This is a Reduced Hysteresis Diac-Triac Phase/Power Control circuit. This circuit is used to control from 5% to 95% of full load power. This circuit can decrease the hysteresis effect to a reasonable region. It can be done because this circuit uses second RC phase-shift network. Beside that, the second RC phase-shift network also extends the range on control. This circuit can be used to vary the supply voltage. Here is the circuit:

Reduced Hysteresis Diac Triac Phase Power Control circuit schematic

The phase-shifted voltage is formed by the large R1 and C1 that is charged primarily through R3. After the diac triggered, the hysteresis is reduced and the C2 is enabled to recharge C1 and the range of phase-shift across C1 is increased. When the R1 reach the maximum resistance, the R3 must be adjusted so that the circuit just drop out of the conduction. [Circuit’s schematic diagram source: littlefuse.com]