Triac 'Flutter' on Power-Up...review/help??

Discussion in 'The Projects Forum' started by tmiuser, May 30, 2011.

  1. tmiuser

    Thread Starter New Member

    Dec 22, 2010
    9
    0
    ok...I basically copied a Triac circuit right off of the datasheet....and it works as intended...except for a slight issue......that I'd like to remove.

    When I first apply power to the entire circuit....the Triac fires for just a millisecond or so....(thus...energizing the load for a few milliseconds as well..)

    Ugh. I don't care for that!!
    I have checked using a O'scope the output to the Opto....(by setting the trigger so that if that line EVER goes above 1V...I'd catch it.)

    It never does. The input to the Opto stays pulled to ground just as I had intended.....but the Triac still does its little 'blip'........

    I did replace the Opto....but to no avail.

    Is there something I should have added to prevent this action?

    Outside of this issue, the circuit operates great.

    Looking forward to comments on this...I am sure you guys have resolved this one before!!

    [​IMG]

    Thanks!!!!
     
  2. #12

    Expert

    Nov 30, 2010
    16,252
    6,751
    It might be dv/dt. The triac has parasitic capacitance from the top connection to the gate. Sudden application of electricity can sneak through this capacitance and turn the gate on. Because R10 seems right, I suspect the triac is weak.
     
  3. tmiuser

    Thread Starter New Member

    Dec 22, 2010
    9
    0
    I have replaced the triac. (several times...actually....)
    I also checked the resistors to ensure they matched the values in the datasheet....and, they are basically dead on the values listed.

    would a lower value for R10 or....possibly another cap out there help me out?
    :-/
     
  4. #12

    Expert

    Nov 30, 2010
    16,252
    6,751
    Yes, lowering the ohms in R10 will allow more of the (false) start pulse to escape to ground. Adding a cap across R10 will do the same thing, but it might interfere with the speed of operation, if speed is important here. Choosing a different number triac might be the answer. Revealing what's on line 23 might show that the false pulse is coming in from that direction.
     
  5. tmiuser

    Thread Starter New Member

    Dec 22, 2010
    9
    0
    Line 23 is staying at Ground Potential....I watched it heavily with my storage scope....and it don't even blip at all....so, I am assuming (virtually sure) the issue ain't coming from that area.

    If I lower R10....and that would eliminate this issue...I'm OK with that.
    Speed is not an issue at all....just as long as the Triac turns ON when I ask it to do so..........(hahha...and OFF when I ask it to!!)

    Changing the value of R10 is the easiest for me...since I already have the PCB created. I have some 330 ohm resistors laying around...that would fit the footprint. I can try one of those, and see how see responds.

    With the Opto OFF...I would think there should be just about nothing as far as AC at the gate of the Main Triac...

    If adding a Cap is the BEST way, I can do it. I just need this sucker to NOT blip on power up.....it's not a good thing...... hahhaaaaaa.....

    Steve
     
  6. #12

    Expert

    Nov 30, 2010
    16,252
    6,751
    R or C? It's a balancing act. Lowering R10 too much will cause excess current problems with the opto or the gate dissipation of the triac. Adding a cap doesn't interfere with the DC current limitations. I don't know the specs on your parts so I will have to leave that up to you.

    However, I could suggest some diagnostics like, physically wiring line 23 to ground and checking for the blip, or temporarily removing the opto to see if the problem is entirely caused by the triac. It's drudge work, but this is how it's done when theory doesn't fix everything.
     
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