How does PWM control LED brightness or motor speed?

 Let’s dive into how PWM (Pulse Width Modulation) actually makes an LED look dimmer or a motor run slower — even though the voltage is always “full ON” or “full OFF.”



 Principle of PWM

  • PWM rapidly switches voltage ON and OFF at a fixed frequency.

  • The duty cycle (%) controls the ratio of ON-time to OFF-time in each cycle.

  • The load (LED, motor, etc.) responds to the average power delivered, not the instantaneous ON/OFF states.

 So, instead of lowering voltage directly, PWM delivers “packets of full power” and lets the load integrate them.

 LED Brightness Control

  • Human eyes cannot perceive fast flickering above ~100 Hz → instead, they see the average light output.

  • Example:

    • 10% duty cycle → LED is ON only 10% of the time → appears dim.

    • 50% duty cycle → LED is ON half the time → medium brightness.

    • 90% duty cycle → LED almost always ON → near full brightness.

 Advantage: LED always gets correct forward voltage → no color shift (unlike analog dimming by lowering current).



 Motor Speed Control (DC motors)

  • A brushed DC motor’s speed depends on average voltage/current supplied.

  • PWM applies bursts of full supply voltage, and the motor’s rotational inertia smooths the pulses into steady motion.

  • Example:

    • 25% duty cycle at 12 V → motor “feels” like ~3 V average → runs slower.

    • 75% duty cycle at 12 V → motor “feels” like ~9 V average → runs faster.

 Advantage: High efficiency, less heating, and strong torque (motor still gets full-voltage pulses).

 Why not just lower voltage with a resistor?

  • Inefficient: Resistors waste energy as heat.

  • Unstable: LED brightness shifts with supply changes, motor torque drops badly.

  • PWM is efficient: Switch is either fully ON (low loss) or fully OFF (no loss).

 Quick Visual (conceptual)

  • 100% duty cycle: ─────────────── (always ON → full brightness/speed)

  • 50% duty cycle: ▓▓▓░░░▓▓▓░░░ (half ON, half OFF → medium)

  • 10% duty cycle: ▓░░░░░░░░░ (short ON bursts → dim/slow)

Summary:

  • For LEDs → PWM controls perceived brightness by changing ON-time.

  • For motors → PWM controls average voltage/current, hence speed, while keeping torque strong.

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