clarification with TRIAC and opti-couplers for A/C circuit

Discussion in 'General Electronics Chat' started by peppy2day, Aug 5, 2011.

  1. peppy2day

    Thread Starter New Member

    Aug 5, 2011
    Hello I am working on a few hobby electronic projects... My background is in software development, but I have taken a few electronics & hardware courses as part of my Computer Science degree.

    I am working on a circuit to switch a/c from a low voltage (dc) logic circuit. I have already built a parallel interface board with led indicators as well as appropriate control/timer software that controls each output line (on or off). It's been working fine with LED's at the end of each line. That was my first step, and now it's time to make each output line switch an a/c source. I am just trying to get a grasp on the basics on how to control the flow/switching of a/c current.

    My initial research pointed towards using a solid state relays. But then after reading a bit more, it seemed to be that I could use an TRIAC and triac driver (moc303. I read through several schematics and figured it would be easy enough to implement, so I ordered the parts:

    my triac:

    my opto-coupler:

    FIRST: I'm stating to think that the opto-coupler i ordered isn't exactly the one I was supposed to order? When I wire up just the opto-coupler to the a/c lines, the device switches ON (when supplied with 2v dc to the input). BUT when I try to wire the output of the opto-coupler to the TRIACs that I have, nothing happens (or the device turns on steady and seems to ignore the switch) I also noticed that when I connected M1 and M2 without wiring the GATE pin.. the device turned on steady -- this is not what I expected (to clarify this - I cut the a/c supply line and connected one end to M1 and 'continued the circuit' through to M2 - so basically no voltage input on the gate - and the device turns on steady when I plug it into the wall). is this supposed to be the case? i thought the two leads would be isolated?

    SECONDLY, I noticed that even if i was able to get the TRIAC to function the way I think it should... I don't think I will be able to shut it OFF? (the opto-coupler turns ON the output device, but doesn't shut it off when supplied with 0... my readings of this point out that the triac will only 'un-latch' when the ac current drops)


    I'm looking for some clarification on my situation. If I want to continue trying with the TRIAC method, what should my next steps be? I'm starting to think that I might need to create a 'reset' loop for the latch? um, something that will drop the a/c current enough that the TRIAC will shut off... I haven't researched this aspect of things because I'm not sure if it was in the right direction.

    --OR-- should I just scrap the whole TRIAC methods and skip to using a relay? I initially ruled out wanted to use relays because of their size and unit cost. BUT it might just end up saving headaches down the road?

    I would also like to point out that I am only in testing stages with these components and have not given any consideration to capacitor or resistor values as of yet and haven't used any in the tests i described. should this matter to the basic function of the devices? I could see the cap on the a/c side being useful if i was using a pulsed dc current to control the opto-coupler. But at this point, that's not in the plans. SHOULD IT BE?

    Thanks for any input you can give.