Zero Crossing Control of a Thyristor

Discussion in 'The Projects Forum' started by roro36, Sep 22, 2010.

  1. roro36

    Thread Starter Member

    Sep 7, 2010
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    My project is to control a 3-phase heater via thyristors. I need to work off the zero point crossing so that when each thyristor is turned on, its at the lowest possible point in the sine wave signal. I have been to about 100 sites and posts about this but none of them describe accurately what should be done.

    Starting right in the beginning I have questions about the detection of the zero points. I believe for me the best was is rectify the AC mains and then use a simple transistor for detection purposes onto a micro.
    Should I or would it be better if I transform the voltage down to say 5V first, then rectify and finally detect the zero crossings?

    Feed these detection pulses into a micro and use the output of the micro to control each thyristor separately, i.e. Obviously have separate transformer/rectifier circuits on each phase and then just pulse the thyristor for the lenght that it needs to be on?

    If you have any links that are related to this, please paste them in. I want a simple but reliable control because the heater is for a aluminum holding pot.
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,789
    945
    http://www.nxp.com/documents/application_note/AN467.pdf


    Pertinent info starts on page 5. It shows a comparator with high R value divider networks fed from a capacitor coupled AC line input.

    The comparator is a 5 volt single supply type and outputs a signal that goes high or low(depends on which input is tied to Hot line) at the zero crossing. There is a small offset in timing if the load is inductive.

    This should give you all the info you need to start breadboarding and tweaking




    The EASIEST way would be to use opto triac drivers with built in zero crossing detection. Cheap solution, unless of course you like the challenge of 'do it yourself' :)
     
  3. eblc1388

    Senior Member

    Nov 28, 2008
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    I would say it is better not to transform down to 5V but to use a dedicated circuit to take care of the zero-crossing detection if you want accuracy and isolation.

    The following circuit is for your reference.

    I did not design the circuit and it is from some design idea column of a magazine. Thanks to the original designer.

    The circuit performs very satisfactorily and give very accurate zero crossing pulses as shown in the simulation.

    A vital feature of this circuit design is other than at the zero-crossing moment, there will not be any major heat dissipation in component.

    [​IMG]
     
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  4. roro36

    Thread Starter Member

    Sep 7, 2010
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    Thanks for the useful post, can I replace r7 with a microcontroller? Or too high voltage?
     
  5. eblc1388

    Senior Member

    Nov 28, 2008
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    No.

    You use an opto-coupler to provide you with electrical isolation between the high voltage AC and the low voltage microcontroller, as shown below:

    [​IMG]
     
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  6. roro36

    Thread Starter Member

    Sep 7, 2010
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    Thanks, thats exactly what i was looking for. Thank you greatly.
     
  7. roro36

    Thread Starter Member

    Sep 7, 2010
    52
    0
    I'm not getting the same results you do on you screen? I get like 1v pulses out at the R7 position.
     
  8. roro36

    Thread Starter Member

    Sep 7, 2010
    52
    0
    I have this circuit, but when I simulate it and change R2's value, it does nothing to the output. Will it work with a 220V input? Or will there be too much voltage on the base of the transistor?
     
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  9. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
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    Do u have a death wish...:eek:

    Where did u study electronics from?
    The things u ask does not add up and somethings u are trying is relatively dangerous.

    I suggest a mod shud lock this thread
     
  10. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,346
    Hello,

    Isolation by the use of an optocoupler or tansformer are a MUST.
    We do not want anybody to be hurt by connecting things directly to the mains.

    Bertus
     
    Last edited: Oct 29, 2010
  11. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Thank you for unlocking the thread, Bertus.

    Roro36, the circuit you have posted is very unsafe, and completely different from what eblc1388 posted. Please do not attempt to build your circuit.

    I've re-drawn eblc1388's schematic with a 4N25 optocoupler on the output. The output is normally 5v, but when the crossing is detected, it falls to 0v. You can detect this using a comparator in a PIC.
     
  12. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Here is the updated drawing. Make sure that there is no electrical connection between the LIVE side and the Safe side, which is the main reason of why an optocoupler is being used in the first place.

    All components and connections must be placed inside an insulated casing with a small hole to let through the two wire(ZCP and 0V) connected to collector and emitter at the right hand side of the optocoupler.

    Use similar arrangement on opposite side of the casing for connection to LIVE and Neutral of the mains AC supply.

    The negative going zero crossing pulse ZCP can easily be detected via the interrupt pin on most MCUs without the use of a comparator.

    [​IMG]
     
    Last edited: Oct 30, 2010
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