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back to back zener diodes does what?

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Discussion in 'General Electronics Discussion' started by Davewalker5, Sep 30, 2014.

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  1. Sep 30, 2014 #1
    Davewalker5

    Davewalker5

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    What does these back to back zener diodes do?

    Why are the reverse polarity back to back?
     

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  2. Sep 30, 2014 #2
    davenn

    davenn Moderator

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    they will symmetrically clip a signal appearing across them to the approx. voltage of the diodes (6.8V)
    so if there is an AC signal coming out of that Op-Amp, through R133. the resulting output past the diodes towards R132 will be a rough looking square wave
     
  3. Sep 30, 2014 #3
    Davewalker5

    Davewalker5

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    I was thinking that also but, isn't U27 a comparator?

    So U27 is not outputting an AC waveform i'm thinking

    U24 is an intergator
     
  4. Sep 30, 2014 #4
    davenn

    davenn Moderator

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    regardless of the function of U27, the zeners are still acting as a symmetrical clipper

    U 24 is also a low pass filter and you wouldn't need a LPF if the signal into U24 was plain DC
     
    Last edited: Sep 30, 2014
  5. Sep 30, 2014 #5
    Harald Kapp

    Harald Kapp Moderator Moderator

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    Even a comparator can output an AC signal. AC is not limited to sinusoidal signal. AC means Alternating Current and encompasses any waveform that is not DC. A square wave from a comparator is AC, too. The zener diodes limit the amplitude of the signal to approx. +-7.5V whereas the output from U27 can be much higher, depending on the power supply +V and -V.
     
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  6. Sep 30, 2014 #6
    davenn

    davenn Moderator

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    yup, agree totally there Harald ... got tied up at work ... doing my job haha
    didn't get a chance to respond with the info you did ... thanks :)

    Dave
     
  7. Sep 30, 2014 #7
    Colin Mitchell

    Colin Mitchell

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    The maximum voltage appearing across them will be 6.8v + 0.7v = 7.5v in either direction
     
  8. Sep 30, 2014 #8
    Harald Kapp

    Harald Kapp Moderator Moderator

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    I think that's what I wrote ;)
     
  9. Sep 30, 2014 #9
    Davewalker5

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    How can you tell from a Low Pass filter to an Intergator? they both look the same to me

    Is it the Capacitor and Resistor values that make the difference?

    I can't tell on schematics if it's a low pass filter or an intergator circuit

    Any hints to know without knowing the input waveform is AC or DC
     
  10. Sep 30, 2014 #10
    davenn

    davenn Moderator

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    they are the same
     
  11. Oct 1, 2014 #11
    Davewalker5

    Davewalker5

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    So they can have the same RC values? so how can you tell which is which? if you don't know what the input waveform is either AC or DC

    How would a tech know by looking at a schematic
     
  12. Oct 1, 2014 #12
    Davewalker5

    Davewalker5

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    I found another back to back zener diode on the op amps input pin

    These diodes must clamp or clip the waveform symmetrically
     

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  13. Oct 1, 2014 #13
    davenn

    davenn Moderator

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    an integrator is just another name for a low pass filter
     
  14. Oct 1, 2014 #14
    davenn

    davenn Moderator

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    no, not symmetrically

    why do you think it would be symmetrical ?
     
  15. Oct 1, 2014 #15
    Davewalker5

    Davewalker5

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    So what does these back to back diodes do?
     
  16. Oct 1, 2014 #16
    davenn

    davenn Moderator

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    they still do clipping
     
  17. Oct 1, 2014 #17
    Davewalker5

    Davewalker5

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    symmetrical clipping but only on the feedback loop?

    The other circuit symmetrically clipped the output of the op amp

    What is the difference between symmetrically clipping the feedback loop compared to symmetrically clipping the output?
     
  18. Oct 1, 2014 #18
    davenn

    davenn Moderator

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    Original question answered

    thread closed
     
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