The idea is: For a typical dimmer, when you apply a pulse on the gate terminal of the TRIAC, it turns on – and stays on until the mains voltage crosses the 0V. So, varying the “firing angle” (“where” on the mains sinusoidal voltage the pulse will be applied) we can vary the amount of power that is delivered to the load. Thus, if the load is a lamp, its brightness will vary according to the TRIACs “firing angle”; the earlier we fire the TRIAC after the mains voltage crosses 0V (lower firing angle), the brighter the lamp will be.
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