LED Series Resistor
Resistor to safely drive an LED.
Formula
Live Calculator
MembersVariables
Example
(5−2)/0.02 = 150Ω
How it works
An LED does not behave like a normal resistor: once it turns on, it needs a limited current to avoid damage. This formula uses Ohm’s law on the resistor that sits in series with the LED, because that resistor must absorb the extra voltage left over after the LED’s forward voltage is accounted for. The result is the resistor value that will set the current to a safe target level.
When to use it
Use this when you are connecting a single LED or a string of LEDs to a DC supply and need to choose the current-limiting resistor. It is common in beginner circuits, indicator lights, and breadboard projects.
Worked example, step by step
Suppose a 9 V supply is used with a blue LED that has a forward voltage of 3.2 V, and you want 15 mA of current. R = (V_s - V_f) / I R = (9.0 - 3.2) / 0.015 R = 5.8 / 0.015 R = 386.7 Ω So the resistor should be about 387 Ω; in practice, you would choose the nearest standard value, such as 390 Ω.
Common mistakes to avoid
- Using the supply voltage directly instead of subtracting the LED forward voltage
- Forgetting to convert milliamps to amps before calculating
- Choosing a resistor too small, which can overdrive and damage the LED
Did You Know?
LEDs need a series resistor because, unlike bulbs, they draw runaway current and burn out without one.
Share this formula
More in Electronics & Circuits
View allOhm's Law
BasicVoltage equals current times resistance.
Electrical Power (VI)
BasicPower dissipated by a component.
Power (I²R)
BasicPower lost as heat in a resistor.
Power (V²/R)
BasicPower from voltage and resistance.
Resistors in Series
BasicSeries resistances add directly.
Resistors in Parallel
IntermediateReciprocals of parallel resistances add.