Control Devices 467
Triac Variable Power Control
The triac is widely used as a variable ac power control device. Con-
trol of this type is generally called full-wave control. Full wave refers to the
fact that both alternations of a sine wave are being controlled. Variable
control of this type is achieved by delaying the start of each alternation.
This process is similar to that of the SCR. The primary difference is that tri-
ac conduction applies to the entire sine wave. For this to be accomplished,
ac must be applied to both the gate and the conduction terminals.
Variable ac power control can be achieved rather easily when the
source is low voltage. Figure 17-11 shows a simple low-voltage variable
lamp control circuit. Note that the gate current of this circuit is controlled
by a potentiometer. Connected in this manner, adjustment of R 1 deter-
mines the value of gate current for each alternation.
Conduction of a triac is controlled by the polarity of TI and T 2 with
respect to the gate voltage. For the positive alternation, assume that point
A is positive and B is negative, thus causing a +T 2 , a –T 1 , and a +IG.. The
value of the circuit gate current is determined by the resistance of R 1 and
R 2. For a high-resistance setting of R 1 , the triac may not go into conduction
at all. For a smaller resistance value, conduction can be delayed in varying
amounts. Generally, conduction delay will occur only during the first 90°Figure 17-11. Variable lamp control