Triac Activation

[maurosmartins]

Hello all,

for a project that I'm starting I need to use triacs to dim lights. After searching I've found two variations of the circuit using an optocoupler, please see image below:



my doubt is in the circuit on the right and relates to the R1 and C2 network, what is the advantage of using this type of circuit instead of only an resistor just like the circuit on the left? the gate current is different in one case and the other, but the final result is the same:


Looking forward your reply,
Best regards, Mauro.

[schmitt trigger]

If your load is purely resistive, then you don't require those snubber components (R1, C1).

[floobydust]

It's in a old Motorola app note AN3003 on driving TRIACs.
The extra R1C1 adds a phase-shift needed for inductive loads. I thought it also gives higher peak gate current to turn on the TRIAC harder, and then fall to a lower holding current.

Your gate drive circuit all depends on your load, and TRIAC selection- sensitive gate or three quadrant type.

In your sims, I'd have the MT1-G resistor to lessen sensitivity to dV/dt; 330R-1k.

[Circlotron]

Without this dc combination you are limited in how close to the voltage zero crossing you can fire the triac because right at the zero crossing there is no voltage available to the gate! With the rc the voltage across the capacitor lags the main voltage so there is still some voltage across it at zero crossing so you should be able to conduct the full 180deg if necessary.

[T3sl4co1l]

It's a noise immunity precaution, so that mains dV/dt isn't coupled directly into the gate by the opto's off-state capacitance.  This is more important for sensitive-gate TRIACs.

If the time constant is relatively large (comparable to the mains period), the phase shift will be noticeable as well, giving the above mentioned features.

Tim