V-F Converter cct

Discussion in 'The Projects Forum' started by Switch_639, Dec 27, 2009.

  1. Switch_639

    Thread Starter New Member

    Dec 27, 2009
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    I didn´t know where else to post, I am new here... if its wrong forum tell me... I am a student and have some questions on this circuit...

    here it is...
    http://i531.photobucket.com/albums/dd351/Switch_639/V-FConverter-1.jpg

    how do I adjust the gain with the 2K rheostat and know its correct? also the optional offset adjust of the OPAMP, when is this correct? at moment I have the cct built... I am getting output at pin3 of LM331... its a small signal around +/- 1V...

    any help will be appreciated... I can not get a square wave pulse on oscilloscope either, but I might be trying to read it in incorrect manner...
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    First of all, get the data sheet for the device. Just Google "lm331". The data sheet should explain how to adjust the composite value of Rs (the 2K control is a variable resistor) for the desired gain. You can never hope to understand a device without its data sheet.

    One suspects that the gain adjustment relates to the frequency scaling according to the applied voltage. That setting will let you set the frequency out as some linear relationship to the input voltage.

    You probably do not have a voltage on the one side of the 10K pullup resistor. The LM331 has an open collector output. If working with logic, that resistor should be connected to the logic Vcc. If not, any old voltage within the working limits (given by the data sheet) will do fine.
     
  3. Switch_639

    Thread Starter New Member

    Dec 27, 2009
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    I have checked the data sheet of the LM331... does not really tell me much... with that pull up resistor... are you saying its the reason I can not see the pulses on the oscilloscope? so its needed for me to read the frequency? anything you can add about the optional offset adjust on the Op-amp? thanks for input on reply...
     
  4. beenthere

    Retired Moderator

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    If the output is open collector, you must add a voltage as I said in order for there to be an output waveform.

    The O/S adjust is optional. Use a reasonably modern op amp like a TL071 and don't worry about it.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Ahh, the LM331 is a voltage to frequency converter. It's output can go to 100kHz, and is open-collector.

    I would change that 10k resistor on the output to 1.4k-1.5k, if you're planning on driving TTL (Vcc=5v) with it.

    It'll be tough to get a commonly available opamp to get to 100kHz when running open loop. It would need to have rail-to-rail I/O and be pretty zippy. I don't see why you would not be able to get the necessary output level by simply selecting the proper pull-up resistor.

    To get you in the ballpark, use this formula:
    Rpullup = Logic_Vcc / 3.5mA
     
  6. Switch_639

    Thread Starter New Member

    Dec 27, 2009
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    LF411A was the recommended op-amp from the data-sheet... so I am using one... I still want to know how to adjust the offset of an op-amp to be zero... I read a lot about the null-offset but have no clue how to... is that optional offset adjust POT there for this purpose?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Yes.

    Connect Vin to ground.
    Then adjust the offset adjustment pot until the output of the opamp is as close to 0v as you can get it.
    The LF411 already has a pretty low offset voltage; under 2mV for the standard LF411, and under 0.5mV for the LF411A.
     
  8. Switch_639

    Thread Starter New Member

    Dec 27, 2009
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    thanks a lot for this info... another question... do I still supply pin4 with -VS and pin7 with +VS when doing so? pin6 is the output correct? cause my output does not change when I adjust the POT... only the voltage on Pin3 changes... I can never notice changes in output voltage...
     
    Last edited: Dec 29, 2009
  9. Switch_639

    Thread Starter New Member

    Dec 27, 2009
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    one more thing I noticed... if I dont drive pin6 its value is at VS... why is this?
     
  10. hgmjr

    Moderator

    Jan 28, 2005
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    There is internal circuitry connected to pin 6 that uses the external components attached to it to establish the operating parameters for the V-to-F conversion.

    hgmjr
     
  11. Switch_639

    Thread Starter New Member

    Dec 27, 2009
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    ok thanks, makes sense... getting back to my previous question also... do I still supply pin4 with -VS and pin7 with +VS when doing so? pin6 is the output correct? cause my output does not change when I adjust the POT... only the voltage on Pin3 changes... I can never notice changes in output voltage... so there´s no way actually to adjust the null offset then?
     
  12. Switch_639

    Thread Starter New Member

    Dec 27, 2009
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    hope someone can answer my previous question please... really in the dark here... would like an answer...
     
  13. beenthere

    Retired Moderator

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    Assuming you are speaking of the op amp, notice the diode between the op amp output and ground. That will clamp the output so it can't go negative.

    Anyway, monitoring the op amp output is not significant to the adjustment. The purpose is to set the offset of the V to F converter. You have the gain adjust to set the response of frequency change to voltage change, and you have the offset adjust, so you can set some frequency to coincide with an input voltage level.
     
  14. Switch_639

    Thread Starter New Member

    Dec 27, 2009
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    appreciate your answer... got my answer then also on the gain adjust, was wondering what is was for...
     
  15. Switch_639

    Thread Starter New Member

    Dec 27, 2009
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    btw... is there any other method to measure frequency without oscilloscope? really struggling to read pulses on the analogue one I have...
     
  16. beenthere

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    You really need a frequency counter to do so. Oscilloscopes are not nearly accurate enough.
     
  17. Switch_639

    Thread Starter New Member

    Dec 27, 2009
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    any suggestions you might have on counters?
     
  18. beenthere

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  19. Switch_639

    Thread Starter New Member

    Dec 27, 2009
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    what peak voltage value does such a V-F converter cct normally provide? like the peak value of the square wave?
     
  20. beenthere

    Retired Moderator

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    That LM331 is open collector, so the voltage out is determined by the voltage applied to the pull up resistor.
     
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