Please help to understand attached circuit!!!

Discussion in 'The Projects Forum' started by paddy611, Feb 18, 2013.

  1. paddy611

    Thread Starter New Member

    Oct 9, 2012
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    Hi,

    Please have a look on attached circuit.

    This circuit is used to detect the gas concentration and depending on concentration it will generate 4-20 mA output.

    I want to know the working of this circuit. Please help me out.

    Thanks,
    paddy
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,446
    3,361
    The picture is not clear. You will have to post the circuit in jpg, png, gif or pdf format.
     
  3. paddy611

    Thread Starter New Member

    Oct 9, 2012
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    0
  4. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
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    Complicated OpAmp circuit. These are used to amplify the signal from the sensor. In the first stage there is also a JFET.

    If you want to understand the circuit, learn OpAmp technology.

    No need unless you specialize on sensors.

    Maybe someone here is willing to produce a detailed explanation.

    Do you need to modify it or something?
     
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  5. paddy611

    Thread Starter New Member

    Oct 9, 2012
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    I have to design a gas detector which gives 4-20 mA output according to gas concentration.

    For that purpose i am trying to understand that circuit.
     
  6. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    Do you have a datasheet of the two sensor elements?

    What is the value of R3 and R6 supposed to be?

    Vref is probably at 2.5V.
    IC is an inverting amplifier, gain can be adjusted with RP1.
    IC4 is an inverting amp also, gain 1. You can choose to use it or not with the jumper LK3.

    IC5 looks like an power amplifier (with TR2) with adjustable offset through RP2.

    What is connected to PL1? It can't be a simple voltage source. Looks like this circuit is changing the power supply input current through TR2 depending on the state of the two sensors...

    You need to know what is connected to PL1 and what are the specs of the 2 sensors. This is not a complete circuit diagram.
     
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  7. MrChips

    Moderator

    Oct 2, 2009
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    SN1 is the toxic sensor.
    SN2 is oxygen sensor.
    IC2 subtracts the two readings to compensate for the effects of O2 on the toxic gas sensor.
    IC1 is the gain amplifier as well as a low-pass filter. It removes high frequency noise.
    Gain is controlled by adjusting RP1.
    IC4 is a unity gain inverting amplifier that is used only with NO2 and Cl2 sensors.
    ZD1 is a 2.5V voltage reference.
    RP2 is the offset adjustment to set the output current to 4mA with 0% toxic gas input.
    IC5 is a voltage to current converter and TR2 is the 4-20mA current driver.
    PL1 is the connection to the current loop output. The load is usually a resistive load in the 4-20mA current receiver. The maximum resistance depends on the power supply voltage. For example, at 24VDC the load resistance must not exceed 825Ω.
    In this circuit, power is supplied by the 4-20mA current loop receiver.


    [​IMG]
     
    Last edited: Feb 18, 2013
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  8. paddy611

    Thread Starter New Member

    Oct 9, 2012
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    Thank you very much praondevou and MrChips.

    I will check out it's results.

    Just one thing i have to ask to MrChips is how you calculate load resistance of 825E for 24V supply??

    Thanks
     
  9. #12

    Expert

    Nov 30, 2010
    16,298
    6,810
    That's when you run out of voltage trying to drive 20 ma through the load.
     
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  10. paddy611

    Thread Starter New Member

    Oct 9, 2012
    25
    0
    Thank you all for your advice.

    I need some more suggestions.

    I have to design circuit for gas detector for toxic as well as explosive gases.

    http://www.gassensor.ru/data/files/pdf/kns/KGS701_CH4_pellistor_Exp.pdf

    I am using the sensor mentioned in above link.

    Can I use instrumentation amplifier to amplify the senor output signal?

    I have to provide potentiometers for Zero and span setting.

    Please advice how to do it.

    thanks,
    paddy
     
    Last edited: Feb 19, 2013
  11. #12

    Expert

    Nov 30, 2010
    16,298
    6,810
    Is that what the teacher told you to do?
     
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