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Exist current source?

  1. Jun 6, 2014 #1
    Which is the real interpretation for current source? Exist some device or electrical object that acts like a current source?
     
  2. jcsd
  3. Jun 6, 2014 #2

    phinds

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    Yes, you can design circuits that act as current sources. The interpretation for "current source" is EXACTLY what it says ... it sources (supplies) current.
     
  4. Jun 6, 2014 #3
    I'm asking for REAL ELECTRICAL OBJECTS...
     
  5. Jun 6, 2014 #4

    adjacent

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    Battery
     
  6. Jun 6, 2014 #5

    phinds

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    No, a battery is a voltage source. Completely different thing from a current source.
     
  7. Jun 6, 2014 #6

    phinds

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    I have no idea what you are talking about.
     
  8. Jun 6, 2014 #7

    ZapperZ

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    How about putting on some effort in explaining clearly what you want? It is obvious even to you by now that what you had written is vague. I can show you a photocathode, which is MY "current source" but I have a feeling that this isn't what you want.

    Remember, we can't read your mind.

    Zz.
     
  9. Jun 6, 2014 #8
    Batteries can be built to provide constant current or constant voltage, it just happens that constant voltage works better for most primary cells. Nuclear batteries are built for constant current because performance is better. Constant voltage is best for alkaline, lead-acid, etc., so that is how it's made.

    A generator, like the turbine in a power plant, can be spun at constant torque for current source operation, but they spin them at constant speed for voltage source operation. Generating full voltage and allowing current to vary with loading produces less loss. If we generate at full current all the time, with voltage varying with load, losses are greater.

    Also, constant turbine speed means constant frequency, allowing synchronous motors to run at fixed speed, and generators on the grin in parallel can be synced. Constant voltage is better, but constant current can be produced at the power plant.

    An inductor is naturally a current source. If an inductor carries a current I, then the impedance across it is suddenly changed, I will remain, V will change abruptly. A capacitor is a voltage source, the opposite.

    Claude
     
  10. Jun 6, 2014 #9

    nsaspook

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    A lightning bolt is close to a perfect current source during the discharge. It will generate whatever potential is needed to maintain current flow, up to a billion volts with currents up to the 100kA range.
     
  11. Jun 6, 2014 #10

    phinds

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    Cool. I did not know that. Thanks.
     
  12. Jun 6, 2014 #11
    My question is simple: if the battery is able to generate voltage, ie, the battery is a voltage source, so which is the electral device that is able to generate current, ie, a current source. Exist current source in the physical world or current source is just a theoretical device?
     
  13. Jun 6, 2014 #12

    Drakkith

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    Cabraham gave some good examples of possible current sources. Note that it is not possible to generate current without generating voltage. A voltage source generates a steady voltage and the current changes depending on the circuit resistance. A current source generates a variable voltage that attempts to keep the current flow in the circuit steady.

    Edit: The following article has some examples and explanations of real current sources: http://en.wikipedia.org/wiki/Current_source
     
    Last edited: Jun 6, 2014
  14. Jun 6, 2014 #13

    CWatters

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    Many NiCad battery chargers deliver a constant current to the battery. It could be dangerous to charge a NiCad using a constant voltage source.

    Constant current sinks are used in some amplifiers designs. For example the input stage of an op-amp typically contains a constant current sink.

    Some motor controllers deliver a constant current rather than a constant voltage.

    Bench power supplies normally deliver a constant voltage but they typically have a current limiter built into them to protect themselves against a short circuit or overload. Sometimes the current limit is adjustable and it's possible to use them as a constant current source. However you need to be careful as it's possible to overheat them if used in this mode.

    Believe it or not some incandescent light bulbs behave a little bit like constant current devices. This is because the resistance of the filament is temperature dependant. If you try and push more current through the filament by turning up the voltage the filament heats a bit more, and the resistance increases. This means the current doesn't go up quite as much as it would if the filament was an ideal resistor. However they are far from "ideal" constant current devices.
     
  15. Jun 6, 2014 #14

    phinds

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    Have you even been reading the answers give to you so far?
     
  16. Jun 6, 2014 #15

    Drakkith

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    I just wanted to touch on this real quick. There are no "ideal" current or voltage sources in real life. An ideal current source would be able to supply a steady current to any load, no matter how high the resistance, implying that it could put out any amount of power. This is obviously not the case in real life.

    An ideal voltage source would function in a similar manner in that it would be able to sustain its voltage across a short circuit, implying that it could generate infinite current and power. This is, again, obviously impossible in real life.

    All real voltage and current sources have limitations as to where and when they can sustain their voltage/current. Within these limitations the source acts as an approximation of an ideal current or voltage source. If the circuit exceeds these limitations then the source can no longer sustain its voltage/current properly.

    Also, there is no clear cut difference between real voltage and current sources. All current and voltage sources behave non-ideally and can have properties of each other given the right circumstances. A voltage source like a battery can be made into a current source by using the correct equipment. Wikipedia's article that I linked earlier gives an example of a battery in series with a resistor and a load device. If the resistance of the resistor is much higher than the load, then the load can vary in resistance and the battery will still supply almost exactly the same amount of current.

    To quote wiki: http://en.wikipedia.org/wiki/Voltage_source#Comparison_between_voltage_and_current_sources

    Since no ideal sources of either variety exist (all real-world examples have finite and non-zero source impedance), any current source can be considered as a voltage source with the same source impedance and vice versa. Voltage sources and current sources are sometimes said to be duals of each other and any non ideal source can be converted from one to the other by applying Norton's or Thévenin's theorems.

    So to answer your question as to whether current sources exist, yes, they do if you build the circuit correctly.
     
  17. Jun 7, 2014 #16

    ZapperZ

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    A thermionic cathode is an example. So there.

    Zz.
     
  18. Jun 7, 2014 #17

    jasonRF

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    http://en.wikipedia.org/wiki/Current_source

    Yes - current sources exist. You can design and build a relatively simple circuit that is powered by a constant voltage source but performs like a constant current source. Also, in transistor circuits it is often desirable to use current sources to bias transistors due to the high impedance (ideal current sources have infinite impedance, real are finite of course). Such a current source can be easily made on a breadboard with a transistor, a diode, and a couple of resistors, but often they are more complex to make more the source more ideal. These are a basic building block in integrated circuits, and I am sure that your computer/phone/tablet that you are reading this on has many many current sources of this kind inside.

    jason
     
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