What is "current source"? Are there true current sources? If not, how do we create artificial current sources? How do we make them perfect?

Maybe your question could be linked to https://www.researchgate.net/post/Does_current_sources_exist

I think current source and voltage source are the same in practical life, only the circuit analysis differs

Quote: "I think current source and voltage source are the same in practical life"

Yes - but I rather would say:....are the same in principle.

That means:

* A voltage source has a low internal resistance (if compared with the load)

* A "current source" is a fixed voltage source with a high internal resistance (in comparison to the load).

* Another type of current source is based on feedback, whereby the value of the driving voltage changes in such a manner that the current does not depend on the load resistance.

*Summary: Ideal sources do not exist - the internal resistance (in comparison to the load) makes the difference.

So a current source is an extremely "bad" voltage source-:)?

Here again, theory and practice make a good mix for a chat.

A current source sets the current at its terminals. (Theory)

A current source keeps the current almost constant for a limited period of time (Power Electronics practice) In that case is can be an inductor.

A current source keeps the current almost constant for a limited voltage compliance (Analog Electronics practice) In that case a transistor with feedback can do the job.

If you go out in the real world to find a Power source supplying a constant current, the closest you'll get is the Photo-Voltaïc cell. (A quick look at wikipedia will show you that it is a coarse approximation)

Then if you go in the wild to find a Power source supplying a constant voltage, don't go far, your car is equipped with such a device giving a constant voltage, much better than 1% stable up to 10A, the Lead Acid Battery (rated 70Ah, 600A pic current)

Or the current source is an "intentionally worsened voltage source"-:)?

I do not agree mixing current source and voltage source.

It is however possible to set the current in a circuit feeding with a variable load using a voltage source as long as it is possible to use a much higher voltage source than the voltage present across the load.

Nevertheless, Thevenin and Norton models represent voltage and current sources.

If you model an unknown source using Thevenin model (E+r) you will not get the same physical aspects as if you model this source by a Norton model (I//r) and this is probably why I believe that current sources are different to voltage sources.

Lutz and Dominique, IMO to make a current source by a fixed voltage source with a high internal (ohmic) resistance is a typical electricity technique. We frequently use it in electronics but, in addition, we have a few more clever techniques. I will try to make a figurative comparison between the electricity and electronics approach. In contrast to Dominique's opinion ("I do not agree mixing current source and voltage source"), I will show the close relation between the two sources by converting a voltage source into a current one. I frequently do this mental experiment with my students.

Well, imagine the IV curve of a perfect (ideal) voltage source producing a constant positive voltage - a vertical line biased to right. In electricity, they begin transforming the voltage source into a current one by connecting a variable resistor (a rheostat) in series and increasing the voltage and the resistance; as a result, the IV curve moves to the right and inclines to the left. When they increase up to infinite the resistance (and the voltage as well), the curve goes extremely far away and lies in an almost horizontal position. This is a silly technique-:) since all the power is dissipated by the internal source resistance (an example - supplying an LED through a resistor by 220V); the IV curve is "perfect" along all the region (from zero up to infinity) but it is not necessary.

In electronics, we are more flexible and clever-:) - we keep the curve horizontal only in a limited useful region (e.g., from 0 to 10 V) and then we bend it to a vertical position (imagine a BJT output characteristic). We do it by a few "dynamizing tricks" - by changing dynamically the resistance or the voltage or the both or by adding an additional voltage...

This was my story about the relation between the two sources...

Regards, Cyril

Nice story of electricians bending curves using active components versus electricians forging currents using I=U/R :)

However, what is the physical difference between a physically built current source and a physically built voltage source ? Norton and Thevenin might confuse or help you.

-:)

A physically built (composed) current source = a voltage source in series to a resistor (Thevenin)?

A physically built (composed) voltage source = a current source in parallel to a resistor (Norton)?

Yes you are getting there.... :) not exactly the way I thought, though. Remember that Thevenin and Norton are exact equivalent in an electrical circuit. So where is the difference ?

The simplest and possibly most widely used "current source" is the conventional iron core current transformer. However, the secondary circuit must be kept closed to avoid very high voltages from being produced.

Voltage and current sources represent energy stored in the electric and magnetic fields respectively if I am not mistaken.

Dominique I think you mean the terminal characteristics of the Thevenin and Norton circuits are equivalent.

As a current source implies energy stored in a magnetic field then materials such as iron are sources of very high magnetic internal fields. In such materials internal magnetic fields some 10,000 greater than in air may be produced through external excitation.

A permanent magnet is possibly close to being a "natural" ideal current source.

Diana, "A permanent magnet is possibly close to being a "natural" ideal current source" sounds quite strange for me...

I`ve got the impression that - in this discussion - we are mixing theoretical and practical aspects (not COMBINING (!) which is appropriate).

For example, the initial questions were "what is the current source" and "are there true current sources"? And in this context, I think that we shouldn`t refer to source MODELS (Norton, Thevenin), which are well-known.

Primarily, I think and feel as an engineer and, thus, my question is:

Is there any "TRUE" current source that is used and/or could be used in electronics for powering a circuit ?

And in this context, I do not know how a "permanent magnet" could be used as an "ideal" current source. How does the corresponding circuit looks like?

I did not anywhere say it could be used nor did I imply this.

The question posed was What is "current source"? Are there true current sources? it did not ask whether they were of any practical use merely whether we thought there were / are any.

Any current source will produce a magnetic field and within a magnetic material there are such "currents" available due to orbital spin. Can they be used in circuit theory and the like I very much doubt it but then that is not what was asked.

Norton and Thevenin are well known for circuit analysis, but do teachers point out that in the Norton model there is power constantly dissipated (unless an ideal short circuit is used for a load) as the Thevenin model dissipate power only when connected to a load.

Not taking into account power (or energy) sometimes leads to erroneous conclusions.

To illustrate this, let's look at a 2D representation for a 3D object : a cylinder and a sphere may look exactly the same is you miss the "z" axis".

Quote:" ...but do teachers point out that in the Norton model there is power constantly dissipated".

My comment: We shouldn`t forget it is simply a MODEL!

What is the importance of the quoted question? I think, this argument would be relevant only if there would be a "true and ideal current source" (which is part of the Norton model).

And that means: We are back again at the basic question - and, for my opinion, the initial question from prof. Mechkov is not yet answered.

What is "current source"?

A two port device imposing the current in the circuit loop it is placed in.

Are there true current sources?

No. (True meaning imposing a current using a single physical phenomenon)

If not, how do we create artificial current sources?

Two ways :

First, the circuit is entirely known so using a voltage source and a selected impedance imposes the current.

Secondly:

The circuit is not entirely known, for example the load behaves randomly, in that case, the use of feedback circuits with a current sensor and a voltage source achieves this.

How do we make them perfect?

Advanced feedback techniques and passive networks for fast transient response.

FYI: Power electronic people call a current source an inductor placed between two voltage sources, one of which, at least, is periodic.

Full agreement!

Also the solar cell current is caused by internal charge separation (commonly called voltage).

Hello,

I was wondering, what would be practical use of current source? Is there any circuitry/system where we need current source, or will it be favorable to replace voltage source by current source in some cases. Can anyone please give some examples?

Mohammad, look at this question as well

Cyril

One example of a laboratory power supply designed primarily to operate as a current source is the HP6177C or HP6181C. These were manufactured by Hewlett Packard, and are now now obsolete (but can still be bought from suppliers of used instruments).

What makes this a good example for anyone interested in studying the design of a practical high-performance current source is the service manual, still available on the Agilent (formerly Hewlett-Packard) website:

Section IV of the service manual mentions some of the techniques used to achieve a highly regulated output current. The most noteworthy feature is the enclosure of the positive output terminal by a 'guard' - that is, a conductive enclosure that is maintained at the same potential as the positive output terminal itself. The guard prevents the regulation of the output current from being degraded by any leakage currents. It also allows a meter to to be provided that monitors the voltage that appears across the load or the output current. The meter is connected to the guard rather than the positive output terminal so that it does not degrade the extremely high output impedance of the current source.

One feature of constant current sources that is not often discussed is their behaviour over a range of frequencies. An ideal current source, of course, has infinite output impedance independent of the frequency of voltage variations that may occur across its output terminals. In practical current sources, this is not so easily achieved. On the HP6181C / HP6177C, the output capacitance is relatively small, which helps this supply to act as a current source even if load variations occur at higher frequencies.

A conventional laboratory power supply with a variable current limit can be set up to operate in current-limiting mode to make a current source. However, most conventional laboratory power supplies will not perform very well as current sources because of 1) low gain in the current limiting control loop, and 2) decoupling capacitors present at the output - these help the laboratory supply to approximate an ideal voltage source (which is its normal function) but degrade its ability to approximate an ideal current source.

Hi Mohammad Islam,

Regarding your questions 'Is there any circuitry/system where we need current source, or will it be favorable to replace voltage source by current source in some cases. Can anyone please give some examples?'

As a brief answer: We need current sources to drive laser diodes, light-emitting diodes, voice coil actuators (such as might be used to control the movement of read/write heads in a disk drive), to drive electromagnets (especially when we need to generate a controlled magnetic field for many kinds of experiments and measurements), and most integrated circuits use one (or several) current sources internally in order to perform their overall function.

I'm sure there are many more examples than I have mentioned here.

As Dominique has already mentioned above it is quite easy to make power supplies to act as current sources and these are already freely available as David points.

By switching a voltage sourced inverter through an inductor and employing closed loop current control one is able to provide a "constant" current and which has long been practiced in the machine tool and variable speed drive industry and elsewhere.

The quality of the output current is a function of the switching frequency and the filtering action of the series inductor which will limit its regulation and frequency response.

However one may also do same using Magnetic Amplifiers or Mag Amps and which has long been used even before the advent of analogue and power electronics.

http://onlinelibrary.wiley.com/doi/10.1002/eej.4390920412/abstract?systemMessage=Wiley+Online+Library+will+be+disrupted+on+18+May+from+10%3A00-12%3A00+BST+%2805%3A00-07%3A00+EDT%29+for+essential+maintenance

a Current source has high output impedance and is ideal for supplying current to loads which do not have impedances that are comparable to the output impedance of the current source...I suspect in industrial instrumentation, 4-20mA current sources are used and this means that we can have long instrument cables, and the current will not change value because it is almost resistance independent...because the instruement cable resistance is very much smaller than the output resistance of the current source within the instrument...This is my interpretation...try reading on current mirrors, i believe they are good current sources because they have high output impedance...

Dominique and Lutz,

I would like to add more thoughts about the Thevenin (V + R) and Norton (I || R) configurations. Looking at them from a "circuit viewpoint", we may think of them as synthetic, artificial sources that are built by cascading two elements - a source and a converter.

Thus, "current source" = "voltage source" + "voltage-to-current converter"...

... and "voltage source" = "current source" + "current-to-voltage converter"

Regarding to the energy consumed (Dominique), the Norton configuration is similar to the voltage divider configuration - both they consume energy even when not loaded. They are similar since the combination "voltage source + resistor R" acts as the current source in the Norton configuration...

Quote Cyril M.:

"Thus, "current source" = "voltage source" + "voltage-to-current converter"..." and

"voltage source" = "current source" + "current-to-voltage converter"

My comment: I am not sure if these analogies really help to understand the principles connected to the initial question of this topic (...are there true current sources?).

To me - from an engineering point of view - the situation is quite clear:

In principle, we can speak only about sources that are able to deliver electrical power (resulting from one out of several methods to change energy forms).

a) If this power is provided with a low internal output resistance (if compared with the connected load) we use the term "voltage source";

b) If this power is provided with a large internal output resistance (if compared with the load) we normally use the term "current source".

From this, it results that there are neither IDEAL voltage nor current sources.

That is not very surprising since nothing is ideal in the area of electronics

(neither parts properties nor formulas). That means, nearly everything is an approximation only, which is not a problem as long as the errors caused by "imperfect" properties or formulas remain within acceptable limits.

Lutz vW

Yes, Lutz! I was ready to say something similar to your conclusion but in a little more figurative way:

We have electrical sources that, generally speaking, can produce (actually, convert) power. By varying the internal resistance in large limits, we can shape, form, model this power P in different proportions between the voltage V and the current I (P = V.I)...

Fine - so we came to a common conclusion.

As just mentioned in another topic, we have only a limited number of words available to describe all the different effects to be observed.

And that is a good reason to use the term "current source" - knowing that, in reality, it is a voltage source with a large internal resistance.

Phew, ohms law pushed to its limits!

Are there true current sources?

No one doesn`t mention photovoltaic system ( panels, DC/DC converters and battery). If the operating point of the photovoltaic cells is controlled near the bend of u-i characteristic (i.e. MPPT-algorithm) the photovoltaic panels would behaviour like current sources. Of cours, there is problem with power efficiency ( max 25%) and reliability. Therefore, the creating current source from voltage source and inductor in tandem connection is most often used technique regarding Power Electronics.

".....the photovoltaic panels would behaviour like current sources"

Yes - would behave like....but it isn`t a current source (physically).

Lutz, I have asked another question about the nature of basic electrical sources. It is closely related to the question, "Does a true current source exist?"

Regards, Cyril

Dear Cyril,

What do you mean exactly by true current sources?

I think one have to precisely define the terms, the words, the concepts and then establish a discussion about their consequences. In this way we can reach to satisfactory conclusion.

There is no ideal electrical signal source.There is practical signal sources. A practical source may behave as a voltage source if its source resistance is much smaller than the load resistance.It may behave as a current source if its source resistance is much greater than the load resistance.

But in general natural electrical sources are characterized by an open circuit voltage which is the electromotive force of the source and a source impedance or resistance. It is thought to be a potential energy source. If it is thought that it is a kinetic energy source, it may be represented by a current source.

Any electrical source is not only a source of voltage but it is also a source of current. It is a source of power. It is a source of the current multiplied by the voltage. For maximum power transfer from the source to the load the source resistance must be equal to the load resistance.

Current sources are built and used intensively in analog integrated circuits as biasing and load elements. Also the colleagues mentioned some applications of the current sources.They became a reality with the advent of the transistors and the analog integrated circuits.

Natural electrical sources are voltage sources.but current sources can be easily built and have many applications in they analog integrated circus.Their existence helped build the advanced analog integrated circuits.

Dear Abdelhalim,

Thank you for the contribution. IMO you should direct your question ("What do you mean exactly by true current sources?") sooner to Lutz since he is the most interested if true current sources exist. Saying "true" here, we (Lutz?) mean "pure" natural (not artificial) current sources; "artificial" means created by modifying initial voltage sources.

Regards, Cyril

Cyril - yes, I agree to the definition as given in your last sentence.

Nevertheless, it was you who has created this topic and it would be interesting to hear if you have the feeling that your initial question ("Are there true current sources"?) has been answered or not.

Lutz

I have asked the new question to see -:)

Dominique also is waiting for an answer:

Like every year at this time of Christmas holidays, my creative activity rises and I begin to think... again and again... on the eternal ideas in circuitry... Thus browsing the web these days, I came across the eternal question of how to make current sources:

I was impressed how sincerely and somehow naively the question was posed:

... and how specific and limited the answers were...

It was a challenge for my ability to understand the complicated things in a simple way... and I started to think again about this problem. I have copied my comment below because I think it can enrich and revive our discussion here...

---------- The comment ----------

"The understanding of specific circuit solutions is based on revealing the basic ideas behind them. So let's see what these ideas are in the case...

To produce current, according to Ohm's law I = V/R, we need only voltage and resistance. So, if the load was pure resistive, we would need only a voltage source to produce current. By changing the voltage, we can set the desired current magnitude.

But if the load behaves as a voltage source (e.g., a rechargable battery, capacitor, Zener diode, short connection, negative resistor, etc.), we need additional resistance in series to set the current. Thus, in general case, the current source is made by two elements in series - a voltage source with voltage V and resistor with resistance Ri... and it is connected to a load with voltage V_L and resistance R_L. These four elements are connected in a circle and each of them affects the magnitude of current determined by the ratio of the total voltage Vt and resistance Rt; I = Vt/Rt = (V ± V_L)/(Ri ± R_L). In this arrangement, the input voltage source tries to set the current by its voltage V and resistance Ri while the load interferes with it by its voltage V_L and resistance R_L. Both the source and load influence the common current and the problem is to eliminate the impact of the load on the current.

The simplest way (typical for electric circuits) is to increase enormously both the voltage and resistance of the input source. They are high but constant (static)... and this is the trouble. The load voltage and resistance become negligible compared to those of the input source. It is clear that making a good current source in this way is associated with big power losses in resistance.

The more clever way (typical for electronic circuits) is to make the source voltage or resistance varying. They are dynamic but low... power losses are low... and this is the profit. We have the illusion of extremely high (differential) resistance but the actual (static) resistance is low. Let's see how this idea is put in practice...

The trick is that when the load increases/decreases its voltage or resistance, the source decreases/increases its voltage or resistance with the same value; so the current does not change.

This compensation can be done without any negative feedback by using a following voltage source (the so-called "bootstrapping") or a current-stabilizing resistor (implemented by a BJT or FET with a constant input voltage). Here we do not monitor the current; we only suppose it is kept constant. When a negative feedback is used, we change the source voltage/resistance so that the current does not change.

One variation of this technique is, instead to change the very source voltage, to add additional voltage in series to the constant source voltage thus compensating for the impact of the load. This idea is realized, for example, in the op-amp inverting current source.

Another more extravagant idea is to inject additional current into the load by connecting an additional current source in parallel to the main input source. It is implemented in the Howland current source.

We can see these techniques in my circuit stories about constant current sources:

https://www.circuit-fantasia.com/circuit_stories/inventing_circuits/current_source/current_source_philosophy/current_source_philosophy.htm"

Cyril, best wishes for a happy and creative 2019.

You are quite right, the break from routine affords time to think afresh about circuit concepts. Here's a few thoughts that come to my mind regarding current sources:

Is there something in nature that behaves as an ideal current source? If there is, the charge sourced by it in unit time must not be affected by anything else that the charge moves through. If the charge moves through something, and there is a potential difference in the path, the source must be imparting energy to that something per unit time. An ideal source must necessarily have the capacity to impart infinite energy if the current provided by it never changes regardless of the the potential difference in the path the charge move in. I don't know of any infinite energy sources in nature, so an ideal current source cannot exist in nature either.

Dear Cyril,

This is how to construct a constant current source practically.

I would like to congratulate you Cyril for your feast. Happy Christmas for you and your family. In this occasion, i would like to wish merry Christmas for all colleagues on the research gate.

Best wishes