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  1. #81
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    Re: Where to get schematics about SCRs full-wave triggering circuits ?

    A diac is in fact a triac, with internal resistors such that it breaks down to a volt or so ( as long as there is sufficient current to keep it on ) once the off state firing voltage is reached



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    Re: Where to get schematics about SCRs full-wave triggering circuits ?

    Quote Originally Posted by KlausST View Post
    Hi,


    When the switch is open, then node may be seen as "floating". Floating means "undefined" .... anything can happen.
    That's why I wrote "2) the switch is of no help. It makes things more difficult.".

    An "open switch" is not the same as "Zero volts".

    Klaus
    Here's another simulation I did, and I think the diac here is continuously in conduction mode, the effect of oscillating as Brain told me that it should happen but it isn't so I think the simulation abilities here isn't perfect.

    My understanding to what Brain told me, is that when the capacitor is charged up with the amount of voltage that should trigger the diac, it discharges and the diac conduct. As the capacitor is charging up again the diac should go back to off state. But that didn't happen, so anyway I don't know how to do it.

    But I can change my experiment to use the UJT circuit or run the diac with an AC voltage source. What you think I should do ?


    Here's the simulation.

    Click image for larger version. 

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    The source voltage is 100V, R3 has a drop voltage of 38.9V the rest is 61.1V and R9 is also 38.9V. So should the voltage that conduct the diac ? so 61.1V - 38.1V = 22.2V. I don't know in the datasheet they mention that DB4 has breakover voltage from 35 - 45V.


    Edit:
    I increased the capacitor value so be able to observe the rise/fall of output voltage. If I return it to 100nF, the rise/fall would be so thin and zooming in the time on the scope actually diminish it and I can't go back.
    Last edited by eagle1109; 4th April 2020 at 16:50.



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    Re: Where to get schematics about SCRs full-wave triggering circuits ?

    Hi,

    My understanding to what Brain told me, is that when the capacitor is charged up with the amount of voltage that should trigger the diac, it discharges and the diac conduct. As the capacitor is charging up again the diac should go back to off state. But that didn't happen, so anyway I don't know how to do it.
    Please tell which of Brianīs posts are you talking about. Canīt remember he talked about oscillation. But maybe...

    Klaus
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    Re: Where to get schematics about SCRs full-wave triggering circuits ?

    With 100V and 10K in series it is possible the current through the Diac is sufficient that it doesn't turn off. That's why I wanted you to try 100K as well so you could see how it works. Instead you have added another 10K resistor where it would limit the discharge current from the capacitor and stop it oscillating.

    Please go back to where I first described the Diac across the capacitor, with a 100K resistor and all you need to monitor is the voltage across the Diac/Capacitor with the oscilloscope. Set it to single shot with positive edge trigger, DC coupling and try a 1 second sweep time.

    Brian.
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    Re: Where to get schematics about SCRs full-wave triggering circuits ?

    OK, I only got one pulse, then it continues to have 22V output.

    Click image for larger version. 

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    I just changed the capacitor value, to have more obvious charging time.

    It charges until 42V ti drop to 22V contiuously.

    - - - Updated - - -

    Quote Originally Posted by KlausST View Post
    Hi,


    Please tell which of Brianīs posts are you talking about. Canīt remember he talked about oscillation. But maybe...

    Klaus
    In #74. He didn't say oscillating exactly :)

    He said a sawtooth waveform .. and I also got the idea before that, that he wants to tell me about the simplest demo to observe the working principle of the diac.



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    Re: Where to get schematics about SCRs full-wave triggering circuits ?

    Hi,

    Eventually we see the voltage at the capacitor rising until the diac triggers.
    But - according datasheet - I can't see why it should stay at 22V. I don't even expect near 22V.

    Out of curiosity I will add the DB4 to my shopping cart. It will take some (many? In times of corona) days...

    In the meantime you may try another diac .... not from the same library.

    Klaus
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    Re: Where to get schematics about SCRs full-wave triggering circuits ?

    Yep, it should work in reality. But I believe the simulator didn't cover this effect.

    I found it here:
    http://www.bristolwatch.com/ele/diac.htm

    It's the diac relaxation oscillator, there's also another one for the UJT.

    So at this point, I think I should continue my work on triggering the SCR, and try to modify the AC circuit to get the proper diac triggering pulses.

    I have now different versions of triggering in my mind:
    1. Ordinary RC circuit
    2. RC with DAIC
    3. RC with UJT
    4. Zero crossing detection connected to a UJT

    These methods I guess are the most basic triggering circuits, so I should start here and until I develop a good experience about triggering systems.

    Of course the world now has more advanced digital triggering systems for the sophisticated systems that have SCR or the other power devices.



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    Re: Where to get schematics about SCRs full-wave triggering circuits ?

    Racing Klaus.....

    This is a DB3 Diac, 100K resistor and 100nF capacitor:
    Click image for larger version. 

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    and this is what I see on the scope:
    Click image for larger version. 

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    Brian.
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    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.


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    Re: Where to get schematics about SCRs full-wave triggering circuits ?

    Hi,

    I have now different versions of triggering in my mind:
    1. Ordinary RC circuit
    2. RC with DAIC
    3. RC with UJT
    4. Zero crossing detection connected to a UJT
    Form the previous discussion you already should know that 1) not useful ... at least problematic
    2) is good for analog phase angle control
    4) is good for digital phase angle control

    I recommend to go on with 2).
    * fixed R, fixed C as timing control
    * diac to get proper gate trigger
    * measure the gate current with a scope

    Don't longer use try and error method.
    Calculate the values.
    * R to get less than 250mW of power dissipation.
    * C to get a useful phase angle

    * measure the gate current with a scope
    * then compare the scope results with the SCR datasheet limits and decide whether or not you need a current limiting gate resistor.

    Show us what you do ... step by step.

    Klaus
    Please donīt contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



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