DIAC drawback; TRIAC four-quadrant operation

Drawbacks of DIAC

• It has no gate terminal, so its switching point cannot be controlled; it turns on only when the applied voltage reaches its fixed breakover voltage (about 30 V).
• It is a low-power device; it cannot handle large load currents, so it is used only as a trigger device for TRIACs/SCRs.
• It has a negative resistance region and a fairly large voltage drop after breakover, and its breakover voltage varies from device to device.

Operating principle of TRIAC

A TRIAC is a bidirectional, three-terminal thyristor (MT1, MT2, gate G), equivalent to two SCRs in inverse parallel with a common gate. It is a five-layer device (N-P-N-P-N between MT1 and MT2), with an N region also near the gate.

        MT2
         |
    +----+----+
    |         |
   SCR1 ->   <- SCR2
    |         |
    +----+----+
         |      G
        MT1 ----+

• MT2 positive w.r.t. MT1: the P1-N1-P2-N2 path is forward biased. A gate pulse turns it ON and current flows MT2 → MT1.
• MT2 negative w.r.t. MT1: the P2-N1-P1-N4 path is forward biased. A gate pulse turns it ON and current flows MT1 → MT2.
• Once ON it latches until the current falls below the holding current (at every AC zero crossing).

V-I characteristic:

              I
              ^     on (quadrant I)
              |    /
              |   |
  -VBO        |   |
 ----|--------+---|-----> V(MT2-MT1)
     |        |  +VBO
     |        |
    /         |
on (quadrant III)

Increasing gate current reduces the breakover voltage in both directions, just like an SCR.

Why four-quadrant operation

Both the MT2 voltage and the gate current can be of either polarity, giving four triggering modes:

Since the TRIAC can be triggered with any of the four combinations of anode (MT2) and gate polarity, i.e. in all four quadrants of the V–IG plane, it is called a four-quadrant device. In practice, modes I+ and III− are preferred because they need the least gate current.