current voltage source dilemna

Discussion in 'General Electronics Chat' started by twainerm, Jan 12, 2016.

  1. twainerm

    Thread Starter New Member

    Nov 30, 2013
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    Hello All,
    I am confused between current and voltage source. I understand textbook definition and that there is no ideal source. What I do not understand is that we can't produce current without voltage, in that case is not current source also a voltage source? If we connect load to a voltage source, current flows through the load, then is not voltage source also a current source.
    Unfortunately none of the textbooks talk about this.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Yes, either supply can generate both voltage and current (and will do so with a load).
    The definitions define which is the controlling function of the supply:

    An ideal voltage source maintains a given voltage independent of the load resistance ( which then determines the current).
    Most real power supplies are of this type within their particular voltage and current limits.

    An ideal current source maintains a given current independent of the load resistance (which then determines the voltage).
    Real current sources are less common than voltage sources but they are used for things like powering LEDs which require a constant current for proper operation.
    A real voltage power supply can act like a current supply if the voltage is set to its maximum and it has a current limit setting.
    In that case the current will remain constant within the voltage limits independent of the load resistance.

    Make sense?
     
  3. dannyf

    Well-Known Member

    Sep 13, 2015
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    An ideal voltage source is one whose voltage output is independent of the load put on it;
    An ideal current source is one whose current output is independent of the load put on it;

    Alternatively, you can think of a current source as a voltage source with infinite output impedance.
     
  4. BillB3857

    Senior Member

    Feb 28, 2009
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    se·man·tics, se·man·tics I think dannyf really means ZERO output impedance. An infinite output impedance would result in zero current available. Maybe I'm wrong, and if so, someone please enlighten me.
     
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  5. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi,

    I've seen this question come up several times in the past when people just start to use batteries and current sources and stuff like that. It's a good question because either power source CAN put out BOTH current AND voltage.

    A simple view is like this...
    An ideal voltage source has a set voltage even with no load.
    An ideal current source has a set current even into a short circuit.

    The internal/external view goes like this...
    Ideal voltage source:
    The voltage of an ideal voltage source depends only on the internal construction of that source, while the current depends only on the external circuit. So the voltage is due to the internals, while the current is due to the externals.
    Ideal current source:
    The current of an ideal current source depends only on the internal construction of that source, while the voltage depends only on the external circuit. So the current is due to the internals, while the voltage is due to the externals.

    So you can see the swapped roles of each source.

    An example of a voltage source is a large 12v battery. If you connect a 1k resistor to it, it puts out 12v and if you connect a 100 ohm resistor to it it still puts out 12v. No change in the voltage.
    There is one exception to this rule but this applies ONLY to ideal sources. If you short circuit the voltage source (sometimes called "killing" the voltage source) the voltage then goes to zero. You can not do this with a real voltage source unless it is current protected, and batteries can blow up if you do this, so this ONLY applies to ideal voltage sources which dont exist in real life.

    Ditto for the current source, except the current never changes not the voltage. And here for the ideal source ONLY the exception is an open circuit, where the current goes to zero (sometimes called "killing" the current source).
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Yes to both questions. All voltage sources produce current, and all current sources exhibit an output voltage. What is confusing you is the technical definition of the word "source."

    In electronics, a voltage source is not just a source of voltage. The term "voltage source" is shorthand for a specific set of characteristics, how the output of the source changes with different loads. Same for the term "current source". At the theoretical or instructional level we talk about "ideal" voltage and current sources to convey the ideas described above. Of course, real world devices are not ideal, and that is where discussions about power sources get down into the circuit design details.

    ak
     
  7. crutschow

    Expert

    Mar 14, 2008
    12,991
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    Sorry, you are wrong. And it has nothing to do with semantics. ;)
    An ideal current source has an infinite output impedance so that any change in output voltage due to a load change has no effect on the output current. Thus the output impedance appears to be infinitely high.
    This can be approximated with electronic feedback in a power supply in constant-current mode, which can appear to have an output impedance of many megohms within its current and voltage limits.

    Thus an ideal current source has infinite output impedance and an ideal voltage source has zero output impedance.
     
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  8. WBahn

    Moderator

    Mar 31, 2012
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    In general, a source supplies both voltage and current (since both are required in order to have non-zero power delivered to a circuit). The difference is that a voltage source, within limits, will produce whatever current, positive or negative, is needed in order to maintain a fixed voltage across it's terminals while a current source, within limits, will produce whatever voltage, positive or negative, is needed in order to maintain a fixed current between its terminals.

    A voltage source is happy supplying zero current and a current source is happy supplying zero voltage. In either case the source provides no power to the circuit it is connected to.
     
  9. WBahn

    Moderator

    Mar 31, 2012
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    Yes and no. In order to think of a current source as a voltage source with an infinite output impedance, you also have to have an infinite source voltage. Since the current is V/R and both are infinite, you have an indeterminate current. You can imagine using L'Hospital's rule to determine the ratio of the derivatives of each and end up with a finite current that happens to be the setpoint of the source. Another way to imagine it is that whatever the resistance is, the voltage is Vo=Io·R. As R goes to infinity so does Vo, but is such a way as to keep Io fixed.

    This is a simple and sometimes sufficient way to make a current source. For instance, imagine you have a load that varies from 10 mΩ to 1 kΩ and you want to have 10 mA of current through it within 1%. What happens if you take a 1000 V source and put 100 kΩ resistor in series with it? Ignoring supply and resistance tolerances, at 10 mΩ you get 10.00 mA of current and at 1 kΩ you get 9.90 mA through it. That's a 1% variations for a load whose resistance varies over five orders of magnitude!

    If your load varies over a much smaller range and your regulation tolerance is not as tight, then what constitutes an "infinite output impedance" and an "infinite voltage" is much more relaxed. This is exactly what we are doing when we size a resistor for an LED -- we are making a poor, but sufficient, current source. It's also how we generally get the current reference for current mirrors.
     
  10. hp1729

    Well-Known Member

    Nov 23, 2015
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    What is voltage?
    Voltage is an imbalance in the atomic structure. More electrons than protons and we have an atom with a negative voltage. Fewer electrons than protons and we have an atom with a positive voltage. Voltage does not flow. It is a static condition at some point in a circuit relative to some other point.
    What is current?
    Current is the flow of electrons from atom to atom in an organized fashion. Electrons are always in motion from atom to atom but in a random direction. When we apply a voltage across a conductor we get the electrons to flow from the more negative side to the more positive side.
     
  11. Picbuster

    Member

    Dec 2, 2013
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    Assume that electricity behaves like water.
    Voltage is the height of the water coulomb sitting there doing nothing.
    Current is when you punch a hole in the bottom water is flowing out.
    The water current is depending the size of the hole and the height .
    It is evident that the hole creates resistance smaller the hole less water.
    So height of water/ resistance = water current. ===> height of water/resistance = water current

    Bck to electronics
    Voltage/resistance = current the rest you can figure out yourself.
     
  12. hp1729

    Well-Known Member

    Nov 23, 2015
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    Bad analogies. Come on ,get down to actual facts. What is voltage, current and resistance at the atomic level?
     
  13. WBahn

    Moderator

    Mar 31, 2012
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    Voltage is due to a separation of charge (which results in an electric field that gives rise to the voltage potential). The charge separation can come about through a variety of processes including chemical, magnetic, photovoltaic. The actual amount of charge separation that is required to create a voltage difference is miniscule in most cases.
     
  14. BillB3857

    Senior Member

    Feb 28, 2009
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    Thanks for clearing that up for me, crutschow! Never too old to learn something. I was looking at it from the point of a constant current source feeding a zero ohm load needing to have a zero ohm impedance, but your explanation cleared it up for me.
     
  15. WBahn

    Moderator

    Mar 31, 2012
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    Even there you need the current source to have an infinite output impedance. Remember, the output impedance of a current source is in parallel with the ideal current source, not in series with it.
     
  16. hp1729

    Well-Known Member

    Nov 23, 2015
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    Good. Not incorrect. Just not complete. Yes, anything that knocks an electron out of place creates a voltage.
     
  17. WBahn

    Moderator

    Mar 31, 2012
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    One thing to keep in mind is that a voltage does not absolutely require a charge separation -- specifically, a changing magnetic field creates an electric field even in a vacuum and the presence of an electric field is sufficient to produce a voltage potential.
     
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