Pulse width modulated

The simplest analog form of generating fixed frequency PWM is by comparison with a linear slope waveform such as a sawtooth. The output signal goes high when the sine wave is higher than the sawtooth. This is implemented using a comparator whose output voltage goes to a logic HIGH when the input is greater than the other.


Pulse-width modulation (PWM), or pulse-duration modulation (PDM), is a technique for getting analog results with digital means. Additionally, it is commonly used for controlling power to inertial electrical devices, made practical by modern electronic power switches.[1]

The average value of voltage (and current) fed to the load is controlled by turning the switch between supply and load on and off at a fast pace. The longer the switch is on compared to the off periods, the higher the power supplied to the load is.

The PWM switching frequency has to be much faster than what would affect the load, which is to say the device that uses the power. Typically switching has to be done several times a minute in an electric stove, 120 Hz in a lamp dimmer, from few kilohertz (kHz) to tens of kHz for a motor drive and well into the tens or hundreds of kHz in audio amplifiers and computer power supplies.

The term duty cycle describes the proportion of 'on' time to the regular interval or 'period' of time; a low duty cycle corresponds to low power, because the power is off for most of the time. Duty cycle is expressed in percent, 100% being fully on. The duration of "on time" is called the pulse width. To get varying analog values, you change, or modulate, that pulse width. If you repeat this on-off pattern fast enough with an LED for example, the result is as if the signal is a steady voltage between 0 and 5v controlling the brightness of the LED.

The main advantage of PWM is that power loss in the switching devices is very low. When a switch is off there is practically no current, and when it is on, there is almost no voltage drop across the switch. Power loss, being the product of voltage and current, is thus in both cases close to zero. PWM also works well with digital controls, which, because of their on/off nature, can easily set the needed duty cycle.

This powerful technique for controlling analog circuits with a processor's digital outputs. PWM is employed in a wide variety of applications, ranging from measurement and communications to power control and conversion.[2]

The logic signal (control) turns the transistor on and off to drive high current loads. For motor speed control or dimming lights, a digital PWM output signal is typically used for control instead of an analog output. Digital PWM is more energy efficient than analog as it significantly reduces the heat dissipated by the transistor (i.e., it is always completely "on" or "off"). For motors, the PWM clock rate is typically in the tens of KHz range. For lighting, it needs to be greater than 50Hz or perhaps 100Hz. Early studies for electric power systems showed that many people have headaches caused by lighting systems that use less than 50Hz AC even if they do not directly perceive a flicker.[3]


  1. Ledwich, G., Pulse Width Modulation (PWM) Basics. 1998. [1]
  2. Michael Barr. Embedded Systems: Introduction to Pulse Width Modulation (PWM). Netrino, LLC. September 2001. [2]
  3. ARM mbed. Drivers, Relays, and Solid State Relays. 20 Apr 2015. [3]


See also[]



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