Power·Jan 1, 2023

AC Dimmer Circuit

A phase-cut AC dimmer that smoothly controls a mains lamp using zero-cross detection and an opto-isolated TRIAC.

Overview

The AC dimmer is one of those circuits where everything looks small on paper but the moment you plug it into the wall it demands a lot of respect. I built mine to actually understand zero-crossing, firing angles and TRIAC drive properly, instead of just memorising them for an exam.

Specifications

RoleCircuit designer and prototype builder

MicrocontrollerArduino-class controller

SensorsZero-cross detector

ModulesTRIACOptotriac driverPotentiometer / control input

ProtocolsGPIOInterrupt timing

Tech tagsAC DimmerTRIACZero CrossOpto-IsolationPower Electronics

Architecture

Zero-cross signal triggers the timing routine. The controller waits a programmed delay and fires the TRIAC to set delivered load power.

Project photos

Videos

Implementation details

Firmware

Firmware detects zero-cross events, calculates firing delay, and triggers the TRIAC gate through an opto-isolated driver.

PCB

Circuit uses isolation between controller and mains trigger path. PCB/layout must maintain safe creepage and clearance for AC use.

Power

Powered from a low-voltage logic supply while switching an isolated mains load through the TRIAC path.

Components / BOM

TRIAC, Optotriac driver, Zero-cross detector, Microcontroller, Lamp/load terminal

Engineering challenges

Doing the timing reliably around the zero-cross point, and keeping the low-voltage controller completely isolated from the mains side.

Solutions

Used an opto-isolated TRIAC driver, handled zero-cross with an interrupt, and made sure the PCB layout respected the creepage/clearance rules for AC.

Results & metrics

Smooth dimming behaviour on a regular AC lamp, no flicker once the timing was tuned.

Future improvements

Enclosure, fuse, EMI filtering, a calibrated knob or small display, and proper closed-loop control of delivered power.