Superposition Thoerem Understanding Problem

[hevsuit]

Superposition Theorem Understanding Problem

Hi Guys,

The last few weeks in my course I've been studying the superposition theorem, where the total effect of multiple sources in a network can be made into a summation of each of the effects of the individual sources each acting on their own inside the network.

The problem I have is when it comes to replacing Voltage sources with short circuits, and Current sources with open circuits. To put it simply, I do not understand intuitively why these replacements are incurred.

This especially bothers me when I'm dealing with a circuit with both a Voltage and Current source working in parallel.

My mind keeps telling me that a Current source is in fact a voltage source that wants to maintain a certain predefined current, and if this is the case, why not short it?

What annoys me most of all is that I can in fact solve problems without difficulty using this method, and the Maths does not lie. I've even carried out simulations of circuits using Multisim 11 in an effort to try and understand the reasoning behind all of this, to no avail.

I hope you guys can somehow enlighten me with some simple logical reasoning.

Cheers

Alex

 

[chaoseverlasting]

I assume you are working with Thevenin's and Nortons theorems on circuit analysis? What these theorems exploit is that from the point of view of the source (current or voltage, they are actually interchangable and both of the theorems are two different ways of saying the same thing), any circuit can be reduced to a 'net impedence'.

This is because, from the point of view of the source, the source doesn't know the exact configuration of the circuit, but it powers it anyway. Thus, from the point of view of the supply, the exact configuration is equivalent to the net impedence.

To find this net impedence, depending on the theorem used, different methods to remove the source and find the required impedence are used.

If you give us a practical example, maybe we can better help you.

 

[uart]

Hi Guys,
The problem I have is when it comes to replacing Voltage sources with short circuits, and Current sources with open circuits. To put it simply, I do not understand intuitively why these replacements are incurred.

Think of it like this. It's not fundamentally about open circuits and short circuits, it's about setting the other sources (that is, other than the one you are currently analyzing) to zero. Have you got that, because it's important, it's not an open circuit, it's not a short circuit, it's just a zero source.

Now it just so happens that a zero-volt voltage source is equivalent to (has the identical V/I characteristics of) a short circuit. And a zero-amp current source is equivalent to (has the identical V/I characteristics of) an open circuit.

Hope that helps.

 

[UR_Correct]

Think of it like this. It's not fundamentally about open circuits and short circuits, it's about setting the other sources (that is, other than the one you are currently analyzing) to zero. Have you got that, because it's important, it's not an open circuit, it's not a short circuit, it's just a zero source.

Now it just so happens that a zero-volt voltage source is equivalent to (has the identical V/I characteristics of) a short circuit. And a zero-amp current source is equivalent to (has the identical V/I characteristics of) an open circuit.

Hope that helps.

Yes, I was going to say this.

This is how I always put it when tutoring: You're essentially setting the source equal to zero. Setting a voltage source equal to zero is like a zero voltage potential drop across something, or like a wire, so you short it. Setting a current source equal to zero means there is no current running through that branch, so you can call it an open circuit.

 

[hevsuit]

Hi lads,

Thanks a million for explaining this in detail. I now understand the logic and analogies behind it. This will also help others in my group who are trying to grasp this concept.
I did some extra maths to satisfy myself, and it now all makes sense!

Thanks to you all!

Cheers

Alex