AN439

TRIAC turn-off description

5/16

Figure 5.

Current and voltage waveforms for resistive loads (phase control)

1.3.2

TRIAC with inductive load

An inductive load induces a phase lag between the TRIAC current and the mains voltage

(see Figure 6).

When the current drops to zero, the TRIAC turns off and the voltage is abruptly applied

across its terminals. To limit the speed of the reapplied voltage, a resistive / capacitive

network mounted in parallel with the TRIAC is generally used (see Figure 13). This

load impedance (Z) and the mains rms voltage. (see. AN437 for RC snubber circuit design)

Figure 6.

Current and voltage waveforms for inductive loads (phase control)

t

I

G

I

T

t

dI/dt

OFF

t

V

Mains

V

T

dV/dt

OFF

t

I

G

I

T

t

dI/dt

OFF

t

V

Mains

V

T

dV/dt

OFF

t

I

G

I

T

t

t

V

Mains

V

T

dI/dt

OFF

dV/dt

OFF

t

I

G

I

T

t

t

V

Mains

V

T

dI/dt

OFF

dV/dt

OFF