Triggering methods

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Triggering methods

1.1

Triggering with synchronization on the TRIAC voltage

The triggering circuit with synchronization across the TRIAC (See Figure 1 and Figure 2)

turns on the component at an angle

β after the current drops to zero, such that

β = ω · Tr.

Time Tr is defined by the time constant (P + Rt)C.

ω = 2 · π · f with f = mains frequency.

Figure 1.

Typical circuit - synchronization across the TRIAC

Figure 2.

Synchronization across the TRIAC - waveforms (general case)

This is the simplest possible circuit but in certain cases it can have an important drawback.

For example, consider a highly inductive load (L

ω / R > 4) where the TRIAC is turned on

with a considerable delay

β, perhaps 100° after the mains voltage zero as inFigure 3.

If the TRIAC is turned on at point A, the conduction (

α) lasts up to about 150°. The TRIAC

turns off at point B at

α + β = 250° after the zero voltage point. At that instant a negative

voltage is applied to the triggering circuit which turns on the TRIAC at point C after an angle

β of 100°, that is, 350° from the starting point.

AC Mains

C

PRt

T

D

Diac

1

2

Mains voltage

T

T

Gate pulse

T

ϕ : Current lag (full angle)

β : Blocking of the component

α : Conduction angle

α

ϕ

full angle

β

TRIAC voltage

TRIAC current