Solution. We have seen that transistor equivalent circuit of an SCR is composed of pnp transistor and npn transistor with collector of each transistor coupled to the base of the other. Since a triac is equivalent to two SCRs connected in inverse parallel (Refer back to Fig. 21.4), the transistor equivalent circuit of triac will be composed of four transistors arranged as shown in Fig. 21.6 (i). The transistors Q1 and Q2 constitute the equivalent circuit of SCR1 while the transistors Q3 and Q4 constitute the equivalent circuit of SCR2. We can explain the action of triac from its transistor equivalent circuit as under :