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- Newsgroups: sci.electronics
- Path: sparky!uunet!melpar!doherty
- From: doherty@melpar (Kevin Doherty)
- Subject: Re: Help: What is an Independent Current Source!?
- Organization: E-Systems, Melpar Division
- Date: Mon, 9 Nov 1992 13:23:18 GMT
- Message-ID: <BxGAIv.3qo@melpar>
- References: <Nov.9.00.11.39.1992.9368@clam.rutgers.edu>
- Lines: 49
-
- 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
-