When to use this calculator
An ideal voltage-drop model provides a starting point; real LED forward voltage changes with current and temperature.
Explore the dc circuits & components calculators for other questions in this topic. For a related calculation, use the Ohms Law Current Calculator: Calculate ideal DC current from voltage and resistance.
PUTTING IT INTO PRACTICE
LED Series Resistor: a worked example
The values below are an illustrative example, not a recommendation. Replace them with measurements or assumptions that match your question.
| Input | Example value |
|---|---|
| Supply voltage (V) | 5 |
| LED forward voltage (V) | 2 |
| Target current (mA) | 20 |
| Result | 150 Ω |
- Check the input labels and enter the example values above.
- Apply the relationship: Resistance = (supply − forward voltage)/(current/1000).
- The result is 150 Ω, rounded for display.
How to interpret your result
Use the LED datasheet, choose a suitable standard resistor and verify power dissipation separately.
Ideal component models omit tolerances, heat and parasitic effects. Distinguish nominal energy from usable energy for batteries. Component ratings and installation requirements need separate manufacturer and design checks.
Frequently asked questions
Can these calculations select a safe installation rating?
No. They explain ideal relationships and do not include every thermal, protective or regulatory requirement.
When is the led series resistor calculator useful?
An ideal voltage-drop model provides a starting point; real LED forward voltage changes with current and temperature.
How do I use the led series resistor calculator?
Enter supply voltage (v), led forward voltage (v), target current (ma). The result updates automatically. Use the units shown beside each input; the example values illustrate the method.
Which formula does this calculator use?
Resistance = (supply − forward voltage)/(current/1000).
What should I check before using the result?
Use the LED datasheet, choose a suitable standard resistor and verify power dissipation separately.
How are results rounded?
Scalar numbers display up to six decimal places, with scientific notation for very large or small values. Text, fraction, date and schedule outputs use the format described on the tool. Intermediate calculations use JavaScript floating-point arithmetic. Keep sensible precision for your measurements.
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