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Pulse Width Modulation (PWM)

  • PWM is a simple method of using a rectangular digital waveform to control an analog variable.
    1. The on-off behavior changes the average power of the signal.
    2. Output signal alternates between ON and OFF with a specific time period.

PWM control is used in a variety of applications, ranging from communications to automatic control.It can also be used to encode information for data transmission.

How it works?​

The period is normally kept constant, and the pulse width (or ON time) is varied.
Duty Cycle: It is defined as the proportion of time the pulse is ON, expressed as a percentage.

formula

Duty Cycle = (pulse ON time) / (pulse period) 100% = t ON/ T 100%

Average value of the signal = t ON*VH + (1 – t on).VL

pwm

Whatever duty cycle a PWM has, there is an average value, as indicated by the dotted line.

  • If the ON time is small, the average value is low; if it is large, the average value is high.
  • By controlling the duty cycle, we can control the average value.

How to Extract the Average Value?​

  • The average value can be extracted from the PWM stream using a low-pass filter.
  • If the PWM frequency and the values of R and C are appropriately chosen, Vout becomes an analog output.
    • Can be used in place of a digital-to-analog converter.

Some Typical Applications​

  1. Control of DC motor: The voltage supplied to the motor is proportional to the duty cycle.
  2. Controlling the brightness of LED: The duty cycle of the voltage source determines the brightness.
  3. Control the temperature (heater): Switch ON and OFF the heater with an appropriate duty cycle.
  4. Many more …