4.2 Driving LEDs

Forward current control

Since LED emits light depending on forward current magnitude, the easiest way how to control LED light source intensity is changing bias current value. Figure 4.2.1 depicts relative luminous flux as a function of forward current value. Change of luminous flux depends on forward-current change nearly linear; therefore control algorithm implementation is very easy.
Relative luminous flux as a function of forward current

Figure 4.2.1: Relative luminous flux as a function of forward current.

 

PWM control

Another way how to control LED light source intensity is pulse-width modulation (PWM) method. Principle of PWM lies on biasing LED by constant nominal current which is periodically switched on and off. Ratio between on-state and off-state defines resulting intensity of the LED. Switching frequency is high enough thus human eye perceives light emitted by LED as continuous luminous flux with intensity depending on PWM duty-cycle. Figure 4.2.3 shows examples of PWM signal with 50 % and 70 % duty-cycle, respectively.

Figure 4.2.3: Examples of PWM signal.

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