- #1
What's the equivalent resistance between point A and B?
- Thread starter oliverlines1234567
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Discussion Overview
The discussion revolves around finding the equivalent resistance between two points, A and B, in a circuit that includes a constant current source. Participants explore the implications of the current source's characteristics on the circuit's behavior, particularly in relation to series and parallel resistances.
Discussion Character
- Technical explanation
- Conceptual clarification
- Debate/contested
Main Points Raised
- One participant seeks a method to find the equivalent resistance without needing an exact value.
- Another participant suggests that an ideal voltage source has zero internal resistance, prompting a discussion about the properties of current sources.
- Several participants propose that an ideal current source has infinite internal resistance, leading to the conclusion that it can be treated as a normal series and parallel resistor arrangement.
- There is confusion expressed about how current can flow through something with infinite resistance, with participants attempting to clarify that no current flows through the current source itself.
- Some participants discuss the concept of source transformation, comparing practical current and voltage sources and their internal resistances.
- Disagreement arises regarding the validity of connecting an ideal voltage source in parallel with an ideal current source, with some arguing it is invalid while others assert it can be valid under certain conditions.
- Participants explore the implications of excess current and how it interacts with the internal resistance of a current source, with some suggesting that this leads to an effective infinite resistance in the circuit.
Areas of Agreement / Disagreement
There is no consensus on the validity of connecting an ideal voltage source in parallel with an ideal current source. While some participants agree on the infinite resistance of an ideal current source, the discussion remains unresolved regarding the implications of this on circuit analysis.
Contextual Notes
Participants mention the internal resistances of practical sources and the conditions under which certain configurations are valid or invalid. There are references to source transformations and the behavior of ideal versus practical sources, indicating a nuanced understanding of circuit theory.
Who May Find This Useful
This discussion may be useful for students and practitioners interested in circuit analysis, particularly those exploring the behavior of current and voltage sources in various configurations.
- #2
Averagesupernova
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Well what do you know about current source? I will give you a hint. An ideal VOLTAGE SOURCE has ZERO internal resistance.
- #3
oliverlines1234567
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Ah, so an ideal current source has infinite internal resistance, and hence its just normal series and parallel resistors that are left? But I don't understand, how does current go through something with an infinite resistance?
- #4
cnh1995
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Ideal current source is modeled as current source with infinite resistance in parallel. This way, no current flows through the parallel resistance and all the current flows through the load. You are asked to find the Thevenin's resistance between A and B. Hence, you need to replace the current source by it's internal impedance, which is infinity in this case. Hence, while calculating RAB, you just open the current source.oliverlines1234567 said:
- #5
Averagesupernova
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It doesn't. That's why the circuit you posted is so easily solved. Just ignore the current source.oliverlines1234567 said:
- #6
oliverlines1234567
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Okay, I almost understand. Am I correct if I say, no extra current (say if I connected a battery across A and B) can go through the current source because the current source is designed so that it maintains a constant current. So because no extra current can go through it, it appears as if it has infinite resistance?
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- #7
cnh1995
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Well, an ideal voltage source in parallel with an ideal current source is an invalid combination in network analysis. Indeed the current source is designed to provide a constant current. If any load is connected to a current source and all the current passes through the load, then there is no "leakage" current and hence, internal (parallel) resistance of an ideal CS is infinite.oliverlines1234567 said:
This is a diagram showing source transformation with 'practical' sources.
Practical current source with parallel resistance is equivalent to practical voltage source with a series(intetnal) resistance. Note that both the resistances are same. In CS, it appears in parallel(current division takes place) while is VS, it is in series(voltage division takes place). If you use load resistance as 2Ω, you can see, in VS diagram, voltage across load is 5V. It is same as in the CS diagram, where current through the load is 2.5A.
Edit: First line corrected by Cabraham. It is valid actually.
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- #8
cabraham
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Regarding a voltage source in parallel with a current source, actually it is quite valid, if I may disagree. If the constant voltage source (CVS) is 1.0 volts, and the constant current source (CCS) is 1.0 amps, they can be connected together in parallel. One will transfer power to the other depending on polarity. If the CVS has the plus terminal on top, CCS has arrow upward, then the CCS charges the CVS, with 1.0 watts transferring from CCS to CVS. Reverse the polarity of just 1 or the other, not both, and the CVS transfers 1.0 watt of power into the CCS. Otherwise I agree with the rest of your post, thanks.cnh1995 said:
Claude
- #9
cnh1995
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Right! I was just about to editcabraham said:
! Actually, two ideal voltage sources in parallel or two ideal current sources in series are the invalid cases. Thanks for the correction!
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- #10
oliverlines1234567
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Okay, yeah that makes sense. So was what I said last time about the excess current can't enter the current source (because the current needs to stay constant), hence it has effectively infinite resistance a good/correct way of thinking about it? Because that's the conclusion I have kind of come up with from what you've said. Because then that would explain why the total resistance of the current source as seen by the other components is infinite, when actually only the internal parallel resistance of the current source is infinite and the part where the current actually travels through has zero resistance.cnh1995 said:
- #11
oliverlines1234567
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Thanks for all the replies by the way :)
- #12
cnh1995
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Sounds right. If the current source is not ideal, the excess current will flow through it's internal resistance. Hence, the voltage source connected across it will "see" the internal resistance instead of infinite resistance.oliverlines1234567 said:
- #13
oliverlines1234567
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Great. Thanks a lot :)cnh1995 said:
- #14
Averagesupernova
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Actually, this would be a battery charger. A real world practical example of such a network.cnh1995 said:
- #15
Averagesupernova
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cnh1995 said:Right! I was just about to edit
I don't see what is invalid about 2 current sources in parallel either. If they were in series I would say that is invalid.
- #16
cnh1995
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That's what I wrote.Averagesupernova said:
- #17
Averagesupernova
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I really need to see my optometrist. LOLcnh1995 said: