SDG #230 Custom AC LED Trailing Edge Dimmer Modules Revisited...

This episode returns to an ongoing in house project: a compact trailing edge AC dimmer module designed to drive modern LED lamps without the buzzing, flickering and dropout problems common to cheap leading edge triac dimmers sold as “LED compatible”.

The key distinction explained here matters to anyone retrofitting dimmable LED bulbs. Leading edge dimmers chop the start of each mains half cycle, delivering a fast rising edge into the switch mode driver inside an LED lamp, which is exactly the kind of stress those drivers handle worst. A trailing edge design instead conducts from the zero crossing and switches off part way through the half cycle, giving a much gentler waveform for the lamp’s input filter and typically far better low end behaviour and less audible noise. Achieving that requires a bidirectional solid state switch rather than a triac, since the device has to be turned off on command, so the module uses MOSFETs with gate drive and a zero cross reference for timing.

The revisit covers what changed since the earlier revision: board layout and packaging so that the module fits behind a wall plate, thermal headroom for the switching devices, snubbing and filtering to keep switching transients from radiating, and firmware control of the dimming ramp and minimum brightness threshold. Bench testing focuses on the practical questions a buyer or builder cares about: whether the dimming curve is smooth, whether cheap bulbs flicker or refuse to strike at low levels, and how much power is dissipated in the module at full load. It is a good case study in why mains dimming is genuinely difficult, and why lamp compatibility varies so widely between brands.