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Input resistance for AC input
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| qwerty 2007-06-27, 1:25 pm |
| I read on a book that when a circuit has an AC voltage source as
the input then it's input resistance is
Ri= ui/ii
where ui is the instantaneous input voltage and ii the
instantaneous input current.
Isn't this false?
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| operator jay 2007-06-28, 3:25 am |
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"qwerty" <qwerty@ihopethisisnotadomain.com> wrote in message
news:Xns995CD8EFC88E9p3ifw90nsdek@193.92.150.76...
>I read on a book that when a circuit has an AC voltage source as
> the input then it's input resistance is
>
> Ri= ui/ii
>
> where ui is the instantaneous input voltage and ii the
> instantaneous input current.
>
> Isn't this false?
It's not a definition that I've seen. It doesn't strike me as a good
definition in general because the math would get a little dicey at the
zero crossings. Otherwise I guess it would be basically correct for a
resistive network, even if resistances changed with time.
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| Govind 2007-06-28, 1:25 pm |
| On Jun 27, 11:19 pm, qwerty <qwe...@ihopethisisnotadomain.com> wrote:
> I read on a book that when a circuit has an AC voltage source as
> the input then it's input resistance is
>
> Ri= ui/ii
>
> where ui is the instantaneous input voltage and ii the
> instantaneous input current.
>
> Isn't this false?
Yes. Its wrong. Input resistance of an ideal voltage source is Zero.
pls click the following link for more iformation
http://en.wikipedia.org/wiki/Voltage_source
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| Don Kelly 2007-06-28, 9:25 pm |
| "Govind" <govindarul@gmail.com> wrote in message
news:1183045721.037694.106430@e16g2000pri.googlegroups.com...
> On Jun 27, 11:19 pm, qwerty <qwe...@ihopethisisnotadomain.com> wrote:
>
> Yes. Its wrong. Input resistance of an ideal voltage source is Zero.
> pls click the following link for more iformation
> http://en.wikipedia.org/wiki/Voltage_source
>
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The input resistance implied is not that of the source but that of the
circuit supplied by the load-as seen by the source and is independent of the
source.
In any case, an ideal voltage source is a convenient fiction useful in
circuit analysis but hard to find in the real world (sure with fast enough
feedback one can come close).
As for the definition- it simply notes that v(t)=R*i(t) rather than
v(t)=L*(di/dt) or i(t)=C*(dv(t)/dt
That is correct.
If R is constant over the range of possible v(t), i(t) then be happy, Ohm's
Law is valid and life is simple(?)
..--
Don Kelly dhky@shawcross.ca
remove the X to answer
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