Why we need to short the input(for voltage supply) when find the output resistance?

[hongzeng]

I believe this is a really basic question to find the Thevenin's equivalent circuit.
When we find the Rth, we "see" from the output of the source and short the voltage supply or open the current supply.
But I want to know the reason of doing it.
Thanks!

 

[harsimran315@gmail.com]

source is a collection of voltage sources, current sources and resistances, while the load is a collection of resistances only. Both the source and the load can be arbitrarily complex, but we can conceptually say that the source is directly equivalent to a single voltage source and resistance so we can determine the value of the resistances and the voltage source, vs by attaching an independent source to the output of the circuit therefor shorten or opened

It is the procedure of thevenin eq ckt that firstly we need to open the load or remove it to find open circuit voltage across it
now the second step is to deactivate the source as we have already found the voltage that is driving the circuit so we no more need the source.
This is done by seeing the internal resistance of the source
For current source internal resistance is infinity therefor open circuited
For voltage source internal resistance is 0 therefor short circuited
the source transformation diag below will make it more clear

Regards

 

[Ratch]

hongzeng,

I believe this is a really basic question to find the Thevenin's equivalent circuit.
When we find the Rth, we "see" from the output of the source and short the voltage supply or open the current supply.
But I want to know the reason of doing it.

You are right, it is basic. The Thevenin impedance is the same as the output impedance. The output impedance depends on the source impedance, and the input impedance depends on the load impedance. Therefore you have to calculate the input or output impedance with the correct source or load impedance.

Ratch

 

[Joe Nicotera]

I believe this is a really basic question to find the Thevenin's equivalent circuit.
When we find the Rth, we "see" from the output of the source and short the voltage supply or open the current supply.
But I want to know the reason of doing it.
Thanks!

"NEED" is an confusing word, reguardless of the language....

1.a)We always "solve" Thevenin within the context of Mr. Thevenin. 1.b)Because it no only does a signal "follow" a load, it also follows the supply. + 2) Although we USUALLY just mark down Vcc on the paper, we also are suckers for rechargable batterys. + 3) Because, two people discussing "how well the impedance's preceedances the signal is too much like telling the bartender to "beam me up Scotty." == Because no matter "how you do it", they are all mathematical parts of the current flow and you will get the same algebra anyhow ==<< Because the circuit models the math, and the math DESCRIBES, existing, known things about the circuit, not the other way around.", has been recalled, it is now =+- "because "the algebra teacher forgot to 'isolate the terms' when the current node was Vcc" >>thev "He'll get my tab"./"Marconi not Morse."

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Because we are only allowed to find the ALGEBRAIC awnser, and that involves always solving for a known variable.

 

[Ratch]

Joe Nicotera,

"NEED" is an confusing word, reguardless of the language....

1.a)We always "solve" Thevenin within the context of Mr. Thevenin. 1.b)Because it no only does a signal "follow" a load, it also follows the supply. + 2) Although we USUALLY just mark down Vcc on the paper, we also are suckers for rechargable batterys. + 3) Because, two people discussing "how well the impedance's preceedances the signal is too much like telling the bartender to "beam me up Scotty." == Because no matter "how you do it", they are all mathematical parts of the current flow and you will get the same algebra anyhow ==<< Because the circuit models the math, and the math DESCRIBES, existing, known things about the circuit, not the other way around.", has been recalled, it is now =+- "because "the algebra teacher forgot to 'isolate the terms' when the current node was Vcc" >>thev "He'll get my tab"./"Marconi not Morse."

- - - Updated - - -

Because we are only allowed to find the ALGEBRAIC awnser, and that involves always solving for a known variable.

Huh? What kind of gibberish is the above?

Ratch