Phase-fired controllers

Phase-fired control (PFC), also called phase cutting or 'phase angle control', is a method for power limiting, applied to AC voltages. It works by modulating a thyristor, SCR, triac, thyratron, or other such gated diode-like devices into and out of conduction at a predetermined phase of the applied waveform. Phase-fired control (PFC), also called phase cutting or 'phase angle control', is a method for power limiting, applied to AC voltages. It works by modulating a thyristor, SCR, triac, thyratron, or other such gated diode-like devices into and out of conduction at a predetermined phase of the applied waveform. Phase-fired control (PFC) is often used to control the amount of voltage, current or power that a power supply feeds to its load. It does this to create an average value at its output. If the supply has a DC output, its time base is of no importance in deciding when to pulse the supply on or off, as the value that will be pulsed on and off is continuous. PFC differs from Pulse-width modulation (PWM) in that it addresses supplies that output a modulated waveform, such as the sinusoidal AC waveform that the national grid outputs. Here, it becomes important for the supply to pulse on and off at the correct position in the modulation cycle for a known value to be achieved; for example, the controller could turn on at the peak of a waveform or at its base if the cycle's time base were not taken into consideration.