Method and apparatus for controlling brightness of light emitting diodes

Abstract

A circuit for driving a light emitting diode (LED) comprises: an alternating voltage power supply, comprising a triac dimmer having a firing angle and output terminals that provide power to the LED: a zero crossing detector that detects a polarity change of the alternating voltage provided by the alternating voltage power supply and provides a zero crossing output signal indicative of the zero crossing; a timer triggered by the zero crossing output signal that generates a timer output signal, during a time period of the timer; and LED power circuitry that reduces current to the light emitting diode based upon timing characteristics of the timer output signal.

What is claimed is: 
     
         1 . A. method of driving a light emitting diode (LED) light source, the method comprising:
 providing an alternating voltage power supply having positive and negative half cycles;   adjusting a dimmer having a firing angle to provide current to the LED when the firing angle, after a beginning of the positive or negative half cycle, has been reached;   waiting a time period after the beginning of the positive or negative half cycle; and   reducing current to the LED at the end of the time period,   wherein the time period does not exceed an end of the positive, or negative half cycle.   
     
     
         2 . The method of  claim 1 , wherein a period of reduced current to the LED is a fixed time period. 
     
     
         3 . The method of  claim 1 , wherein a period of reduced current to the LED is a variable time period. 
     
     
         4 . The method of  claim 3 , wherein the period of reduced current to the LED is set to be proportional to the firing angle of the dimmer.