Control Devices 463
are all connected to two pieces of crystal. Conduction will take place only
through the crystal polarity that is forward biased. When T 1 is negative,
for example, N 1 is forward biased and P 1 is reverse biased. Terminal selec-
tion by bias polarity is the same for all three terminals.
The schematic symbol of the triac is representative of reverse-con-
nected diodes. The gate is connected to the same end as T 1 , which is an
important consideration when connecting the triac into a circuit. The gate
is normally forward biased with respect to T 1.
When ac is applied to a triac, conduction can occur for each alterna-
tion, but T 1 and T 2 must be properly biased with respect to the gate. For-
ward conduction occurs when T 1 is negative, the gate (G) is positive, and
T 2 is positive. Reverse conduction occurs when T 1 is positive, G is nega-
tive, and T 2 is negative. Conduction in either direction is similar to that of
the SCR.Triac I-V Characteristics
The I-V characteristic of a triac shows how it responds to forward
and reverse voltages. Figure 17-8 is a typical triac I-V characteristic. Note
that conduction occurs in quadrants I and III. The conduction in each
quadrant is primarily the same. With 0 IG, the breakover voltage is usu-
ally quite high. When breakover occurs, the curve quickly returns to the
center. This shows a drop in the internal resistance of the device when
conduction occurs. Conduction current must be limited by an external re-
sistor. The holding current or IH of a triac occurs just above the knee of the
IT curve. IH must be attained or the device will not latch during a specific
alternation.
Quadrant III is normally the same as quadrant I and thus ensures
that operation will be the same for each alter-nation. Because the triac is
conductive during quadrant III, it does not have a peak reverse voltage
rating. It does, however, have a maximum reverse conduction current val-
ue the same as the maximum forward conduction value. The conduction
characteristics of quadrant III are mirror images of quadrant I.Triac Applications
Triacs are used primarily to achieve ac power control. In this appli-
cation the triac responds primarily as a switch. Through normal switch-
ing action, it is possible to control the ac energy source for a portion of
each alternation. If conduction occurs for both alternations of a complete
sine wave, 100% of the power is delivered to the load device. Conduction