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Wikipedia writes this about calculating the output current \$I_{AB}\$ when finding the Thevenin equivalent:

2a. Replace voltage sources with short circuits, and current sources with open circuits.

2b. Calculate the resistance between terminals A and B. This is \$R_{Th}\$.

But! What if the circuit contains the following (independent) current source and resistance:

+--R---o
|
I
|
+------o

I think they could be replaced with a voltage source with the potential difference \$V=I*R\$.

If we calculate the output current before replacing and after replacing the above, we would get different answers! What am I misunderstanding?


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Sorry, how can I make the question better? –  Karin Sep 28 '11 at 19:17
    
I tried to use latex, but can only get $$ to work (not $). –  Karin Sep 28 '11 at 19:37
1  
I don't know why you've been downvoted. However, I do know that, on our site, we have quite a few questions involving prices, so we switched our MathJax delimiters to \$ and $$. I've edited your question to use these. –  Kevin Vermeer Sep 28 '11 at 19:52
    
Thank you Kevin! –  Karin Sep 28 '11 at 19:57
    
@karin, you get 5 for an upvote and -2 for downvote, so you make decent reputation gains as long as you are breaking even. –  Kortuk Sep 28 '11 at 20:02
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2 Answers

up vote 2 down vote accepted

That could only be replaced by an infinite voltage source.

Current sources operate on the basis that they will output their current no matter the load. When you leave it open circuit your open circuit voltage would be infinity. IE, this circuit cannot exist.

The standard replacement will be a current source with a parallel resistance to a voltage source with series resistance. As shown in this wikipedia image.

Image from wikipedia at-http://en.wikipedia.org/wiki/Th%C3%A9venin's_theorem

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Thank you for correcting my misunderstanding! –  Karin Sep 28 '11 at 20:44
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There is nothing wrong with your answer - you just have to work through it and then see the implications.

You come up with something similar to what Kortuk said except that you don't need to change what is asked IF that was what was asked.

Following your rules you get Rth = infinite.
The resistance of a perfect current source is infinite.
It's voltage will rise to infinity if necessary to cause its define current to flow. Open circuiting a perfect current source is on the dangerous side compared to sticking a bent nail into a near-perfect voltage source such as the AC mains.

So your instructions work.

ie Thevenin's laws should work for anything linear, but a current source is a bit of a strange beast and needs an apparently strange answer to make sense of it.

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a current source always needs a parrallel discharge path to be physically realizable for any output conditions, and as we both know, it is still never a true current source, just a voltage source with a different kind of feedback network for regulation. –  Kortuk Sep 30 '11 at 15:37
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