An electronic dimming apparatus includes a semiconductor power controller which couples a lamp to an alternating current source and variably controls the average power supplied to the lamp by adjusting the relative proportion of each half-cycle in which the power controller is in a conductive condition. The semiconductor power controller is controlled by the output of a transition controller which in turn is connected to a triggering circuit. The triggering circuit is responsive to a first input condition corresponding to desired average power and also responsive to the zero-crossing of the alternating current waveform. The transition controller gradually changes the condition state of the semiconductor power controller between non-conduction and conduction and controls the voltage across the lamp load to maintain a stable rate of voltage change despite lamp impedance variations, thus reducing electromagnetic interference.