Thevenin and Norton

[stevenal]

http://www.doonesbury.com/strip/dailydose/index.html?uc_full_date=20060504

http://www.doonesbury.com/strip/dailydose/index.html?uc_full_date=20060510

The question is: Am I the only one who thinks Alex, the Cornell professor and G.B. Trudeau seem to have missed the whole point of Thevenin and Norton equivalent circuits?

 

[GregLocock]

Open circuit the ideal current source box would get warm as it would still have a current running through the resistor in parallel with the source, whereas the ideal voltage source would just sit there.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[IRstuff]

This goes well with the question about simulators and simulations. While the two configurations are identical with respect to terminal characteristics, they behave quite differently thermally and power-wise.

In a similar vein, a DC model of a BJT is not the same or even relevant to the AC model; yet, they both purport to model the BJT.

It's a very good reminder that not all equivalents are equivalent.



TTFN

 

[davidbeach]

I think it's great that something that esoteric and highly specific to electrical engineering would wind up in the daily comics. I also agree that should it actually be possible to build those two black boxes (no power cord allowed, otherwise it isn't a two terminal black box), the current source would have to dissipate power in the resistor while the voltage source would do nothing while open circuited. Short circuit the two black boxes and the current source will cool off and the voltage source will heat up.

 

[Warpspeed]

Any power source can be regarded as either a current source or a voltage source, the question has no meaning.

Unless the source impedance is either an impossible zero, or equally impossible infinity, it must then be something finite. It can then be measured, and reduced to a simple equivalent of either model.

 

[Skogsgurra]

I agree, warpspeed.

The Thevenin/Norton equivalents are not physical circuits. They are mathematical (or mental) representations that simplify analysis of circuits.

The idea that they are "boxes" probably stems from class-room demonstrations, like the situation in the comic strip. But, as already said, they are not. And the question can not be asked.

But, it is perfectly OK to ask the student if he can tell whether a circuit consists of a current source and a parallel resistor or if it is a voltage source and a series resistor. He shouldn't be able to tell which is what - and that demonstrates the equivalency between Thevenin and Norton.

Gunnar Englund

http://www.gke.org

 

[stevenal]

I'll agree with Skogsgurra. The resistor in a Norton circuit is needed to make the ideal current source behave like a real source. The same real source has a similar thevenin circuit. The problem statement says nothing about these added resistors, so the ideal current source is easily determined by the arcing across the open terminals. If real sources are assumed, one behaving more like a current source and the other behaving more like a voltage source, they are certainly not mathematically equivalent and test tools can find the difference. If sources and circuits are both present in the boxes, then a resistor in parallel with a voltage source can be just as effective in producing heat.

I would suggest Alex rephrase the question: Two black boxes are found to have identical parameters using test tools. One is known to include a current source, the other known to include a voltage source. ...

 

[Warpspeed]

But the whole point of this exercise is, suppose you have three power sources, a real world one, and the EXACT Thevenin and Norton models of that real world supply.

There would be no physical way of distinguishing the difference by any testing you could possibly do across just the load terminals.

That is why the original question is meaningless. They truly are fully interchangeable equivalents, both mathematically, and in practice.

 

[ScottI2R]

Jeez,
And I thought the entire point of the exercise was to determine which college to attend.......no wonder I am going to McDonald's (see my laptop screen post)

Enjoy everyone,
Scott

In a hundred years, it isn't going to matter anyway.

 

[Warpspeed]

Yes indeed Scott.

Not only will Mc Donald's hamburger university give you a well rounded education, you can also get a good square meal.

http://en.wikipedia.org/wiki/Hamburger_University

 

[ScottI2R]

WOW!
Now I wonder what happened to my original post?

I think I am losing my mind!!!!!


Scott



In a hundred years, it isn't going to matter anyway.

 

[bacon4life]

I got asked this question in one of my first internship interviews.

 

[ScottI2R]

bacon4life ,
And was your answer an acceptible one?

Scott

In a hundred years, it isn't going to matter anyway.

 

[OperaHouse]

Alex should go to RIT. All the people I knew going to Cornell never graduated. They all got within 6 credit hours of graduating and would then changed thier major (several times). Ithica is a tough place to leave.

 

[Warpspeed]

It is an excellent question to ask at an interview because it separates those that just remember things, from those that understand the real meaning behind it.

 

[ScottI2R]

Yes,
I agree with Warpspeed. And to all, my original post about my laptop LCD was deemed "inappropriate for this forum" and has been removed. When they answer me, I will re-word it without the humor and post it again.

Thanks,
Scott

In a hundred years, it isn't going to matter anyway.

 

[stevenal]

So it's worse than I thought? Employment decisions are being made on the basis of a meaningless question? And I thought it was just a cartoon artist who didn't get it. Or was some other variation of the question asked in these interviews that actually made sense?

 

[Warpspeed]

Oh, I think you could be pretty sure that the interviewer would pose the question in a rational manner, and the response of the interviewee assessed fairly.

If the candidate truly knows his stuff, he is not going to be rattled by something like that.

The last job interview I had there were two interviewers, and it lasted NINE HOURS, It was pitch dark when I finally emerged. But I did get the job, and they paid me for the day, hehehe. That was with a telecommunications company in Melbourne.

 

[ScottI2R]

stevenal,
This a continuing problem that occurs when management dictates that prospective employees transgress through HR. Lower mgt. proposes the questions, and then the results are relayed back to the "hiring manager." That's my life anyway. Multiple interviews with the knowlegable persons involved could (and should) prevent this problem. I personally don't see this happenning any time soon.

Thanks,
Scott

In a hundred years, it isn't going to matter anyway.

 

[Warpspeed]

That may unfortunately be true in large corporations and government service.

But in smaller companies, and privately owned businesses, the interviewer is most likely to be the chief engineer.