AN308

Triggering methods

7/16

Figure 8.

Circuit with triggering by pulse train synchronization by mains voltage

This results in a train of pulses being applied to the gate of TRIAC, T. In this way T is

retriggered into conduction instead of remaining blocked as in the previous circuit.

The pulse train continues until the mains voltage crosses the zero point, at which time

TRIAC Ts, supplied through a resistive load, turns off.

For the following half-cycle, the time constant is once more determined by the capacitor and

the potentiometer. This half cycle is then symmetrical to the previous one..

To sum up, the improved triggering circuit synchronized by the mains voltage has a number

of advantages.

●

Low cost

●

Excellent control accuracy

●

No need for auxilliary power supply

●

Operation with all types of loads (different cos

ϕ or variable cos ϕ values)

●

No risk of failure over the whole adjustment range

This circuit has been developed by the STMicroelectronics applications laboratory and used

with success for a wide range of equipment.

1.4

Conclusion

The tricky control of an inductive load requires a critical choice of triggering circuit. The first

two circuits described here give the user a very limited adjustment range. A universal circuit

can be obtained by taking into account two main factors:

●

To reach perfect symmetry of the first gate pulses in positive and negative half-cycles,

the triggering circuit should be synchronized by the mains voltage.

●

The phase angle variation allows perfect symmetry of the current if the TRIAC is

continuously triggered.

The circuit described in the Section 1.3: Triggering synchronized by the mains voltage and

suitable for industrial applications on page 6 combines these two principles in a very simple

way. It allows complete variation of power with an inductive load without particular problems.

It can thus serves as the basis for a universal circuit for control an inductive load by phase

splitting.

AC Mains

C

T

D

Ts

Rt

P

1

2