Technician Handbook ! 673 Electronic & Computer Controlled Systems Duty Cycle The terms pulse-width modulation and duty cycle are often confused or used incorrectly. Pulse-width modulation is a function an ECU can perform to turn a circuit on and off rapidly to regulate the amount of ON time. As the pulse width changes, the frequency of the pulses might or might not change depending on the circuit design and intended operation. When a circuit is switched on and off rapidly at a constant frequency, duty cycle measures the percentage of ON time compared to total cycle time. If the circuit is ON 75% of the time, it is operating at a 75% duty cycle.
When a circuit is duty-cycle controlled, the pulse frequency does not change – only the percentage of ON time. An ECU varies the duty cycle to control the speed of a motor or the brightness of a lamp by switching the circuit ON and OFF hundreds of times per second. Human senses can’t perceive a lamp or motor being cycled on and off that quickly. Nonetheless, the amount of power to the component increases or decreases depending on how much of the time the circuit is ON versus OFF. As OFF time increases, the net power supplied to a component decreases resulting in the lamp becoming dimmer or the motor running slower.
As ON time increases, power increases and the lamp becomes brighter or the motor runs faster. NOTE When the circuit is ground-side controlled, voltage before the load is always battery voltage, and voltage after the switch is zero, or near zero. To observe voltage modulation, place the positive probe between the load and the switch (which may be an ECU). Technical Training 15