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Zero crossing question.

Discussion in 'General Electronics Chat' started by alphacat, Jun 28, 2009.

  1. alphacat

    alphacat New Member

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    Two days ago I didnt manage to trigger the scope when I turned on the light bulb, so today I've overcome this problem.
    Instead of using the light bulb's voltage as the trigger source, I used the relay's coil as the trigger source.
    Its much more easier to use a DC signal as a trigger source than an AC signal. :)

    Here are pics of unsuccesfull zero crossing code and after I fixed it up :)
    (the other signal you see there is the coil's voltage).

    Could you tell me please (or direct me to an article), why is switching the appliance at zero cross points so important?
    Most of us dont switch the device at zero cross points.

    Unsuccessful zero crossing:
    [​IMG]


    Successful zero crossing:
    [​IMG]
     

    Attached Files:

    Last edited: Jun 28, 2009
  2. crutschow

    crutschow Well-Known Member Most Helpful Member

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    It's mainly to reduce EMI. For example, light dimmers, which do not use zero crossing. often will generate the familiar buzz interference in AM radios. The fast risetime that occurs in a non-zero turn-on (as in your first picture) generates many high frequency harmonics which can interfere with other electronic devices.

    Also, if you are controlling power, such as a heater, by cycling the power on and off, zero-crossing maintains a higher power factor then using a phase-control (dimmer) type controller. The power companies like a high power factor to minimize reactive power current, which generates IR power losses in the power lines.
     
  3. alphacat

    alphacat New Member

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    Thank you very much, it was explained well.
    By the way, is there a course in university (in EE degree) where they teach you about harmonics?
     
  4. dave

    Dave New Member

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  5. Hero999

    Hero999 Banned

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    Every degree should cover harmonics. I did an HND (High National Dimploma) which is only half a degree and it covered harmonics.
     
  6. alphacat

    alphacat New Member

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    Well, i've finished two years in my EE degree, and amongst the courses I took, the ones who dealt with electronics (Semiconductors, linear circuits, electromagnetic fields) didnt speak about harmonics (we did learn about harmonics in Fourier Series course but didnt relate it to electronics).
     
  7. Hero999

    Hero999 Banned

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    Come to think of it, I don't think we applied it to electronics, that must've automatically happened for me. This is because I understand it, even though I'd be totally lost if you set me a maths problem for me to solve.
     
  8. Augustinn

    Augustinn New Member

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    EMI is main reason. However, if you are turning loads in other angle that 0 or 180, you will cut off current which will directly result in huge voltage spike and vise versa. In or around 0V, your voltage jumps only few volts instead of few hundred.
     

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