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How to get a constant current source?

Discussion in 'Electronic Basics' started by [email protected], Jun 17, 2005.

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  1. Guest

    I have been frustrated for months working through an introductory
    circuits book, as I want to build many of the circuits, but many of
    them include constant current sources. Why is it so easy to find
    voltage sources at your local store (i.e., batteries!), but searching
    for current sources on the web leads to a complicated bunch of circuit
    diagrams?

    Here is my naive question: using thevenin-norton equivalent circuits,
    couldn't I transform a battery (i.e., voltage source) into the desired
    equivalent current source using Vth=InorReq. That is, can I put a
    voltage source in series with a resistor (as opposed to its equivalent,
    a current source in parallel with the same resistor)? What is the
    problem with doing that?

    Does anyone know where I could buy a cheap but reliable current source?
    Why is this so hard?
     
  2. Tom Biasi

    Tom Biasi Guest

    A constant current source would keep the current constant as the load
    changed. If the load were a small percentage of the voltage source's
    internal resistance the small changes in load would be small compared to the
    source resistance and the current would change very little.
    Examine a 12 volt source with a series resistance of 1 Meg. Ohm. How would
    the current change as the load varied from 100 ohms to 1K ohms?
    Tom
     
  3. redbelly

    redbelly Guest

    Because batteries involve chemistry, and it is the nature of the
    chemicals or matierals used to maintain a constant potential
    difference. This is simply how nature works.

    To make a constant current source requires (usually) some sort of
    active feedback loop, to monitor the current output and adjust the
    power source accordingly.

    Mark
     
  4. Guest

    Sorry I posted the same message twice. THe first time something
    happened with my (Windows) browser and I thought it didnt' go through.
    Thanks for your help, and first post looks like it generated some
    suggestions I will have to work through.
     
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