AN308

Triggering methods

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The second turn-on occurs at a very low voltage and the angle

α’ is much smaller than α.

The following period begins under similar conditions and the unbalance persists. This type

of asymmetrical operation is not only unacceptable but can be dangerous (high current due

to load magnetic saturation due to the dc content of the waveform).

The unbalance is illustrated for a particular case, starting from zero of the mains voltage.

Other causes also produce this behavior

●

variation of the load impedance, for example, with motors, due to torque variation

●

modification of the control turn-on angle

This phenomenon is due to the fact that the circuit does not take its time reference from the

mains zero voltage. Rather, the synchronization is taken from the voltage across the TRIAC,

which is dependent on the load current, that is, on the load phase shift.

Figure 3.

Synchronization across the TRIAC - waveforms (delayed turn on)

To sum up, this first very simple triggering circuit, synchronized by the voltage across the

TRIAC, has the following characteristics:

●

Advantages:

–

Simple design and low cost

–

Connection by two wires, without polarity issue

–

Absence of a separate power supply

–

Little power dissipated in P and Rt

●

A serious disadvantage:

–

Because of its principle, this circuit cannot be used for highly inductive loads with a

narrow conduction angle because it can result in unacceptable asymmetrical

operation.

This very simple triggering circuit should be reserved for low-cost applications with the

following characteristics:

●

Resistive or slightly inductive loads

●

No stringent requirements concerning the accuracy of regulation

●

Highly inductive loads where the power varies between 85 and 100% of the maximum

power

α

full angle

β

Mains voltage

Gate pulse

β

B

ϕ

A

α'

C

T

T

T

TRIAC voltage

TRIAC current