One of the incandescent lamp life extension circuits

Incandescent lamps are most likely to burn out when the power is turned on, because the resistance of the filament in the cold state is very small (about 9 times smaller than in the red-hot state).
The power it consumes at the moment of power-on greatly exceeds the rated value, and the filament cannot withstand the current impact and burns out. Burn out.
This is often the case with expensive high-power (up to 500w) light bulbs such as those used in projection equipment.
    In order to prevent the bulb from being impacted by a large instantaneous current, an initial voltage can be given to the bulb to preheat the filament when powering on, and then
the voltage can be raised to the rated value after a certain period of time.
(1) One circuit of the incandescent lamp life extension circuit is shown in Figure 2-29. It adopts two-level voltage power supply mode, the first level is half wave and the second level is full wave. When the switch SA is closed, the voltage across the lamp EL is about 110V. After a few tenths of a second to a few seconds (adjustable), 220V voltage is applied to both ends of the EL , and it turns on.