Abstract: An alternating current signal is applied across a series connected switch and load and also to a synchronous ramp generator. The switch is closed by a comparator when the ramp generator output signal is greater in a given sense than an analog reference signal. The reference signal is produced by a digital-to-analog converter responsive to the count in an up-down counter so that the period during which the switch is closed each cycle is controlled by the count in the counter and the shape of the synchronous ramp signal. The ramp signal may be a linear or non-linear function of time thereby providing linear or non-linear incremental changes in the switch conduction period for given incremental changes in the count.