LED Resistor Calculator
An LED needs a resistor in series to set its current. Without one, it can draw more current than its data sheet allows. This page works out the resistance from the supply voltage, the voltage the LED drops, and the current you want.
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- Works offline after your first visit
- Saves on this device
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- Updated
How it works
The resistor takes the voltage left over, which is the supply voltage minus the LED's forward voltage. Ohm's law then gives the resistance: that leftover voltage divided by the current in amps. Milliamps are divided by 1,000 first.
The power in the resistor is the leftover voltage times the current. Its data sheet gives the forward voltage and the current to use.
Worked example
A 5 volt supply with an LED that drops 2 volts at 20 milliamps needs a resistor of about 150 ohms, which takes 0.06 watts.
Questions
Where do I find the forward voltage?
It is printed in the LED's data sheet, often at a stated current. Red LEDs are commonly near 2 volts and white ones near 3 volts, but the data sheet is the figure to use.
Why does the current matter?
A higher current makes the LED brighter, and it also shortens its life. Stay with the current its data sheet gives.
What if the supply is lower than the LED's forward voltage?
The result shows 0, because no voltage is left over to drop. The LED will not light properly from that supply without a different design.
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