Hello all, this is my first thread post.

So I'm modifying a table top oven to be digitally controlled by a pid loop using an Arduino nano. I just have a few questions about triac Control.

I'm aware I need to isolate the mains from the MCU using an optocoupler. I have this one
78-VOT8121AD
A rough schematic of what I think will work is attached.
I want to know how to calculate R2 in this circuit in order to switch the triac on early in the half cycle, and also, to not push too much current through the phototriac in the optocoupler. Also, I read somewhere that you can tie the gate of the main triac to the MT1 pin through a resistor in order to "keep the triac from switching when it's not supposed to". I'd like to know how that works.

Thanks!

 

Welcome to AAC!

Rather than reinvent the wheel I suggest that you buy a DC logic input to AC output solid-state relay. Get one that also includes zero-crossing switching.

Here is an example.
https://www.sparkfun.com/products/13015

 

Please check the links.
https://www.fairchildsemi.com/application-notes/AN/AN-3006.pdf

 

Use a MOC3041 or Other Zero crossing type..

 

Check out Fairchild AN-3006, AN-3003 and also Microchip, AN-958, range controller.
Max.

 

Mrchips. The solid state relay sounds like a fine idea! Although I currently am still interested in reinventing this particular wheel, haha.

Dodgy Dave. I should've clarified I'm not making a dimming circuit. All I want to do is switch on and off, so I shouldn't need a zero cross detector.

And about other optocoupler chips, what's wrong with the one I have? Is it the maximum current through put on the output?

 

It is actually a Zero cross detector version you NEED then, if wanting burst firing , not a phase angle/random phase type.
Max.

 

I plan to switch on and off and very low frequency. Just about 1 or 2 seconds a cycle. From my understanding, zero cross is for high frequency switching and or dimming right?

 

No, you want to switch at the zero crossing point, as I understand it.
Dimming uses the random phase version.
The Fairchild AN-3006 explains it as I quoted.
Max.

 

Oh, thank you for setting me straight, I had them backwards! So what about calculating the current limiting resistor? Basic ohms law? Also with a random phase optocoupler, would the rating of that resistor affect at what voltage the triac switched?

 

Even for low frequency burst mode, or simple on/off, switching off at the zero-cross point should result in less interference generation from inductive voltage spikes.