This circuit allows the effective power in a load to be controlled by varying the duty cycle of the on/off ratio of l
oad current. No power is dissipated in the switching circuit. U1A generates a bipolar square wave that is integrated into a triangle by R4 and C2. Reference voltage from R6 is fed to a comparator. The triangle wave on C2 goes to the comparator as well. By varying the reference voltage (R6), the output waveform is a variable width pulse, that drives Ql. R6 controls on/off ratio and therefore load power. R5 sets the offset of the triangle wave across C2.