Diac-Triac Phase/Power Control without Hysteresis

Hysteresis in lamp dimmer or in other electric appliance might cause some trouble if you need a fine tuning, something like out of control. This is a circuit of Diac-Triac Phase/Power Control without Hysteresis. This circuit can eliminate the hysteresis effect. Here is the circuit:

The initial condition of the timing capacitor is uniform because the (a) circuit reset timing capacitor to the same level after each positive half-cycle. However, this circuit suitable only for resistive loads which the firing angle is not symmetrical throughout the range. The (b) circuit is used when the symmetrical firing is needed. [Circuit’s schematic diagram source: littlefuse.com]