The calculation, without hidden assumptions
A simple low-current LED string needs a current-limiting element because LED current rises steeply once the junction conducts. For a basic DC series-resistor circuit, the resistor drops the voltage left after the LED forward voltages and any entered driver/switch drop. MAXScanner calculates the ideal resistance, rounds upward to a selectable E12 or E24 standard value, recomputes actual current, shows resistor dissipation and also translates the chosen resistance into a conventional four-band color code.
How to use this calculator
Enter the DC supply voltage, one LED forward voltage, number of identical LEDs in series and target current in mA.
Add any known transistor/driver/switch voltage drop, then choose E12 or E24 for the next standard resistor value at or above the ideal result.
Review actual current, resistor power and the four-band color code for the selected standard value, then verify LED datasheet and real thermal limits.
Where people use it
- •Indicator LEDs and breadboard circuits
- •Checking a simple series LED string
- •Comparing E12 versus E24 resistor choices
- •Identifying the selected standard resistor by color bands
Example: one 2 V LED at 15 mA from 5 V
The resistor must drop 3 V. R = 3/0.015 = 200 Ω. E24 keeps 200 Ω, which uses red-black-brown-gold bands for a typical ±5% four-band resistor.
What the result does not assume
- •LED forward voltage varies with current, temperature and manufacturing spread. Use the actual datasheet range rather than a color-based guess when the result matters.
- •The displayed four-band code describes the selected nominal resistance and assumed E-series tolerance; always read the real component marking when identifying an unknown resistor.
- •This simple series-resistor model is not a substitute for a constant-current driver for high-power LEDs, and parallel LED branches should not be assumed to share current evenly through one resistor.
Frequently asked questions
Why round the resistor upward?+
In this simplified fixed-Vf model, a resistor at or above the ideal value does not increase LED current above the target solely because of rounding.
Does the calculator show resistor color bands?+
Yes. It converts the selected E12/E24 nominal resistance to a conventional four-band code and shows the tolerance band used for the chosen series.
Can several LEDs share one resistor?+
Identical LEDs can be placed in one series string if the supply has enough voltage. Separate parallel branches generally need their own current-limiting path or an appropriate driver.
How do I choose resistor wattage?+
The calculator shows dissipation and applies the headroom percentage you enter, but the final component choice must also account for tolerance, ambient temperature, package derating and real operating conditions.
Semantic next steps
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Keep the result connected to the real job
MAXScanner keeps the formula and assumptions beside the result so you can verify the number before using it in a drawing, estimate, specification, worksheet or document. Where a supplier, manufacturer, drawing or applicable standard owns a requirement, that source remains authoritative.