Triac

The triode AC switch (TRIAC) is a power-switching device as is the SCR. The TRIAC conducts currents in both directions while the SCR allows current in only one direction. A common application is for lighting controllers. In response to a trigger, the triac conducts until the AC voltage applied reaches zero, then blocks flow until the next trigger occurs. Since a trigger can cause it to trigger current in either direction, it is an efficient power controller from essentially zero to full power.

Fig 1. Triac Symbol

The triac is similar in operation to two thyristors connected in reverse parallel but using a common gate connection. This gives the triac the ability to be triggered into conduction while having a voltage of either polarity across it. In fact it acts rather like a "full wave" thyristor. Either positive or negative gate pulses may be used.

Triacs are mainly used in power control to give full wave control. This enables the voltage to be controlled between zero and full power. With simple "half wave" thyristor circuits the controlled voltage may only be varied between zero and half power as the thyristor only conducts during one half cycle. The triac provides a wider range of control in AC circuits without the need for additional components, e.g. bridge rectifiers or a second thyristor, needed to achieve full wave control with thyristors. The triggering of the triac is also simpler than that required by thyristors in AC circuits, and can normally be achieved using a simple DIAC circuit. A simplified triac control circuit is shown in Figure 2. The operation will be explained after introducing the Diac.

Figure 2. Simplified AC Power Control Circuit using a Triac

Note that in practical control circuits using Thyristors, Triacs and Diacs, large voltages are switched very rapidly. This can give rise to serious RF interference, and steps must be taken in circuit design to minimise this. Also as Mains is present in the circuit there must be some form of safe isolation between the low voltage control components (e.g. the Diac and phase shift circuits) and the mains "live" components, e.g. the Triac and load. This can easily be achieved by "Opto-coupling" the low voltage control circuit to the high voltage power control (Triac or SCR) part of the circuit.

The Opto Triac

The materials used in the manufacture of Triacs and SCRs, like any semiconductor device, are light sensitive. Their conduction is changed by the presence of light; that's why they are normally packaged in little chunks of black plastic. However, if an LED is included within the package, it can turn on the high voltage device output in response to a very small input current through the LED. This is the principle used in Opto-Triacs and Opto-SCRs, which are readily available in integrated circuit (IC) form and do not need very complex circuitry to make them work. Simply provide a small pulse at the right time and the power is switched on. The main advantage of these optically activated devices is the excellent insulation between the low power and high power circuits, (typically several thousand volts). This provides safe isolation between the low voltage input and high voltage output.