duty cycle

A duty cycle is the fraction of time that a signal, switch, or device spends in its active state during a repeating cycle. In a square wave or pulse train, it is expressed as a percentage: the time the signal is high divided by the total period, multiplied by 100. A 50% duty cycle means the signal is on for half the period and off for the other half. A 75% duty cycle means it is on three-quarters of the time. This metric applies to clock signals, PWM outputs, relay switching, solenoid actuation, and any intermittent load.

Duty cycle is fundamental to pulse-width modulation (PWM), the standard technique for controlling power delivery in motor drives, LED brightness, and switching power supplies. By varying the duty cycle of a fixed-frequency square wave, the average voltage or current delivered to a load changes without wasting energy as heat. A motor driven by a 25 kHz PWM signal at 60% duty cycle receives the same average power as one fed a constant voltage scaled proportionally, but with lower switching losses and better control resolution.

Temperature and component life

Duty cycle directly affects thermal stress and component lifespan. A relay or solenoid operated at continuous 100% duty cycle generates steady heat and ages through thermal cycling and mechanical fatigue. The same device run at 30% duty cycle allows cooling time between activations and extends service life significantly. Electrolytic capacitors in power supplies degrade faster under continuous high current than under intermittent load at the same average current. Manufacturers specify both continuous and intermittent ratings; duty cycle is the bridge between them.

Measuring duty cycle requires an oscilloscope or a meter with pulse measurement capability. For a simple square wave, divide the pulse width (the time the signal is high) by the period (the time from one rising edge to the next). The result is often called the mark-space ratio in older equipment documentation. In practice, duty cycle is rarely perfectly stable; ripple, jitter, and component tolerance create small variations that usually matter only in precision timing or high-frequency applications.

The term originates in mechanical systems, where duty cycle referred to the fraction of time a machine was under load. The concept transferred directly to electronics: a device under electrical load (the duty) for a portion of its operating cycle. Today it is essential vocabulary in every domain where power is switched rather than continuously applied, from industrial automation to consumer electronics to aerospace.