phasecutting

Phase cutting is a method of regulating alternating current (AC) power by removing portions of each half-cycle of the waveform with a bidirectional switch. By controlling the timing of conduction in every half-cycle, the average or RMS voltage delivered to the load can be adjusted without changing the supply voltage. This contrasts with purely resistive or resistive-capacitive dimming, which modify voltage in other ways.

There are two common forms: leading-edge and trailing-edge phase cutting. In leading-edge dimming, conduction is delayed after each zero-crossing, so the switch turns on at a programmable delay and remains conducting until the current falls to near zero at the end of the half-cycle. In trailing-edge dimming, conduction starts at the zero-crossing and is cut off before the end of the half-cycle, shortening the delivered portion of the waveform. The switching elements used include triacs or silicon-controlled rectifyers (SCRs) for traditional phase-cut devices, and, in more modern implementations, MOSFETs or IGBTs with appropriate drive circuitry. The choice of method affects load compatibility, acoustic behavior, and electromagnetic interference.

Phase cutting is widely used for incandescent lamp dimming, motor speed control, and certain heating controls. Its applicability to LEDs and electronic power supplies depends on the driver’s compatibility with chopped waveforms. The technique introduces harmonic content and potential flicker, requiring careful design of snubbers, filters, and EMI considerations. Safety and proper thermal management are essential, given the high voltages and power levels involved.