Who This Guide Is For
Use this guide when you need a simple current-limiting resistor for an indicator LED or a short series string from a stable DC supply.
Formula Summary
R = (Vsupply - N × Vf) ÷ I
P = Vresistor × I = I²R
Convert LED current from milliamps to amps before calculating resistance or power.
Worked Example
For one 2 V LED at 20 mA from 12 V:
R = (12 - 2) / 0.020 = 500 Ω
The resistor dissipates 10 V × 0.020 A = 0.2 W, so a 0.5 W resistor gives better margin than a 0.25 W part.
Choose a Standard Value
Select the next higher standard value when the exact result is unavailable. Choosing 510 Ω instead of 500 Ω slightly reduces current and is generally preferable to exceeding the target.
Check Resistor Power
P = Vresistor × I = I²R
The example resistor dissipates 10 V × 0.020 A = 0.2 W. A 0.25 W resistor has little margin, while a 0.5 W part runs cooler.
Forward Voltage Is Not Fixed
Vf changes with current, temperature, LED color, and manufacturing variation. Use datasheet curves and evaluate current at minimum and maximum supply voltage.
Separate parallel LED strings generally need separate current limiting. High-power LEDs should use an appropriate constant-current driver.
Common Mistakes
- Using milliamps directly instead of amps.
- Selecting the exact calculated resistance when the next higher standard value is safer.
- Ignoring resistor power dissipation and temperature rise.
- Putting parallel LEDs behind one shared resistor and expecting equal current.
Reference Notes and Assumptions
This guide uses Ohm's law and resistor power formulas. LED forward voltage, current limits, and thermal constraints should come from the specific LED datasheet.