltage. That leftover voltage must go somewhere, and yes it goes into the resistor, otherwise the LED will die.

So voltage across resistor is simply supply voltage minus LED total voltage. If this value becomes zero or negative, then calculator immediately tells you that. It means you are trying to run too many LEDs from too little voltage, so nothing magical can fix that.

Now once we have voltage across resistor, we use the most basic but most trusted formula. Resistor

Value = Voltage Across Resistor / LED Current.

Resistor Power

Then we also calculate resistor power. This is because resistor also heats, and we do not want smoke. So we multiply voltage across resistor with LED current which gives power in Watts. Calculator shows this value so you know what minimum rating you need.

But here we do one human common sense step. We never choose exactly that watt rating, rather always go higher. If calculator says 0.3W, then we choose 0.5W or 1W, because real life is not ideal life.

So now the calculator finally shows three things. Total LED Voltage, Series Resistor value and Minimum Resistor Power. That is all we need to safely connect LEDs without killing them.