Three LEDs in series, one branch
- Input
- 12V 3.1V 20mA E12 x3
- Expected output
- 150R 18.0mA
The LEDs drop 9.3 V, leaving 2.7 V; 135 Ω becomes 150 Ω.
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resistor for multiple leds series parallel
With more than one LED, the question stops being "which resistor" and becomes "how many, and where". The calculator opens with 12 V and two branches of three white LEDs, and shows one resistor per branch, the total current and the power drawn from the supply.
The LEDs drop 9.3 V, leaving 2.7 V; 135 Ω becomes 150 Ω.
The resistor is the same per branch; the total rises to 36 mA and about 0.43 W from the supply.
8 V is left; 400 Ω is not in E12, so it goes up to 470 Ω.
The math gives 250 Ω; the next E24 value is 270 Ω, passing 18.5 mA.
That is the most you can fit on 5 V: three 1.8 V LEDs would add up to 5.4 V, above the supply.
Subtract the sum of the series LEDs' Vf from the supply voltage and divide by the desired current in amps: R = (Vsupply − N × Vf) ÷ I. For 5 V, a 1.8 V red LED and 20 mA, R = (5 − 1.8) ÷ 0.02 = 160 Ω.
One per branch, not per LED. LEDs in series share a single current and use one resistor. LEDs in parallel need separate resistors, one for each branch, so the current does not pile onto the lowest-Vf part.
The largest N where N × Vf is below the supply voltage. On 24 V with 2 V LEDs that is 11 (22 V), since 12 would add up to 24 V and leave nothing for the resistor. In practice, keep at least a few volts of headroom.
It works, but poorly. Because Vf varies from part to part, the current splits unevenly and the hardest-worked LED fails first. With one resistor per branch each LED gets the current you planned.
Multiply the real current of one branch by the number of branches: 2 × 18.0 mA = 36 mA. Supply power is the voltage times that current, 12 V × 0.036 A ≈ 0.43 W. The source has to handle that total with room to spare.
If the real Vf of your LED falls between 2.7 and 3.5 V (the range for this color), the current is between 10 and 26 mA with this resistor.
The calculation runs in your browser; no value is sent to a server. The tool is for low-voltage direct current only, never for wiring an LED straight to the mains.