🚀 go-pugleaf

steuer@clam.rutgers.edu (robert Steuer) writes:

>I understand an Independent Voltage source is like a battery and I
>understand that an Independent Current source can supply a current,
>but what is a REAL-LIFE example of an Independent Current Source?
>I have asked fellow students, my teachers, even a teacher with a
>Doctorate Degree and still I get hmmm, thats a good question!

If you're asking about a constant current source, there are quite a
number of ways of doing it.

For relatively low currents, an op amp will do the trick:

                    +-----------------------O
          I1 -->    | |\               <--\
                    | |  \                | I2
    Vref O---\/\/\--+-+ -  \            --/
               R      |     >---------------O
                +-----+ +  /
                |     |  /
                |     |/
               ---
                -
  To keep the negative input at a virtual ground, the amp outputs a
current equal and opposite to the input current (ie, I2=I1).  By using a
well regulated reference voltage one can set the output current with R
(Iout = Vref/R).

  A higher current alternative (up to 1.5 amps) can be kludged out of a
common 78XX series voltage regulator.  These devices are feedback based
linear voltage regulators; the regulator tries to keep its output pin
exactly XX volts above its reference pin.  The following circuit:

                   +---------+
                   |         |    R
     Vin O---------+IN    OUT+--/\/\/--+-------O Iout
                   |   REF   |         |
                   +----+----+         |
                        |              |
                        +--------------+       
will take an unregulated voltage Vin and provide a regulated Iout; it does
this by generating whatever output voltage is necessary to maintain its
rated voltage across the resistor R.  The output current is thus the rated
voltage divided by the feedback resistor.  Some restrictions: the voltage
compliance (ie, max voltage it'll generate to try to force the desired
current) is only Vin minus the rated voltage minus about 3 volts; also, the
78XX family is only good for maybe 32 volts Vin.  Also-use a power resistor.

							KMD