Could not turn on triac/ how to test it

Discussion in 'General Electronics Chat' started by Vindhyachal Takniki, Jun 8, 2016.

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  1. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
    1. I have attached a circuit in which triac is driven using MOC3021. Load is connected as shown in ckt attched . It is pure resistive load of 350mA.
    2. Ac voltage is 220Vac,50Hz, +5V to MCU is driven by using capacitive supply.
    3. Attached fine picture of zero cross & signal to MOC3021. There is some logic by which I have to drive triac on after 1ms of zero cross pulse detect. Triac pulse is of 85us .

    4. Problem is there is no load current flow, even I turn on moc3021 after zero cross detect. What is the problem?
    I have checked the circuit connection multiple times all are ok.

    Mod edit: removed and archived schematic with tranformerless power supply
    Last edited by a moderator: Jun 8, 2016
  2. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
    To learn more about the location of the problem, you can:
    1. Ensure that MOC3021 is switched on and see whether that turns on the triac. Disconnect R8 from the PIC and connect it to +5V instead. If the PIC is in a socket you could remove the PIC and then link pins 1 & 5 on the socket.
    2. Ensure that that the triac has gate drive and see if switches on then. Short together MAIN1 and MAIN2 on the MOC3021.
  3. #12


    Nov 30, 2010
    I wonder if 85 microseconds is enough time to fire the 2 triacs.
    also, can't find the spec sheet for your larger triac.
  4. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
    Attached is image of zero cross detect & pulse at MOC3021 anode. Now I give triac pulse 2ms after zero cross is detected & triac pulse is 340us long.
    I am not driving another larger triac by T1M5F-A, just connected load in series with it. But current does not flow thorugh load
  5. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
    The larger triac specification says typical turn on time 2uS. I couldn't find an equivalent specification for the MOC device but it shows a graph of pulse width going down to 10uS, so it looks as if 85uS should be OK.
  6. benta


    Dec 7, 2015
    The MOC3021 needs at least 8 mA (worst case 15 mA) of LED current to trigger, which it is not getting. Don't forget to subtract the LED forward voltage as well as the output voltage drop on the 12F675 when calculating R8. Either make R8 smaller (if the 12F675 can supply more current), or use MOC3023 instead, then it should work.
    BTW, even with MOC3023 you'll probably need to reduce R8.

    Last edited: Jun 8, 2016
  7. DickCappels


    Aug 21, 2008
    Hello and welcome to All About Circuits.

    Discussion of transformerless power supplies such as the one you are working on is not allowed according to the Terms of Service that we all read carefully before agreeing to abide by it.
    1. Restricted topics. The following topics are regularly raised however are considered "off-topic" at all times and will result in Your thread being closed without question:
      • Any kind of over-unity devices and systems
      • Devices designed to electrocute or shock another person
      • LEDs to mains
      • Phone jammers
      • Rail guns and high-energy projectile devices
      • Transformer-less power supplies
      If You come across a thread or post discussing any of the above topics please report the thread so We can close, and if necessary remove, the offending thread/post.
    As a result, this thread will be closed to further posts.
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