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Definition: PWM

(Pulse Width Modulation) A modulation technique that generates variable-width pulses to represent the amplitude of an analog input signal. Like its fixed-width pulse density modulation (PDM) cousin, the output switching transistor is on more of the time for a high-amplitude signal and off more of the time for a low-amplitude signal. The digital nature (fully on or off) of the PWM circuit is less costly to fabricate than an analog circuit that does not drift over time. See PDM.

Power Supplies, Motors and Bulbs
PWM is widely used in the common "switch-mode" power supplies that convert AC power to DC for computers and other electronic devices. It is also used to control the speed of a DC motor and the brightness of a bulb. For example, if the line were closed for 1ms, opened for 1ms and continuously repeated, the target would receive an average of 50% of the voltage and run at half speed or half brightness. If the line were closed for 1ms and open for 3ms, the target would receive an average of 25%.

Audio Amplifiers and LCD Screens
PWM is used in audio amplifiers to generate output signals for cellphone speakers to high-power stereo systems. Due to the fully on/off nature of the PWM output, PWM amplifiers produce less heat than traditional analog amplifiers. See Class D amplifier.

In concert with amplitude modulation, PWM is used to deliver the required intensity to the liquid crystals in the pixels of an LCD panel. See LCD.