Red LED, E12 series
- Input
- 5V 1.8V 20mA E12
- Expected output
- 180R 17.8mA
The math gives 160 Ω; the next E12 value is 180 Ω, passing 17.8 mA.
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5v led resistor
On 5 V, the resistor for a red LED at 20 mA works out to exactly 160 Ω, a value that does not exist in the E12 series. The calculator below opens with that case already filled in (5 V, red LED with a 1.8 V forward voltage, 20 mA) and shows the stock resistor you end up with, the current it really passes and the heat it dissipates.
The math gives 160 Ω; the next E12 value is 180 Ω, passing 17.8 mA.
The E24 series has 160 Ω, so the current lands on exactly 20.0 mA.
2 V remain; 2 ÷ 0.020 = 100 Ω, a value already in the E12 series.
The math gives 95 Ω, which rounds up to 100 Ω and passes 19.0 mA.
Half the current, double the resistor: 3 ÷ 0.010 = 300 Ω, and the E12 series sends it up to 330 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.
For 20 mA the calculation is 160 Ω. In the E12 series use 180 Ω (17.8 mA); in E24 use 160 Ω. If the LED is a high-brightness type or you want thermal margin, 220 Ω also works, at about 14.5 mA.
No. Without a resistor, nothing limits the current except the LED's tiny internal resistance. The current climbs until the LED burns out, and it may overload the pin or port along with it. The series resistor is what sets the current.
Small indicator LEDs typically have current ratings from about 1 mA to a little above 20 mA. Twenty milliamps is a common starting point, not a guarantee: confirm it in your LED's datasheet, and use 5 or 10 mA when you want less brightness and less drain.
Almost never. Power is the voltage across the resistor times the current: 3.2 V × 17.8 mA comes to about 0.057 W. A common 1/4 W resistor has plenty of headroom; the tool shows the power and the smallest rating with a 2x margin.
If the real Vf of your LED falls between 1.63 and 2.03 V (the range for this color), the current is between 16.5 and 18.7 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.