Characteristic curve

The characteristic curve of an LED is strongly non-linear. An LED is non-conductive if no external voltage is applied. The LED starts to conduct when the applied forward voltage U_F is at least as high than the conducting voltage U_D and the band gap is overcome by the electrons. The forward current is not proportional to the applied forward voltage. A small change in voltage can cause a large change in current. A small voltage change can lead to a strong change in light emission due to the proportionality of luminous flux and current intensity. This means that LEDs must generally be operated with a current limiter of some form or other, otherwise even slight fluctuations in the applied voltage can destroy the LED.

A gradient triangle is drawn on the example characteristic curve. On the basis of this gradient triangle it can be seen that a small change in the voltage of 0.5 V from 3 V to 3.5 V results in a large change in the current intensity of 200 mA from 100 mA to 300 mA. In this example case, a voltage change of less than 17 % results in a current change of 300 %.

This example shows that small voltage fluctuations lead to large changes in the current through the LED and thus to large changes in the luminous flux.