Red LED at 10 mA
- Input
- 3.3V 1.8V 10mA E12
- Expected output
- 150R 10.0mA
1.5 V remains and E12 has exactly 150 Ω.
Carregando ferramenta… / Loading tool…
3.3v led resistor
Boards running at 3.3 V (ESP32, Raspberry Pi, STM32) leave little room for the LED, and that is where 5 V habits break down. The calculator opens with 3.3 V, a red LED and 10 mA, a lower current than the usual 20, and shows the resistor and the real current it delivers.
1.5 V remains and E12 has exactly 150 Ω.
Only E24 includes 75 Ω; the resistor dissipates about 30 mW.
With no 75 Ω in E12, the resistor steps up to 82 Ω and the current falls to 18.3 mA.
Only 0.2 V across the resistor: 20 Ω becomes 22 Ω, but any Vf drift changes the current a lot.
The math gives 130 Ω; E12 jumps to 150 Ω, passing 8.7 mA.
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 10 mA, 150 Ω. For 20 mA, 75 Ω in E24 or 82 Ω in E12. The closer Vf sits to 3.3 V, the smaller the resistor and the touchier the current becomes.
It depends on the part. Published ranges are 2.48 to 3.7 V for blue and 2.7 to 3.5 V for white. An LED with a Vf above 3.3 V will not light; one at 3.0 V lights dimly and unevenly. Check the Vf in the datasheet.
Yes, with the series resistor, as long as the current stays inside the pin limit in the chip's datasheet. Each pin has its own limit; when in doubt, size it for 5 or 10 mA, which already gives visible light on modern LEDs.
Because Vf varies from one LED to the next and with temperature, and at 3.3 V the gap between supply and Vf is small. A 0.1 V error in Vf already changes the current by a large fraction. The tool shows the current range across the color's published interval.
If the real Vf of your LED falls between 1.63 and 2.03 V (the range for this color), the current is between 8.5 and 11.1 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.