456 Electrical Power Systems Technology
ac applied to an SCR, it will
automatically turn off during
one alternation of the input.
Control is achieved by alter-
ing the turn-on time during
the conductive or “on” alter-
nation.
SCR I-V Characteristics
The current-voltage
characteristics of an SCR
tell much about its opera-
tion. The I-V characteristic
curve of Figure 17-4 shows
that an SCR has two conduc-
tion states. Quadrant I shows
conduction in the forward
direction, which shows how
conduction occurs when the
forward breakover voltage
(VBO) is exceeded. Note that
the curve returns to approxi-
mately zero after the VBO has
been exceeded. When con-
duction occurs, the internal
resistance of the SCR drops to
a minute value similar to that
of a forward-biased silicon
diode. The conduction cur-
rent (IAK) must be limited by
an external resistor. This cur-
rent, however, must be great
enough to maintain conduc-
tion when it starts. The hold-
ing current or IH level must
Figure 17-3. Representative
SCR packages