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Protection circuitry for opto-triacs
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vector:
Hi!
I'm designing a board to control the heaters and the boiler in my apartment with a Raspberry and I'd need some advice related to safety and protection circuitry.

The boiler is already controlled by a relay (coil working at 230 V), so the circuit I came up with, after looking a bit online, is the following (pin 1 is for the mains line, pin 2 goes to the relay):



The MOV is supposed to protect the opto-triac from the voltage spikes due to the relay coil, but I have some doubts:

* Is this necessary if there is already an RC filter on the relay? And if there is no RC filter?
* Is it better to connect the MOV between pin 1 of J3 and ground?
* In the latter case, should I put one MOV also on pin 2?While writing this I realized that might be a good idea to put a fuse on the mains line here :D

Then the heaters are controlled with the so-called "pilot wire": you set their mode (comfort, eco, nofrost, off) depending on the type of signal you feed here (no signal, full 220 V sine wave, positive/negative half sine). The circuit I developed is the following:



Here I get the mains lines in J1, then with two opto-triacs I select the half-sine I want to send to the heater (the MOSFET is there only because I want the negative half sine to be on if the raspberry is off). Here I put MOVs to ground and a PTC fuse (the smallest I found, since the pilot wire pulls a very small current). In this case I'm more confident about the protections.

Then, for the layout, I'm keeping 0.8 mm between tracks and 2.5mm (where possible) between exposed pads. I will also cover everything with insulating lacquer. Of course the mains part is completely separated from the low voltage part.
Do you think this is enough?

If someone is interest in the full project, I can share it, of course.

Thank you!

EDIT: I should have fixed the images now...
T3sl4co1l:
Put the fuses on the other side of the MOV, just in case.  The MOVs are correct, and not a bad idea.  Make sure the peak MOV voltage drop is below the TRIAC's rating, at the load current (so, 1A or thereabouts).  MOVs are pretty sloppy limiters, but I think the voltage ratings should work out okay here.

Why diodes for half cycles?  Can get a zero-crossing MOCxxxx part, or just time it yourself on the rPi (needs a line frequency/phase detect signal from somewhere).

Tim
wraper:
These optotriacs are not intended to drive any load directly, even if it's something like relay coil. They are only suited for triggering triac/SCR, nothing else. And when you use triac, snubber should be placed to protect triac, not optoisolator.

http://www.onsemi.com/pub/Collateral/MOC3163M-D.pdf

--- Quote ---This optoisolator should not be used to drive a load directly. It is intended to be a trigger device only.
--- End quote ---
james_s:
There's a Big Clive video discussing design of robust opto triac circuits. He was at the time designing fairground lighting controllers but the same principals will apply.
vector:

--- Quote from: T3sl4co1l on June 06, 2018, 08:58:04 pm ---Put the fuses on the other side of the MOV, just in case.  The MOVs are correct, and not a bad idea.  Make sure the peak MOV voltage drop is below the TRIAC's rating, at the load current (so, 1A or thereabouts).  MOVs are pretty sloppy limiters, but I think the voltage ratings should work out okay here.

Why diodes for half cycles?  Can get a zero-crossing MOCxxxx part, or just time it yourself on the rPi (needs a line frequency/phase detect signal from somewhere).

Tim

--- End quote ---

Ok, I see. Thanks for the suggestion. So, you would put the fuses before the MOV so it can limit the MOV current in case of a surge? Is there the possibility that the PTC would fail short in this case?
I used diodes because they are simpler than a zero-crossing circuit, and also because I'm not sure if I can get such a precise timing with the RPi. Using diodes allows me to write a much simpler code.
Also, I wouldn't know how to get the half-wave output if the raspberry is off/rebooting.



--- Quote from: wraper on June 06, 2018, 09:15:52 pm ---These optotriacs are not intended to drive any load directly, even if it's something like relay coil. They are only suited for triggering triac/SCR, nothing else. And when you use triac, snubber should be placed to protect triac, not optoisolator.

--- End quote ---

I assumed that since they have a max current of 1A they would have been suitable for such a small load. What can be the differences between using just the optotriac and using a second triac (let's say a Z0103, that has the same max current https://datasheet.lcsc.com/szlcsc/STMicroelectronics-Z0103MA-5AL2_C58459.pdf )? (this is the first time I try to use triacs, so it might be a stupid question)



--- Quote from: james_s on June 06, 2018, 09:25:33 pm ---There's a Big Clive video discussing design of robust opto triac circuits. He was at the time designing fairground lighting controllers but the same principals will apply.

--- End quote ---

I will look for it, thanks!
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