Here's a simplified block diagram of the funcionality expected. The meter IC, CS5463, is powered from 5V0 and 3V3 generated from the AC/DC brick and linear regulators. The microcontroller uses 3.3V only. The unregulated 12V are needed for the IGBT section of the dimmer. The IGBT section is used to limit the inrush of current when the SCRs are triggered, hence reducing the need for a bulky inductor.