Introduction To Triac

  TRIAC

In this tutorial, we will learn about some of the basics of TRIAC. In the process, we will understand the structure, symbol, working, characteristics, applications of TRIAC.

Introduction

As we know that the SCR as a unidirectional device and has a reverse blocking characteristics that prevents the current flow in reverse biased condition. But for many applications, bidirectional control of current is required, particularly in AC circuits. To achieve this with SCRs, two SCRs must be connected in anti-parallel to control over both positive and negative half cycles of the input.

However, this structure can be replaced by special semiconductor device known as a TRIAC to accomplish the bidirectional control. The TRIAC is a bidirectional switching device that can control the AC power efficiently and accurately. These are often used in motor speed controllers, AC circuits, pressure control systems, light dimmers and other AC control equipments.

TRIAC Basics

The triac is an important member of the thyristor family of devices. It is a bidirectional device that can pass the current in both forward and reverse biased conditions and hence it is an AC control device. The triac is equivalent to two back to back SCRs connected with one gate terminal as shown in figure.

TRIAC is an abbreviation for a TRIode AC switch. TRI means that the device consisting of three terminals and AC means that it controls the AC power or it can conduct in both directions of alternating current.

The triac has three terminals namely Main Terminal 1(MT1), Main Terminal 2 (MT2) and Gate (G) as shown in figure. If MT1 is forward biased with respect to MT2, then the current flows from MT1 to MT2. Similarly, if the MT2 is forward biased with respect to MT1, then the current flows from MT2 to MT1.

The above two conditions are achieved whenever the gate is triggered with an appropriate gate pulse. Similar to the SCR, triac is also turned by injecting appropriate current pulses into the gate terminal. Once it is turned ON, it looses its gate control over its conduction. So traic can be turned OFF by reducing the current to zero through the main terminals.