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April 2004

AN442

APPLICATION NOTE

TRIAC & MICROCONTROLLERS: THE EASY CONNECTION

REV. 2

INTRODUCTION

The aim of this note is to show how to connect an ST triac and an ST microcontroller.

CONVENTIONAL SOLUTION

For many years the triac has been used to switch load on the AC mains and thanks to the low cost of mi-

crocontrollers (µC) this solution is widely used in the appliance market.

All the system use a buffer transistor between the output port of the microcontroller and the triac as shown

in the Figure 1.

Figure 1. Drive in the 1st and 4th quadrants

Because of the low sensivity of the triac in the 4th quadrant this type of drive is often unpractical, and is

replaced by the topology of the Figure 2.

Figure 2. Conventional drive in the 2nd and 3rd quadrants

To save cost, manufacturers want to use fewer and fewer components and of course want to remove the

buffer transistor, but a problem arises.

LINE

µC

0V

LINE

0V

G

A2

A1

µC