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How to build 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. Your desire to build many circuits but not any of the
    simpler current sources is a puzzling combination.
    Maybe nothing. How much of a current source do you want?
    How constant does it have to be versus terminal voltage?
    Your subject indicates you want to build a current source.
    If that is out of the question, one of these parts,
    http://www.rohm.com/products/shortform/26rstr/rstr_index.html
    together with a battery could be called a "current source".

    You can use a JFET, with its gate connected to its
    source, as a current source. Put a variable resistor in
    series with the source to adjust its value downward.
    Somebody used to sell a two-terminal current source
    which was just a JFET hooked up that way.
    It's not.
     
  3. Rich Grise

    Rich Grise Guest

    If a current sink will do, then you can do it with an NPN transistor,
    a couple of diodes, and a couple of resistors.

    "output"
    |
    |= x mA
    |
    C
    +V ---[R1]--+--B
    | E
    V |
    --- |
    | [R2]
    V |
    --- |
    | |
    GND GND

    Where R2 is sized to give .6V at your desired current, R1 is maybe
    1K or so, and you should recognize the two diodes. I think a 2N4401
    can go up to almost a half an amp.

    Turning that into a source, with a PNP transistor, is, of course,
    left as an exercise for the reader. ;-)

    Have Fun!
    Rich
     
  4. Voltage sources produce a fairly constant output voltage for all loads
    from infinite resistance (zero current) to some minimum resistance
    (more than zero current), but they are only approximate voltage
    sources, because their output voltage does sag and current passes
    through them. This is because they have some internal resistance.

    Practical current sources hold a very nearly constant current for all
    loads from zero ohms (zero voltage drop) to some maximum value of
    resistance (some maximum voltage drop called the compliance voltage).

    Ideal current sources would produce a constant current, regardless of
    the load resistance, including infinite resistance, by producing a
    compliance voltage up to infinity volts.

    It is a lucky accident that we have discovered chemical cells that
    produce something close to a voltage source over useful ranges of load
    current. There is no similar simple power sources that provide a
    constant current output.
    The problem is that it is not a current source, but a current source
    in parallel with a fairly low value of resistance. Mathematically
    transforming a voltage source that has a little series resistance to
    the equivalent current source with parallel resistance does not
    actually alter the characteristics of the source, at all.

    Almost all practical current sources are actually active variable
    resistances that waste all the extra voltage from a voltage supply
    that is not needed to force the specified current through the load.
    This active circuit has to be designed to pass the required current,
    while measuring it, in some way, and using some sort of feedback, vary
    an active resistance as needed to hold the measured value of current,
    constant, till the load uses up all the available voltage. These
    active current regulators are specified with their effective parallel
    resistance (equivalent to a perfect current source in parallel with a
    resistor), their voltage compliance, their frequency response, and
    sometimes how long it takes for them to recover current regulation
    after being exposed to an excessively high load resistance and then
    the load falls to within the normal operating range.

    Name your specifications, and people, here will help you design
    current sources (regulators) that meet your needs.
     
  5. Bob Monsen

    Bob Monsen Guest

    You can buy a constant current source. I have a cheap one myself. It is
    basically a power supply with a voltage source and current limit. Turn
    the current to 0, short the output, turn the voltage all the way up, and
    then adjust the current to your liking. You can get them here:

    http://www.mpja.com/

    You can also build a constant current source, given a voltage source. An
    LM317 voltage regulator makes a good one. It tries to keep the voltage
    between it's output and adj terminal at 1.24V. Thus, if you put a
    resistor of value R on the output, and connect the adj terminal after
    it, before the load, the LM317 will try to adjust the voltage so that
    there is a constant 1.24/R A across the load. If you use an R = 100, for
    example, then it will try to keep 12.4mA across the output by varying
    the output voltage. Note that it will only be able to adjust the voltage
    within the limits of the original voltage source, minus some for
    whatever the LM317 + resistor needs, probably 3 or 4 volts.
     
  6. Guest

    Wow, you have all been very helpful. I will print these suggestions out
    and try to figure out which would be best. As far as specifics, I would
    be happy with something that could generate a single DC current of
    ~100mA. In the best of possible worlds, I would love something that
    would let me, through a turnpot, vary the DC current between 0 and 1
    Amp. I don't want anything fancy, just something so that I can play
    with my breadboard while working through an electronics book.

    I guess we are lucky to have batteries. I wonder why no companies sell
    a cheap current source that takes AC in or even uses batteries to power
    it. They could probably make a KILLING with all the EE departments
    around the country! Maybe I'll make one and provide the supply!
     
  7. John G

    John G Guest

    Nobody as asked:-
    Why do you want a constant current supply?

    Most of the electronics experiments you will encounter will require
    constant Voltage and these supplies are every where because everyone
    uses them for most everyting.
     
  8. Take a look at page 16 of the data sheet for the very common voltage
    regulator LM317:
    http://www.national.com/ds/LM/LM117.pdf

    All you need for a variable current regulator is an isolated voltage
    source (battery or wall wart) and a variable, low value resistor, or a
    selector switch that selects various low value resistors.

    The current setpoint is 1.2 volts/resistance. The LM317 may well need
    a heat sink for currents above 100 mA. At 1 amp, the resistor will
    also produce some significant heat ((1.2 volts^2)/resistance).
     
  9. Have you ever gone through basic electronics text book? They are full
    of hypothetical circuits that contain voltage sources and current
    sources, to demonstrate the analytical principles. If you are an
    experimentalist, you need both sources to verify that they are not
    lying to you.
     
  10. Guest

    Have you ever gone through basic electronics text book? They are full
    Exactly, my main motivation. Secondarily, I am a neuroscientist. We
    inject current into neurons all the time, and I want to stop treating
    the stimulator as a black box!
     
  11. kell

    kell Guest

    Whose neurons?
    Hey has anybody seen my foil hat!!?
     
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