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Wien-bridge oscillator questions

Discussion in 'Datasheets, Manuals or Parts' started by hadoque, Jul 24, 2007.

  1. hadoque

    hadoque New Member

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    Hi
    I'm very new to electronics, and I thought a fitting first project would be a Wien-bridge oscillator. Since I have a lm386 amplifier lying around, I thought I'd use the circuit discribed in the datasheet of this component (attached, see page 5). Now I have some questions about this diagram.
    What is RL?
    What is the "Bypass"? What voltage should I use and where do i connect the output (speaker or whatever)?

    Many thanks for help with answering these probably simple questions.
     

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  2. audioguru

    audioguru Well-Known Member Most Helpful Member

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    RL is the oscillator's load. It can be an 8 ohm speaker or the input resistance of an amplifier.

    The optional Bypass capacitor is used to reduce hum when a wall-wart AC adapter is used to power the circuit.

    Look at the datasheet. The supply voltage of the LM386 is from 4.0V to 12.0V. Try a 9V alkaline battery.

    It is RL in the circuit.

    You might find it difficult to find a low voltage and low current light bulb. Try a Christmas Tree small light bulb, the kind that are in series.
    Try a 120V/4W "night-light" bulb.

    The changing resistance of the light bulb makes the signal BOUNCE and BOUNCe and BOUNce and BOUnce and BOunce and Bounce and bounce.
    Hee, hee.
     
  3. Optikon

    Optikon New Member

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    RL is the "load."
    By placing a ceramic capacitor on this pin, the AC characteristics of the amplifier are improved.

    This thread should really be in general electronics BTW. This sub-forum is mostly for information requests on specific parts etc..
     
  4. dave

    Dave New Member

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  5. hadoque

    hadoque New Member

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    Thanks for the help everyone. Here are some pictures of the final construction.
     

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  6. audioguru

    audioguru Well-Known Member Most Helpful Member

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    What kind of light bulb did you use?
    Does the oscillator works well?
     
  7. hadoque

    hadoque New Member

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    I found a 3V 10mA microbulb.
    I don't think it's a perfect sinuswave coming out of it, because it's not a clear signal. It's more like a very fast "Bop-Bop-Bop".
    The next time I'm in school I might hook it up to an oscilloscope to see what the wave looks like.
     
  8. audioguru

    audioguru Well-Known Member Most Helpful Member

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    When it makes a sound like Bop-bop-Bop then we say it is motorboating.
    It is probably caused by the fairly high current into a speaker causing a weak battery's voltage to drop then it stops oscillating, the battery voltage rises because then there is a low current then it starts oscillating again and stops again, over and over.

    It can be fixed by adding a supply bypass capacitor from the + of the battery on the pcb to the - of the battery on the pcb to keep the battery voltage from changing too much. Use 220uF or 470uF or 1000uF and use a new name-brand 9V alkaline battery.
     
  9. hadoque

    hadoque New Member

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    Well, I put a resistor on the powersupply and hokked it up to the soundcard of my computer and recorded the signal. The attachement shows what it looks like, hardly a sine wave.
    I tried putting a 1000uF capacitator parallell to the battery, but that did actually not change anything. Any ideas?
     

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  10. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Please double-check your parts values and connections.
     
  11. hadoque

    hadoque New Member

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    Everything seems to be in order. There are a few things though. There is one capacitator which I'm a bit uncertain about. It's marked:
    Front:
    Code (text):
    10
    Back:
    Code (text):

    WIMA
    0,047
    63-
    P4
     
    That capacitator is supposed to be 0.05uF. There is also another capacitator that is supposed to be a 50uF, for which I used a 47uF cap. Do you think this might be the source of the problem? Otherwise I might try a different bulb and experiment with resistors...
     
  12. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    No that's not it, 47 and 50 are the 'same' thing.
     
  13. hadoque

    hadoque New Member

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    Ok, now I've found the problem.
    I replaced the bulb with a 6V 40mA bulb and a potentiometer, and found a clear signal at about 10 ohms (the weakest I had was a 1k potentiometer). On monday I'm going to buy a 10 ohm potentiometer, to find the perfect resistance. Thanks for the help everyone, I'm posting the values monday.
     
  14. hadoque

    hadoque New Member

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    Back again. I hooked the oscillator to my soundcard again, and analysed the clear tone. Seems it isn't that clear after all, the signal looks exactly like the previous one, so I guess there is only a difference in frequency that gives the clearer signal. IK don't have a clue what it could be now...
     
  15. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Usually a Wien bridge oscillator has equal values for the frequency determining resistors. That circiut doesn't.

    I modified it so the resistors are almost equal.
    Try it like this then try it without the 10uF capacitor at pins 1 and 8.
     

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  16. hadoque

    hadoque New Member

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    ok, thanks. I'll get back to you when it's finished...
     
  17. hadoque

    hadoque New Member

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    Ok, after the alteration, the signal looks moore like a sine, actually most likeasquare wave. For me, it's close enough. Thanks a lot for the help.
    Just one moore newbie question. I connected the plus, from the battery, to V_s and minus to V_o. This is the correct way, right? I'm not supposed to connect the minus to the ground, right?

    thanks again for all the help
     

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  18. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The signal is terrible. It is not supposed to be modulated like that.

    NO!
    That is the cause of the problem. The (-) of the battery is supposed to connect to ground. The 1000uF capacitor is supposed to connect to Vs and to ground so that it is connected across the battery.
    Everything with the ground symbol must be connected together and connected to the (-) of the battery.

    Vo is the output that connects to the 'scope and/or an 8 ohm speaker.
    The 'scope and speaker also connect to ground. RL is the speaker.

    Luckily nothing is burnt.
    Connect the (-) of the battery to ground and you will have a very nice sine-wave.
     
  19. hadoque

    hadoque New Member

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    Wow, now I feel kind of dumb...
    Anyways, that did the whole trick. Now, there's a clear sine signal. Check it out!

    Thanks again for all the help, without it, I would have gone nowhere...
     

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  20. hadoque

    hadoque New Member

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    Ok, nevermind the last picture, here is a better one...
     

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  21. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Your signal is a low level square-wave instead of a high level sine-wave.
    If you added a resistor in series with the light bulb then remove it.

    EDIT:
    Your new pic is better but it is still very distorted and low level.
     
    Last edited: Jul 31, 2007
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