AN3168

Triggering circuits

Doc ID 17193 Rev 1

10/20

Figure 10.

Experimental validation of SCR triggering

2.6

Diac control circuit

A Diac-based circuit can be used for non-insulated control circuits. Such a circuit is very

simple and known since the 70's. It was traditionally used for light dimmers or universal

electromagnetic noise coming from Triac switch-on at voltage levels different from zero. The

circuit will be similar for a universal motor speed control circuit. The filter is then placed

before mains input as the noise comes mainly from motor brush commutations.

Figure 11.

Diac-based light dimmer

The operation of the dimmer circuit shown on Figure 11 is as follows:

1.

The capacitor C is charged through the resistive potentiometer (P)

2.

decreased by

discharge current which is applied to the Triac gate. Rs is used to enlarge the gate

current pulse width. T is then switched on. Here, for the positive line polarity, T is

triggered in Q1.

3.

T remains on until the next zero current crossing point. As T is on, there is no more high

voltage applied between A1 and A2, and the Diac is not charged anymore

V

CTRL

(10V/div)

V

D

(5V/div)

V

T

(100V/div)

I

A

(10mA/div)

I X1 @

L

-20 ‹C

7 mA

)

)

)

)

-‹

t

p

C

P

D

L

F

C

F

T

Vac

I LAMP

VC

R

Rs

Vac

300 V/Div.

V

BO

DV

Vac

300 V/Div.

V

BO

DV