Duty cycle

In engineering, the duty cycle of a machine or system is the time that it spends in an active state as a fraction of the total time under consideration.

The term is often used pertaining to electrical devices, e.g., switching power supplies. A 60% duty cycle means the power is on 60% of the time and off 40% of the time. The "on time" for a 60% duty cycle could be a fraction of a second – or for say, irrigation pumps, days – depending on how long the device's period is. Here one period is the length of time it takes for the device to go through a complete on/off cycle. The term "duty cycle" has no agreed meaning for aperiodic devices.

Definition

The duty cycle D is defined as the ratio between the pulse duration ($\tau$) and the period ($\mathrm {T}$) of a rectangular waveform

In a periodic event, duty cycle is the ratio of the duration of the event to the total period.

duty cycle $D={\frac {\tau }{\mathrm {T} }}\,$

where

$\tau$ is the duration that the function is active

$\mathrm {T}$ is the period of the function.

Examples

Electrical machines

A motor runs for one out of 100 seconds, or 1/100 of the time, and therefore its duty cycle is 1/100, or 1 percent.

Electronics

In an ideal pulse train (one having rectangular pulses), the duty cycle is the pulse duration divided by the pulse period. For example, a pulse train in which the pulse duration is 1 μs and the pulse period is 4 μs has a duty cycle of 25%. The pulse duration is normally calculated for positive pulses unless "negative duty cycle" is specified.

The duty cycle of a non-rectangular waveform, such as a sine or triangle wave, is defined as the time that the waveform spends above zero volts as a fraction of the period.

Digital signal processing

In a continuously variable slope delta (CVSD) modulation converter, the mean proportion of binary "1" digits at the converter output in which each "1" indicates a run of a specified number of consecutive bits of the same polarity in the digital output signal.

Electronic music

Music synthesizers vary the duty cycle of their audio-frequency oscillators to obtain a subtle effect on the tone colors. This technique is known as pulse-width modulation (PWM).

Welding

In equipment such as a welding power supply, the maximum duty cycle is defined as the percentage of time in a 10 minute period that it can be operated continuously before overheating. Duty cycle is the percentage of time that a device or signal is ON, compared to its period.

Printers and copiers

In the printer / copier industry, the duty cycle specification refers to the rated throughput (that is, printed pages) of a device per month.