AN308

Triggering methods

5/16

The second triggering pulse occurs at point C when

θ + α = 240°. It has no action on the

TRIAC which is still conducting. The TRIAC is not turned on for the next half-wave. As in the

previous case, the operation is totally asymmetrical, and thus unacceptable.

Figure 6.

Synchronization by the mains voltage - asymmetrical operation

θ < ϕ

To prevent this fault, it is necessary to limit the turn-on angle to maintain

θ > ϕ. This is

possible for loads whose L and R parameters remain strictly constant.

Experience shows that for the majority of inductive loads used in industrial applications (like

motor controls and transformers) the values of L and R are not constant and vary a great

deal during operation. For these types of applications it is not possible to limit the turn-on

angle without considerably reducing the voltage excursion.

To sum up, this simple triggering circuit, synchronized by the mains voltage, is more

developed than the previous one. It has the following charactersitics:

●

Advantages:

–

Simple design

–

More accurate control than the previous circuit

–

No auxiliary power supply or transformer required

●

Disadvantages:

–

Connection of the circuit by 3 marked wires, instead of 2 without polarity in the

previous circuits

–

Higher power dissipated in passive components P and Rt (since the mains voltage

is continuously applied across them)

–

Operation becomes completely asymmetrical if the control angle

θ is less than ϕ.

This triggering circuit can be used only for applications in which the phase shift of the load

remains constant (air inductor) or if operation is restricted to values of

θ much higher than ϕ,

that is, for low load voltage operations.

full angle

Mains voltage

Gate pulse

B

ϕ

T

T

T

α

θ

C

θ

TRIAC voltage

TRIAC current