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While researching TRIACs, I came across this question: What's the difference between 3 and 4 quadrant triacs?

Ok, straightforward enough, and that matches Wikipedia's explanation.

However, I came across the following Littelfuse datasheets, which seem to include 3-quadrant TRIACs that are listed as standard (not alternistor) TRIA

So why would a 3-quadrant TRIAC not be snubberless (aka an Alternistor aka high-commutation)? Or am I simply misreading the datasheets?

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  • \$\begingroup\$ Good question! :-). A=? Voltage is lower than alternistors. (So?). | It's all 3 Qxx15's. | You've duplicated the 15A datasheet. | Qxx10 series datasheet here. The plot thickens. This datasheet has a 4th quadrant trigger columns, unlike your cited one. ALL the std versions EXCEPT the L5 also have a 4th quadrant trigger spec, and the L pkg alternistor has a 4th quadrant trigger spec BUT R & N pkg alternistors do not !!!!!! This seems seriously weird. \$\endgroup\$
    – Russell McMahon
    Dec 13, 2019 at 1:49
  • \$\begingroup\$ I suggest that asking Littlefuse may be instructive. \$\endgroup\$
    – Russell McMahon
    Dec 13, 2019 at 1:49
  • \$\begingroup\$ Please do note me in a comment if you find out more. \$\endgroup\$
    – Russell McMahon
    Dec 13, 2019 at 1:55
  • \$\begingroup\$ Whoops, good catch! Fixed the second link. :D \$\endgroup\$
    – MRiddickW
    Dec 13, 2019 at 16:27

2 Answers 2

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Triacs are inherently more difficult to trigger in Q-IV compared to the other quadrants, by typically about 2:1. The manufacturer may simply have decided not to characterize the standard triacs for that quadrant of operation since few experienced designers use it- it doesn't mean that they won't work if you hit them in Q-IV with double the 50mA or whatever positive gate current.

There is a clear difference in the commutation specs between the two types- about 6:1 in the Littelfuse datasheet.

If you refer to the older Teccor (before the Littelfuse acquisition) databook here you'll see that there are standard triacs with and without Q-IV specs, and of course all Alternistors have no Q-IV spec.

enter image description here

Conclusion: Alternistors(TM) or Snubberless(TM) triacs lack Q-IV operation, but not all triacs which lack a Q-IV specification are Alternistors.

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    \$\begingroup\$ Just for fun: I agree with your answer, but note, as I said in my answer that the QX10LH5 is shown as an alternistor AND has a Q4 gate trigger current specified. The L is a pkg spec and the alternative pkg parts - RH5 and NH5 do not have a Q4 spec. I suspect that this is a datasheet typo / error - but interesting. \$\endgroup\$
    – Russell McMahon
    Dec 13, 2019 at 10:04
  • \$\begingroup\$ @RussellMcMahon Strange. Bad things sometimes happen when graphic artists revise datasheets of mature products after an acquisition, so my instinct is to go back to the original because things seldom get better. I was quite surprised and pleased to see TI issue a remarkable app note (SLOA277) on the very old and multiple-sourced LM324 dated January 2019! \$\endgroup\$ Dec 13, 2019 at 11:51
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    \$\begingroup\$ Indeed. even better is when suppliers crib datasheets from other manufacturers and alter what they hope are enough figures to make them look real. I've seen too many of those. \$\endgroup\$
    – Russell McMahon
    Dec 13, 2019 at 15:54
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This does NOT answer your question but is, I think, useful as an answer in that it shows that the "mystery" is even deeper than you indicate. ie anyone with a similar question in future will be informed by this "answer" even if nothing is added to it.

All of the 3 Qxx15 package variants have the cited characteristic.
This is significant when compared to the Axx10s - see below.

You've duplicated the 15A datasheet. The Qxx10 series datasheet is here.

The plot thickens.
This datasheet has a 4th quadrant trigger columns, unlike the QX15 one.
ALL the standard QX10 standard thyristors EXCEPT the L5 package also have a 4th quadrant trigger spec,
and the L pkg alternistor has a 4th quadrant trigger spec BUT R & N pkg alternistors do not !!!!!!
This seems seriously weird.
I suggest that asking Littlefuse may be instructive.

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