AN3168

Triggering circuits

Doc ID 17193 Rev 1

11/20

4.

T Triac switches off when its current reaches zero. Then the mains voltage is applied

back between A1 and A2. So Capacitor C is then recharged in negative bias. This will

5.

Triac T is switched on as explained on point 2 but here in Q3 (current sunk from the

This means that ACS can not be triggered by such circuits as they need that their gate

current is always sunk from the gate (i.e. only Q2 and Q3 triggering is possible).

The main point to check with Diac-based circuits is that the gate current pulse width lasts

for further information on this parameter). Figure 12 explains this point. The gate current has

current, higher than the maximum latching current (according to datasheet maximum value,

This will ensure a correct Triac turn on.

Figure 12.

Required gate pulse width, with Diac circuit, to trigger the Triac

used to define the Rs and C values. For example, if a 15 µs pulse width is required, a 33

Ω

resistor Rs is required with a 150 nF capacitor C.

It should be noted also that resistor Rs is helpful to keep Diac current below its maximum

Figure 13.

Typical pulse width duration versus Rs and C values

time

Load current

Gate current

time

Zoom

IL MAX

IGT MAX

t

p

time

Load current

Gate current

time

I

I

15 µs

150 nF