Choosing a Resistor for an LED
An LED needs a resistor in series. Without one, only the LED's small internal resistance limits the current, and the LED burns out quickly.
Numbers from the simulation
Computed by the simulator from the part values in the schematic.
- LED current
- 14 mA
- LED forward voltage (Vf)
- 1.92 V
- Voltage across R1
- 3.08 V
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How it works
A red LED drops about 1.8 to 2 V when lit. The rest of the supply voltage drops across the resistor.
The LED current is the resistor voltage divided by its value: I = (Vs − Vf) / R. To pick a resistor, turn it around: R = (Vs − Vf) / I.
A standard 5 mm LED is safe up to about 20 mA. With 220 Ω the current stays below that, and the LED is still bright.
Try changing
- Lower R1 to 100 Ω. The current goes past 20 mA, and the simulator warns you with a suggested resistor value.
- Raise R1 to 1 kΩ. The LED still lights, but dimmer, at only about 3 mA.
- Turn the LED around. No current flows, because an LED is a diode.