# test sources and circuits without independent source

Discussion in 'Homework Help' started by jorgemotalmeida, Jun 28, 2008.

1. ### jorgemotalmeida Thread Starter Member

Oct 23, 2007
23
2
Hi

I tried to find out in your online book any clues/hints for doubts I have about test sources and circuits without independent sources... but it doesn't mention these cases:

a ) I don't have a clue about the reason that a circuit WITHOUT any independent sources has a Vth(évenin)= 0 V... Why?

b) And why do we always must use a test voltage V(test) source for circuits with independent and dependent sources to calculate the Rth?

c) May you explain why when we have an infinite load resistor we are in an alternate current situation?...

d) Please fix me if I'm wrong: we NEVER can take off any DEPENDENT sources in the circuit because they rely on variables of the circuit?

Last edited: Jun 28, 2008
2. ### jorgemotalmeida Thread Starter Member

Oct 23, 2007
23
2
ok. I will give a try in the first question.

a friend of mine gave a good clue and it makes sense: as we don't have any independent sources, we don't have a source that delivers current. Even with dependent sources they depend on other variables like current or voltage. Is it correct?

Last edited: Jun 28, 2008
3. ### studiot AAC Fanatic!

Nov 9, 2007
5,003
523
Can you show us a circuit example of what you mean?

Oct 23, 2007
23
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5. ### jorgemotalmeida Thread Starter Member

Oct 23, 2007
23
2
"And why do we always must use a test voltage V(test) source for circuits with independent and dependent sources to calculate the Rth?"

Possible answer: because without any independent sources (they were discarded..) the DEPENDENT source would BE 0 V.. ? is it correct?

Please enlighten me on these 4 questions.

Oct 23, 2007
23
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7. ### prussell226 New Member

May 15, 2008
3
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a) If there is a circuit without either an independent voltage source or independent current and you wanted to find the thevenin equivalent, the thevenin equivalent voltage will be zero across across that desired point. Remember, to theveninize a circuit for example the circuit you posted, if you wanted to thevenize the circuit as far as R10(1Meg Ohm resistor) is concerned, it is removed from the circuit and Eth and Rth is calculated respectively.After r10 is removed the voltage Eth will exist across the terminals where r10 was. Rth will be the total resistance seen looking outside from those terminals at the circuit.(Rth=Eth/Isc-sc:short circuit) Eth will be the voltage across those terminals after the network has been solved. If there are no independent sources the dependent source value will be zero.The dependent source is dependent on a particular voltage across or current through an element in the circuit. For example looking at the circuit you posted if Vs is 0v then no current will flow in the network and there wont be anyvoltage drops across the elements hence the voltages Vx and current ix will be zero due to ohms law. Ex=3Vx.....Vx=0.....3(0)=0V
For c) I'm not sure what you are saying but I think I get what your asking. When creating circuit we would like the circuit to be 100% efficient i.e an ideal current source or voltage source to be used in the construction. A current source is ideal when its internal resistance connected in parallel is infinity which is an open circuit. This means no current flows though the internal resistance all flows through the network.
Dependent sources cannot be removed like you said because its value and depends on another variable value somewhere else in the network. To solve the network you have to work with it just the way it is using different methods to solve the network. Dependent sources can be remove. The operation will just change due to the modification.

8. ### jorgemotalmeida Thread Starter Member

Oct 23, 2007
23
2
thank you. this really helped me.