TRIAC (Triode for Alternating Current) is a semiconductor device consisting of a four-layer silicon structure that allows current to flow in both directions through three electrodes. A TRIAC shows a PNPN function in the positive direction and an NPNP function in the negative direction. This feature makes it an ideal switch for controlling alternating current (AC) signals in both directions with a single device.

TRIACs are typically included in the class of devices such as silicon-controlled rectifiers (SCRs) and thyristors. Some of the key characteristics of TRIACs include:

1. Power Amplification: It can provide power amplification at low to medium voltages.
2. Bidirectional Current Flow: It allows current to flow in both directions, making it superior to traditional SCRs.
3. Two States: Like any thyristor, the TRIAC operates in only two distinct states: on and off.
4. Gate Signal Triggering: Like SCRs, TRIACs are triggered by a gate signal, making them suitable for various control applications.

^{TRIAC Voltage-Current Characteristic}

Operating Principle and Structure

The operating principle of the TRIAC is derived from the combination of two thyristors (SCRs) connected in antiparallel. This structure enables it to handle both directions of current. Regardless of the current's direction, the device can switch the current on, allowing a single TRIAC device to control both directions of AC currents. However, the sensitivity of triggering is highest when the polarity at both ends is in the same direction.

^{TRIAC Symbol}

Applications

TRIACs are primarily used as switches in AC circuits. They are commonly preferred in applications like lamps, fans, and motor speed control systems at low to medium power levels. TRIACs are also used in temperature and liquid level control systems. In circuits where the TRIAC functions as a switch, current starts flowing as the device is triggered, and the conduction continues until the current approaches zero.