Why do real circuits look different compared to circuits people study

  • #1

Main Question or Discussion Point

Doing circuit analysis everything comes in rectangular loops and it just adds more rectangles. In my Circuit Analysis studies we don't even use the ground symbol. But my teacher referred to the ground symbol once as the reference node which we use in the Node Voltage Method.

In reality circuit just have wires everywhere with not much rectangular loops. How am I meant to apply what I have learnt to circuits like this.

Why is this?

(I have yet to study Thevenin theorem and Norton theorem, will that enable to analyze more complex circuits? Also we have only dealt with resistors not really any other electronic element, we study that later I think)
 

Answers and Replies

  • #2
FactChecker
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The analysis is done on a symbolic representation of the real circuit. A real circuit can be a "can of worms", which would be impossible to analyze.
 
  • #3
Delta2
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The exact geometry of the circuit (whether it is rectangular or circular or ellipsoid or some other geometry) plays an important role for the study of the electric and magnetic fields inside and around the circuit, but plays minimal role if we study the circuit in terms of voltages and currents, that is it doesn't affect as much the voltages and currents throughout the circuit. That is there can be many different configurations of electric and magnetic field inside and around the circuit (each configuration for each different geometry) that all have approximately the same configuration for voltages and currents. If you want to know more as to the "why" the geometry affects the EM field but not as much the voltages and currents just ask.

In circuit theory we are interested mainly for the voltages and currents in various points throughout the circuit, therefore the circuit geometry plays a minimal role.
 
  • #4
jim hardy
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Doing circuit analysis everything comes in rectangular loops and it just adds more rectangles.
Draftsmen like order. In the early days everything was hand drawn using T-Squares .
Good draftsmen took pride in making a product that was both pleasing to the eye and led the mind to a quick understanding.

In reality circuit just have wires everywhere with not much rectangular loops. How am I meant to apply what I have learnt to circuits like this.
THAT my friend takes a lot of concentration. It is seldom easy to translate between a schematic and a physical layout.
When you start experimenting and building circuits you will at first lay them out just like the schematic, well, as close as the pin arrangement will allow.
As you "learn the language" you too will build 'rat's nest' circuits.
See this fun link:
http://www.jackdollhausen.com/shebang/quest/howto/how.htm


Why is this?
It takes less wire and physical space to build a rat's nest. .
Observe that a :"Node" might be many inches of a zigzag line on a schematic but physically only needs to be one dot.

old jim
 
  • #5
eq1
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In reality circuit just have wires everywhere
It depends on the circuit. Some circuits are simple. But most circuits do complex things so it should follow that they themselves are complex.

As one gains experience usually they stop thinking in terms of circuit elements and instead you analyze in terms of groups of elements (subcircuits) and systems. You find the common patterns that repeat over and over again and fall back on the equations that describe that pattern. It makes the complexity much more manageable. Similar to the way you don't think in letters when writing. You think in sentences and words.
 
  • #6
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Circuit is a broad word. I like to think of the North American Power Grid east of the Rockies (including everything that's plugged in to it) as a circuit.
:wink:
 
  • #7
Svein
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In reality circuit just have wires everywhere with not much rectangular loops. How am I meant to apply what I have learnt to circuits like this.

Why is this?
Finicky electronic technicians used to put wires in nice bundles and place them in nice rectangular "cable ways". That worked fine until the cables started to carry high frequency (>10MHz) signals - and then trouble started.
  • Placing cables close to another meant that there was a capacitive coupling between them
  • A long cable carrying a digital signal suddenly had to be treated as a transmission line ("long" being related to the rise and fall time of the signal)
  • "Signal ground" stopped being equivalent to "0V power"
Therefore the "rats nest" in the lab (with wires going randomly all over the place) would suddenly stop working when tidied up...
 
  • #8
sophiecentaur
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Why is this?
You can regard it as a piece of real luck that the range of possible component values is so huge. Engineers are very good at identifying which are the relevant variables in any Engineering problem and know what they can ignore in most circumstances. (This also applies to Mechanical and other branches of Engineering) Resistance values can cover a range of around ten orders of magnitude; Inductance and Capacitance, a few orders less, in practice. The 'parasitic' values of R,L and C that are commonly found in many 'real' circuits are usually way outside the range of values of the components that are used for low frequency or DC circuits. When a parasitic component shows up, it can very often be characterised by, say, a small amount of series R or parallel C and the conventional a circuit representation can have that element added to it.
As has already been mentioned, at higher 'radio frequencies' we can't get away with this and the 'parasitics' can be the dominant elements - e.g in a transmission line or antenna.
Strangely, it is still possible to minimise many of the parasitic effects in extremely high frequency digital circuits. The one thing that you cannot ignore is the speed that signals travel around a processor and it is essential to make connecting paths of equal lengths so that signals will coincide at, say, the input of an OR gate. There are some relatively simple circuit layout rules that can be followed to avoid such 'hazards'. Then you are back with the simple schematic diagram again.
 
  • #9
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In reality circuit just have wires everywhere with not much rectangular loops. How am I meant to apply what I have learnt to circuits like this.
Those rectangular loops like the first step for a baby. Just keep on learning, and you will be able to run: later on you might get your first car.

Maybe one day you will fly a plane too.
 

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