This applet shows the basic idea of pulse width modulation (PWM). The average voltage Uavg is linearly dependent on the on-time. The normalized on-time is called the 'duty cycle', shown as 'D' in the applet. In a technical realization, a power switch will change periodically between the on- and off-state, and the output voltage of this switching cell is then filtered (see e.g. buck converter).