digital firing angle using 555 timer

Discussion in 'Homework Help' started by makleo, Dec 18, 2010.

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  1. makleo

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    Dec 18, 2010
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    i am asked to make a module for digital firing angle for thyristors.i intend to do so using zero-crossing detector and 555 timer.plz do give me ur useful suggestions.thnx
     
  2. Alberto

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    Nov 7, 2008
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    Once detected the zero crossing you activate the 555 delay and when the delay elapsed the circuit fires the triac. Remember that delay should be from 0 up to 10 millisecs max with a 50 Hz main or 0 up to 8 millisecs for 60 Hz main.

    At delay = 0 your duty cycle will be 100%, while at 10 millisecs your duty cycle will be zero. So if you want to devide by 100 you will need increments of 100 microsecs in your delay system (555).

    Alberto
     
  3. beenthere

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  4. makleo

    Thread Starter New Member

    Dec 18, 2010
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    thank you all for your precious time & suggestions.for the aforementioned project,i need to interface:
    1)zero-crossing detector
    2)555 timer
    3)optocoupler
    4)thyristor

    how should i proceed with designing and simulation of this on proteus?any circuit diagram/helping material?:confused:
     
  5. thatoneguy

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    Feb 19, 2009
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    I'm not sure what you mean by "digital", a 555 is an analog device.

    What is the digital requirement?
     
  6. makleo

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    Dec 18, 2010
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    @thatoneguy:i have to design an alternative to microcontroller based digital firing angle.say,if the input=0v(logic0),it detects the zero-crossing.then after a certain delay,when the input jumps to 5v(logic1)....thyristor is fired!!!
     
  7. thatoneguy

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    So while the input is low nothing happens, when the input is high, the chopper(triac cutting up sine wave) is active at a preset xx mS delay, or is the delay adjusted digitally as well?
     
  8. makleo

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    Dec 18, 2010
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    delay adjusted digitally as well.
     
  9. bertus

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    Apr 5, 2008
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    Hello,

    What do you want, digital or analog?
    A microcontroler is digital, a 555 is analog.

    Bertus
     
  10. thatoneguy

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    Feb 19, 2009
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    Is this a homework assignment?

    What is the exact need, so useful information can be given.

    A Digital solution would use a microcontroller (or a lot of logic ICs), a transformer, regulator, and opto-isolated Triac, and various passive components.
     
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  11. Alberto

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    Nov 7, 2008
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    makleo, you are making some confusion. Detecting the zero crossing is not a digital function but an analog function, since you are exploring the sin wave hunting for the zero value. You will generally use a comparator, a bridge rectifier and a transformer for this purpose. Now if your circuit works you will see on your scope a pulse rising every time the sin wave cross the zero. Now, to dim your lamp (mind you got to be a resistive load) you need to delay the firing of the triac and for this purpose you have suggested to use a 555, (here again an analog component) setting up the proper RC you will handle the required delay to obtain your dimmer.

    So you see that there is no digital staff but only analog. If you want a digital dimmer then you will need to use a microcontroller or digital ICs but this is another story :)

    Alberto
     
    Last edited: Dec 18, 2010
  12. DickCappels

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  13. makleo

    Thread Starter New Member

    Dec 18, 2010
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    hnx 2 all 4 their time.let me clear one thing.we(group of 4) were askd to make a 'digital firing angle'.......actually we do not have the code for microcontroller 8051.so,we decided to make analog firing angle using:
    1)zero-detector
    2)555 timer
    3)thyristors
    nw plz tell me how 2 proceed.i've simulated zero-detector on Proteus.project has to be submitted on PCB.the issue is to generate adjustable delay via 555 & then fire the gate of thyristor.i am sure the ambiguity is over now.
     
  14. thatoneguy

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    What you are making is not a digital solution, but if the prof approves of your method, your solution is workable.

    ZCD starts one shot timer which triggers triac at xx.xx mS. This would allow chopping at any point in the phase.

    What frequency mains is this for?
    Is the timer/phase adjustment via potentiometer OK as it is an analog solution to start with?
     
  15. makleo

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    Dec 18, 2010
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    let me clear the ambiguity.i need to write code in C for 8051.using time delay and keypad interfacing,design a digital firing angle 4 thyristors...........i've simulated the zero-detection on proteus.plz explain how to proceed further.thnx a lot;)
     
  16. thatoneguy

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    Is there a reason you can't use the microcontroller for the delay and control the thyristors directly/through an optocoupler?

    Using a 555 for this I guess has us confused.
     
  17. mike prrior

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    Dec 20, 2010
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    Makelo u can sonsult to Dr.Habshi Najashi for programming purposes........
     
  18. makleo

    Thread Starter New Member

    Dec 18, 2010
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    i hope the confusion is over now........8051 with keypad and LCD interfacing is used for delay.........optocoupler 4 firing thyristors.i think it'll be put together as follows:
    "AC input(60Hz)--->zero-detector--->8051microcontroller--->optocoupler--->thyristor"....
    isn't it?time 2 simulate it on proteus................!
     
  19. makleo

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    Dec 18, 2010
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    i am waiting for your urgent reply...........time running out!!!
     
  20. thatoneguy

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    Yes, that is a way to do it, you only need to write the code to provide the input of delay, then timers for triggering the triac, it's mostly software/programming work from here.
     
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