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Wien oscillator

Discussion in 'General Electronics Discussion' started by zeZinho_maluco, Nov 8, 2011.

  1. zeZinho_maluco

    zeZinho_maluco

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    Nov 8, 2011
    Hi all,

    I am trying to build a sine wave generator. I use this schematic: http://en.wikipedia.org/wiki/Wien_bridge_oscillator. The op-amp I have used is the TL082 and the power supply of 9V. However, I do get a nasty sound, indicating that I am not getting a sine wave but rather a distorted waveform. Any idea why this is happening? Do you know any list of components for the wien oscillator that can generate accurate sine waves?

    Thanks,
    zeZito
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    The most likely problem is that the current through the bulb is either to high or too low.

    You need a current where the resistance of the bulb changes significantly in response to small changes in current.

    Note that the explanation is that the self heating causes the resistance of the bulb to stabilise at around Rf/2. You need to choose Rf such that Rf/2 is an achievable resistance for the filament at a reasonable (small) current.

    Here and here are more detailed explanations.
     
  3. davenn

    davenn Moderator

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    Sep 5, 2009
    Hi there
    welcome

    I was having all sorts of problems with one of these ~ 6 months ago
    I finally found and built a circuit that works.
    BUT is wasnt as easy as one would imagine... I found them to be very touchy oscillators which required very tiny adjustments of the 1k feedback trimpot.

    here's the circuit that I finally used... it does work.

    [​IMG]

    the nasty buzz you are hearing is probably the osc producing a square wave signal ... this it does very easily
    as I said its a very fine adjustment. You WILl need to monitor the output with an oscilloscope so you can watch the signal go from square to sine :)


    cheers
    Dave
     

    Attached Files:

    Last edited: Nov 9, 2011
  4. zeZinho_maluco

    zeZinho_maluco

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    Nov 8, 2011
    Thanks Dave and Steve! I bought a linear 1k potentiometer to use as Rf, but I'm still not getting a sine (I keep getting the nasty sound). The bulb I am using is 12V 65mA. Is the small current in the fillament enough to cause the bulb to emit visible light?
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Typically it does not.

    I would be expecting a current through the bulb of around 8 mA, and that suggests that the 1k pot is too large in value.

    At what point in the travel of the 1k trimpot does oscillation cease? If it is right up against the end stop (near zero ohms) then you should replace the trimpot with one of a much smaller resistance.

    The correct adjustment of the 1k pot is somewhere between the point where there is no sustained oscillation and where the output is a square wave. You see one of these extremes, do you see the other? It may even help to place a smaller value trimpot in series with a larger one so you have expanded control around the critical point.

    The critical point may vary slightly between ranges. The latitude in adjustment (caused by the non-linear nature of the bulb) should allow you to find a setting which works for all ranges.

    People have lost hair trying to get these oscillators going. Don't feel special :)
     
    Last edited: Nov 11, 2011
  6. davenn

    davenn Moderator

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    Sep 5, 2009
    hi again zeZito

    did you monitor the output with an oscilloscope as I previously suggested ?
    did you try the circuit I posted ... I know it definately works

    [​IMG]

    NO the lamp WONT glow, not even a little bit

    The adj of that 1k feedback pot is VERY critical, I recall that I ended up using a 10 turn pot just to get the fine adjustment.


    cheers
    Dave
     

    Attached Files:

    Last edited: Nov 11, 2011
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Ah, I was looking at your diagram Dave. Do you recall where the 1k pot eventually got set?

    Looking at some values I pulled off the web, the filament resistance would be around 20 to 60 ohms, suggesting that you would have the trimmer set to between 40 and 120 ohms -- of course that's highly dependant on the bulb characteristics.

    The choice of 1K seems to be based on the "hot" resistance of the bulb (mid setting is about double that resistance).
     
  8. davenn

    davenn Moderator

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    Sep 5, 2009
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Got back Tuesday morning. But it was fun.
     
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