# Calculating resistors in circuit with optocoupler and triac

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?

• What is the CTR? Commented Feb 17, 2023 at 1:15
• @user263983 Current transfer ratio. Commented Feb 17, 2023 at 1:19
• that parameter not for triac. Commented Feb 17, 2023 at 10:26
• @user263983 Oh, so CTR is only for optoisolator with phototransistor? Commented Feb 17, 2023 at 12:01
• @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. Commented Feb 17, 2023 at 18:36