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conversion of gas concentration into 4-20 mA output!!!!!

Discussion in 'General Electronics Discussion' started by paddy, Feb 18, 2013.

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  1. paddy

    paddy

    81
    0
    Sep 11, 2012
    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
     

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    11,517
    2,652
    Nov 17, 2011
    Can you exchange the figure for one with less white space and more legible schematic, please?
    Where is that circuit from? You should find an explanation on the original website.
     
    Last edited: Feb 18, 2013
  3. paddy

    paddy

    81
    0
    Sep 11, 2012
  4. shrtrnd

    shrtrnd

    3,821
    519
    Jan 15, 2010
    I would assume the manufacturer has a patent on their sensing circuit.
     
  5. paddy

    paddy

    81
    0
    Sep 11, 2012
    Thank you all for your advice.

    I need some more suggestions.

    I have to design circuit for gas detector.

    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
  6. Electrobrains

    Electrobrains

    259
    5
    Jan 2, 2012
  7. paddy

    paddy

    81
    0
    Sep 11, 2012
  8. Electrobrains

    Electrobrains

    259
    5
    Jan 2, 2012
    That circuit would take some time to analyze...

    Roughly and quickly (don't blame me if I'm wrong):

    The right part is a smart combination of a two-wire current loop, with power supply and 4-20mA output at the same time.
    The top (IC3) supplies 5V for the circuit and ZD1 a reference voltage.
    IC2 is probably MAX4236 (difficult to read), that works as the core sensor amplifier.
    TR1 (SST177) probably works as a constant current source (?)
    Why two sensors (oxygen and toxic) are combined and exactly how, is not obvious to me at this glance (a Chemical Engineer should rather explain that function).
    Anyway it's clear that the combined sensor signal is brought to pin 4 of IC1
    There it's filtered, amplified and level shifted (with IC4 it could also be inverted)
    IC5, R14, TR2 and the components in that region form a voltage/current converter and set the output current in the range of 4-20mA

    You could probably make the circuit easier if you use an external power supply instead of feeding from the loop itself.
    Yes, you could use an instrumental amplifier for your sensor. That would probably also simplify the circuit a bit.

    EDIT: Looking again at the sensor part, it seems as if the sensors are connected in a modified sensor bridge circuit, with IC1 supplying the current through R3 to balance the bridge (putting one node at pin4 of IC1 at a virtual VREF level). RP1 adjusts the full scale (20mA) and RP2 the zero point (4mA).
     
    Last edited: Feb 19, 2013
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