Electrical Circuits
LED Series Resistor
An LED needs a current-limiting element because its forward voltage changes only slightly with current. With a series resistor, the resistor drops the difference between the supply voltage and the LED forward voltage, and the target current sets the resistance. The forward voltage is device-specific and depends on colour and technology, so it must be taken from the datasheet rather than assumed.
LED Series Resistor
R = (V_s − V_f) ÷ I_f
Variables and units
| Symbol | Quantity | Unit |
|---|---|---|
| R | Series resistance | Ω |
| V_s | Supply voltage | V |
| V_f | LED forward voltage | V |
| I_f | LED forward current | A |
Conditions of application
The LED forward voltage and target current must come from the device data. The supply must exceed the forward voltage. Check the resistor power rating and the LED thermal limits.
Worked example
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