build own circuit with op amp or voltage current converter IC

Discussion in 'General Electronics Chat' started by kevin0228ca, Jun 5, 2015.

  1. kevin0228ca

    Thread Starter Member

    Jun 5, 2015
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    0
    I am trying to create a constant current output for my project between 0 - 10 mA.
    My output signal is 5-30v square wave and I am trying to convert to 0-10 mA.
    I read we can convert voltage to current by using op amp
    http://www.allaboutcircuits.com/textbook/semiconductors/chpt-8/voltage-to-current-signal-conversion/
    but I also found IC http://www.ti.com/lit/ds/symlink/xtr110.pdf
    so for voltage current conversion do I have to use a IC like xtr110 or should I use a op amp?
    Assuming I am new to general electronics, and have budget.
    Also, is there anything I should keep in mind when using IC

    Also, I am using AD822 op amp, and I am seeing weird behaviour with a simple circuit.
    [​IMG]
    for inverting I get correct Vout = (Rf/Rin) x Vin
    but for

    [​IMG]
    My Vin is 5v, Rg = 10K, Rf = 20K, should Vout should max at Rf/Rg x Vin + 1 = 11V
    but my Vout is exceeding 11V as Vcc increase, and - pin is also increasing with Vcc but max at 5V.
    I read that op amp +pin -pin should have same voltage.

    Sorry for my newbie question.
    Can anyone clarify?
    Thank you.
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    I would recheck your non inverting equation.
    E
    Vo = Vin(1+Rg/Ri)
     
  3. kevin0228ca

    Thread Starter Member

    Jun 5, 2015
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    oops, just verified and Vout max at 15v so should be correct.
    also, I just read wiki
    Equilibrium will be established when Vout is just sufficient to "reach around and pull" the inverting input to the same voltage as Vin.
    So I guess is normal that - pin voltage is changing and max at 5V?
     
  4. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
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    The AD822 is a R2R OPA, so with 5Vin and Vcc of 15V, the Vo max should be close to +15V, when Rf=20K and Rg=10K.
     
  5. Bordodynov

    Active Member

    May 20, 2015
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    Do Rg = Rf = 10kOhm.
    Vin = 5V-> Vout = 10V.
    Operational amplifiers often use to align the dynamic range.
     
  6. kevin0228ca

    Thread Starter Member

    Jun 5, 2015
    30
    0
    Thank you, I think my op amp is working.
    However, when I build
    [​IMG]
    I use 200 ohm instead of 250, and my Rload is 20K, Vin = 5V
    But V at -pin becomes 0, and 0 mA instead of 5/200 = 25mA.
    So my op amp cannot act as voltage current converter?
     
  7. kevin0228ca

    Thread Starter Member

    Jun 5, 2015
    30
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    oops 0 mA should be correct assuming all current flow through loop.
    but I get 1 mA loop instead of 25mA.
     
  8. Bordodynov

    Active Member

    May 20, 2015
    643
    188
    Use not 200 Ohm, use 20kOhm resistor.
    If you used a perfect operational amplifier with supl. voltage 550V,
    your electronic circuit would have worked.
    In load would be allocated power
    P = (25mA * 20kOhm) * 25mA = 500V * 25mA = 12500mW = 12.5W.
     
  9. kevin0228ca

    Thread Starter Member

    Jun 5, 2015
    30
    0
    I just found answer
    op amp should at least have enough Vcc
     
  10. kevin0228ca

    Thread Starter Member

    Jun 5, 2015
    30
    0
    Thank you.
    I just read that Vcc should be enough to give Rload 25 mA.
    So suppose I want 25mA, and my Rload is 10K, My vcc should be 250.
    Since my Vcc is 30V, my Rload max is 1200.
     
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