464 Electrical Power Systems Technology
for half of each alternation permits 50% control. If conduction is for one-
fourth of each alternation, the load receives less than 25% of its normal
power. It is possible through this device to control conduction for the en-
tire sine wave, which means that a triac is capable of controlling from 0%
to 100% of the electrical power supplied to a load device. Control of this
type is efficient as practically no power is consumed by the triac while per-
forming its control function.
Static Switching. The use of a triac as a static switch is primarily an
on-off function. Control of this type has a number of advantages over me-
chanical load switching. A high-current energy source can be controlled
with a very small switch. No contact bounce occurs with solid-state
switching which generally reduces arcing and switch contact destruction.
Control of this type is rather easy to achieve. Only a small number of parts
are needed for a triac switch.
Two rather simple triac switching applications are shown in Figure
17-9. The circuit in Figure 17-9(a) shows the load being controlled by an
SPST switch. When the switch is closed, ac is applied to the gate. Resis-
tor R 1 limits the gate current to a reasonable operating value. With ac ap-
plied to the gate, conduction occurs for the entire sine wave. The gate of
this circuit requires only a few milliamperes of current to turn on the triac.
Practically any small switch could be used to control a rather large load
current.
The circuit of Figure 17-9(b) is considered to be a three-position
switch. In position 1, the gate is open and the power is off. In position 2,
gate current flows for only one alternation. The load receives power dur-
ing one alternation, which is the half-power operating position. In posi-
tion 3, gate cur-rent flows for both alternations. The load receives full ac
power in this position.
Start-stop Triac Control
Some electrical power circuits are controlled by two push-buttons or
start-stop switches. Control of this type begins by momentarily pushing
the start button. Operation then continues after releasing the depressed
button. To turn off the circuit, a stop button is momentarily pushed. The
circuit then resets itself in preparation for the next starting operation. Con-
trol of this type is widely used in motor control applications and for light-
ing circuits. A triac can be adapted for this type of power control.
A start-stop triac control circuit is shown in Figure 17-10. When elec-
trical power is first applied to this circuit, the triac is in its nonconductive