Design Ideas: October 13, 1994
Applying ac power to the circuit in Fig 1 causes it to gradually increase the power to the ac load, effecting a "soft start."
The components connected to 555 timer IC1's pin 2 provide a trigger synchronized with the ac input's zero crossing. In conventional 555 fashion, R1, R2, and Cdetermine IC1's period. However, the RC delay network connected between IC1's pins 1 and 5 causes IC1's period to approach its full value only gradually, rather than instantly at power-on. Select a resistor-capacitor combination that suits your load.
IC1's pin-3 output, via the transistor Q1, drives the Motorola MOC 3021 triac driver. Any general-purpose, small-signal, npn transistor works in the circuit. (DI 1578)