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Thevenin and Norton

Thevenin and Norton

RE: Thevenin and Norton

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.

RE: Thevenin and Norton

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



RE: Thevenin and Norton

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.

RE: Thevenin and Norton

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.

RE: Thevenin and Norton

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
www.gke.org

RE: Thevenin and Norton

(OP)
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. ...

RE: Thevenin and Norton

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.

RE: Thevenin and Norton

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.

RE: Thevenin and Norton

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.

RE: Thevenin and Norton

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

RE: Thevenin and Norton

bacon4life ,
And was your answer an acceptible one?

Scott

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

RE: Thevenin and Norton

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.

RE: Thevenin and Norton

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.

RE: Thevenin and Norton

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.

RE: Thevenin and Norton

(OP)
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?

RE: Thevenin and Norton

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.

RE: Thevenin and Norton

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.

RE: Thevenin and Norton

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.

RE: Thevenin and Norton

Why is that meaningless?  

Is it any more irrelevant than figuring out the threshold voltage of a TTL input?  Which the applicant failed to answer THREE times, where he was shown the answer after both the first and second times?

TTFN



RE: Thevenin and Norton

Ha!  Good point IR.

After learning the Thevenin and Norton stuff I have never ever used it. Pish-shaw!

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Thevenin and Norton

I made up a test to give to perspective technicians at an interview.  Every one of the applicants said they worked from schematics and repaired boards at a component level.  It was 20 questions with four multiple choice answers.  Simple things like he would find in the job.  Example;  A transistor with .75V on the base of a transistor 0V on the collector sinking current from a LED.  LED is not on and 12V is on the otherside of it is 12V.  We  have a lot of LEDs that get put in backwards.  Another id a voltage on a resistor and figure out the current, nice even number stuff.

Where you could get 25% just by chance, I found a score of 35% was a good candidate.  Some people ran out of the room.  The only one that ever passed it said he wouldn't work at such an idiotic place.  I had to reduce the test to 4 questions because they were taking close to an hour to answer 20 simple questions!

RE: Thevenin and Norton

OperaHouse;  Ah, that is some choice stuff there!

I once took a test for the electrician's union placement.  I aced the math and was in the top 5% for the the ridiculous "draw lines between the lines" of hundreds of concentric circles with both hands at the same time.  Some sort of mind numbing dexterity test.  Then I got to the verbal.  After we all laughed about the my then recent news worthy arrest for driving around my town and over some hundred foot trestles on the railroad tracks they asked me how badly I wanted to be an electrician. I said I wanted to actually be an Electrical Engineer.. That was a conversation stopper. Hahaha . We all decided I shouldn't bother with the electrician path.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Thevenin and Norton

Keith - And the rest, as they say, is history...  

RE: Thevenin and Norton

I had just finished my first semester of circuits class, where we of course covered the idea of equivalents.  We had also just touched on voltage dependant current sources and current dependant voltage sources.  Since we had never combined the two ideas, I figured I could try putting one on the equivalent circuit.  Maybe an active device would react differently than the passive RLC components we talked about in class?  It turns out that if you put a voltage controlled current source on the terminals, and then proceed work through some math you can prove the difference as long as you ignore a minus sign on the first step.  I figured I had also better also try out a current controlled voltage source, which I was actually did correctly.  As I was redoing the math for the first case, the interviewer finally took pity on me an explained about the heat/power cord technique.

I felt really bad, until I went back to my dorm and asked many of peers who were all as stumped as I had been.

RE: Thevenin and Norton

The Thevenin & Norton Equivalents are only meant to be equivalent at their two terminal outputs, not necessarily so from the power supply end.  One could just as easily claim (correctly) that you could just open the box and look!

By the way, it might confound the line-cord heat analyzers if they were not allowed to "peek" at the two output terminals (fair's fair!)  How?  Easy.  If the two ouput terminals were shorted (the dual of the open circuited case) the Norton Equivalent box would remain cool as a cucumber....

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