Forward Voltage Basics
Forward voltage is the drop across an LED when it conducts.
What forward voltage means
Forward voltage (Vf) is the voltage across an LED while it is conducting and lit. Below it, almost no current flows. Above it, current rises very quickly for a small increase in voltage.
Typical values at about 20 mA
- Red, orange, yellow: roughly 1.8 to 2.2 V
- Green: about 2.0 V for older types, 3.0 V or more for modern ones
- Blue, white, ultraviolet: roughly 3.0 to 3.6 V
- Infrared: roughly 1.2 to 1.6 V
Why it varies
Vf differs between batches, falls slightly as the LED warms, and rises with current. Two LEDs of the same type may differ by a few tenths of a volt, so use the datasheet's typical and maximum values.
Why it matters
Your supply must be higher than Vf, and the difference is what a resistor must absorb. A 3 V coin cell may light a red LED but often struggles with blue or white.
Try it yourself
Sketch the circuit on paper first, calculate the values, then build it on a solderless breadboard using a low-voltage supply. Measure with a multimeter before you make anything permanent.