# Why use only oversimplified/ideal equivalent circuits?

Most circuit analysis books use ideal resistor, inductor, capacitor and don't even bother to show practical circuits of these components, let alone any solved examples with practical circuits.

For diodes, all of these books give examples for ideal and practical diodes (to some extent). My question is if it is just because of the complexity involved (although students should be helped to solve at least a few of these to understand it better) or is it because results using ideal components are very close to original/practical in case of R, L, C than in case of diodes?

• I think it has to do with the degree of accuracy required and the application scenario. For most cases the non-complex or more-idealistic treatment suffices !
– vyi
Commented May 20, 2014 at 5:58
• @vvy: Don't you think students should at least know by text book that what is actual behavior compared to ideal, and solve a few examples to know the difference. Commented May 20, 2014 at 6:04
• I think its conventional to start with the simple and gradually add on the complexity. On a basic level, I've seen/read textbooks which have hints about the ideal vs non-ideal models of components. There are topics like tolerence values which are meant to signify these considerations. The advanced level course books do consider these intricacies.
– vyi
Commented May 20, 2014 at 6:19

The real world is infinitely complex, so all descriptions are approximations. There is no "final and definitive truth", or if it exists we don't know it yet. (A colloquial term is "it is turtles all the way down": you never get to the root cause.)

In engineering we always use an abstraction (= simplified description) of the components we use. That abstraction should be valid within the limits of the problem we are solving. What people (and teachers, and textbooks, and worse: engineers) sometimes forget is to state or be aware of these limits. (This accounts for a fair amount of funny questions we get here at S.E.!)

I don't know your textbook, I can only hope that

• the level of description it gives is appropriate for the example problems it shows and for the problems the students have to solve
• it states the limits of the description (probably mainly frequency, but also extreme current, voltage, temperature, etc.)

To finally answer your question: The type of problems that the students are expected to solve is the rationale for the level of description.

• +1 for What people (and teachers, and textbooks, and worse: engineers) sometimes forget is to state or be aware of these limits. Commented May 20, 2014 at 6:58

The R, L and C also show non-ideality. But their effect becomes significant only at high frequencies, when the signal wavelength is comparable with circuit element size.

But in Network Theory, we use lumped model. The assumption is that the signal wavelength is not comparable with the circuit elements. So you can use passive elements as ideal there.

But if you are playing at higher frequencies, you have to consider these non-ideality. Think that you haven't came across electromagnetic theory.

But active elements like diodes' deviation from ideal is significant at low as well as high frequencies.

• I am myself aware of the fact about higher frequencies. To answer I would say diode non-ideality is significant on low voltages only. Point being why we can't see non-ideal equivalents in most circuit text books (not even in AC section), even if used at very high frequencies. These are found only in specialized books and most students are not aware of it. Commented May 20, 2014 at 5:59
• Most of the components are manufactured assuming that they going to be operated at specific voltage range (~10V), normal frequency range (audio frequency) so text books also talk about a model which is valid in this scenario. Commented May 20, 2014 at 6:08
• Nah, if you look close enough parts are non-ideal at any frequency. Resistors have Johnson-Nyquist noise, and if you model that they have excess noise. They have a temperature coefficient, and if you model that, it's not linear, and if you model that there is hysteresis and if you model that it's time dependent etc.etc. It's a never-ending series of matryoshka dolls. Commented May 20, 2014 at 10:45
• @nidhin whoever said that was not an engineer ;) jokes! Commented May 20, 2014 at 13:30
• @Waqar yes he was. an engineer with long sightedness. :-) Commented May 20, 2014 at 13:38