LED is a device which is driven by a current, and its brightness is proportional to the forward current. There are two ways to control forward currents.
The first is to use the LED I curve to determine the voltage to be applied to the led to produce the desired forward current. Its realization method generally uses a voltage power supply and a town flow resistor. As described below, there are a number of deficiencies in this approach. Any change in the LED forward voltage will lead to changes in the LED current. If the rated forward voltage is 3.6V, the current of the LED in Fig. 1 is 20mA. If the voltage changes to 4.0V, this is a specific pressure change due to temperature or manufacturing changes, then the forward current is reduced to 14mA. A positive voltage change of 11% results in a larger positive current change of 30%. In addition, according to the available input voltage, the pressure drop and power consumption of the town flow resistance will waste power and reduce the battery life.
The second method is also the preferred led current adjustment method is the use of constant current power supply to drive the LED.