IA problem

Discussion in 'The Projects Forum' started by gurupush, Apr 7, 2014.

  1. gurupush

    Thread Starter New Member

    Mar 21, 2014
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    first of all i would like to thank all you people for helping me out till now with my problems..but still i am unable to run the project,,,..i am posting a picture with what i am trying to do so that it would be easy to understand...i have a load cell whose output is connected to the INA118 and whos output is again connected to LM311(used as an comparator). ihave a dual rail supply with +12v gnd and -12v .

    so what is going wrong, i want to further drive a relay with this circuit using a MOSFET(IRF540) but i cant get there because there is still some problem with this circuit,why that comparato doesnt give output +Vsat and -Vsat...?
    the IA is working Fine


    Theres another thing that i want to mention i created the dual rail supply myself,,so when i put my DMM on DC mode i can see +12 regulated vtg an the leads but when i swith to AC mode for the same leads i get 24v reading on my DMM ,,,Why?
     
  2. gurupush

    Thread Starter New Member

    Mar 21, 2014
    13
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    heres the diagram
     
  3. #12

    Expert

    Nov 30, 2010
    16,248
    6,745
    I think we better start with a drawing of the power supply.
     
  4. gurupush

    Thread Starter New Member

    Mar 21, 2014
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    ok heres the supply circuit
     
  5. #12

    Expert

    Nov 30, 2010
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    That looks perfectly reasonable, but where the 24 volts of AC is coming from can wreck the deal. You really must look into the difference between the drawing, which looks perfectly good, and practice, which gives you 24 volts of AC on the 12 volt output. There are a lot of mistakes that can happen between the drawing and the circuit board.

    First, the tiny capacitors that keep the regulators from oscillating must be connected close to the regulator chips,
    second, they must be correct according to the datasheet for that exact regulator.
    .22 and .1 are right for the LM7812
    The 7912 needs more capacitance.
    While you're in there, be sure to look for simple mistakes.
     
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  6. gurupush

    Thread Starter New Member

    Mar 21, 2014
    13
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    ok i will check again
     
  7. gurupush

    Thread Starter New Member

    Mar 21, 2014
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    it looks like circuit is ok...theres no mistake....but i just dont get it that how am i getting 12v regulated then??? and at same terminals 25v AC?.... i mean the AC between +12 and -12 is 50v....how??
     
  8. RamaD

    Active Member

    Dec 4, 2009
    254
    33
    On normal low cost DMMs in AC mode, the input voltage is rectified, averaged and the resultant DC is measured and displayed. With a DC input at the terminals of the DMM, and DMM in AC mode, Higher figures are normal.
     
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  9. gurupush

    Thread Starter New Member

    Mar 21, 2014
    13
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    ok so ita a DMM prob then,,so i am getting 12v dc then....so why my circuits not working?
     
  10. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    Because you left out the resistor between pins 1 & 8 of the INA.
     
  11. #12

    Expert

    Nov 30, 2010
    16,248
    6,745
    That part fooled me. Even the old Simpson 260's (in the 1960's) had a coupling capacitor for AC measurements to stop the DC from getting in.
     
  12. gurupush

    Thread Starter New Member

    Mar 21, 2014
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    @eirne
    No I didnt..I just forgot to draw it....bt I mentioned there 10ohm for gain 5000
     
  13. RamaD

    Active Member

    Dec 4, 2009
    254
    33
    Cost cutting! Saving One coupling cap rated higher than the highest AC voltage allowed and the contact for switching it through the switch!
     
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