TRIAC and DIAC
A TRIAC conducts in both directions for AC power control; a DIAC triggers it symmetrically.
Intermediate
A TRIAC is effectively two SCRs in anti-parallel with a shared gate, so it controls both half cycles of AC — the heart of light dimmers and fan regulators. A DIAC breaks over at about 30 V in either direction and gives the TRIAC a clean, symmetrical trigger pulse.
Opto-TRIAC drivers (MOC3021, or zero-cross MOC3041) safely couple a microcontroller to mains-switching TRIACs.
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