Online Wikipedia Definition of TRIAC:
TRIAC, from triode for alternating current, is a generic trademark for a three terminal electronic component that conducts current in either direction when triggered. Its formal name is bidirectional triode thyristor or bilateral triode thyristor. A thyristor is analogous to a relay in that a small voltage and current can control a much larger voltage and current. The illustration on the right shows the circuit symbol for a TRIAC where A1 is Anode 1, A2 is Anode 2, and G is Gate. Anode 1 and Anode 2 are normally termed Main Terminal 1 (MT1) and Main Terminal 2 (MT2) respectively.
TRIACs are a subset of thyristors and are related to silicon controlled rectifiers (SCRs). TRIACs differ from SCRs in that they allow current flow in both directions, whereas an SCR can only conduct current in a single direction. Most TRIACs can be triggered by applying either a positive or negative voltage to the gate (an SCR requires a positive voltage). Once triggered, SCRs and TRIACs continue to conduct, even if the gate current ceases, until the main current drops below a certain level called the holding current.
Mostly TRIACs are used when you need to switch high voltage AC loads (motors, lights, solenoids, and such) ON and OFF from a microcontroller's pin. AC voltage canbe switched ON and OFF by a relay or a TRIAC. The relay is easy to use, but has the disadvantages of being slow and consuming a lot of current (to keep the coil energized). Due to the fact that the relay contacts are mechanical, they will also wear out over time and make switching noise. On the other hand, the TRIAC is a small solid-state device that can switch really fast, hence you can use it to dim lights, control motors speed and heating elements. To turn the TRIAC on a positive voltage has to be supplied to the Gate termina.
The best and safest way to trigger a TRIAC is to use a Triac Driver Optocouple like MOC3051M. It consist of a LED and a phototriac that isolate the DC part of your circuit from the AC part.
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