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In page-8 to 9 in Art of Electronics by Horowitz and Hill, the concept of current sources and voltage sources are discussed. Batteries are given as example of ideal voltage source but there seem to be no examples given of an 'ideal' current source, so what is an ideal current source?
My question is different from this question as here I ask about ideal sources, not real sources.
An 'ideal' current source would have to have infinite voltage range. Likewise, an 'ideal' voltage source would have to have an infinite current range.
In other words, these ideal sources don't exist. We can approximate them by using a power source (like a battery, or a floating power supply) with either current or voltage regulation on its output.
Current sources (and sinks) are nevertheless widely used in electronics. They always have a voltage compliance range limitation that defines how non-ideal they are.
Solar Cells, Batteries with large internal resistances (e.g. button cells), LED constant current supplies...
Note that this question doesn't really take you super far. A battery isn't very much of an ideal voltage source, and it doesn't take much more "idolization" to treat other power sources as current sources as much as you treat batteries as voltage sources.
It's really more of a question of how much "non-ideality" you accept.
Batteries in general behave as voltage sources due to their chemistry. When current flows and charge is taken from one electrode to the other, this upsets the internal electrochemical balance and creates an opportunity for redox reactions to occur, which replenish the used charge. And the more current flows, the more reactions occur, which means the battery tends to produce a stable voltage. It also depends on state of charge and internal resistance of course.
On the other hand, a solar cell or a photodiode really is a current source. Each photon it receives results in a certain amount of charge being generated. Maximum output current, which is flow of charge, is directly proportional to the number of photons received per second. This is somewhat hidden by the fact that, if the solar cell is open circuit and no current is drawn from it, voltage will rise to the point where charge leaks back internally, so it looks like a constant voltage source. But internally, it is a voltage-limited current source.
In real life a current source is a combination of a voltage source that can provide enough voltage for the particular application, and some electronics that seek to control the current- this is often done by attempting to maintain a constant voltage across a current sense resistor. For example in LED lighting it’s common to use 36V LED arrays, so a power supply with a capability of 40V is adequate, and an electronic circuit limits the current to a desired value.
The ideal voltage source is capable to deliver it's desired voltage independent of the current. This is in practice impossible, every voltage source has it's current limits. The same for a current source (but reversed). May be your question should be: when is a current source needed. A example is for charging batteries, the output current for charging is always the same value (untill the charged battery is at it's maxium open voltage) so the voltage changes to maintain that current.
