AN308

Solutions summary for inductive load control

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A.2

Triggering synchronized by the mains voltage

See Figure 12 part B for the schematic diagram.

A.2.1

Resistive load

No fault over the whole adjustment range.

A.2.2

Inductive load

The following two cases should be considered:

●

Turn-on delay angle

θ > load lag, ϕ (in full-cycle control)

Correct synchronization of the triggering pulses enables balanced conduction for all

variations up to the load lag angle. Some applications use this principle (for example,

200 V – 100 V rms step-down circuit).

●

Turn-on delay angle

θ < ϕ.

Triggering occurs before the lag angle is reached. The TRIAC will conduct for an angle

α > 180°. It is blocked after the gate pulse of the following half-cycle. The current does

not flow for next bias cycle. The circuit thus acts as a rectifier.

A.2.3

Circuit characteristics

Advantages of the circuit:

●

Accuracy of the triggering pulses.

●

Current operation with a resistive load (but circuit complex, previous circuit preferred)

●

Excellent operation for power variation circuits limiting conduction to small angles with

inductive loads.

Disadvantages of the circuit:

●

Connection by 3 wires - it is necessary to have access to the mains terminals.

●

Power is dissipated permantly by the passive components.

●

Impossible to adjust the delay angle to values close to or lower than the load phase

shift. This circuit cannot be used for inductive loads where a variation close to the

highest conduction angle is required.