Being my first time to use a Triac and a Diac to control an AC signal, I read many questions/answers in this forum related to DIACs controlling TRIACs, all being very helpful.

Now, I see in a MOC302 DIAC's datasheet page 8, something that I dont understand:


Won't the Triac conduct if the R1 is low enough? I do not understand the reason to use R1 there, it should be an open circuit in R1, so that the DIAC could be in series with Triac's Gate.

And I saw the same conficuration at this question:

enter image description here

But no one commented why his R6 is there.


Regarding mainly @Spehro Pefhany's answer, since the R1 makes the Base less sensitive,

I guess R1 has to be high enough so that It wont cause the Triac to conduct when the photo Diac is off and AC voltage is on this stage:

enter image description here

So if Triac conducts at Holding current= 20mA, we need the Resistor to be at least (more than) R=100/0.002=50kOhms, Right?

(As long as the voltage on its gate does not exceed (lets say) 1.3V where it conducts, the 50Kohms will do the job, but still, I do not know how to calculate if the Voltage at the gate will surpass 1.3V on the above schematic with R1=50K)

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    \$\begingroup\$ Won't the Triac conduct if the R1 is low enough? No, the opposite will happen (the TRIAC will never trigger), if R1 has a low value, there will not be enough voltage across R1 to trigger the TRIAC when the DIAC starts to conduct. The trigger pulse has to some from the "upper" side of the TRIAC, where R3 (1st schematic) is connected (or R13 in the 2nd schematic). Mind that a TRIAC's A1 and A2 connections are not identical! \$\endgroup\$ Mar 26, 2021 at 14:41
  • \$\begingroup\$ Thank you but I was saying that R1 needs to go away (open circuit), not to lower it. Did I get your answer wrong? You say "if R1 has a low value" \$\endgroup\$ Mar 26, 2021 at 15:25
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    \$\begingroup\$ but I was saying that R1 needs to go away (open circuit) Again no, R1 can still be present. It depends on R1's value what will happen. If R1 is 1 ohm, the TRIAC will never trigger. If R1 is 1 kohm the TRIAC can easily trigger assuming that R2 and R3 have a much lower value than 1 kohm. Resistor present/not present does not matter. One resistor being much larger/smaller than another resistor does matter. \$\endgroup\$ Mar 26, 2021 at 15:47

1 Answer 1


That's not a "diac", it's a photo triac.

The gate-MT1 resistor is to deal with leakage and bias current in the photo triac and/or zero crossing circuit. It effectively lowers the sensitivity of the triac a bit by conducting some of the current that would otherwise go to the gate.

Many triacs are insensitive enough that it won't make any difference.

Worst case will usually be at high ambient temperature (for both the triac and the opto-isolator). So you want to pick a value that is low enough that the triac cannot trigger when it is supposed to be off, but does not conduct so much current that the circuitry cannot drive it. If you make the resistor too low, then R2/R3 (1st schematic) or R13 (2nd schematic) will burn up because the triac won't trigger at all.

  • \$\begingroup\$ I understand, so I added a "Edit" part on my question, where I try to calculate the R1. What do you think of that? \$\endgroup\$ Mar 26, 2021 at 15:21
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    \$\begingroup\$ You can't calculate R1 with the information in that datasheet. The datasheet is incomplete- a Taiwan copy of a Motorola (now Onsemi) part. Find the original datasheet and look for the leakage current at maximum temperature and relate that to the maximum vs. typical leakage at room temperature. And hope that Lite-On is more-or-less the same as Onsemi (which means add some safety factor). And you'll need a good datasheet for the triac. \$\endgroup\$ Mar 26, 2021 at 15:37

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