# DIAC datasheet's circuit is not in series with TRIAC's Gate

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:

But no one commented why his R6 is there.

EDIT:

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:

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)

• 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! Mar 26, 2021 at 14:41
• 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" Mar 26, 2021 at 15:25
• 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. Mar 26, 2021 at 15:47