In our time the bulbs are available with very low headroom secure. The inrush current that occurs when the lighting often brings the bulb failure due to the small resistance in the cold state. And the cost bulbs when our salaries "bite".
Therefore, when the lighting bulb must heat a small current and then to turn on full power and It makes the scheme sparing inclusion shown in figure.
When applying voltage (SB1 included) thyristor VD3 closed the lamp HL1 is fed through the diode VD4 transmits only the negative half-wave so on the bulb is served half the operating voltage. The voltage is rectified diode VD1 and through the divider R1 R2 begins to charge the capacitor C1.
When reaching a certain voltage, the Zener diode VD2 and breaks the tension from the chain R1 R2C1 applied to the control electrode of the thyristor VD3. It opens and begins to pass positive half of the mains voltage. With that moment the light bulb goes full line voltage and the light comes on full intensity.
The incorporation process takes approximately 2 this is enough to heat up light bulb When using the diode VD4 type D226 you can apply light bulbs power up to 120 watts. To achieve a power of 700 watts, the author used the diode KD202 and the thyristor is put on a small radiator.
For 10 years the author has not failed no light bulb. And that's make conclusions.
Author: O. G. Rashitov, Kiev