I have recently seen a opto-isloated triac driver circuit. The Gate terminal of triac is connected to a live wire through physical switch. So when the physical switch is turned on, triac conducts and gives the signal to opto-isolator which then trigger gpio on the Microcontroller.

Now my question is: As far as I know opto-isolator used in the circuit can handle the mains voltage also. What is the necessity of triac here. Why can’t I directly connect the mains voltage to opto-isolator and skip the triac all together?

Let me know if the question is not clear. I will try to come up with circuit diagram soon.

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    \$\begingroup\$ Yep, the circuit diagram is necessary. \$\endgroup\$ Jan 6, 2018 at 12:22
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    \$\begingroup\$ Don't expect people to put more work into helping you than you are willing to do yourself; post a diagram, datasheet what you have and meny people here will be more than willing to help you with your question. \$\endgroup\$
    – user173292
    Jan 6, 2018 at 12:25
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    \$\begingroup\$ Why can’t I directly connect the mains voltage to opto-isolator and skip the triac all together? There are opto-isolators with a build-in Thyristor / TRIAC so sure you can do without an external TRIAC. But ask yourself: what does the TRIAC do? How much current can an opto coupler switch and how much can a TRIAC switch? Before asking if a certain component is really needed in a circuit you first need to fully understand the circuit. If it was not needed, why would it be there? \$\endgroup\$ Jan 6, 2018 at 12:28
  • \$\begingroup\$ Keep testing and validating your assumptions . One of them is wrong. E.g not all design sources are good ideas \$\endgroup\$ Jan 6, 2018 at 12:55
  • \$\begingroup\$ @Vinzent I understand. The reason I couldn’t post a circuit diagram right away is because, the details that I mentioned here are by reverse engineering a product. Since I’m completely new to electronics I am not entirely sure about representations if Triac and Opto-coupler so it’s taking some time for me to draw the diagram. My apologies. \$\endgroup\$
    – Raghav
    Jan 6, 2018 at 12:59

1 Answer 1


The triac can handle a lot more current than the optoisolater can. It is in a package that can dissipate a significant amount of heat.


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