Current Source.

Discussion in 'General Electronics Chat' started by electron_prince, Oct 24, 2012.

  1. electron_prince

    Thread Starter Active Member

    Sep 19, 2012
    What is a current source? Don't give definition. I know it.
    Battery is a voltage source. so what is a current source? transistors?
  2. bertus


    Apr 5, 2008
  3. MrChips


    Oct 2, 2009
    A voltage source should have low internal resistance while a current source has high internal resistance.
  4. GetDeviceInfo

    AAC Fanatic!

    Jun 7, 2009
    inductors passing through a magnetic field.
  5. crutschow


    Mar 14, 2008
    That would make a poor current source since the resistance is low. A current source has a very high (ideally infinite) output impedance.

    An example of a near ideal current source that isn't electronic is a static machine such as a Van de Graaff generator. They generate a small constant-current which builds up voltage in the collector until it arcs over.
  6. cabraham

    Active Member

    Oct 29, 2011
    Current source examples are abundant. The power plant generator is forced to spin at constant speed resulting in constant voltage, as well as constant frequency. If the generator was forced to spin at constant torque, the output would be constant current w/ variable voltage & frequency.

    A constant current battery can be produced, but its performance is much less than that with constant voltage. Nuclear batteries, however, exhibit better behavior with constant current. But nuclear batteries, likely for safety reasons, are not on the market.

    A photodiode across a low impedance is a current source. It is usually connected across the inputs of an op amp. It delivers a constant current proportional to the incident light intensity. I've used many of them. They have a high shunt impedance, tens to hundreds of megohms, which is desirable for a good current source.

    An inductor is a crude current source. It maintains its present value of current. If the load resistance suddenly changes, the current is maintained while the voltage varies. Eventually it gives up its internal energy. An LED driver works based on this behavior.

    A lightning stroke is a current source. Typically it can be 20,000 amps, or more. Did I help at all?
    Last edited: Oct 24, 2012
  7. electron_prince

    Thread Starter Active Member

    Sep 19, 2012
    yes you helped. I am just wondering why current sources are used so frequently in circuit analysis when we don't have any good current source.

    I was confused because circuit analysis is laid on two basis laws
    1) Law of conservation of energy
    2) Law of conservation of charge.

    If an ideal current source exist, then people would exploit it for producing large potential difference by connecting a large load across it. But as energy can't be created it's impossible to deliver infinite amount of energy.

    so i doubt the existence of current source or any source which is close to it.

    btw, lightening and the turbine which is allowed to spin at constant torque are good examples.
  8. crutschow


    Mar 14, 2008
    Current sources are used in circuit analysis because they can simplify the simulation of the circuit, for example injecting an AC current into a closed-loop circuit to check the loop stability without changing the loop dynamics.

    In actual integrated circuits constant currents are often provided by current mirrors.

    Of course the law of conservation of energy and charge applies to current sources, the same as voltage sources. A current source requires energy to generate the current.

    A Van de Graaff generator is actually quite close to an ideal current source. It will generate current (in the mA region) to charge the sphere capacitance until the breakdown voltage of the air is reached.
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  9. ramancini8

    Active Member

    Jul 18, 2012
    A large value resistor (1M) acts like a current source when connected between Vbb ans The base of a transistor (emitter grounded).
  10. Jeffrey Samuel

    New Member

    Oct 25, 2012
    Yeah very true but the internal resistance is parallel in current source to maximise the admittance to the output
  11. cabraham

    Active Member

    Oct 29, 2011
    How can you doubt the existence of something that has been produced many times? Like its voltage source counterpart, it cannot output more power than its own input receives. If a generator is spun at constant torque for constant current, the output voltage is determined by current source value times load resistance.

    The output power is I*V = P. To maintain this power, the turbine spinning the generator must provide mechanical power equal to P plus enough to cover losses. Likewise to achieve this mechanical power some fuel must be burned. If the load power demand is more than what the turbine can provide, then the output will fall out of regulation, i.e. the current value drops since there is not enough power driving the generator to maintain the target current.

    With a voltage source, it is just the same. If the demanded load current times the fixed voltage, i.e. power demanded, is greater than what the turbine can provide, the output will fall out of regulation. Conservation laws apply to both types of power sources, CC or CV.

    Does this help?

    Last edited: Oct 25, 2012
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  12. JMac3108

    Active Member

    Aug 16, 2010
    There are lots of very good current sources and they're used in practical designs all the time.

    No. Just as a real voltage sources can not deliver infinite current, a real current source can't deliver infinite voltage. Eventually the real voltage source runs out of ability to deliver current, and its output voltage is reduced. In the same way, a real current source runs up against the max voltage it can supply, and its output curent is reduced. In the current source this voltage limit is called compliance.
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