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Ideal sources

Introduction to the voltage source and current source. Created by Willy McAllister.

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  • blobby green style avatar for user oscarsamuel
    What software did you use to do the videos?
    (2 votes)
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  • starky sapling style avatar for user Vatsal Kejriwal
    How can current be constant it is changing everywhere due to resistance
    (3 votes)
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    • leaf green style avatar for user _phidot_
      IMHO it can. :)

      (my personal) Definitions

      Voltage sources : Deliver fixed voltage regardless of current needed.

      Current sources : Deliver fixed current regardless of voltage needed.


      Resistance change? No problem for the sources, their power delivery systems (power control circuit/regulator) ensures that it will works just fine. Example : electrical lab power power supply (http://my.rs-online.com/web/c/test-measurement/bench-power-and-power-measurement/bench-power-supplies/).

      Since P = VI

      A voltage source will supply the needed power by varying the supply current, and fixing the supplied voltage, depending on the total resistance encountered. A current source will do the same by varying the supply voltage, and fixing the supplied current, depending on the total resistance encountered.

      hope that helps.
      (7 votes)
  • leaf green style avatar for user Robert Opalko
    According to the plot of the current and voltage it appears they can have negative values? What does it mean to have negative voltage or current?
    (1 vote)
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    • spunky sam orange style avatar for user Willy McAllister
      Voltage is a relative measurement, just like measuring elevation is a relative measurement. When you measure elevations the habit is to say sea level = 0 elevation. If you go up a mountain you get positive elevation. If you take a submarine under the ocean you have negative elevation (also called "depth"). Voltage works the same way.

      Current can be positive or negative. If you look at the symbol for the ideal current source you see it has an arrow. That defines which way positive current flows. If you assign a value of I = -2mA to the ideal current source, it means 2mA flows in the opposite direction of the arrow.
      (8 votes)
  • starky ultimate style avatar for user Elijah Gnuse
    I have a question.

    At about , you start talking about graphing the lines of the ideal sources. You mention that the voltage can fluctuate even though the current stays constant, and the current can fluctuate but the voltage stays constant. How is this possible?
    In my understanding (which is unfortunately limited), the two are linked. The greater the voltage, the greater the current, because there is more charge pushing and therefore the charge moves faster. This must be wrong if one can change while the other stays constant. Did I misunderstand the definition of voltage?
    (1 vote)
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    • spunky sam orange style avatar for user Willy McAllister
      You are mixing together the concept of an ideal source (v or i) with the concept of a resistor. These are two different animals.

      A resistor obeys Ohm's Law, v = i R. If R is constant, then v and i scale up and down together.

      An ideal source is a mathematical model of a thing that holds either voltage or current constant. It is not a resistor, so Ohm's Law does not have to be true.

      A battery is an example of a real voltage source. Over a reasonable range of currents, the voltage of a battery does not change. A small AA battery producing 1.5v will hold that voltage constant for currents between 0 and say 20ma. At higher currents the voltage may droop down a little, but not too much. The ideal mathematical model for a voltage source does a pretty good job describing a battery, as long as you don't try to draw a giant current out of it.
      (4 votes)
  • duskpin ultimate style avatar for user UnrealDreamer989
    What applications would be needed for either constant voltage or current source?
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    • purple pi purple style avatar for user APDahlen
      Hello Dreamer,

      A lithium ion battery charger is a good example that contains both a Constant Current (CC) and a Constant Voltage (CV) source. See a data sheet here:


      When charging begins the current is initially limited to a safe value via a circuit operating as a CC source. For this particular cell look for a CC of 1375 mA in the data sheet. Later in the charging cycle the voltage will reach 4.2 volts. When this happens the charger will switch to CV mode and hold at 4.2 volts until the current drops below 100 mA. You can see these curves in the "charge characteristics" section of the data sheet.

      For another answer you may want to look here:


      In particular, take a look at the DIY op-amps:


      In this discussion you will find advanced discussions about CC sources.

      Please don't get discouraged. It takes years of study to understand all of the implication on this website.

      Perhaps you could build one of these amplifiers. The ZEN would be an interesting amplifier to construct as some versions are built using a CC source.

      Happy soldering!


      P.S. While we are talking about audio you may want to look at these links:


      (2 votes)
  • leaf blue style avatar for user Junaid Shaikh
    what does the term constant means in the circuit ?
    (1 vote)
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  • leaf green style avatar for user Alexander Wu
    Do current sources produce an emf like voltage sources? Do they produce electric field in a wire? Or does the usual electromagnetism no longer apply here?
    (1 vote)
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    • leaf green style avatar for user _phidot_
      current sources produce an emf?
      > All charge interaction (generation/flow/change) WILL produce emf.

      produce electric field in a wire? >
      > if there is a current flow in the wire, there IS an electric field, regardless of source type.

      the usual electromagnetism no longer apply here?
      > electromagnetism applies everywhere, here is included.

      p/s : just because it was not mentioned, doesn't mean it is not there. For the sake of syllabus, simplicity is preferred. When we deal will the real world, we cannot ignore earth gravity in our experiment/machine due to its magnitude. But this doesn't mean that there is no gravity effects from the sun and moon. It is there, just a question of scale/applicability/device accuracy/sensitivity.
      Eg. LHC + CERN team have to calculate earth rotation speed in their experiments. But in our school lab, assuming earth is static seems applicable for the scale we work at. Good to know though. :)
      (3 votes)
  • blobby green style avatar for user bluedog1205
    can i build a battery with your help?
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  • blobby green style avatar for user GOMEZHEC000
    In order to pass a circuit you need a positive and negative charge
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  • piceratops sapling style avatar for user Neal0804
    Hi for constant current must you say II or. Just draw the symbol
    (1 vote)
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Video transcript

- There's two kinds of ideal sources we're gonna talk about. One is an ideal voltage source, and the other is an ideal current source. A ideal voltage source, the symbol is, looks like a circle like that. We put a voltage indication right inside there and that's called V. And this is a constant voltage, what I've shown here is a constant voltage. And that come from a power supply or it could come from a battery. When it comes from a battery, we have a special symbol for that, the battery symbol looks like this and the convention for this, we also label it V. The convention for the polarity of a battery symbol is the long line there is the plus terminal and the short line right there, that guy is the minus terminal. So that's the convention for a battery. The other type of ideal source is called a current source and it also has a symbol with a circle and this one we put an arrow and it goes in the direction of the current. The current symbol is I. It can look like that or we could point the arrow the other way, like that, depending on how the application goes. And that's an ideal current source. And those are the two symbols for constant current. One of the things we can do, we can plot these, we can plot theses two voltage sources, the voltage source and the current source. We can plot them on a curve that has coordinates as voltage and current. So this would be called an IV curve or an IV plot that we're about to do here. For a constant voltage source, the voltage doesn't change. The current goes up and down, depending on what the rest of the circuit demands, but the voltage is the same everywhere, so it plots something like this. That would be the IV plot of a constant voltage, where V equals some constant V. And if we wanna plot our constant current source on this kind of IV plot, this would be something where the current is always the same, the current is the same, independent of the voltage. And so a plot of that for a positive current would look like this and we would say something like I equals a constant I. So that's the IV plot of a current source and the IV plot of a voltage source. Alright, these are the two basic ways we deliver power or signals into circuits. Now we have a complete set of elements that we can build things with.