Thyristor Triac

SCR stands for Silicon Control Rectifier is a unidirectional device that can conduct from anode to cathode only and not from cathode to anode while A Triac can conduct in both the directions. 

A TRIAC is thus bidirectional thyristor with three terminals device. Thyristor used extensively used for the control of power in the AC circuit. 
Circuit Symbol 

The word of Triac stands for is comings from by combining the capital letters from the word TRIode and AC. When in operation, a Triac is equivalent to two SCRs connected in anti-parallel to shown in a figure. The circuit symbol and its characteristics are shown in the figure given below.

Characteristics of TRIAC
This article also gives some advantages and disadvantages of TRIAC to know more learn about TRIAC device in power electronics engineering. 

Advantages of  TRIAC :
  • It needs a single fuse for protection
  • It can be triggered with positive or negative polarities of glass pulses
  • When the voltage reduced to zero the TRIAC turns off
  • It needs a single heat sink of slightly larger size compared to other
Disadvantages of TRIAC :
  • It is not suitable for DC application
  • It has a very high switching delay
  • It can be triggered in any direction 
  • It is not much reliable than SCR
  • It has a low rating compared to SCR
  • dv/dt rating is very low than SCR