unable to trigger 555 with ZCD

Discussion in 'The Projects Forum' started by onlyvinod56, Mar 23, 2010.

  1. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    362
    1
    Hello,

    In short.....

    Iam designing a full bridge thyristor based controlled rectifier....

    I need to trigger two 555 monostables with the zero crossing detector..
    see the attachment 1

    In the attachment 2; see the signals at the four points shown.
    They are practically observed.

    I know that a negative going pulse can trigger the 555,...

    How can i modify the two signals to 'negative going pulses'.....??????




    thankyou
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,765
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    Don't forget you should use a capacitor to couple the pulses to the 555 (see 555 Monostable). One of the pulses is already negative. For the other pulse use something like a transistor.

    [​IMG][​IMG]
     
  3. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    362
    1
    Hello bill_marsden

    The article is very good and i have seen your articles and projects. they are pretty good. "Managing the resistors in groups" is a good idea in projects.

    coming to the point, In your article its mentioned that 'The timer starts when the input goes low or switched to the ground level, and the output goes high.'.

    What is meant by INPUT GOES LOW?

    for example..... i know that a pulse going 5v to zero can trigger the monostable...

    but a pulse from 0v to -5v can trigger the monostable?????

    In your reply, the second circuit is inverting the signal using a transistor....and its ok..... but the first picture, i didnt understand...

    I thought a clamping circuit is required to clamp the [0 -5]v pulse to [5 0]v
     
    Last edited: Mar 24, 2010
  4. Wendy

    Moderator

    Mar 24, 2008
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    If you couple it with capacitor as shown it will be from +5 (or whatever) going low. The capacitor removes DC bias on the input and allows you to substitute your own DC bias, which in this case is Vcc.

    The transistor version will likely also need a capacitor in front of it for similar reasons.
     
  5. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    362
    1
    Thankyou Bill_marsden....


    you cleared my doubt and.......

    what would be the R1 and C1 values?

    Thankyou
     
  6. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    C1= 10 to 100nF, R1= 10k.
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    If the pulse is sharp (fast) almost anything will work. What Bernard said.
     
  8. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    362
    1
    Thankyou Mr. Bernard and Bill_Marsden
     
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