of the led is completely cut off so 2 5v can be set as the driving voltage of disabling the led and 3 6v is the driving voltage of enabling the led in addition the driving voltage supplied to the led can be also supplied to a functional ic and other circuits since the driving voltage still has a value as high as 2 5v when the led is disabled the functional ic and other circuits can normally work even when the led is dark as shown in fig3 a led dimming system according to the present invention includes a functional ic 12 to provide a dimming signal dpwm a voltage setting circuit 22 to provide two driving voltage setting signals ea 1 and ea 2 a selector 20 to select one of the driving voltage setting signals ea 1 and ea 2 according to the dimming signal dpwm for a power source 24 to determine its output voltage for the led and the functional ic 12 is the driving voltage vo 1 or vo 2 each of the driving voltages vo 1 and vo 2 is as large as enough to drive the functional ic 12 when the dimming signal dpwm is high the selector 20 sends out the driving voltage setting signal ea 1 and the output voltage of the power source 24 is the driving voltage vo 1 which enables the led when the dimming signal dpwm is low the selector 20 sends out the driving.