Frequently Asked Questions
How do I size an LED series resistor?
R = (V_supply − n × V_forward) ÷ I, where n is the number of LEDs in series and I is the desired current. Example: (12 − 2×2.0) ÷ 0.02 = 400 Ω; pick the next standard value.
How is this different from a resistor color-code calculator?
A color-code tool decodes the bands of an existing resistor. This computes the resistance you need to safely drive an LED, plus the power the resistor must dissipate.
What resistor power rating do I need?
P = I² × R. A typical 20 mA LED resistor dissipates well under ¼ W, but always pick a rating at least 2× the calculated dissipation for margin.
Can I put LEDs in series or parallel on one resistor?
Series LEDs share one resistor (sum their forward voltages). Parallel LEDs should each have their own resistor - shared resistors cause unequal current and current hogging.
Why does an LED without a resistor burn out right away?
An LED does not regulate its own current. Connect it straight across a supply and the current climbs without limit until the junction is destroyed. The current-limiting resistor turns the excess voltage into heat at a controlled current.
What if the supply voltage is lower than the total forward voltage?
The LEDs cannot light up. The calculator flags this with an error message. You need to reduce the number of LEDs in series or use a higher-voltage supply.
Why use E12 values instead of the exact value?
Resistors are manufactured in preferred-value series (E12, E24, E96). The nearest E12 value is the one you can readily buy. The resulting current difference is usually under 10%, well within the design margin for indicator applications.
Does this calculator work for high-power LEDs?
Only for low-power LEDs (typically up to 100 mA). High-power LEDs (1 W and up) need a constant-current driver that holds the current steady despite temperature and voltage variations.
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Estimates for informational purposes only.
Important Disclaimer: Estimates for informational purposes only.
This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.