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Old 13th September 2012, 07:50 AM   #1
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Default Voltage Sources & Current Sources

Hi Everyone,

Am having some trouble understanding ideal voltage sources and ideal current sources. I have read up on the articles on the internet - on Wikipedia and I find them lacking. My question is this

(1) Ideal voltage source
Wikipedia says that the series resistance is zero but does not emphasize on the impedance of the voltage source itself. If one were to argue that it should have zero resistance then this argument will violate Ohm's law which states V = I x R. I have said in the past that since it is a voltage source it should have a very high impedance.

(2) Ideal current source
Wikipedia says that its output resistance is at infinity. Naturally if you put a load less than that of the output resistance there will be current flow in the load. What if the load has a resistance that is also at infinity? Then there will be no current flow to the load. If the current source is at 0.1A where will it go?

Hope you can help me out with these questions. Thank you.

Last edited by rascal101; 13th September 2012 at 07:52 AM.
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Old 13th September 2012, 10:21 AM   #2
wwenze is offline wwenze  Singapore
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30 views and no replies? Don't mind if I do...

@rascal101, an ideal voltage source is just a two-pin element with a voltage across it, and that voltage will be whatever it has been set to be. The voltage source will source/sink as much current as possible to achieve that voltage.

Similar, the amount of current flowing through a current source is whatever you set it to be.

Voltage of an ideal voltage source remains unchanged no matter how much current you throw into/draw from it. It behaves as if series R = 0. (Otherwise voltage develops across R and voltage at output pin depends on current.)
Current through a current source does not change even if you put infinity volts across it. So it behaves as if there is an infinite R.

Last edited by wwenze; 13th September 2012 at 10:25 AM.
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Old 13th September 2012, 10:38 AM   #3
DF96 is offline DF96  England
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Quote:
Originally Posted by rascal101
Wikipedia says that the series resistance is zero but does not emphasize on the impedance of the voltage source itself.
There are not two resistances. Just one. The output impedance of a real voltage source can be modelled by adding a series resistance to an ideal voltage source. The series resistance is the impedance.

Ohm's Law applies to the resistance, not the voltage source.

Quote:
What if the load has a resistance that is also at infinity?
If you put an infinite resistance across a perfect current source then you get an infinite voltage. Simple! (One exception - this assumes the current source produces a non-zero current. A zero current source is otherwise known as an open circuit: infinite resistance, zero current)
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Old 13th September 2012, 01:56 PM   #4
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Thanks guys
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