AN440

Conclusion

7/8

4

Conclusion

In the case of controllers supplied by positive voltage this solution allows the replacement of

conventional TRIACs used in the 1st and 4th quadrants by Snubberless or LOGIC LEVEL

TRIACs which operate only in the first three quadrants. This solution only requires the

addition of a capacitor and a diode to control each TRIAC.

With inductive loads (motor, transformer, etc...) a pulse train can be used because of the

phase lag between current and voltage.

In the case of logic or transistor failure, the capacitor C operates as an open circuit for dc

current and avoids all triggering. This factor acts as a safety feature.

But this trigger circuit can not be effectively used to drive small loads (like valves, fan etc...)

because the latching current value is quite high compared to the load current. In this case a

dc gate current is required.

directly from the gate with the control circuit. There is then no need of a supplementary

capacitor and diode. This solution is then easier and cheaper.

5

Revision history

Table 2.

Document revision history

Date

Revision

Changes

May-1992

1

Initial release.

23-Apr-2004

2

Style sheet update. No Content change.

10-Mar-2008

3

Reformatted to current standards. Full technical review