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Can someone help me understand how dimming works on 3-quadrant triacs? I understand they are great because they have higher reliability and don't need snubbers. What I don't understand is how 0-100% dimming can work if you can never operate in quadrant IV.

Ok so on a standard TRIAC if I want 50% dimming I would fire the TRIAC at the peak and trough of the sine wave, the start of quadrant II and quadrant IV. If I want 40%, I would fire even further into quadrant II and quadrant IV, and so on down to 0% dimming, otherwise known as off.

But if I can't fire in quadrant IV, how am I able to achieve dimming of 50% or less? Seems I would only be able to fire in quadrant II.

I have looked for waveform diagrams and other learning resources to try and understand how it is done but I haven't been able to find anything. Can anyone offer some insight here? What am I missing?

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Quadrants refer to the polarity of the gate current vs. the polarity of MT2 vs. MT1. Image from here. Polarity of gate current is more-or-less irrelevant to what current goes through the load.

enter image description here

Simple dimmers and opto-isolated switches derive the trigger current from the voltage across the triac so they always trigger in quadrant I or III.

Triacs that are only rated to operate in three quadrants don't work well in quadrant IV so we use a negative gate current to trigger them, say if an MCU is being used to generate the timed pulses. So they are used in quadrants II and III.

Even triacs that are guaranteed to trigger in all four quadrants generally have inferior sensitivity in quadrant IV.

In each case, you generally can get the whole AC waveform across the load (give or take a bit of losses). Or none (give or take some leakage).

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  • \$\begingroup\$ Good answer, though typically most circuits operate in Quadrant I and Quadrant III ...see a good explanation here: electronics-tutorials.ws/power/triac.html. \$\endgroup\$
    – Jack Creasey
    Commented Dec 1, 2021 at 5:45
  • \$\begingroup\$ The even quadrants occur when motors reverse acceleration with back EMF. (I think) wikiwand.com/en/TRIAC \$\endgroup\$
    – D.A.S.
    Commented Dec 1, 2021 at 5:51
  • \$\begingroup\$ There are even TRIAC where one quadrant (III IIRC) is actively suppressed by an internal circuit to be immune to transient \$\endgroup\$
    – Lorenzo Marcantonio
    Commented Dec 1, 2021 at 7:35
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    \$\begingroup\$ Ok I get it now thanks for your answer. I was confusing the TRIAC quadrants with the 4 quadrants of AC voltage and current. \$\endgroup\$
    – humanistscience
    Commented Dec 2, 2021 at 18:37

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