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zero crossing

Discussion in 'General Electronics Chat' started by be80be, Dec 25, 2014.

  1. be80be

    be80be Well-Known Member

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    Z.png
    I was just thinking about this today would it be usable to use 2 resistors and to diodes a zener to clamp to max of 5 volts like the sch.
     
  2. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    I would use an opto
     
  3. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi burt,
    D2 will clamp it at 0.7V , not the zener.
    E
     
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  4. dave

    Dave New Member

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  5. alec_t

    alec_t Well-Known Member Most Helpful Member

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    D1 and D2 are both redundant.
     
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  6. crutschow

    crutschow Well-Known Member Most Helpful Member

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    You want isolation if this is from the mains voltage. A optisolator, as 4pyros mentioned, is a cheap way to provide that as well as give you the zero crossing signal. Just add a series resistor and a inverse connected diode across the opto input.
     
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  7. be80be

    be80be Well-Known Member

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    I was playing with it just to see how it looks in a simulator it's giving crazy reading in mV like -47mV then +47mV
    I was thinking without the zener I would read a 0 to 60 volt maybe up to 75 But that's not showing up
     
  8. crutschow

    crutschow Well-Known Member Most Helpful Member

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    What simulator?
    What diodes are you using?
    Post the simulated circuit and sim results.
     
  9. be80be

    be80be Well-Known Member

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    Im using Multisim I figured out the bad readings Im getting what it should be -754mV to +754mV I was using there virtual diode
    testReadings.png

    Thats with a 4006 diode its lower then the virtual but still what I figured is should be
     
    Last edited: Dec 26, 2014
  10. killivolt

    killivolt Well-Known Member

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    I shouldn't but; does the last zener clip the signal?

    kv
     
  11. be80be

    be80be Well-Known Member

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    Thats what it dose and It shows in the pic But I was thinking Id get half a cycle 0 to a choped peak but its showing it to go below ground
     
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  12. crutschow

    crutschow Well-Known Member Most Helpful Member

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    A zener will clamp to its rated voltage when reverse biased and clamp to a about 0.7V negative when forward biased (with the anode grounded).
     
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  13. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi burt,
    D2 will start conducting long before the Zener diode reaches it zener breakdown and clamp the output at ~0.7V
    Remove D2
    Eric
     
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  14. be80be

    be80be Well-Known Member

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    This is what I think id go with. Let me know what you see wrong with it. Hope you all had a wonderful Christmas
    Thanks Burt
     

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  15. alec_t

    alec_t Well-Known Member Most Helpful Member

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    D2 may clamp the voltage too low and prevent Q1 turning on. You could replace D2 with two diodes in series, or simply eliminate it, to overcome that.
    BTW R3 has much too low a value. Try 220k or more.
    For safety's sake the 120V supply should come via a transformer.
     
    Last edited: Dec 27, 2014
  16. be80be

    be80be Well-Known Member

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    R3 should only drop 120 mA In simulator it shows It at about 119mA But I'm with you On that I think a bigger 100K or more would be safer but that didn't let the Q1 pull to ground.
    I'm trying for a short low at At Q1 when the AC line hits zero.

    I do all the live testing with a transformer I was just looking to make this small.
     
  17. alec_t

    alec_t Well-Known Member Most Helpful Member

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    120mA x 120V = 14.4W !! You'll need a mighty big resistor and a heatsink.
    Not possible. If the AC volts are zero then Q1 will be switched off, not on.
     
  18. dr pepper

    dr pepper Well-Known Member Most Helpful Member

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    Hi Burt nice to hear from you again.

    Just so happens I'm working on something remotely similar, my mrs's pyrography wood burner power controller.
    A lot of designs use an opto, however you can do it with a transistor like I have (wit a diode to protect the base from inverse voltage), then you dont need a zener, and theres a low risk of the voltage going above vcc, of course a transistor doesnt give isolation, but then whats the point when my circuit is powered by a line transformerless supply?

    One thing to bear in mind is the voltage rating of the resistors, might be ok in the US on 110, however here in the UK its better to use 2 47k resistors in series than 1 100k, if there was any kind of spike on the mains theres a risk of flashover.
     
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