triac app need an opto-coupler?

[Mr.Cool]

Hi all,

i have an application where a triac is turning on a load which is powered by 24VAC. the analog circuitry that determins when to turn ON the triac is referenced to its own 15Vdc supply. because the triac is a current controlled device, do i need to have the interface between analog circuitry and triac opto isolated?

Mr.Cool

 

[IanP]

It is always good idea to separate AC side from DC circuit and you can do it by using an optocoupler such as MOC3021 or similar ..
Link to data sheet:
https://www.alldatasheet.co.kr/datasheet-pdf/view/TI/MOC3021.html

 

[Mr.Cool]

yes, that exact datasheet is what got me started thinking if i needed an opto.

correct me if i am wrong, the opto coupler is used in this application because the earth ground of the mains AC supply may not be at the same potential as the ground reference of the analog circuitry. if the references are not tied together, then current leakage could occur. and if the traic is a sensitive gate type, as mine is, the leakage current might be enough to enable the triac. this is why optos are used with current controlled device here.

now, to provide more detail. upstream, i have AC mains connection at 115Vac. i use a step down transformer to generate the 24Vac mentioned above. that SAME 24Vac i also rectify and run through a 15V regulator to provide the analog voltage for the drive circuitry. so.. technically, the drive circuitry AND the 24Vac applied across the triac have the SAME reference, that being the secondary side of the transformer. hmmm... actually, this is not entirely true. it is one diode drop away due to the rectification stage after the transformer.

now do i need an opto?

Mr.Cool

 

[IanP]

Could you post the schematic of your circuit?
There is a chance that if you use half-wave rectifiction for your drive circuitry and treat the bottom of the secondary winding as a local 0V dc, you may not need an (opto) isolation ..

 

[Mr.Cool]

you'll have to use your imagination... see attached:

V3 is the secondary of the transformer (primary is conected to 115Vac mains).

none of the control circuitry is on this sheet (its not finished yet anyway), but you can easily see how the 15Vdc & reference are generated.

Mr.Cool

 

[IanP]

First of all, the diodes you use in the bridge are signal diodes with limited current.
I would suggest something like 1N4002/4/...

I think you should include an opto-coupler in you circuit. They are not expencive and you will have all possible problems off you mind ..