KerimF
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The usual convention for the gate positive current is the direction of the current in the triac when the gate is more positive than MT1. In other words, a positive gate current enters the gate.
We refer that a triac is in (the voltages are relative to MT1, Vg on the X-axis and Vmt2 on the Y-axis):
Quadrant I (one) when Vg is positive & Vmt2 is positive.
Quadrant IV (four) when Vg is positive & Vmt2 is negative.
Quadrant II (two) when Vg is negative & Vmt2 is positive.
Quadrant III (three) when Vg is negative & Vmt2 is negative.
Concerning the gate sensitivity, the worst quadrant is quadrant IV (four).
Vmt2 is AC hence it can be positive or negative. So it is preferred to drive the gate with a negative current to keep the triac in quadrant II and III (avoiding IV). To do this (in the direct triggering), we usually connect MT1 to the positive Vcc and let a transistor (npn or n-MOS) to sink a negative current from the gate via a limiting resitor to ground (groung=the negative terminal of the Vcc supply).
Kerim
We refer that a triac is in (the voltages are relative to MT1, Vg on the X-axis and Vmt2 on the Y-axis):
Quadrant I (one) when Vg is positive & Vmt2 is positive.
Quadrant IV (four) when Vg is positive & Vmt2 is negative.
Quadrant II (two) when Vg is negative & Vmt2 is positive.
Quadrant III (three) when Vg is negative & Vmt2 is negative.
Concerning the gate sensitivity, the worst quadrant is quadrant IV (four).
Vmt2 is AC hence it can be positive or negative. So it is preferred to drive the gate with a negative current to keep the triac in quadrant II and III (avoiding IV). To do this (in the direct triggering), we usually connect MT1 to the positive Vcc and let a transistor (npn or n-MOS) to sink a negative current from the gate via a limiting resitor to ground (groung=the negative terminal of the Vcc supply).
Kerim