This is a standard circuit which can be used to adjust the brightness of mains lights and the speed of AC motors. It uses a triac, diac and has a radio-frequency interference (RFI) noise suppression circuit built into it as well.The circuit here controls the average power to a load through the triac by phase control. The ac supply is applied to the load for only a controlled fraction of each cycle. The triac is held in an OFF condition for a portion of its cycle then is triggered ON at a time determined by the circuit.
The main problem with this circuit is radio frequency interference (RFI.). Each time the triac is turned on the load current changes very quickly – a few micro seconds – from zero to a value determined by the lamp resistance and the value of the mains voltage at that instant in time. This transition generates RFI. Since there may be long lengths of mains wire between
the triac and the lamp load which will radiate this RFI an L-C RFI suppression network is usually built into these types of circuits. You may detect this RFI by bringing an FM radio close to the dimmer circuit. Short out the choke coil and notice that the RFI increases. The wire-wound coil and C1 provide the RFI suppression network.