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zero crossing detector circuit using lojic gates

Discussion in 'General Electronics Chat' started by ricepatties, Sep 12, 2008.

  1. ricepatties

    ricepatties New Member

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    I am new to circuit design so forgive my question if it sounds silly. I am just playing around with some ideas but I was looking at some schematics for a zero crossings detector. Can a two input 'and' lojic gate be used for this type of circuit? Have one input tied to ground. The other input would be tied to a low voltage ac power supply. Then when the ac voltage crosses zero both inputs to the and gate would be zero and would therefor provide a high output. Or am I missing something? Thanks in advance for any feedback i may receive.

    Ricepatties
     
  2. Boncuk

    Boncuk New Member

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    Zero crossing detetector

    Hi Ricepatties,

    the problem with logic gates, in this case it must be an 'AND'-gate is the logic level for the function. Both inputs must be high to achieve a positive output state.

    In order to to that you require a power supply for the logic gates. The AC must then be adapted not to exceed the allowed input voltage (CMOS-ICs) can handle up to 15V provided they are supplied with a 15VDC power supply.

    for the steady ground you require a steady high as one input to the 'AND'-gate and for the AC-input it must be inverted, causing high on the other input when AC reaches zero Volt.

    Boncuk
     
  3. Willbe

    Willbe New Member

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    Try this; unless you attenuate it, Vp-p of the incoming AC waveform has to be < +V of the CMOS gate.
    The resistor/zener string should pass current >5x more than the gate input current.

    Thanks, Mr. G., for telling me how to do a thumbnail.
    :)
     

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    Last edited: Sep 13, 2008
  4. dave

    Dave New Member

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  5. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi Willbe,
    If you Google for: ExpressPCB you can download a free electronics schematic and pcb layout program.:)

    Also get free PDFCreator program.
    and free ScreenHunter 5.

    The ExpressSCH is ideal for quickie drawings, then use either PDFCreator or get a clipped GIF with ScreenHunter.
     
    Last edited: Sep 13, 2008
  6. Willbe

    Willbe New Member

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    Don't you know that computers are The Work of Satan???

    Can it also put meters everywhere? Meters are handy when ideas fail.
     
    Last edited: Sep 13, 2008
  7. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    Dont understand the use of 'meters' in this context.?:)

    Computers are just a tool and only as good or bad, as the people who use them.
     
  8. KMoffett

    KMoffett Well-Known Member

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    I agree, it's a great schematic tool. I use ExpressPCB's Export function to transform schematics to a .bmp file (@ 300 pixels per inch). Then right click on the file and "Open with >" Microsoft Office Picture Manager. Easy to crop out frame, resize, and export to many formats...usually .gif. I think the best part is the ease in creating any schematic symbol you want...or modifying any in their large library.

    Ken
     
    Last edited: Sep 13, 2008
  9. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I make schematics with Microsoft Paint program.
    Parts of other schematics and datasheets are copied and are pasted into the Paint program. Straight lines are made with the SHIFT key down. Simple and look good.
     
  10. rezer

    rezer New Member

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    I agree. I have used ExpressPCB for some time now. It was a lot easier to learn than Eagle. The components are easier to manage (at least right now). I downloaded Eagle because a number of people had suggested it and the price was right:D I'm still working with Eagle, trying to learn the ins and outs. Is there a "sticky" somewhere that could give me some helpful hints? Or maybe a tutorial on-line?
     
  11. Willbe

    Willbe New Member

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    I was joking about Satan! :D

    Some people have posted schematics with meters showing readings that were evidently calculated using math models for the components shown in the schematic.
    How'd they do that?
    :confused:
     
  12. rezer

    rezer New Member

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    The may have been using MultiSim. There are different instruments you can put into your schematic to make "simulated" measurements.
     
  13. Willbe

    Willbe New Member

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    Thanks; I think you're right.
    http://en.wikipedia.org/wiki/NI_Multisim
    Maybe earlier versions are free? :confused:
     
  14. rezer

    rezer New Member

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    It comes from National Instruments. They have an eval version free for 30 days. I've used it and as far as I know does everything the licensed one will do. You should be able to reinstall to extend it. The license cost $40.00 as of last Fall.
     
  15. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You can spot a Multisim schematic because its parts are spread out all over your neighbourhood and it has measles dots all over it.

    There is another schematic software that makes its schematics as a negative, white lines on a black background. Weird.
     
  16. Ubergeek63

    Ubergeek63 Well-Known Member

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    oy! a resistor, with diode clamps if you are paranoid, to the input of a schmidt trigger is all you need.
     
  17. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi there,

    A comparator makes a much better zero crossing detector because you can control
    the hysteresis much better. If you must use a logic gate then use a high voltage
    AC signal through a resistor to drive the input, and use two diode clamps (one to
    positive and one to ground) to clamp the input to a safe level for the gate. This
    will get you closer to the true zero crossing.
     
  18. Willbe

    Willbe New Member

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    I think hysteresis muddies when the crossover occurs.
     
  19. Blueteeth

    Blueteeth Well-Known Member

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    You're right, thats the point.

    With a definate cross-over point noise on a slow moving signal will cause multiple crossings. It's a conpromise between sensitivity and relability. You don't have to have a lot of hysteresis, a few mv should be enough to curb any noise issues.

    Also, zero-crossing detectors are designed to cope with a certain bandwidth. What frequency are you dealing with here? If it is a datastream from some form of telecommunications, then the bandwidth would be fixed, or at least within a narrow band. This way you can tune the zero crossing detector. You can't expect it to cope with a sine wave of 5Hz, AND say 100khz.

    Blueteeth
     
  20. Boncuk

    Boncuk New Member

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    KILOMETERS!

    Don't forget computers are individuals. Hug them from time to time to carry on working. :)

    Hans
     
    Last edited: Sep 14, 2008
  21. Boncuk

    Boncuk New Member

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    Zero crossing detector

    Hi ricepaddies,

    if you just want to detect the zero crossing of mains AC here is a circuit which works with mains.

    It's output is half cycle of mains frequency. (full wave rectifier) You can control a triac directly from the output transistor.

    Boncuk
     

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