Triggering methods

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1.2

Triggering with synchronization by the mains voltage

This triggering circuit of Figure 4 is synchronized by the mains voltage. The pulses are

always shifted by 180° with respect to each other, whatever the type of load.

Figure 4.

Typical circuit - synchronization by mains voltage

Figure 5.

Synchronization by the mains voltage - waveforms

Angle

θ is the delay between the mains zero voltage and the triggering pulse. It can be

adjusted by means of potentiometer, P, from 0 to 180° to vary the load voltage. The current

in an inductive load (L.R) lags the voltage by an angle

ϕ: (tan ϕ = ω · L/ R).

For triggering angles,

θ, higher than ϕ, operation is perfectly symmetrical and stable.

This simple circuit can still present the risk of a fault in the case where

θ is smaller than ϕ, as

shown in Figure 6.

As an example, take the case of a highly inductive load and an angle

θ = 60°. The TRIAC is

turned on at point A (60°). It conducts over more than 180°, up around 230°. It is blocked at

point B: (290°).

AC Mains

C

T

D

P

Rt

1

2

Mains voltage

T

Gate pulse

ϕ : Current lag (full angle)

β : Blocking of the component

α : Angle of conduction

T

β

T

ϕ

full angle

α

θ

θ

A

A

θ : Triggering delay angle

TRIAC voltage

TRIAC current