AN871 APPLICATION NOTE

6/13

Figure 6. Example of application of SCR in safety equipment. The thyristor is turned on when the

sensor is activated and discharges the capacitor through the powder firing system

Bidirectional switch applications

In a car, motors and various actuators require generally the control of the current in both directions: the

function is activated and inactivated by reversing the current.

To achieve this bidirectional switching with the conventional technics it is necessary to implement a full

bridge: four power MOS transistors associated with the heavy gate drive circuit inherent to this topology.

We propose a "one-device" solution: the triac. It is the unique switch able to control the current in both

directions.

A good example is the central door lock system shown below.

Central door lock circuit. In this application two relays with change over contact achieve the function

"locking" and "unlocking".

In some new models, a switch is connected in series with each lock motor in order to control separately

each door (see Figure 7).

+V

SENSOR

POWDER

FIRING

SYSTEM