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PWM

A PWM signal switches rapidly between a high and a low voltage with a given duty cycle, the fraction of the period the signal is high. The load or filter after the signal effectively only sees the average voltage, which is proportional to the duty cycle – this is used to control everything from LED brightness to motor speed.

V_{avg} = D\cdot V_{high}

average voltage from duty cycle D

Symbols

$D$	duty cycle (fraction of the period the signal is high)
$V_{high}$	voltage level when the signal is high	V
$V_{avg}$	average voltage	V

Example

5 V PWM with 25% duty cycle:

$V_{avg}=0.25\cdot5=1.25$ V.

Duty cycle is the pulse length divided by the whole period, not by the time the signal is low.

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