The LED

An LED emits light when forward current passes through it. Its current–voltage relation is nonlinear. In reverse it normally conducts very little, but excessive reverse voltage can damage it.

An LED’s quoted forward voltage is measured at a particular current and temperature. Typical examples are about 2 V for red and about 3 V for blue/white, but the values vary by device. It is not an exact on/off threshold: current and light rise gradually and steeply.

Above the threshold, the LED behaves very differently from a resistor: a tiny rise in voltage causes the current to shoot up steeply. That sharp, almost-vertical jump is precisely why an LED can’t be trusted to limit its own current — it will happily draw enough to destroy itself.