Talk:Equivalent circuit

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Differences between "reality" and "Equivalent circuit"[edit]

Are there methods of measuring of at what extend our abstract model of electric circuit differs from "real system function" of question. Can we detect whether principles of functioning of elements of our "equivalent system" are based on the same fundamental physical principle? Is it necessary in adding a theme about methods of measuring of quality of this Equivalent circuit. I mean to add in article not only methods of mathematical estimation but also connection between our model of abstraction ("Equivalent circuit") with principals of functioning of real system. Thank You 178.66.233.84 (talk) 08:44, 30 October 2014 (UTC)[reply]

There are two meanings to equivalent circuit and the answer depends on which meaning you mean. One meaning is that the circuit is electrically equivalent in every way to some other abstract circuit. Usually this means that the impedance between every pair of poles (terminals) of the circuit are equal between the two equivalents. This of course means we must define beforehand which nodes of the circuit are to be considered poles. For instance, two resistors, R1 and R2, in series have an equivalent circuit of a single resistor equal to R1+R2. There is an unstated assumption here that the node at the junction of the two resistors is not considered a pole. See the article equivalent impedance transforms for some not so trivial examples. Equivalents of this sort are arrived at analytically, they are mathematically exact equivalents.

The other meaning (which I suspect is the one you mean) is that the equivalent circuit is a model of some real system. One cannot even begin to answer questions on the accuracy of the model without knowing the specific details of the system being modelled. However, the two most common simplifying assumptions leading to modelling errors are that the system is linear, and that it can be represented by a lumped element model. As for measuring the errors, this is a simple matter once we have some idea where the model starts to depart from reality; is it at high or low voltage, current, frequency, magnetic flux, physical size, etc etc. SpinningSpark 14:59, 30 October 2014 (UTC)[reply]

Equivalent circuits and models[edit]

@Chetvorno: your most recent series of edits have conflated model and equivalent circuit. I have reverted the changes for now. The changes (in part and with references moved for clarity) are:

Original (and current) text

In electrical engineering and science, an equivalent circuit refers to a theoretical circuit that retains all of the electrical characteristics of a given circuit. Often, an equivalent circuit is sought that simplifies calculation, and more broadly, that is a simplest form of a more complex circuit in order to aid analysis.

Reverted text

In electrical engineering an equivalent circuit is a theoretical electric circuit that is different from but shares important electrical characteristics with a given actual circuit. Equivalent circuits are used as aids in circuit analysis. Often, an equivalent circuit is sought that has similar behavior to a real circuit under limited conditions but is easier to analyze, so solving the equivalent circuit gives the behavior of the real circuit under those conditions. For example, an AC equivalent circuit is a circuit that emulates the behavior of a real circuit only for alternating current (AC) inputs. A small-signal equivalent circuit is a circuit which is used to calculate the behavior of a nonlinear circuit when small AC voltages and currents are applied to it.

An equivalent circuit is not an approximation. It is an exact equivalent of the thing being compared. If it were not, we would be forced to say approximate equivalent circuit. A model may well not be an exact representation of a physical system. Almost by definition it is not. But an equivalent circuit of the model will be an exact representation of that model. For instance, the Thevenin equivalent is an exact equivalent of the models chosen to represent a set of interconnected generators, but the model of those generators may be very approximate indeed. Your hiding of small-signal model through a pipe to small-signal equivalent circuit rather bears this out.

Admittedly, the article is already confused on this point, but these changes made it so much worse. SpinningSpark 10:41, 30 November 2016 (UTC)[reply]