We shall now see that a triac is equivalent to two separate SCRs connected in inverse parallel (i.e. anode of each connected to the cathode of the other) with gates commoned. Fig. 21.3 (i) shows the basic structure of a triac. If we split the basic structure of a triac into two halves as shown in Fig. 21.3 (ii), it is easy to see that we have two SCRs connected in inverse parallel. The left half in Fig. 21.3 (ii) consists of a pnpn device (p1n2 p2n4) having three pn junctions and constitutes SCR1. Similarly, the right half in Fig. 21.3 (ii) consists of pnpn device (p2n3p1n1) having three pn junctions and constitutes SCR2. The SCR equivalent circuit of the triac is shown in Fig. 21.4.