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Current source

  1. Jan 13, 2010 #1
    hi all
    I want to know that,do 'current sources '(which supply current) really exist in the world or it is only theoretical concept? if it exists than please give some example.
    thanks
     
  2. jcsd
  3. Jan 13, 2010 #2
    Yes they exist, just like voltage sources. Battery charger is a current source for example.
     
  4. Jan 13, 2010 #3

    berkeman

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    Staff: Mentor

  5. Jan 13, 2010 #4
    Vacuum photodiodes, photomultipliers, and vacuum tube (especially pentode) anodes are good examples of current (meaning electron current) sources.
    Bob S
     
  6. Jan 14, 2010 #5
    Or, if you prefer semiconductors, under appropriate conditions a Bipolar Junction Transistor will behave as a current source (collector current), as will most Field Effect Transistors (Drain current).

    Solid-state photodiodes also behave as current sources, if they are reverse-biased, or at least not allowed too far into forward-bias.
     
  7. Jan 14, 2010 #6
    An inductor tends to maintain its present value of current. When a current is established in an inductor, then a switch is activated forcing the current to change paths, the current does not change abrupltly when the terminal impedance is changed. The voltage abruptly changes. Switching power supplies work in this manner.

    A typical electrical generator can output constant current or constant voltage. Forcing the generator at constant speed results in constant voltage. Forcing constant torque results in constant current. An added benefit of constant speed/voltage is constant frequency as well. This makes synchronous devices feasible, like clocks, or phonographs.

    A battery can be produced for constant current operation, but constant voltage works better. However, nuclear batteries tend to be optimized to produce constant current.

    Current sources are not produced for a good reason. Conduction losses are greater than insulation losses because insulators approach the ideal better than conductors. When turning on a lamp, the wire has a conduction loss due to resistance, and an insulation loss due to leakage in the insulator. But the conduction loss is much greater.

    The power grid is constant voltage because it results in much lower losses. A constant current power grid would mean that full current is always being generated and distributed. To turn a device on, we would open a switch in parallel with the device, and a voltage would develop.

    This would be very lossy. If high temperature superconductors ever become available and cheap, this might happen. For now, constant voltage is way better and all batteries, and generators will be designed as such.

    Does this help?

    Claude
     
  8. Jan 14, 2010 #7

    sophiecentaur

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    There is no such thing as a pure current or voltage source but it is a convenient shorthand (black box) description of many practical devices. Engineers just love the concept of "near enough for Jazz" and there are many working approximations which are used with full justification most of the time.
    Using an amplifier with lots of feedback, you can arrange for a given amount of current to flow over a wide range of voltages - that is what a (practical) current source is; it behaves as if its source resistance were infinite . We are, possibly, more familiar with the idea of a constant voltage source, which will maintain the same voltage across it for a wide range of currents. A car battery is a pretty good voltage source (no appreciable internal resistance - until you actually try to turn a starter motor, at which point it becomes relecant) and so is a stabilised power supply - which, again, uses an amplifier with lots of feedback to behave as it there were no source resistance.
    Over a wide range of operating volts, even the anode of a humble triode valve behaves like a current source because the stream of electrons is already on its way and the Anode can't do a lot to alter that current, which has been set by the other electrodes.
     
  9. Jan 15, 2010 #8
    yes now I have some idea about practical current source.
     
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