468 Electrical Power Systems Technology
of the alternation. If conduction does not occur by this time, it will be off
for the last 90° of the alternation.
Variable control of the same type also occurs during the negative al-
ternation. For this alternation, point A is negative and point B is positive,
thus causing a – T 2 , a +T 1 , and a –IG. Gate current will flow and cause con-
duction during this alternation. The resistance setting of R 1 influences IG
in the same manner as it did for the positive alternation. Both alternations
will therefore be controlled equally. Variable control of this type applies to
only 50% of the source volt-age. If conduction does not occur in the first
90% of an alternation, no control will be achieved.
DIAC POWER CONTROL
A diac is a special diode that can be triggered into conduction by
voltage. This device is classified as a bidirectional trigger diode, meaning
that it can be triggered into conduction in either direction. The word diac
is derived from “diode for ac.” This device is used primarily to control the
gate current of a triac. It will go into conduction during either the positive
or the negative alternation. Conduction is achieved by simply exceeding
the breakover voltage.
Figure 17-12 shows the crystal structure, schematic symbol, and I-V
characteristics of a diac. Note that the crystal is similar to that of a transis-
tor without a base. The N1 and N 2 crystals are primarily the same in all
respects. A diac will therefore go into conduction at precisely the same
negative or positive voltage value. Conduction occurs only when in-put
voltage exceeds the breakover voltage. A rather limited number of diacs
are available today. The one shown here has a minimum VBO of 28 V. This
particular device is a standard trigger for triac control. Note that the volt-
age across the diac decreases in value after it has been triggered.
ELECTRONIC CONTROL CONSIDERATIONS
Electronic power control with an SCR or a triac is efficient when used
properly. These devices are, however, attached directly to the ac power
line. Severe damage to a load device and a potential electrical hazard may
occur if this method of control is connected improperly. As a general rule,
SCR and triac control should not be attempted for ac-only equipment such