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Voltage Controlled Current Source

  1. Aug 26, 2008 #1
    guys, i need some help...

    i'm looking for a voltage-controlled current source...

    i found this: http://electronicdesign.com/Articles/ArticleID/9018/9018.html

    but when i try it for a maximum current of 500mA (using LM324N for the opamp and 2N2222 for the transistor), it doesn't work...

    help me pls...
  2. jcsd
  3. Aug 26, 2008 #2


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    does the current source sink to ground? or are both terminals floating?
  4. Aug 26, 2008 #3
    you mean the output terminals?

    i was thinking of inserting an ammeter on the output side to get the maximum current reading..
    +Vout > Ammeter > -Vout > Zener > -Vcc > Ground

    or you're talking of something else? sorry...
  5. Aug 26, 2008 #4


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    Staff: Mentor

    Several issues.... First, if you want 500mA, you will need a lot bigger transistor than the 2N2222. That transistor is more like a 100mA transistor (if that). Second, you mention a LM324, but the app note shows an LM321 precision opamp. The LM321 is meant for split supply operation, and can handle the +/- 16V supplies in the diagram. The LM324 is optimized for single supply operation, and can only handle +/-15V max. If you gave it +/-16V, it is toast.

    Also, the output reference voltage of that figure is a bit strange. Can you just have your current source work with respect to ground? What output voltage range does the current source need to be able to work with (that's called its "compliance")?
  6. Aug 27, 2008 #5
    attached is the modification i made to the circuit, on the ammeter it reads 490+ mA... but when i try to do it on the breadboard, i get 0 A... pls advice :(

    Attached Files:

  7. Aug 28, 2008 #6
    guys, if any of you have a design or circuit for a voltage controlled current source having a maximum current output of 500mA... no matter how simple or tedious, i would highly appreciate it... honestly i'm next to being desperate :(

    thanks for the replies though...
  8. Aug 28, 2008 #7
    Try this,
    Hook up a precision resistor to monitor the current being consumed(drop).
    Use a precision opamp as differential amplier for feedback/stabilisation.
    You can use a transistor to sink the current at the output.
  9. Aug 28, 2008 #8
    Or else you can go with a combination of LM723 and 2N3055. you should be able to tune the circuit for 500mA.
    correct me if wrong
  10. Aug 28, 2008 #9
    I tried some more modifications. Now, though a little less than theoretical, it outputs something...

    For better accuracy, I'll try what you suggested...

    Thanks :)
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