This is circuit of soft light dimmer. This circuit uses the IGBT STGP10N50A and the TS555 timer as main components. Here is the circuit:

Soft Light Dimmer circuit schematic
The timer is triggered on the zero crossing voltage pulse. The time constant determined by C5/R14+R15 that is used to determine the conduction angle. The T2 – T3 inhibits the power switch until the auxiliary supply voltage reaches 8V to guarantee that the gate of the IGBT receives correct voltage level.

R5 is the current sense resistor, that is used for the short circuit and over-current protection. The gate of the IGBT  will taken low and turn off, when the voltage across R5 reaches the sensitive gate thyristor’s gate trigger voltage.  At the same time the timer will reset. This current limiting also provides  protection against excessive in-rush current and automatic soft-start. [Source: STMicroelectronics Application Note]