Electel Blogspot

Pulse Skipping Modulation Green - Input AC Yellow - Output AC after Pulse Skipping Modulation Pink - Gate Drive Signal Pulse skipping modulation (PSM) or cycle control or burst fire is a method of power control where whole cycles of voltage are applied to the load. Here I’ll talk about PSM involving a thyristor, specifically a triac. The triac connects the AC supply to the load for a given number of cycles and then disconnects the AC supply for another given number of cycles. It has of course become quite obvious from the title that the purpose of PSM is to control or limit power to the load. We know that the thyristor is a latching device – when the thyristor is turned on by a gating signal and the current is higher than the holding current and the latching current, the thyristor stays on, until the current through it becomes sufficiently low (very close to zero). The thyristor turns off when current through it becomes zero, as happens at the AC mains zero crossing. This is the natura...

Principle of Phase Angle Control Top - Output Voltage Bottom - Gate Drive Signal Image source: Wikipedia (http://en.wikipedia.org/wiki/File:Regulated_rectifier.gif) The photo above clearly illustrates phase angle control: output voltage controlled by the gate drive signal applied to a thyristor. What is phase angle control? That is what I'm going to talk about in this article. Phase angle control is a method of PWM applied to AC input voltages, usually the mains supply. Of course, the AC supply could be from a transformer or any other AC source, but the mains supply is the most common input – this gives the phase angle control method its greatest usefulness. It has of course become quite obvious from the title (and I’m sure most of you reading will already know this) that the purpose of phase angle control is to control or limit power to the load. The power device used in phase angle controllers is a thyristor – mostly triacs or SCRs. (There are methods of phase controlling employi...