I'm trying to build circuit with optocoupler and triac, but i dont know how to calculate R2.

For R1 it was easy, since Led Trigger Current is 15mA-30mA, so with 330Ω and 5V i will get 15mA (or am i wrong?).

Apperently, to calculate R2 i need CTR of MOC3020, but i cant find it in documentation, so how can i do it?


  • \$\begingroup\$ What is the CTR? \$\endgroup\$
    – user263983
    Commented Feb 17, 2023 at 1:15
  • \$\begingroup\$ @user263983 Current transfer ratio. \$\endgroup\$
    – Alkyone
    Commented Feb 17, 2023 at 1:19
  • \$\begingroup\$ that parameter not for triac. \$\endgroup\$
    – user263983
    Commented Feb 17, 2023 at 10:26
  • \$\begingroup\$ @user263983 Oh, so CTR is only for optoisolator with phototransistor? \$\endgroup\$
    – Alkyone
    Commented Feb 17, 2023 at 12:01
  • 1
    \$\begingroup\$ @Alkyone Current transfer ratio is only meaningful for optoisolators that have a linear input-current-to-output-current relationship. Triacs are about as nonlinear as it gets; once the input current is high enough to trigger it, it turns on as hard as it can and stays that way until the AC line has a zero crossing. \$\endgroup\$
    – Hearth
    Commented Feb 17, 2023 at 18:36

1 Answer 1


Maximum voltage is 322V. Maximum current for MOC is 1A. The closest resistor value 330 Ohm, that is minimum. Bigger value, the triac will open later from zero voltage. Choose properly resistor wattage.

  • \$\begingroup\$ So R2 only matters in terms of current capabilities of optocoupler? And what about Q1 gate? How can i tell if provided current will be correct? \$\endgroup\$
    – Alkyone
    Commented Feb 17, 2023 at 0:27
  • \$\begingroup\$ Describe correct current. Maximum gate current is 2A. So bigger is better. But it can not exceed max MOC value. Understanding how triac works may help. That current flows only short time. Then triac is on, voltage drops. \$\endgroup\$
    – user263983
    Commented Feb 17, 2023 at 0:53
  • \$\begingroup\$ I thought correct current for gate was 10mA. What if i want triac to be on for long time, wouldnt providing 0,69A(with 330 Ohm resistor) damage triac in long term? \$\endgroup\$
    – Alkyone
    Commented Feb 17, 2023 at 1:18
  • \$\begingroup\$ Triac is used to commutate alternative voltage. It sinusoidal. 230V is RMS voltage. Peak voltage is 322V. No, it will not damage and does not affect life span. \$\endgroup\$
    – user263983
    Commented Feb 17, 2023 at 10:40
  • \$\begingroup\$ So to trigger BTA316-800E gate i need to provide at least 10mA and maximum 2A of current? I'm trying to learn something new and thats why i need details. \$\endgroup\$
    – Alkyone
    Commented Feb 17, 2023 at 12:04

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