SDG #359 Dodgy AliExpress MOSFET-based multichannel AC Dimmer

This episode examines a three channel DMX controlled AC dimmer module from AliExpress as a candidate output stage for the Waveshare ESP32-S3 touchscreen scene controller, which sends DMX over RS485 and reports button presses to Home Assistant over MQTT. The module is an aluminium extrusion with plastic ends, a seven segment display and two buttons, with mains in, three dimmed outputs and DMX in and out.

Inside, a microcontroller board with an STC processor, shift register and EEPROM sits above a power board using the same dual MOSFET per channel arrangement as the channel’s own trailing edge dimmer. Each channel has an opto coupler driving the gate circuit through an isolated 12 V to 9 V DC-DC converter, with TMF20N50 MOSFETs at 0.26 ohm, an opto coupler and bridge rectifier for zero crossing detection, and a single RS485 transceiver with the DMX in and out simply bridged rather than repeated. The reverse engineered schematic is sound in principle: the zero crossing detector draws about 2.3 mA through 33k resistors, dissipating around half a watt continuously, and the gate drive uses discrete transistors rather than a gate driver.

The implementation fails on isolation. The live side of the zero crossing circuit runs beside the electronics ground plane, which is also the DMX ground, before any opto isolation, and the metal enclosure is not earthed. Tested through an isolation transformer, standalone dimming from the front panel works, and driven from the touchscreen controller the three outputs dim correctly, though the LED lamps used only begin to respond at around 50% and would need gamma correction. The form factor is attractive, but the isolation failure and unearthed case make it unsuitable as supplied, so the plan is to design a safe board for the same style of enclosure.