Triggering methods

AN308

6/16

1.3

Triggering synchronized by the mains voltage and suitable

for industrial applications

This new circuit is derived from the previous one by improving the triggering pulse

generator. The improvement consists of maintaining the triggering signal during each half

wave between values

θ and 180°. This is done simply by sending a pulse train after the

initial pulse so as to maintain the triggering order, as shown in Figure 7.

Figure 7.

Triggering by pulse train synchronization by the mains voltage

For example, suppose that angle

ϕ is equal to 85° and θ is equal to 60°. At the first pulse,

the TRIAC is turned on at point A (60°). It conducts for angle

α1 greater than 180° and close

to 240°. It is blocked at point B, but is immediately triggered at point B’ by the next gate

pulse. During the first half-waves, operation is slightly asymmetrical but gradually the

durations of conduction become balanced (refer to the dotted line curve in Figure 7).

Figure 8 gives the circuit diagram. A small sensitive auxiliary TRIAC, Ts, is used to produce

the required pulse train. The delay time constant, defined by capacitor C, compensating

resistor Rt and potentiometer P, sets the angle

θ. The capacitor charges from 0 V. DIAC D

triggers TRIAC T as soon as the capacitor voltage reaches the DIAC breakover voltage

(Vbo). This time is the same for both half-waves, it just depends on Vbo symmetry.

A first pulse is applied to the gate of the main TRIAC, T. A voltage pulse occurs across Rd

and triggers sensitive TRIAC Ts. Once turned on, this Ts bypasses potentiometer P. Thus

the remaining charging cycles of the capacitor have a much shorter time constant Rt · C.

Mains voltage

Gate pulse

ϕ : Current lag full angle

β : Blocking of triac

α1 : 1st angle of conduction

α2 : 2nd angle

T

T

T

β

α 1

A

B

full angle

α 2

θ

θ

ϕ

θ : Triggering delay time

TRIAC voltage

TRIAC current