Pulse Width Modulation (PWM) is a technique in which the width of the periodic sequence pulses is varied in accordance with the baseband signal. PWM is also known as Pulse Duration Modulation. The leading edge of the pulse is held constant and the change in pulse width with signal is measured with respect to leading edge. In PWM, the pulse width is proportional to the amplitude of the signal. By varying the duty cycle of the pulse, the speed of the fan can be controlled. Duty cycle may be defined as the amount of time in a particular period during which the pulse is active or high. The speed is made slow, medium, fast, very fast and zero by having different duty cycles. Fig 2 shows the pulses with varying duty cycles: