circuitry that provides alternating current to a load from a unipolar power supply . a current source in the circuitry controls current applied to the load . a first switch and a second switch are connected between the load and the current source and allow current to flow from the current source to the load in a first direction responsive to the first switch and the second switch being closed . a third switch and a fourth switch connected between the load and the current source allow current to flow from the current source to the load in a second direction responsive to the third switch and the fourth switch being closed .