SDG #359 Dodgy AliExpress MOSFET-based multichannel AC Dimmer

This is a teardown and safety assessment of a compact three channel DMX512 trailing edge dimmer sold on AliExpress, intended for dimming mains powered LED retrofit lamps. It was sourced as an off the shelf alternative to a custom ESP32-S3 based touchscreen scene controller project, where DMX frames are sent over RS485 while button presses are also published to Home Assistant over MQTT.

Internally the unit is better thought out than expected: an STC microcontroller board with shift register driven seven segment display and external EEPROM for storing DMX addressing, plus a power board using two back to back MOSFETs per channel (TMF 20N50, roughly 0.26 ohm RDS(on) at 10V gate drive), opto isolated gate drive, three isolated 12V to 9V DC-DC converters and a bridge rectifier based zero crossing detector. The topology closely mirrors a proven discrete trailing edge dimmer approach, though the gate drive uses discrete transistors rather than a dedicated driver, and the 9V rail slightly under drives the MOSFETs.

The problem is layout, not concept. Live mains tracking for the zero crossing detector runs immediately adjacent to the logic ground plane, that same ground plane is exposed on the DMX connector, clearance under the opto couplers is marginal, and the aluminium enclosure is not earthed while the PCB rests against it. Tested through an isolation transformer, standalone button dimming and DMX control both worked, with some unevenness and the connected lamp only striking around 50 percent, so gamma correction would be needed. Functionally usable, but not something to wire directly to mains.