Hi again,

Yeah I like that better too, it's actually mesh analysis. It works on a lot of networks, but from what I understand it does not work on some networks that are non-planar. It's great for these simpler ones though.

I usually use Nodal Analysis though even though it could result in one more equation.

There are several methods that can be used. Some, like nodal and mesh analysis, are more formal and general (nodal is more general than mesh) and can be applied algorithmically to most circuits. Others are more ad hoc and can greatly simplify the analysis of some circuits, but are very difficult to apply to arbitrary circuits. The reduction of the circuit to a two-mesh circuit which allows the analysis by a single, very simple node equation is the approach I showed above. Once that voltage is known, the determination of the three loop currents in the original problem is trivially easy. This is also a problem that lends itself particularly well to superposition. But none of that is relevant because none of those will produce the matrix equation for the loop currents, which is what the problem specifically requests.

 

There are several methods that can be used. Some, like nodal and mesh analysis, are more formal and general (nodal is more general than mesh) and can be applied algorithmically to most circuits. Others are more ad hoc and can greatly simplify the analysis of some circuits, but are very difficult to apply to arbitrary circuits. The reduction of the circuit to a two-mesh circuit which allows the analysis by a single, very simple node equation is the approach I showed above. Once that voltage is known, the determination of the three loop currents in the original problem is trivially easy. This is also a problem that lends itself particularly well to superposition. But none of that is relevant because none of those will produce the matrix equation for the loop currents, which is what the problem specifically requests.

Hello,

My point was that doing it two different ways gives us at least one way to double check the original results when first done the way the problem statement asked for.

I thought about superposition too, and also about simulation. It would be weird to me to see any student that did not know how to use a simulator at least at some point in their studies. Although simulators are not the end of all ends, they did come a long way since I first started analyzing circuits.

 

I thought about superposition too, and also about simulation. It would be weird to me to see any student that did not know how to use a simulator at least at some point in their studies. Although simulators are not the end of all ends, they did come a long way since I first started analyzing circuits.

Circuit simulation should definitely be taught and students should be expected to become reasonably proficient, but I don't think that using a simulator to check results on problems at this level is appropriate. The purpose of problems at this stage is to develop the ability to apply fundamental concepts and techniques to a problem. It's about problem solving skills far more than it is about solving the problem. Using a circuit simulator is counter productive to that goal at this level.

 

Circuit simulation should definitely be taught and students should be expected to become reasonably proficient, but I don't think that using a simulator to check results on problems at this level is appropriate. The purpose of problems at this stage is to develop the ability to apply fundamental concepts and techniques to a problem. It's about problem solving skills far more than it is about solving the problem. Using a circuit simulator is counter productive to that goal at this level.

Hi,

"at this level" what does that mean.

Using a simulator is not counter productive unless the student relies on the outcome of the similator, which is not really possible due to the nature of the resolution of simulators. You usually can only get approximate results to see if your results are in the ballpark, although the ballpark is often very small.
For example:
We calculate 10.12345 volts, the simulator shows 10.0 volts or 10.2 volts, neither is as accurate as the calculation. But even if it was 10.12 volts, that may not be enough, and if the student relies on that it's not what I would tell them to do.
Checking the result is not about stealing the answers from a test, it's about having a second opinion. The idea is to FIRST calculate, then simulate. If there is a significant difference, then something MAY have gone wrong with the calculation. Zeroing in on what went wrong is what helps get it right.

I would find it very strange if they were taught NOT to use a simulator, EVER
I guess it is a possibility though, but not what I would tell anyone. I would make sure they knew the proper use though and how it can differ from an accurate calculation.

 

"at this level" what does that mean.

At the point in their educational journey were these are the kinds of problems that they are being asked to work.

The goal at this point is not to find how much current is flowing in a particular resistor in some contrived problem. It is on understanding the fundamental concepts behind several analysis techniques and being able to evaluate how to apply those concepts, something which is only accomplished by them doing the work. The more practice they get at doing so, including solving the same problem by multiple approaches, the better they will understand them and be able to apply them.

By using a simulator to check the work, they are missing out on valuable practice and not getting anywhere near the proficiency that they could if they had to check their work using the techniques being taught.

I saw this happen when I was an undergrad. PCs were just barely being made available in schools and they were very limited in their abilities. The semester that I took Circuits I one of the instructors showed their class how to use PSPICE (from a command line running simulations on manually written circuit files) and encouraged his sections to use it. The rest of the sections weren't shown it at all (I was introduced to it a year later when I took the Electronics I course). So we had to check our work using the techniques available to us at that point. All sections took the same exam (it was on paper at a common exam time) and his sections did substantially worse than all of the others -- I think the average on the final exam for his sections was something like 15 percentage points lower and the vast majority of people that failed the course were from his sections. And it wasn't because he was a poor instructor -- his track record was extremely solid and, when he dropped the use of the simulator from his course the next semester, he sections were right up at the top again. In later courses it was usually pretty evident when I was working with someone that had had the misfortune to take his section that semester as their understanding of the fundamental analysis concepts tended to be very weak. Of course, there were exceptions both ways, as there always will be.

[QUOTE[
Using a simulator is not counter productive unless the student relies on the outcome of the similator, which is not really possible due to the nature of the resolution of simulators. You usually can only get approximate results to see if your results are in the ballpark, although the ballpark is often very small.
[/QUOTE]

It's counter productive if it results in them not learning the material as well as they would have without it. That's pretty much the definition of counter productive.

As for not being possible -- the resolution and precision of simulation results for the kinds of problems that would be being asked, namely problems that are expected to be done by hand, is far more than what is needed.

For example:
We calculate 10.12345 volts, the simulator shows 10.0 volts or 10.2 volts, neither is as accurate as the calculation. But even if it was 10.12 volts, that may not be enough.

Show me an example of a legitimate problem in which the student would need to give an answer to seven significant figures.

By legitimate problem, I mean a problem that would be typical of the kind of problems given to students that are just learning the basic analysis techniques such as mesh, nodal, and superposition.

Show me an example of a legitimate problem in which a decent simulator, such as LTSpice, isn't going to give an answer that is accurate to more sig figs than the student is going to be expected to report their results.

Let's consider this problem as an example. If we do the analysis to find the voltage across the vertical 2 Ω resistor we discover that it is

Va = (860/121) V = 7.1074380165289.....

Putting that circuit into LTSpice and running an operating point analysis yields

Va = 7.107438 V

Checking the result is not about stealing the answers from a test, it's about having a second opinion.

And just where did I say that using a simulator was somehow stealing the answers from a test????

I said that checking answers with a simulator is counter-productive to the goal of gaining understanding of and proficiency in applying basic analysis techniques.

I would find it very strange if they were taught NOT to use a simulator, EVER
I guess it is a possibility though, but not what I would tell anyone. I would make sure they knew the proper use though and how it can differ from an accurate calculation.

And just where did I say that they should ever be taught to use a simulator?

I seem to recall saying, "Circuit simulation should definitely be taught and students should be expected to become reasonably proficient."

 

At the point in their educational journey were these are the kinds of problems that they are being asked to work.

The goal at this point is not to find how much current is flowing in a particular resistor in some contrived problem. It is on understanding the fundamental concepts behind several analysis techniques and being able to evaluate how to apply those concepts, something which is only accomplished by them doing the work. The more practice they get at doing so, including solving the same problem by multiple approaches, the better they will understand them and be able to apply them.

By using a simulator to check the work, they are missing out on valuable practice and not getting anywhere near the proficiency that they could if they had to check their work using the techniques being taught.

I saw this happen when I was an undergrad. PCs were just barely being made available in schools and they were very limited in their abilities. The semester that I took Circuits I one of the instructors showed their class how to use PSPICE (from a command line running simulations on manually written circuit files) and encouraged his sections to use it. The rest of the sections weren't shown it at all (I was introduced to it a year later when I took the Electronics I course). So we had to check our work using the techniques available to us at that point. All sections took the same exam (it was on paper at a common exam time) and his sections did substantially worse than all of the others -- I think the average on the final exam for his sections was something like 15 percentage points lower and the vast majority of people that failed the course were from his sections. And it wasn't because he was a poor instructor -- his track record was extremely solid and, when he dropped the use of the simulator from his course the next semester, he sections were right up at the top again. In later courses it was usually pretty evident when I was working with someone that had had the misfortune to take his section that semester as their understanding of the fundamental analysis concepts tended to be very weak. Of course, there were exceptions both ways, as there always will be.

[QUOTE[
Using a simulator is not counter productive unless the student relies on the outcome of the similator, which is not really possible due to the nature of the resolution of simulators. You usually can only get approximate results to see if your results are in the ballpark, although the ballpark is often very small.

It's counter productive if it results in them not learning the material as well as they would have without it. That's pretty much the definition of counter productive.

As for not being possible -- the resolution and precision of simulation results for the kinds of problems that would be being asked, namely problems that are expected to be done by hand, is far more than what is needed.



Show me an example of a legitimate problem in which the student would need to give an answer to seven significant figures.

By legitimate problem, I mean a problem that would be typical of the kind of problems given to students that are just learning the basic analysis techniques such as mesh, nodal, and superposition.

Show me an example of a legitimate problem in which a decent simulator, such as LTSpice, isn't going to give an answer that is accurate to more sig figs than the student is going to be expected to report their results.

Let's consider this problem as an example. If we do the analysis to find the voltage across the vertical 2 Ω resistor we discover that it is

Va = (860/121) V = 7.1074380165289.....

Putting that circuit into LTSpice and running an operating point analysis yields

Va = 7.107438 V



And just where did I say that using a simulator was somehow stealing the answers from a test????

I said that checking answers with a simulator is counter-productive to the goal of gaining understanding of and proficiency in applying basic analysis techniques.



And just where did I say that they should ever be taught to use a simulator?

I seem to recall saying, "Circuit simulation should definitely be taught and students should be expected to become reasonably proficient."
[/QUOTE]

Hi again,

I think you are reading too much into my replies. I never said you said using a simulator was somehow stealing the answers from a test, I was using that as a comparison just to say it's not that bad.

"Show me an example of a legitimate problem in which the student would need to give an answer to seven significant figures."
Example:
Connect a 12v battery source to a resistor of 9.72 Ohms. Express the resulting current to 7 significant figures.
Cheat: +1.234568, but not +1.234567 and that is to see if they know to round the last digit from a 7 to an 8.

I am not sure I said that though. All I meant was that a simulator is ANOTHER way to test our calculation results. It can't be used as a substitute because it probably gives different results, but off by only a little bit unless the spice models used are much different.
I good example I think is a problem that uses a diode with zero voltage drop. Most diodes in spice have at least some voltage drop, so they would have to realize that too. There are ways around it, but they probably don't know that yet

I'm all for using two different analysis methods using math too.

 

"Show me an example of a legitimate problem in which the student would need to give an answer to seven significant figures."
Example:
Connect a 12v battery source to a resistor of 9.72 Ohms. Express the resulting current to 7 significant figures.
Cheat: +1.234568, but not +1.234567 and that is to see if they know to round the last digit from a 7 to an 8.

In what universe would that ever be a legitimate problem?

You might as well insist that they identify that it is a repeating decimal or that they must give the answer as 100/81 A.

Have you EVER been asked to take a 2 sig fig number, divide it by a 3 sig-fig number, report the result to 7 sig figs, and then be marked correct or incorrect based on how you rounded the 7th sig fig?

Even asking them to do that is poor pedagogy because it reinforces the tendency of students to report answers to more sig figs that are justified merely because some calculator or simulator or spreadsheet happens to display them. Instead, students that reported an answer to seven sig figs instead of a much more reasonable two or three are the ones that should lose credit.



Furthermore, how does this in any way distinguish whether the person used a calculator manually or used a simulator -- in either case it is up to them to round the last digit appropriately.

[/QUOTE]
I good example I think is a problem that uses a diode with zero voltage drop. Most diodes in spice have at least some voltage drop, so they would have to realize that too. There are ways around it, but they probably don't know that yet
[/QUOTE]

First off, diodes are probably a non-issue at this level because these analysis techniques are almost always taught in a first-year (typically sophomore year because of the math/physics prereqs) linear circuits course. Diodes are a non-linear component and some of the analysis techniques, such as superposition and techniques based on it, like mesh current analysis, are not valid (without more knowledge and skill than is expected at this point).

When nonlinear components are presented, usually in a third course, Electronics I, following Circuits I (Linear Circuit Analysis) and Circuits II (Transform Methods), then if circuit simulations that match ideal hand calculations are desired or expected, the students would be provided with models or told how to create them. It's really quite trivial, you just put in a .model statement, give it a name, and use the D model specifying the basic parameters you want. For instance, to create an ideal 4.7 V Zener diode, you would use.

.model Dzener D(Vfwd=0, Vrev=4.7)

A basic ideal diode is just

.model Dideal D(Vfwd=0)

An ideal LED with a 2 V forward voltage is just

.model Dideal D(Vfwd=2)

If you want your ideal diodes to have a 0.7 V forward drop, then it's

.model Dideal D(Vfwd=0.7)

You can also specify forward and reverse resistances to match those models as well.

You can do similar ideal transistor models. LTSpices basic opamp model is already an ideal model.

 

"In what universe would that ever be a legitimate problem? "

In my universe
I think you are being too pedantic with something so subjective.

 

"In what universe would that ever be a legitimate problem? "

In my universe
I think you are being too pedantic with something so subjective.

Well, since I'm not interested in problems that are only legitimate in your fantasy universe, I see no reason to continue the discussion.

 

Well, since I'm not interested in problems that are only legitimate in your fantasy universe, I see no reason to continue the discussion.

Ha ha, I like that. Ok no problem