Pulse Width Modulation (PWM) & Duty Cycle

Automotive circuits use different methods to control components, such as motors, LEDs, and other devices. One such method is pulse width modulation (PWM). PWM is a technique that varies the waveform’s duty cycle to control the average power delivered to a device. To understand PWM, it is important to also define the terms: duty cycle and frequency. You can also refer to the diagram below for graphical explanations.

Duty cycle refers to the percentage of time that a signal is high, compared to the total time of the signal. It typically ranges from 0% to 100%. A high signal refers to the ON state, while a low signal refers to the OFF state. Duty cycle is directly proportional to the power delivered to the device.

Frequency, on the other hand, is the number of waveform cycles that occur in one second. It is measured in hertz (Hz). Frequency determines how often the waveform switches between ON and OFF states.

Defining PWM

Now, let’s tie these concepts together. PWM involves generating a waveform with a fixed frequency but varying the duty cycle of the waveform. By changing the duty cycle of the waveform, the average power delivered to the device can be controlled. For example, if a circuit switches between 12 V in its ON state and 0 V in its OFF state, and it has a duty cycle of 50%, the device will receive half of the maximum power, or 6 V.

PWM can be used on either the power or the ground side of the component being controlled. If the controller is placed on the power side, and the component is connected directly to the ground, then Duty+ is modulated to control the component. If the component is connected to a fuse, or other constant power source, and the controller is on the ground side, Duty- is modulated to produce the same effect.

PWM Applications

PWM is used to control variable valve timing systems, HVAC blower operation, and even the brightness of the headlamps on some vehicles. These are only a few examples of a vast number of applications. Knowing how these circuits operate is essential to fixing the vehicle.

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