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The TRIAC can be triggered by the current going into the gate. To create a triggering current, a positive or negative voltage has to be applied to the gate with respect to the MT1 terminal. Once triggered, the device continues to conduct until the current drops below a certain threshold called the holding current. When the gate current is discontinued, and if the current flowing between the two main terminals is more than what is called the latching current, the device keeps conducting.
After the TRIAC is turned on, removing the voltage at the gate will not turn the TRIAC to the off state. The only way the TRIAC can be turned off is by reducing the gate current below the holding current or reducing the current to zero by removing the voltage across MT1 and MT2.
When sufficient current of the internal LED (MOC3041) is present, the TRIAC will re-trigger every half cycle of AC line voltage until the internal LED switches off and the power TRIAC has gone through a zero current point. The internal LED is turned on by the user from the PC, which in turn triggers the corresponding TRIAC, and the device connected across the TRIAC is switched on. Thus, each electrical device or appliance is controlled through a solidstate switch using a TRIAC and an opto-TRIAC.