Please help me in AC Dimmer Circuit Using Optocoupler.

Discussion in 'The Projects Forum' started by Jigar_Patel, Mar 23, 2012.

  1. Jigar_Patel

    Thread Starter New Member

    Mar 23, 2012
    Hello Guys,
    I am working on a AC dimmer circuit using Optocoupler (MOC 3041 )and Triac(BT136 / BTA16 ).
    but still I am not getting Required result.
    The circuit works in simulator but doesn't work on PCB.
    I am attaching details regarding it.

    when I switch on AC supply the load remains ON although I do not provide +5V to Opto. The load should start only when I provide +5V DC.
    Please help me regarding it.

    Thank you
    Jigar Patel
  2. DickCappels


    Aug 21, 2008
    You seem to have replicated the circuit as published in the datasheet, so it would probably work unless there is something "funny". Some thoughts:

    1. Ground loop -perhaps snubber current gets into the LED circuit -you show both the LED circuit and the triac circuit sharing a common ground. If you float both diode leads and it doesn't come on when power is applied, then its probably something related t the LED wiring.

    2. The Traic is turning on because of high dv/dt when the mains switch is thrown -there could even be some arcing. You can try adding a small inductance, say 100 uH, between the snubber and the load, and if that eliminates the problem, then you know you have a dv/dt problem. Similarly, you can try lowering R4 to see what effect that has.

    3. The best bet is that something is not connected correctly. For example, are MT1 and MT2 swapped?
  3. t06afre

    AAC Fanatic!

    May 11, 2009
    You may have zapped the LED if you have used a common ground for the LED and the mains voltage. Disconnect from the main. And measure the LED current.
  4. bertus


    Apr 5, 2008

    The values of the resitor and capacitor in the snubber circuit are quite different from the application note:


  5. Jigar_Patel

    Thread Starter New Member

    Mar 23, 2012
    Thnks bo..
    I have used different ground of r both sections in my application but i mademistake in schematic. for DC +5V I have used -ve of Adaptor and for load I have used Neutral of AC supply.

    @ bertus I have also tried the same circuit which you suggested.
    perhapes first I tried that one and the came to the next schematic.

    @ DickCappels
    I have implemented this ckt for 4-5 times so chances of wrong connection is not there in my case. but still I will try Inductor bet. snubber and load.
  6. n1ist

    Active Member

    Mar 8, 2009
    You can't dim using an opto with a built-in zero-cross detector, but you should be able to turn it on or off.

    You mention using an adapter to power the input side of the opto; measure its voltage as the no-load voltage is often much higher than the nominal voltage, in which case you may have blown the LED in the opto. Measure the LED current to see if it's OK.

    Are you sure your triac is OK? They tend to fail shorted. Also, is the triac's metal tab connected to anything in your circuit? Internally, it's tied to MT2.

  7. Jigar_Patel

    Thread Starter New Member

    Mar 23, 2012
    Thanks mike,
    I checked My I/p part, it's ok. Led works fine and I am actually using this circuit for switching ON and OFF the AC load.
    I replaced 4-5 Triacs and Metal cap is not connected anywhere still I am not getting required o/p.

  8. n1ist

    Active Member

    Mar 8, 2009
    Try removing the resistor from gate to MT1 (R4 in your schematic). Also, post a picture of each side of your board.

    What's your load?