Q.Describe the static VI characteristics of TRIAC.

The static V-I characteristic of a TRIAC is symmetrical in the first and third quadrants because the device conducts in both directions, blocking until the break-over voltage in each direction, and the break-over voltage falls with gate current. …

[!TLDR]

The TRIAC has an SCR-like curve in both the first and third quadrants (symmetric), blocking to $\pm V_{BO}$ then conducting; $V_{BO}$ falls with $I_g$.

The TRIAC is a bidirectional device (equivalent to two SCRs connected in inverse parallel), so its static V-I characteristic is symmetrical in the first and third quadrants, with voltage $V$ on the horizontal axis and current $I$ on the vertical axis and the origin at the centre.

• Forward direction (first quadrant, MT2 positive with respect to MT1): the TRIAC blocks and carries only a small leakage current as the voltage is increased, up to the forward break-over voltage $V_{BO}$. At $V_{BO}$ it switches (through a negative-resistance transition) into conduction, the voltage falls to a low value and a large current flows, limited by the external circuit.

• Reverse direction (third quadrant, MT2 negative with respect to MT1): the behaviour is a mirror image; the device blocks with a small leakage current up to the reverse break-over voltage $-V_{BO}$, then switches into conduction in the opposite direction. …