SDG #068 Designing a Trailing Edge Dimmer - Lighting Controller Project - Part 2

The second instalment of the lighting controller project moves from concept to a working trailing edge dimmer design, and it is a useful case study in why this topology has largely replaced the old leading edge triac dimmer for modern lamps. Where a triac chops the front of each mains half cycle and delivers a sharp current step into the load, a trailing edge design switches on softly at the zero crossing and turns off part way through the cycle. That gentler slope means far less conducted noise, less audible buzz from transformers and windings, and much better behaviour with LED and electronic drivers that respond poorly to fast edges.

The design walkthrough covers the parts of the circuit that decide whether a dimmer works or fails dangerously: a bidirectional switching element built from MOSFETs so that the AC waveform can be blocked in both directions, gate drive referenced to a floating node, snubbing and dissipation considerations, and the zero cross detection that gives the microcontroller a timing reference for phase angle control. A clean timing reference matters more than most people expect, because jitter in the zero cross signal appears as visible flicker at low brightness settings.

Measurement discipline is the other half of the story. Probing a live mains switching node with a conventional single ended oscilloscope probe is a fast route to a damaged oscilloscope or worse, so a differential probe is used to examine gate drive and switch node waveforms safely without a mains referenced ground. Anyone planning to build their own dimmer should treat that isolation as mandatory rather than optional, alongside careful PCB clearances between mains and control side circuitry.