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I have seen both current source and voltage source in many electrical circuits (in paper). When it comes real hardware battery can be considered as a voltage source. Then what is a current source ?

Edit : With reference to some answers and comments here I will try to make my question more clear.

Voltage source (battery) produce constant voltage (for near ideal condition). Current through the circuit only depend on the load resistance. Is there any such current source devices other than circuits which virtually behave like current source by using a voltage source ?

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    \$\begingroup\$ Fun fact: Current sources rank right up there with unicorns and the Tooth Fairy. \$\endgroup\$ – Ignacio Vazquez-Abrams Apr 4 '14 at 5:21
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    \$\begingroup\$ An inductor can be used as a "temporary" current source, and is analogous to how a capacitor can be used as a "temporary" voltage source. \$\endgroup\$ – helloworld922 Apr 4 '14 at 5:29
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    \$\begingroup\$ One might consider a switching power supply with current-limiting enabled to be a current source. \$\endgroup\$ – JYelton Apr 4 '14 at 5:30
  • \$\begingroup\$ @IgnacioVazquez-Abrams - Do unicorns have their own tooth fairy? \$\endgroup\$ – John U Apr 4 '14 at 8:04
  • \$\begingroup\$ @tollin: It's not clear what category of devices you are trying to explore. Any actual device that makes a voltage or current appear in a circuit has to convert that energy from somewhere else, such as chemical, heat, light, or electrical sources. The answers have proposed a number of these, but you've ruled out some as being "virtual". So which are of interest, only those with a chemical source? \$\endgroup\$ – gwideman Apr 5 '14 at 0:16
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I'm making this answer a community wiki so anyone can add to it.

The current mirror circuit is one very common current source used mostly within ICs:

schematic

simulate this circuit – Schematic created using CircuitLab

A linear regulator can be used to make a current source:

schematic

simulate this circuit

It's also possible to make a current source from an op-amp:

schematic

simulate this circuit

Most simple current source can be made of just two parts, JFET and resistor. This circuit bahave mostly like above by utilizing cutoff voltage of JFET as reference.

schematic

simulate this circuit

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  • \$\begingroup\$ But note that none of these accurately mimic the schematic symbol called "current source" since none of these can actually force the load to consume all of the current supplied, nor can they cause the voltage across the load to go above that of the voltage source if the load consumes more power than the source can supply. \$\endgroup\$ – Ignacio Vazquez-Abrams Apr 4 '14 at 5:59
  • \$\begingroup\$ It would be great if JFET+resistor circuit is also added to your answer. sub.allaboutcircuits.com/images/05269.png \$\endgroup\$ – Vovanium Apr 4 '14 at 16:04
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    \$\begingroup\$ @Vovanium, I made my answer community wiki to encourage you to edit it and add additional types of current sources. \$\endgroup\$ – The Photon Apr 4 '14 at 16:54
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When textbooks say "voltage source" or "current source", they are implying constant voltage or constant current sources.

If you have a constant voltage, then the circuit's current is dependent on the load. A voltage source should be able to provide whatever current the circuit demands.

Constant current is opposite: For the circuit to have a fixed current, then the current source must be able to provide whatever voltage is required to push the current through the load.

People are used to the idea of a battery, or a power supply, being considered a voltage source. But they aren't actually constant. A battery's voltage will start to collapse when you draw too much current from it, a switching power supply has ripple, etc.

So, if we are are willing to stretch the expectations in a similar way, there are some common devices that could be called "current sources":

  1. A constant current LED driver. A BuckPuck, for example, will drive (say) 350mA through a string of LEDs in series, no matter how many LEDs there are. Unless you exceed it's maximum output voltage, of course.

  2. A benchtop power supply, with a current limiting setting. If you dial it down to 1 Amp, and short the output, it will still only deliver one amp. The voltage will collapse to almost nothing, but you'll get 1 Amp.

  3. A solar panel! If you look at a V/I chart of any given panel, you'll see that they output a fixed current over a large set of load conditions. The current doesn't start to fall until the load gets big enough. You seldom think about this, though, because we always want to operate a solar panel with a useful load.

And so on...

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  • \$\begingroup\$ Even the common battery is, when used with a low-resistance load, a reasonable approximation of a current source. \$\endgroup\$ – Wouter van Ooijen Apr 4 '14 at 5:59
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    \$\begingroup\$ J5xx constant-current diodes are another example, as are LM334s. \$\endgroup\$ – user207421 Apr 4 '14 at 5:59
  • \$\begingroup\$ "Is there any such current source devices other than circuits which virtually behave like current source by using a voltage source?" - There really is no difference, even though it's hard to get our heads around that. You could equivalently say, "Is there any such voltage source device other than circuits which virtually behave like voltage sources by using a varying current?" A voltage source varies its current to keep the load voltage at a fixed level... Does this help at all? \$\endgroup\$ – bitsmack Apr 4 '14 at 17:25
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  1. Several electronic circuits are to be current sources. Most simple one consist only of JFET and resistor. In schematics current sources are implemented this way. Note, commonly voltage sources are alike (see Zener diode and band-gap)

  2. Radioisotopic, say alphavoltaic and betavoltaic cells are (exponential) current sources. Radioisotope in this cell emits fixed amount of charged particles per time, so there could be fixed current between its terminals.

  3. Same as previous for some chemical cells, which have its current defined by chemical process.

  4. Photovoltaic cells, as well as phototransistors could have current defined by amount of light.

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    \$\begingroup\$ The JFET and resistor is known as a "Constant-current diode". The simplest arrangement is to short the gate to the source and omit the resistor, although this can result in a range of currents "Due to the large variability in saturation current of JFETs". \$\endgroup\$ – Tut Apr 7 '14 at 11:20
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A lithium coin cell is sort of a current source.. you can test LEDs with them, which is reasonable application of a current source.

Transformers are usually designed to be AC voltage sources, but those designed to be current sources exist- current transformers that are quite accurate current sources up to a few watts out- and transformers designed with magnetic shunts for use operating gas discharge tubes (e.g. neon).

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A device which could unconditionally supply a constant amount of current (or a constant voltage for that matter) would need to be able to supply an unlimited amount of power. That is, of course, impossible. It is possible to construct devices which will either pass a relatively constant amount of current over a range of voltages that would push energy into them, or to construct devices which will drive a relatively constant amount of current over some range of voltages which pull energy out of the device so long as the device has energy available to give (either stored internally or supplied externally). In general, when a schematic shows a constant current source, the intention is that it will take the form of a device that will pass or drive a constant amount of current over the range of voltages it have to deal with for the overall circuit to achieve its intended function.

Note that even when a voltage source or current source is used within the intended operating range, the voltage across it and current flowing through it will be affected by each other. For example, a passive (energy-absorbing) current source might be specified as passing at least 99.5uA with 1 volt across it, at most 100.5uA with 10 volts across it. In general, circuits will be designed so they can work "perfectly" if current sources (as well as all other components) behave as they ideally should, but "acceptably" for any combination of behaviors within required tolerances.

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Let us imagine a very simple circuit - for example a DC voltage source which provides a potential difference of 1 volt across a resistance of 1 ohm which causes a current of 1 ampere to flow. If you then broke the circuit on the low side (between the resistor and ground) and then added a multi-meter in series with the resistor you would measure a current of 1 ampere.

schematic

simulate this circuit – Schematic created using CircuitLab

If the circuit I described is a Thevenin Equivalent, then from a "circuit analysis" point of view the current source is more like a Norton Transform.

If you then thought about the whole system, including the voltage source as a "black box" and as only having two outputs (the terminals which the multi-meter connects to) then you could call it a current source.

However, if you then tried to connect a load to this current source, you would find that the current would change (because of Ohms law acting within the black box).

So there are practical circuits which can be built out of collections of op-amps or other semiconducting materials, which have this "constant current effect". However none of them are a pure "current source".

You say that a battery is a "voltage source".

However without a resistor attached to allow current to flow the battery cannot dissipate electrical energy, so your voltage source cannot do anything useful without allowing current to flow. The definition of current is rate of flow of charge. One good definition of voltage is electrical energy per unit charge. Separating the two into "sources" is hard, because:

(a) the bundles of electrons we call a coulomb is not going to move and create the phenomena we call "current" without the electrons moving.

(b) the potential difference, or work done against the e-field in moving a charge from physical point a to physical point b is also requiring movement.

What I am trying to say is that voltage and current do not happen independently when a source {whether we are calling it current or voltage} is providing energy to a load, which is why I think it is hard to conceptually "think" about voltage and current sources as if they were/are different.

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  • \$\begingroup\$ It's incorrect to say that voltage and current do not happen independently (as a general rule). The example of a battery or a charged capacitor is a voltage that does not depend on current. Electrons traveling through free space need not gain or lose energy...a current without a voltage. \$\endgroup\$ – Joe Hass Apr 4 '14 at 15:03
  • \$\begingroup\$ Thank you Joe Hass, I have edited my post to make the context of that comment clearer. \$\endgroup\$ – AugustCrawl Apr 7 '14 at 9:24

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