TRIAC: What is it? (Definition, Operation & Applications)

What is a Triac?

A TRIAC is a three-terminal thyristor that can conduct current in either direction after it is triggered. A single TRIAC can therefore control both half-cycles of an AC waveform, unlike unidirectional silicon controlled rectifiers. Supported gate-current polarities depend on the device family and trigger quadrant.

This bidirectional semiconductor switch has main terminals MT1 and MT2 plus a gate. TRIACs are rated by RMS current, repetitive blocking voltage, gate trigger current, surge current, thermal limits and commutation performance. There is no universal 16 kW maximum; usable load power depends on the selected device, circuit, waveform and cooling.

The symbol shows main terminals MT₁ and MT₂ plus the gate terminal. Its opposed thyristor shapes represent bidirectional conduction.

Construction of Triac

A TRIAC is often modelled as two SCRs connected in inverse parallel with a common gate, although a real device uses one integrated multi-layer structure. Because current can flow in either direction, the main terminals are called MT₁ and MT₂ rather than anode and cathode. The third terminal is gate G.

The construction diagram identifies main terminals MT₁ and MT₂ and the gate terminal.

Operation of Triac

A TRIAC is normally turned on by a gate-current pulse while its main-terminal voltage is within the rated blocking range. The pulse must meet the selected device’s gate-trigger current and duration requirements long enough for main current to exceed the latching current. A fixed 35-microsecond pulse is not universal. Once latched, the TRIAC remains on until main current falls below its holding current.
Four trigger quadrants are defined by MT2 voltage and gate-current polarity relative to MT1:

1. MT₂ positive and gate positive relative to MT₁
   This is quadrant I. In the simplified layer model, current is associated with the path P₁-N₁-P₂-N₂. The P₁-N₁ and P₂-N₂ junctions are forward biased while N₁-P₂ is reverse biased. Positive gate current relative to MT₁ injects carriers near P₂-N₂ and initiates regenerative turn-on.
2. MT₂ positive and gate negative relative to MT₁
   This is quadrant II. The main-current path is represented as P₁-N₁-P₂-N₂. Forward bias at P₂-N₃ injects carriers into P₂ and triggers the regenerative structure.
3. MT₂ negative and gate negative relative to MT₁
   This is quadrant III. The simplified path is P₂-N₁-P₁-N₄. The P₂-N₁ and P₁-N₄ junctions are forward biased while N1-P1 is reverse biased. Quadrants I and III commonly provide the best gate sensitivity.
4. MT₂ negative and gate positive relative to MT₁
   This is quadrant IV. Carrier injection near P₂-N₂ can trigger a standard four-quadrant TRIAC, but this quadrant usually has the lowest gate sensitivity. Many snubberless and logic-level TRIAC families do not support quadrant IV, so the gate circuit must follow the selected data sheet.

Characteristics of a Triac

The triac V-I characteristic resembles an SCR characteristic in both positive and negative voltage directions. Trigger-quadrant names use the polarities measured relative to MT1:

First Quadrant Operation of Triac

MT₂ is positive relative to MT₁, and gate current is also positive relative to MT1.

Second Quadrant Operation of Triac

MT2 is positive relative to MT1, while gate current is negative relative to MT1.

Third Quadrant Operation of Triac

MT2 is negative relative to MT1, and gate current is also negative relative to MT1.

Fourth Quadrant Operation of Triac

MT2 is negative relative to MT1, while gate current is positive relative to MT1.

After turn-on, main current is limited by the load and circuit impedance. A series resistor is appropriate only where required by the load design. The circuit must keep RMS current, surge current, rate of current rise and junction temperature within ratings. A phase-control trigger such as a diac can set the firing angle. Gate-pulse amplitude and duration must follow the TRIAC data sheet.

Advantages of Triac

1. A standard four-quadrant device can accept positive or negative gate-current pulses, subject to quadrant ratings.
2. One bidirectional device can replace two inverse-parallel SCR switching paths in many AC circuits.
3. A single package can simplify mounting, gate drive and overcurrent protection.
4. It provides rated symmetrical off-state blocking in both directions without an external antiparallel diode.

Disadvantages of Triac

1. Commutation with inductive loads can be more demanding than with separate SCRs.
2. Static and commutating dv/dt limits can require a snubber or a snubberless device family.
3. Very high current and voltage applications may offer a wider choice of SCRs than TRIACs.
4. Gate-drive polarity, isolation, noise immunity and supported quadrants require careful design.

Uses of Triac

1. AC on-off control circuits.
2. Lamp switching and dimming within the selected device ratings.
3. Phase-angle control of heaters and universal motors.