When the circuit operation begins initially, the energy is stored in the capacitor until it reached the required threshold value to activate the diac ( D1). Once that happens, the diac ( which acts as the voltage regulator) generates the required gate voltage to activate the triac which is the main component contributing to the voltage regulation across the load.
The variable rheostat is the pointer regulator under manual control. By adjusting the rheotstat, we can control the charge in the capacitor which can further control the voltage drop across the load. And this regulation in voltage finally leads to change in the brightness of the bulb. Hence, when not doing any heavy activity like reading or writing, the bulb or tube light dimness can be attained and hence can lead to less power consumption. Also, if we use a purely resistive circuit for this purpose ( using a variable resistance or rheostat), the ohmic resistance of the circuit increases. Hence, using triac and diac switching devices help in reducing the ohmic losses and increase the efficiency of the circuit.
I wanted to know how efficient this concept would be in power saving. Will it contribute to a substantial level in reduction of power consumption if applied to an entire house?
Link to the circuit: http://www.eleccircuit.com/wp-content/uploads/2010/09/light-ac-dimmer-120watt.jpg