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# How to choose resistor and capacitor for a snubber circuit used in a triac controlled with an opto-triac

Associate

Hello,

I want to design this kind of circuit using a Vline of 600V:

I would need to use Triac model T2550-12T.

For the R1, R2, R3 and C1, I know what are the values, but I was wondering what is the power of the resistors and the capacitors that I need to use for 600V nominal in entry. If I use the power equation, I find a power that is ridiculous.

Im a student so I am trying to understand.

Thanks for any help!

Antoine

3 REPLIES 3
ST Employee

Welcome, @Community member​, to the community!

you will find an answer to exactly your question in AN5114, e.g. section 3.6.

If the problem is solved, please mark this thread as answered by selecting Select as best, as also explained here. This will help other users find that answer faster.

Good luck!

/Peter

Associate

Thanks for your answer. I really dig this document, but some formula was missing for me to complete my circuit.

As I was saying im using 600VAC RMS to supply a transformer and I am using a triac to control when I want the transformer to be on and off. Im controlling this triac using a phototriac. For the circuit that I put on my first comment, I wan't to understand and know how to find R2 and R3, as there is no formula for them. How am I suppose to design this circuit using a 600Vline? Is this possible with a opto-triac limited to 600Vpeak?

Thanks for any help again!

Antoine

ST Employee

If you read through the above AN5114, especially section 3, you will find many reasons why there cannot be a simple equation for calculating R3 (RGK in AN5114) an d R2. However, you will find many hints on how to select them depending on the conditions.

And to the last question: the optotriac "sees" the same voltage as the main triac. If you want to control 600V, which I assume is 600VAC, then the peak voltage is what?

This then determines the required blocking voltage of both the triac and the optotriac. By the way, the AN4363 goes into more detail about their selection.

Regards

/Peter