One 1.8 V LED, E12 series
- Input
- 12V 1.8V 20mA E12
- Expected output
- 560R 18.2mA
The math gives 510 Ω; E12 only offers 560 Ω, passing 18.2 mA.
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12v led resistor
On 12 V the resistor stops being a footnote: it has to absorb more than 10 V and it really does get warm. The calculator opens here with a 12 V supply and a string of 3 white LEDs, and works out the resistor, real current and power with a safety margin. It is meant for low-voltage DC only.
The math gives 510 Ω; E12 only offers 560 Ω, passing 18.2 mA.
The exact value exists in E24: 510 Ω and 20.0 mA, close to 0.2 W.
2.7 V remain; 135 Ω becomes 150 Ω and 18.0 mA.
The LEDs drop 10 V and leave 2 V; 2 ÷ 0.020 = 100 Ω, exact.
With 10 V across the resistor, 1 kΩ gives exactly 10 mA and 0.1 W.
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.
For a red LED at 20 mA, 510 Ω (E24) or 560 Ω (E12). For a 3.1 V white LED the math gives 445 Ω and E12 rounds up to 470 Ω. If the LED is a "12 V" part with a built-in resistor, it needs no other.
Multiply the voltage across the resistor by the current. For a single LED that is 0.19 to 0.2 W, so use 1/2 W. With three LEDs in series it falls to 0.05 W and 1/4 W is plenty. The tool reports the smallest common rating with a 2x margin.
Yes, whenever it fits. The LEDs share one current, so total draw is that of a single LED, and the resistor runs cooler. The one condition is that the sum of the Vf values stays below the supply voltage.
No. The tool covers 0.1 to 60 V DC and is for low voltage only. Running an LED from mains calls for a different kind of circuit and risks that this page does not address.
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.