Wien bridge Oscillator

Discussion in 'The Projects Forum' started by Cheslyn, Mar 25, 2014.

  1. Cheslyn

    Thread Starter New Member

    Jan 18, 2013
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    0
    Hi,guys

    Can anyone help me with a Wien bridge oscillator that does not Osillate
    I'm using a LM324 Op amp,and the supply volatge is a 9 V battery.
    The inverted input of the Op Amp is conected to a divider network of a 68 ohm resistor and a 12 V 50mA filament lamp.
    The non inverted input is connected to a feed back network consisting of the following:a 100nF capacitor(I've used polarised and Non Polarised capacitors)in parralel with a 10 k ohm resistor.This resistor and capacitor that's in parallel with each other is conected to a 10 k ohm resistor in series with a 100 nF capacitor which is conneted to the output of the Op Amp.The one end of the filament lamp is connected to the other end of the Parallel network consisted of a Capacitor and Resistor.The other end of the resistor witch forms the divider network with the filament lamp is connected to the output of the op amp.When power is supplied nothing happens.No osillation noting.I'll post the schematics when I'm at my PC I'm using my BB.

    If anyone can help it would be appreciated.I don't know maybe the way I've setted up the circuit on the bread board is wrong.I really don't know.
    Oh and the resistance of the lamp when it is off is 35.5 ohm witch is half of the resistor in the divider network for the inverted input of the op amp.
     
  2. LvW

    Active Member

    Jun 13, 2013
    674
    100
    The underlined sentence sounds a bit unclear to me.
    I suppose, we all know how the WIEN oscillator looks like.
    But the main queston is: Are you familiar with the restrictions connected with single supply operation?
    Try to show us the whole circuit and tell us how you have calculated the 68 ohm resistor.
     
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  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    First, the basics.

    Here is a previous thread:
    http://forum.allaboutcircuits.com/showthread.php?t=74807

    Here is another tutorial covering the circuit but not the lamp:
    http://www.electronics-tutorials.ws/oscillator/wien_bridge.html

    Here is a design using diodes instead of a lamp. Stability is much better and no warmup, but distortion is a bit higher:
    http://www.ecircuitcenter.com/Circuits/opwien/opwien.htm

    Attached is a pdf clip from a National Semi app note and a jpg of another version. When working with a new Wein design, usually the problem is with the lamp. It must be a low current lamp so the opamp output can drive it. Notice in the pdf that the resistance of the lamp (10 / .014 = 714 ohms) is slightly less than the series feedback resistor. This is because the lamp resistance increases as it heats. This is what gives this circuit its unique combination of very low distortion plus reasonable amplitude stability.

    In your case the resistance of the lamp is 240 ohms and the series feedback resistor is 68 ohms. This won't work because the resistance of the lamp never will be so low that the bridge comes into balance. Also, in your description you do not say which parts are tied to ground.

    Separate from all of this are the circuit changes that must be made because you are using only one power supply. Because of the relatively low impedance of the negative feedback loop, setting a virtual ground is not simple. Show us your sch and we'll see.

    ak
     
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  4. LvW

    Active Member

    Jun 13, 2013
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    I think, in the above circuit diagram the opamp inputs have to be exchanged.
     
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  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I agree. The filter network on the + input. Timing cap on - input.
     
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  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Hey, it was on the innergoogle so I just assumed it was awesome.

    ak
     
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  7. Cheslyn

    Thread Starter New Member

    Jan 18, 2013
    4
    0
    Hi Guys thanks for the replies I'll try those suggestion
    but the lamp's resistance before it lights up is about 35 ohms
    Here is the site that i got this circuit from http://www.learnabout-electronics.org/Oscillators/osc34.php all the information is there.I've followed the circuit design IT JUST DOES NOT oscillate.It should produce a oscillation frequency of 159Hz but every where I PROBE on the circuit i get a 12Vp-p 50Hz signal and when i connect the ground connection of the probe to the ground of the circuit i get nothing.

    I hope this site will help you guys in diagnosing what could possibly be wrong.

    Thanks
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Are you in Europe or anywhere else where the AC mains is 50 Hz? If so, that is the usual pickup when a scope probe is ungrounded. The site is pretty convincing about a 324 being able to function with such a low feedback reistance, so the first suspect is the wiring. If you are using a protoboard, take everything off and reassemble the circuit from scratch. That can catch a lot of errors that just looking at it over and over will miss.

    ak
     
  9. LvW

    Active Member

    Jun 13, 2013
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    Another good example that it is wise not blindly to trust web-based information.
     
  10. LvW

    Active Member

    Jun 13, 2013
    674
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    Please tell us:
    - What are the values (R and C) you have used, and
    - What is the power supply? Single or double (read again my question in post#2, why didn´t you answer?)
     
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