Dear Cyril Mechkov

Real voltage sources consist of ideal voltage sources in series with resistance called internal resistance. This resistance doesn't really exist so we can see it. This is the resilience that is concluded by the behavior of real voltage sources.

However, The ideal internal resistance of the voltage source is zero; It is able to supply or absorb the amount of currents. The current through the ideal voltage source is fully determined by external circuits. When connected to the open circuit, there is zero current and thus zero power.

For more, this links might be useful :https://electronicsarea.com/internal-resistance-voltage-source/ and https://en.wikipedia.org/wiki/Voltage_source

Kind Regards

Qamar Ul Islam

And what about this source :-)

Josef, a voltage stabilizer with an overload protection?

Dear @Qamar Ul Islam,

Thanks for the explanation. I only wonder how "real voltage sources can consist of ideal voltage sources in series with resistance called internal resistance"... and, at the same time, "this resistance doesn't really exist so we can see it"? How can a source consist of something ... and we can't see that thing?

The output resistance of the voltage source is a subtle problem.

If the source consists of one controlled transistor ("in series" with load; conventional laboratory sources), the output resistance is defined only to draw current out of the source. From the point of view of circuit theory, this is not an "ideal voltage source" - it is not able to absorb currents. However, if the output of the voltage source is complementary, it is also able to absorb currents - and that is OK. In addition, the resistance may vary depending on the current and voltage ratios. In the previously mentioned figure (Cyril correctly named the source type) there is first a small output resistance, then an almost infinite output and finally a "completely different" output resistance - the voltage on the control transistor has exceeded the limit value - the power loss of the transistor is reduced.

Josef, very profound answer about artificial voltage "sources" (stabilizers) in electronics...

I only wonder what is the role of the "completely different" output resistance. Isn't it enough to limit the current?

And we can think :-)); it saves "power" and "heat".

Josef,

Looking at your weird IV curve, I can say that, in the final part, it represents a negative resistance.

The source output behaves as follows: first, as an "ideal" voltage source (in the vertical IV segment)... then, as an "ideal" current source (in the horizontal IV segment)... and finally, as a negative resistor (in the segment with reverse slope).

The measurement of MAA78XX series stabilizers fed me for 8 years (in TESLA Rožnov). The described method of protection against power overload had one vice. If this protection was a bit "sensitive", the circuits did not want to "start" to capacity - and this is a bit of a stupid property of the voltage source :-) When measuring the volt-ampere characteristic, such "sensitive" circuits behaved as shown (the load resistance changed).

And now it's clear. At the moment of switching on, the filtering capacity represents a short circuit. An electronic circuit with a certain consumption - ie equivalent resistance - is connected in parallel. If this resistance is less than a certain critical value (and our circuit is "rude") - we are unlucky - the stabilizer "does not start" - remains in "protection mode".

Cyril Mechkov

If it looks like a duck, swims like a duck and quacks like a duck, then it probably is a duck. (Duck-test).

It is not entirely clear what you mean by "real resistance". Electrical resistance is defined as the coefficient of proportionality between voltage and current, i.e. by Ohm's law for a section of an electrical circuit. In the case of a real circuit, an expression is used that includes the "internal resistance of the current source". Of course, it does not really exist, as the overwhelming majority of model values, including "real resistance". The model equation (within certain limits of currents, etc.) includes these quantities. There are "reality" of these?

Hi Josef,

I am glad to know that before university you worked in industry and probably came up with interesting ideas...

It was interesting for me to look at 7805 internal structure, in particular at the circuit of current protection. Even a year ago, I asked such a question at StackExchange (https://electronics.stackexchange.com/questions/483339/what-is-the-idea-behind-the-ingenious-widlar-current-limiter) and then answered it myself. A really interesting circuit solution...

@Vadim S. Gorshkov,

My question is intended for people (like me) for whom resistance, particularly, source internal resistance, is not just a ratio between two letters (V/I, a/b, c/d, etc.)... but they want to see what is behind it.

My idea is that, in the case of a "natural" real (imperfect) voltage source like a battery, there is only a varying (dynamic) voltage source but not any physical resistance… while, in its equivalent circuit, there is only physical resistance.

To continue this chain of thoughts:

Mains voltage source. Someone will say that wires have resistance. But they are so short that their resistance is negligible. The connection terminals (for example of a secondary battery) also have a contact resistance ... but it is outside the source. The same is valid for the line resistance...

However, in the case of a mains power supply, the line resistance dominates. Since, for us, the voltage source is the wall socket, the line resistance serves as an internal source resistance (the power plant is the source). Thus both circuits - real and equivalent, are the same... and the source resistance is a physical resistance.

Electronic voltage source. In electronics, we use mainly artificial voltage sources (voltage stabilizers). They consist of a constant voltage source (power supply) and electrically-controlled "resistor" (the collector-emitter part of a transistor) in series. In addition, this "resistor" is dynamic (depends not only on the input voltage but on the load voltage as well). An emitter follower is a typical example.

Because of the finite transistor gain, the output voltage slightly decreases when the load current increases. This gives the illusion of constant internal (output) resistance... but actually this is changing (dynamic) resistance.

Cyril Mechkov

That is fine, can you determine, please, what is resistance, if not V/I ratio, especially "real resistance"?

I have nothing against the definition of resistance given by Ohm's law in its third form R = V/I... but I want to show and discuss its various “material” manifestations and implementations in order to better understand the phenomenon. People do not understand things through definitions written in such a compact form simply because they are not computers but human beings who need much more - associations, analogies, imagination, intuition, common sense, previous experience… If you take a look at the Wikibooks story (https://en.wikibooks.org/wiki/Circuit_Idea/Walking_along_the_Resistive_Film) we created with my students in 2008, you will get some idea of what it is all about. This story is based on Ohm's law but presented in an unusual way, not as a sterile definition.

Saying “physical”, “true”, “genuine”, “tangible”... maybe and “real” resistance, I mean rather various “resistors” (material things) that possess the property of resistance. But we can imitate (mimic) this property in a quite unusual way - by a variable voltage source. If I connect it in series and in an opposite direction to the main (input) voltage source, and begin varying its voltage V so that V = I.R (Ohm's law in its second form), you will have the illusion of a “resistor” with resistance R = V/I. Is this a resistor? This is a voltage source but acting as an artificial, “virtual resistor”.

Awareness of this idea allowed me years ago to explain what the true negative resistance really is - https://en.wikibooks.org/wiki/Circuit_Idea/Revealing_the_Mystery_of_Negative_Impedance. For example, one of its implementations - the so-called VNIC (https://en.wikibooks.org/wiki/Circuit_Idea/Negative_Impedance_Converter) is the same varying voltage source as above, but reversed so that its voltage is added to the input voltage instead of subtracted from it.

That is how we came to my question... It is a bit strange that I realized this truth so late - the "natural" voltage source (for example, a battery) is just a source without any internal resistance in any material form. It is just a varying voltage source whose voltage depends on the current (a kind of a "current-to-voltage converter")... and thus creates the illusion of some material resistance like in the equivalent circuit...

A cell has a resistance of its own – its internal resistance. ... If the external resistance is R and the internal resistance is r, the total resistance of the circuit is R+r, so that the current that flows is E/(R+r). https://phys.libretexts.org/Bookshelves/Electricity_and_Magnetism/Book%3A_Electricity_and_Magnetism_(Tatum)/04%3A_Batteries%2C_Resistors_and_Ohm's_Law/4.07%3A_Electromotive_Force_and_Internal_Resistance

Check https://courses.lumenlearning.com/physics/chapter/21-2-electromotive-force-terminal-voltage/

Check this paper.

https://digestiblenotes.com/physics/circuits/resistance.php

@Kiprotich Kiptum, nice tutorial.... but quite formal... I sent them the following comment on what a modern ammeter is:

"Modern, digital ammeters use an integrated circuit within the meter to measure the current" is not precisely said. The integrated circuit is an analog-to-digital converter that measures the voltage drop V = R.I across a resistor R inserted in the path of the current I.

So, "modern, digital ammeters" consist of a resistor (acting as a current-to-voltage converter) and voltmeter in parallel.

In contrast, old magnetoelectric voltmeters consist of a resistor (acting as a voltage-to-current converter) and ammeter in series.

@Chinaza Godswill Awuchi,

Here is my reply to your first comment and link:

Nice explanation of the electrical equivalent circuit presented as a constant voltage source with a constant "electromotive force" (voltage) E... But this is only a (possible) model of the real voltage source where the voltage decrease is explained by subtracting a voltage drop across an internal "resistor" from E.

Why not explain what a real voltage source is through another model - just as a "varying voltage source"? Or as a circuit of series-connected constant voltage source E and oppositely connected varying source with voltage Vint = I.R?

And here is my reply to your second link:

Extremely interesting tutorial… only I (again) can not agree with the first statement in the Section Summary:

"All voltage sources have two fundamental parts—a source of electrical energy that has a characteristic electromotive force (emf), and an internal resistance r."

This is only a model (an electrical equivalent circuit) where, for convenience, the real voltage source is presented as consisting of two elements in series - a constant voltage source and a resistor.

In fact, there is only one element - a varying (current-dependent) voltage source.

So, the statement should be something like this:

"All voltage sources can be thought of as having two fundamental parts—a source of electrical energy that has a characteristic electromotive force (emf), and an internal resistance r."

"Can be thought of as" is quite different from "have"...

Unfortunately, I just discovered this discussion today.

First a short comment on the sentence:....

(Quote): "Or as a circuit of series-connected constant voltage source E and oppositely connected varying source with voltage Vint = I.R?"

My comment: Do you believe that such a "oppositely connected varying source" helps to better understand a real voltage source - more than an internal resistor?

I am not sure.....

Next, a general comment:

Cyril, what bothers you about the equivalent circuit with a source resistor (even if it does not show up as an identifiable element)?

Isn't it the task of an equivalent circuit diagram to reproduce the behavior of a system using well-known parts to make it accessible to computation ?

More than that, it is common practice to use the term "source resistance" for the mentioned effect. Hence, is it not quite logical to use the symbol of a resistor to model this effect in the equivalent diagram ?

Example: Transistor (BJT) with controlled current source. Inside the transistor there is of course no current source - neither ideal nor real.

Hi, Lutz... Nice to see you here again. There are so few people who want to think "philosophically" on the basic circuit phenomena...

First of all, let me remind you that I use equivalent electrical circuits mainly for the purposes of understanding and not so much for the purposes of calculation.

I expected you to draw a parallel with the topic "re" in an emitter follower. Because just as here there is no "physical Rin" inside the real voltage source, so there is no "physical re" inside the transistor there.

My idea is to tell those who think there are some "resistors" inside these devices that they are just varying sources of voltage (in the case of a real voltage source) or current (in the case of an emitter follower).

"I use equivalent electrical circuits mainly for the purposes of understanding and not so much for the purposes of calculation."

OK - accepted.

So let`s cancel the last part of the sentence. Nevertheless, I think for the purpose of understanding it is best to explain the real (physical) reason for the behaviour of the non-ideal source.

Example: Early effect which is responsible for the d(Vce) when the load current increases.

Will an (artificial) equivalent circuit diagram with an "oppositely connected varying source" help to better understand?

I rather think that it is quite obvious that the finite slope of the Ic=f(Vce) curve can best be interpreted as a ratio of two dynamic quantities which allow the definition of a dynamic resistance rce.

Dear Lutz,

Only to clarify that I have proposed the model with an "oppositely connected varying source" for those who want to see something in series to the voltage source. I do not need it. My intuitive model of a real voltage source is extremely simple: it consists only of a varying voltage source whose voltage decreases when the current increases. There is nothing else...

I explain the Early voltage (https://www.researchgate.net/post/Can-the-Early-voltage-be-physically-explained) by presenting the collector-emitter part of BJT as a "self-varying" ("dynamic") resistor whose resistance increases when the voltage increases.There is no some internal voltage source "producing" the Early voltage:-)

The fact that the original real voltage source and its "source + resistor" model produce the same voltage and current values ​​does not mean that they are internally equivalent. For the sake of understanding, however, this is important because we want to answer the question, "How does it do it?"

"A real voltage source is a source that changes its voltage in proportion to the current" is a better answer than "A voltage drop proportional to the current is subtracted from the voltage of a constant voltage source."

Hi Cyril,

* Of course, I agree to your comments to the Early voltage. It is a pure artificial parameter for the purpose of describing (approximately) the slopes of the set of curves Ic=f(Vce). Remember, in my comments I spoke about the Early effect only (not about the Early voltage).

* It would be interesting to see how your model (described verbally, up to now) looks like using symbols and control functions.

Lutz, I am glad that we think alike...

In my imagination, resistance is something material that resists current. As a result, voltage is lost and power in the form of heat is dissipated. So, if we connect an element (resistor) with such a property in series to a constant voltage source, the voltage after the resistance will linearly decrease when the current increases.

But the opposite (that if the voltage decreases linearly when the current increases, it is a result of the presence of resistance) is not always true. The voltage may decrease just because the voltage source is varying. No resistance, no power losses... only changing voltage...

Voltage sources, whether batteries, generators, etc., are not ideal (perfect). A real voltage source is composed of an ideal voltage source in series with a resistance called internal resistance. This resistance does not really exist so that we can see it. It is a resistance deduced by the behavior of the real voltage sources.

in other terms, Internal resistance is the resistance within the battery or any other voltage source that causes a drop in source voltage when current is present. On the other hand, external resistance is the resistance present in the external circuit.