Constant current source

Discussion in 'Homework Help' started by drago, Sep 21, 2009.

  1. drago

    Thread Starter Member

    Aug 12, 2009
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    I have built constant current source using ua741 amplifier and npn BJT bc639. Circuit gives constant current of 100mA. My new problem is how to control that output current from let say from 20 to 100mA by output voltage. The whole project goes like this; the circuit has to give constant current (max. 100mA) which amount has to be regulated from outside with voltage.
     
    Last edited: Sep 21, 2009
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Post an image of your schematic, preferably in .PNG format.

    You can attach it to your first post by using the Edit button, and then click the Go Advanced button below the text box. On the next screen, click the Manage Attachments button, navigate to your image on your computer, select it, and then click the Upload button.
     
  3. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
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    constant current sources are typical in that they regulate voltage over a fixed resistance. It then becomes a matter of summing a variable reference voltage into your circuit.
     
  4. drago

    Thread Starter Member

    Aug 12, 2009
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    The load is changing between 0.5 and 2kΩ. the basic scheme and file for multisim simulator is in attachment.the current through load is constant and the problem is to control the amount of that current (max 100mA) with voltage.i think that should be an another circuit that control current trough the load.

    i have no idea how to build that and mentor will not help me he said the task is:constant current source max 100mA the load variate between 0.5 and 2kΩ the amount of that current must be regulated by voltage 0 to 1V from another source, something like input signat to the circuit.
     
  5. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    When you say 2 K Ohms at 100 mA the voltage must be at least 200 Volts for the driver circuit.
    This can not be done with the simple 741, as that can work on max ± 15 Volts.

    Greetings,
    Bertus
     
    Last edited: Sep 21, 2009
  6. drago

    Thread Starter Member

    Aug 12, 2009
    14
    0
    sorry guys it is 10mA
     
    Last edited by a moderator: Sep 21, 2009
  7. SgtWookie

    Expert

    Jul 17, 2007
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    To get 20mA current through a 2k resistor will require a Vcc of 40v.
    To get 100mA current through a 2k resistor will require a Vcc of 200v.

    You wouldn't be able to use the current source schematic, as the 741 wouldn't tolerate a high enough Vcc.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    So, what is the new current range? 2mA to 10mA?

    Oh, is this a school homework assignment?
     
  9. drago

    Thread Starter Member

    Aug 12, 2009
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    Yes it is may first home work!!! and yes again 2mA to 10mA Vcc is +-15V
     
  10. beenthere

    Retired Moderator

    Apr 20, 2004
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    The numbers still won't work. I = E/R. 15 volts and 2000 ohms can only produce 7.5 ma.
     
  11. Wendy

    Moderator

    Mar 24, 2008
    20,765
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    Drop the emitter resistance. Use the + side of the op amp as a control signal (voltage in, current out).
     
  12. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    There is a way to get 10 mA ....

    see attachment
     
  13. drago

    Thread Starter Member

    Aug 12, 2009
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    0
    Vcc for bc639 is 15V.Vcc for 741 is +-15V. Rc is 100Ω and Re vary from 0.5k to 2kΩ. The current Ic is constant and i am getting 11mA. What am i missing...
     
  14. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Let's see ... check your measuring device's specification ... or ... it's probably out of calibration.

    15V / 2000 ohms = 7.5 mA ... it's not called Ohm's suggestion, it's Ohms LAW


    Post your diagram.
     
    Last edited: Sep 22, 2009
  15. Mu86neer

    Member

    May 1, 2009
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    HERE IT IS ,,,
    try to built the circuit in the attachment and then i can show you the calculations.
     
  16. hgmjr

    Moderator

    Jan 28, 2005
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    Isn't the gain of the first stage -10 rather than -11?

    hgmjr
     
  17. Mu86neer

    Member

    May 1, 2009
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    yes ,as long as its connected to the non inverting input of the opamp ( Av=-Rf/Rin)
     
  18. hgmjr

    Moderator

    Jan 28, 2005
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    If you take a look at this presentation the inverting gain is Av = -Rf/Rin while the non-inverting gain is Av = 1+ Rf/Rin. I think you may have it backwards.

    hgmjr
     
  19. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
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    Mu86neer,

    Nice that you are using virtual devices. Try it with a spice model of something you'd really use ... complete with the +/- voltages.

    If using the virtual devices were allowed, the sky would be the limit ... if any limitations were imposed.
     
    Last edited: Sep 23, 2009
  20. Mu86neer

    Member

    May 1, 2009
    23
    0
    WOW that was impressive..hehe
    well, could u plz tell me what if i feed the input with 12V whereby the gain is -10 so isn't it irrational to get an output of -120Volts...?????
    i could have been killed in the lab if it was so.!!!!!
     
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