TRIAC Circuit Explanation

Discussion in 'The Projects Forum' started by Randa Mohamed, Aug 27, 2015.

  1. Randa Mohamed

    Thread Starter New Member

    Jun 19, 2015
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    Hello everyone

    I have a project where im using TRIACs to switch several AC voltages from a transformer.
    I found the following circuit and I need help in explaining how it works assuming that LIVE and NEUTRAL are from the transformer.

    [​IMG]
     
  2. Lestraveled

    Well-Known Member

    May 19, 2014
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    Your circuit uses a PIC micro controller. It has to be programmed. Do you have the program and a programmer?
     
  3. Randa Mohamed

    Thread Starter New Member

    Jun 19, 2015
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    Actually i don't want to use this PIC. My plan is to control the TRIAC via an opto-coupler. I'm concerned about the TRIAC and transformer connections. I'm interested in TRIAC functionality. Please explain to me how it should work in such case. Do I need to use a zero crossing detection circuit ?
     
  4. ian field

    Distinguished Member

    Oct 27, 2012
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    You need the right kind of opto-coupler with AC output (opto diac) connected from gate to the MT furthest from the gate via a current limiting resistor - the value could be 100 - 180R depending on the mains voltage.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Look up Fairchild App note AN-3006.
    Shows the use of Zero crossing and random phase drivers and opto's.
    Max.
     
  6. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    How are you going to sense the Zero crossing, can you not use a simple triac dimmer circuit....?
     
  7. Randa Mohamed

    Thread Starter New Member

    Jun 19, 2015
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    Thank you all for replying

    @MaxHeadRoom Looking at the Fairchild App was very helpful and it clarified some concepts. @ian field I think the below circuit is what you were talking about.
    I have some questions regarding this circuit from the app note a3006

    zzzz.png

    Is the snubber RC is for the triac only or do i need another snubber to protect the optocoupler. Why they have connected the snubber circuit that way? Why not use an RCD snubber ? And how they got the values for the RC circuit ? do these values depend on the load only and how is that ?

    [/QUOTE]

    @Dodgydave I'm currently trying to design a voltage zero crossing circuit to turn the triac on and a current zero crossing circuit to turn it off. I'm planning to use comparators.
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The snubber is used to prevent spurious switching noise on the AC line, however with zero cross turn on and off type switching it may not be relevant.
    Max.
     
  9. ian field

    Distinguished Member

    Oct 27, 2012
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    Wherever they come from - transients can unintentionally trigger a triac, they have a Dv/Dt specification stated in the datasheet.

    The marketplace has had for some time - "snubberless triacs". Probably worth a look.
     
  10. Lestraveled

    Well-Known Member

    May 19, 2014
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    Here is a simple zero crossing circuit I like. It uses a bi-polar opto-isolator. You can use two normal opto-isolators in parallel and get the same result.

    [​IMG]
    Are you looking for just on/off, or are you looking for a dimming function?
     
  11. ian field

    Distinguished Member

    Oct 27, 2012
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    Those AC input optocouplers were fairly common in modems.

    Look for any PC internal modem cards that don't have a bridge rectifier in the input section.
     
  12. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The OP mentions full phase on/off zero-switching, no phase angle control.
    You need a sync circuit as per the PDF when using a Pic.
    The MOC316x are zero crossing opto's.
    Max.
     
    Last edited: Sep 4, 2015
  13. Randa Mohamed

    Thread Starter New Member

    Jun 19, 2015
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    From my research the on/off switching noise depends on the load, if the load is resistive then the voltage and current are in phase so having a voltage or current zero crossing circuit is enough. My aim is to switch on/off an inductive load where the voltage and current aren't in phase. Basically the triac should switch on at zero volts and switch off at zero amps.

    @ian field First time to hear about "snubberless triacs" I'll research about

    Well this circuit should do, my aim was to convert the voltage and current signals into squarewaves using a comparator then detect the falling edge part using an xor gate, i think its gonna be more precise.
     
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