What is pulse-width modulation (PWM) and how is it used in power control?

• A. PWM varies the amplitude of a fixed-frequency waveform to control instantaneous voltage.
• B. PWM varies the duty cycle of a fixed-frequency waveform to control average load voltage; used in DC motors, power supplies, and audio amplifiers.
• C. PWM changes the frequency of the waveform to regulate power.
• D. PWM encodes digital data for transmission.

Answer: (B)

PWM works by keeping the switching frequency fixed and adjusting the fraction of time the signal stays high (the duty cycle). By changing how long the signal is on versus off, you change the average voltage that the load sees, without changing the peak voltage or the switching frequency. This lets you control the power delivered efficiently, since the electronics are either fully on or fully off, minimizing losses.

This approach is ideal for power control in DC motors, power supplies, and audio amplifiers because you can smoothly vary the effective voltage or current with high efficiency, and then filter or directly use that average value as needed. The average load voltage is essentially the duty cycle multiplied by the peak voltage.

Why other ideas don’t fit: varying the amplitude of a fixed-frequency waveform would change instantaneous voltage levels and can cause more heat and less predictable control; changing the frequency isn’t how PWM modulates power, it’s the duty cycle you adjust; encoding digital data describes a communication function, not power control.

So, PWM controls average load voltage by adjusting duty cycle at a fixed frequency, and is widely used for efficient power regulation.