PWM Control is the most practical way to control the fan speed, without harming the fan. An input duty cycle percentage is given to the fan through a brown or white leadwire, and the fan’s internal PWM duty versus speed response curve will tell the fan at which speed it should be operating. The PWM duty cycle percentage equates to the width of the pulse signal being input to the fan. A 100% duty cycle is equal to a constant high signal, and a 0% duty cycle is a constant low signal. The PWM duty cycle can be varied anywhere in-between these two extreme points. A 75% duty cycle means that the signal is high 75% of the time, and low 25% of the time, for each pulse cycle. It is important to note that a 50% cycle does not necessarily correspond to the fan running at 50% of maximum speed. The slope of the fan speed versus PWM duty cycle response curve will dictate to what speed a duty cycle corresponds.