A Triac is a three-terminal electronic component that functions as a gate-controlled bidirectional switch, primarily for AC circuits. Its structure is basically equivalent to two oppositely-facing silicon-controlled rectifiers (SCR's) connected in parallel, with their gates connected together.  Whereas an SCR is capable of conducting current in only one direction, a triac can conduct current in both directions because of its dual-SCR configuration.

The main terminals of a triac are often designated as 'MT1' and 'MT2', while its input control terminal is referred to as a 'gate', as mentioned earlier.  Whenever a sufficient positive or negative voltage is applied at the gate of a triac, one of its two SCR's turn on, causing current to flow through the triac. Which SCR is conducting at any one time depends on the polarity of the voltage across the triac.

Figure 1.  Photo of a triac (left) and the circuit symbol for a triac (right)

Just like an SCR, a triac will continue to conduct once it is turned on, even if the triggering gate voltage is removed.  However, the current flowing through the triac must remain above a certain level, known as the 'holding current', in order to keep the triac conducting.  If the current through the triac falls below the holding current, the triac switches off, and needs to be triggered again in order to conduct. 

A triac is a good switching device for AC loads, such as incandescent bulbs and AC motors. In normal AC applications, a triac turns off when the sinusoidal current crosses the zero level. Note that when a triac is used to drive inductive loads, the current is more difficult to drive to zero (since inductors oppose instantaneous changes in current), and this might present some issues during switch-off. Thus, this phenomenon must be considered when designing an application wherein a triac is used to power an inductive load.

The point within the cycle of the sine wave at which a triac is triggered may also be precisely timed, such that the percentage of power delivered to the load may also be controlled with a triac.

Examples of applications for triacs include:  1) switching and dimming for AC incandescent bulbs; 2) speed controls for appliances with electric motors, e.g., electric fans; and 3) interfacing of AC appliances to digital computer systems.

The following parameters must be considered when selecting a triac: 1) forward and reverse breakover voltage; 2) maximum load current; 3) minimum holding current; 4) gate voltage and current trigger specifications; 5) switching speeds; and 6) maximum dV/dt.

See Also:   Silicon-Controlled Rectifier (SCR);  Active Components;  More Articles