466 Electrical Power Systems Technology
state. The load does not receive any power for operation. All the supply
voltage appears across the triac because of its high resistance. No voltage
appears across the RC circuit, the gate, or the load device initially.
To energize the load device, the start pushbutton is momentarily
pressed. C 1 charges immediately through R 1 and R 2 , which in turn causes
IG to flow into the gate. The VBO of the triac is lowered and it goes into
conduction. Voltage now appears across the load, R 2 - C 1 , and the gate. The
charging current of R2-C 1 and the gate continue and are at peak value
when the source voltage alternation changes. The gate then retriggers the
triac for the next alternation. C 1 is recharged through the gate and R 2. The
next alternation change causes IG to again flow for retriggering of the triac.
The load receives full power from the source. The process will continue
into conduction as long as power is supplied by the source. To turn off
the circuit, the stop button is momentarily depressed. This action immedi-
ately bypasses the gate current around the triac. With no gate current, the
triac will not latch during the alternation change. As a result, C1 cannot be
recharged. The triac will then remain off for each succeeding alternation
change. Conduction can be restored only by pressing the start button. This
circuit is a triac equivalent of the ac motor electrical contactor.
Figure 17-10. Start-stop control