LED Circuit - Pulsed LED Operation

Pulsed LED Operation

Many systems "pulse" LEDs on and off. This leads to the incorrect belief that "pulsing" an LED makes it appear brighter than a simpler DC circuit that supplies constant power to the same LED.

Most LED data sheets specify a maximum DC current that is safe for continuous operation. Often they specify some higher maximum "pulsed" current that is safe for brief pulses, as long as the LED controller keeps the pulse short enough and then turns off the power to the LED long enough for the LED to cool off. Often a pulsed maximum current of 4 times the DC maximum current is allowed for brief pulses when the LED is turned on 1/4 of the time or less. However, LEDs are less efficient at turning electric energy into light at high currents above the maximum DC current. A simple circuit that drives an LED at a constant current equal to the maximum rated DC current will be better -- drawing slightly less power and producing slightly more light -- than a more complicated circuit that drives the same LED at 4 times the current for 1/4 of the time (the same average current).

In a typical multiplexed display, only LEDs on one row at a time can be turned on at any instant. With 8 equal rows, that means that the LED driver turns on any one LED at most 1/8 of the time. (It is possible to build LED arrays that can turn on all the LEDs all the time -- such a display would appear at least twice as bright, but such designs typically cost more than a multiplexed display).

Many systems only occasionally require the full (maximum DC) brightness of the LED. They typically "dim" the LED by PWM pulsing the LED, alternating between driving it at the full (maximum DC) current and turning it completely off. (It is possible to build LED drivers that drive the LED at some constant "dim" current, but such dimming hardware typically cost more than PWM dimming.)