an led driver circuit operating from the ac power line providing high efficiency , good line and load regulation , high power factor , low line current harmonics , low conducted emi , high led utilization , and lamp dimming compatibility , while consisting of a minimal number of components . no inductors , nor capacitors , nor high current switching transistors are employed . the top of a string of series connected led segments is connected to the output of a rectifier , which in turn is connected to an ac sine wave power source . the string is tapped at various locations , including the bottom of the string . each segment can consists of any number of serial or parallel connected leds . current control elements or regulators sink current at each tap and are sequentially turned on and off one at a time , tracking the rectified sine wave voltage . voltage across each regulator and current when conducting is individually controllable . power loss in the regulators is minimized by keeping regulator voltage to a minimum . the regulators may control current in a multitude of ways , including a constant current , or a current dependent on voltage across the regulators including a resistor , or a combination . the driver is self - commutating , with the sequencing of the current control elements an inherent feature closely integrated with the current control elements and providing optimal performance over variable operating conditions . given the large number of design variables , the driver circuit can be optimized for various performance criteria including input voltage range , line / load regulation , output power / current , efficiency , power factor , line current harmonics , dimmer compatibility , and led utilization .