Direct Capacitance to voltage convertor ?s

Discussion in 'General Electronics Chat' started by Shayan22h, Oct 24, 2012.

  1. Shayan22h

    Thread Starter Member

    Oct 24, 2012
    30
    3
    guys i am doing my pro where i need to convert my capacitance sensor input to voltage, i know there re two methods of direct and indirect, i would prefer the direct one where i can directly convert capacitance to AC voltage then using AC to DC covertor to get the DC voltage as the output.
    so I searched through the internet and found a good circuit
    http://www.google.com/url?sa=t&rct=...3YD4DA&usg=AFQjCNF7YbHY9BKljN6Ggmcd1SrBtVsbTw

    the circuit is in page 4 figure 4, could someone please explain a little bit the circuit for instance i dont see any input voltage ?
    thank u so much for ur time and help !;)
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    What do you have for power supply for the opamps?
    There should be a + and a - power supply available.

    [​IMG]

    I have attached the document as a PDF and extracted the fig 4 from it, so others have a more convenient view.

    Bertus
     
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  3. Shayan22h

    Thread Starter Member

    Oct 24, 2012
    30
    3
    thank u so much for ur response, i know that the power supplies for the OPAMP re missing and most likely some other stuff such as voltage input and i searched alot but couldnt find a single circuit for direct conversion of C to V, if u have any circuit or webpages, could u please recommend me, i guess i am going no where with this circuit !:(

    cheers mate,
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    Wich opamp did you use in your set-up?
    The left opamp makes a wienbridge oscillator.
    This generates an AC signal.
    The right opamp makes a rectifier circuit.

    The power supply should be connected
    between the ground and +V for the positive side
    and between ground and -V for the negative side.

    Bertus
     
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  5. Shayan22h

    Thread Starter Member

    Oct 24, 2012
    30
    3
    I am using LM741, or is it recommended to us another one ?

    again thanks for ur help,
    cheers mate ;)
     
  6. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    The LM741 is a rather old opamp with not so good specifications.
    How did you power the circuit?
    The + power supply should go to pin 7 of the LM741.
    The - power supply should go to pin 4 of the LM741.
    The power supply should be + and - 12 Volts.
    You can also try to power it with 2 9 Volts batteries.

    You could try to make the circuit using a TL072, wich is a dual opamp.

    I have attached the datasheets for the LM741 and the TL072 to compare.

    Bertus
     
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  7. Shayan22h

    Thread Starter Member

    Oct 24, 2012
    30
    3
    i use the laboratory power supply, LM741 could be good for the rectifier but not the oscillator i have no choice it is the only amplifier left in the lab :( now i just calculated that the range of frequency i need to be oscillated is between 32khz - 382khz cuz i need the impedience of 1-5kohm and since my input capcitance is 500pF-1000pF so i used this formula to calculate the frequency f= 1/[2pi*C*(impedience)] . unfortunatelly the oscilator cicuit has been designed for 1.6KHz so my only challenge is to change the capacitance value in the oscillator circuit in the way to get my desired frequency, any suggestions mate ?
    thanks loads for ur help,
    Best Regards,
    Shayan
     
  8. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    The LM741 most probably will have problems with creating the 382 kHz sinewave signal of enough amplitude.
    Can you get hold of a TL072 or even better a NE5532?
    (I have attached the datasheet of the NE5532 for you).

    Bertus
     
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  9. Shayan22h

    Thread Starter Member

    Oct 24, 2012
    30
    3
    awesome :)
    I am gonna ask laboratory to get me those, really thanks for ur help,
    today is my first day in this fourm and i feel like i have been registered in such a great website,
    friendly environment, great concept .

    Dank,

    Best Regards,
    Shayan
     
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