Red (1.8 V)
- Input
- 5V 1.8V 20mA E12
- Expected output
- 180R 17.8mA
Red has the lowest forward voltage and therefore the largest resistor.
Carregando ferramenta… / Loading tool…
led resistor by color red green blue white
Color is what sets an LED's physics: shorter wavelengths need more voltage to light. That is why one resistor does not fit every color. This page opens the tool with a blue LED on 5 V at 20 mA, and the examples compare the resistor for each color under identical conditions.
Red has the lowest forward voltage and therefore the largest resistor.
Both sit in the same range, near 2.1 V; 145 Ω becomes 150 Ω.
A happy coincidence: the calculation lands exactly on 100 Ω.
Blue and white share the 3.1 V midpoint and the same resistor.
Only 1.2 V is left for the resistor; 60 Ω becomes 68 Ω.
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.
Forward voltage is tied to the energy of the emitted photon, which is higher for short-wavelength light. That is why red sits near 2 V while blue, violet and UV sit above 3 V.
With the midpoint values, on 5 V it comes out nearly the same: 100 Ω for both. But the published ranges are wide and overlap, so check each LED's datasheet before assuming the Vf matches.
It uses 3.1 V, the midpoint of the published 2.7 to 3.5 V range, rounded to 0.1 V. The field is editable: replace it with the datasheet value when you have it.
No. Power LEDs (several watts) need a constant-current driver, not a resistor, because the high current and the drift of Vf with temperature make a resistor unstable and wasteful. This calculator is for small indicator LEDs.
If the real Vf of your LED falls between 2.48 and 3.7 V (the range for this color), the current is between 13 and 25.2 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.