Page 420 - Power Electronics Handbook

Heating and Lighting 409

1 I
(b) Load
Figure 14.59 Simple phase-control circuits for resistive loads: (a) triac arrangement;
(b) single-thyristor and diode bridge arrangement

sufficiently high value before the trigger diodes break over. Zener diodes can
be used in place of the trigger devices but then the magnitude of the trigger
pulse is reduced, once the diodes start conducting, and the range of control
is also less. Power loss in the controlling potentiometers of both these circuits
is low, since once the power device turns on the voltage across the
potentiometer falls to a very low value, the loss being minimum at full load,
the delay angle being small.