A phase-control power controller converts a line voltage to an RMS load
voltage and includes a phase-control clipping circuit that clips a load
voltage to provide the RMS load voltage. The power controller may be in a
base of a lamp and connected to a lamp terminal. The phase-clipping
circuit establishes a phase conduction angle that determines the RMS load
voltage for the lamp and includes a transistor switch and a
microcontroller that operates the transistor switch, where ON/OFF periods
of the transistor switch define the phase conduction angle. The
microcontroller senses a load voltage at the lamp terminal, compares the
sensed load voltage to a reference RMS voltage, and adjusts the ON/OFF
periods of the transistor switch in response to the comparison to cause
the load voltage to approach the reference RMS voltage. The circuit may
be used for reverse, forward, or forward/reverse hybrid phase clipping.