462 Electrical Power Systems Technology
tion they would be the anode and cathode. For the other alternation they
would respond as the cathode and anode. It is common practice therefore
to use the terms main 1 and main 2 or terminal 1 and terminal 2 to describe
these leads. The third connection is the gate. This lead determines when
the device switches from its off to its on state. The gate G will normally
go into conduction when it is forward biased, and is usually based on the
polarity of terminal 1. If T 1 is negative, G must be positive. When T 1 is
positive, the gate must be negative. This means that ac volt-age must be
applied to the gate to cause conduction during each alternation of the T 1 -
T 2 voltage. The schematic symbol and the crystal structure of a triac are
shown in Figure 17-8. Notice the junction of the crystal structure simpli-
fication. Looking from T 1 to T 2 , the structure involves crystals N 1 , P 1 , N 2 ,
and P 2. The gate is used to bias P 1. This is primarily the same as an SCR
with T 1 serving as the cathode and T 2 the anode.
Looking at the crystal structure from T 2 to T 1 , it is N 3 , P 2 , N 2 , and
P 1. The gate is used to bias N 4 for control in this direction, which is simi-
lar to the structure of an SCR in this direction. Notice that T 1 , T 2 , and G
Figure 17-8. I-V characteristics of a triac