Comment fonctionne le calcul
The resistor value is Ohm's law applied to the voltage the LEDs do not use: R = (Vs − n × Vf) ÷ I, with the milliamp figure converted to amps first. At the defaults of a 9 V supply, 2.0 V forward voltage, 20 mA and one LED, that is 7 V ÷ 0.02 A = 350 Ω. The tool then rounds up to the next standard value in your chosen series, 390 Ω in E12, recalculates the real current through it at 17.9 mA, and reports P = V_R × I.
Forward voltage moves the answer most, and the preset row fills it in: Red 2.0 V, Yellow 2.1 V, Green 2.2 V, Blue and White 3.2 V, IR 1.5 V, UV 3.5 V. Use your LED's datasheet where you have one. LEDs in series multiplies Vf, not the current. The resistor series only decides which stock value you are offered, with E12 (10%) giving coarse steps and E24 (5%) landing closer to the exact figure, and both round upward so actual current lands at or below your target.
One resistor, one series string. Putting LEDs in parallel behind a single resistor is not modelled, and should not be built that way, since the lowest-Vf LED takes more than its share of the current. Forward voltage is treated as a constant, but real Vf shifts with current, temperature and production batch, so measured current will differ from the figure shown. The recommended wattage is simply twice the calculated dissipation, with no allowance for a hot enclosure or a constant-current driver.
FAQ
What resistor do I need for a red LED on a 9V battery? +
With the Red preset at 2.0 V and 20 mA, the resistor has to drop 7 V, so the exact value is 350 Ω. The nearest E12 part is 390 Ω, which brings current down to 17.9 mA, slightly dimmer but comfortably inside the LED's rating.
How many LEDs can I run in series from one supply? +
As many as fit under the supply with headroom left for the resistor. Four 3.2 V white LEDs need 12.8 V, so a 12 V supply fails and the tool reports Supply too low; three need 9.6 V and leave 2.4 V across the resistor.
Should I pick the E12 or E24 resistor series? +
E12 is the 10% tolerance range with twelve values per decade, which is what most beginner kits contain. E24 is the 5% range with twenty-four values, so it lands closer to the exact figure, offering 360 Ω instead of 390 Ω against a 350 Ω target.
Why is the suggested resistor always bigger than the calculated value? +
Rounding up is the safe direction. More resistance means less current than your target, whereas the next value down would push past the LED's rated forward current. The Current at nearest row shows what you give up, 17.9 mA against a 20 mA target on the defaults.
What power rating does the resistor need? +
The tool doubles the dissipated power for headroom and picks a standard rating from that. On the defaults the resistor burns 126 mW, so doubling gives 251 mW and it recommends a 1/2 W part rather than a marginal 1/4 W one.
