Rth in a circuit containing both dependent and independent sources

Thread Starter

janatka22

Joined Apr 16, 2017
1
In order to find Thevenin equivalent of a circuit, I first set open circuit between a and b to find a voltage. Since the circuit consists both dependent and independent sources, in order to find equivalent resistance, can I make the circuit passive - change voltage sources as short and current sources as open? Or is the only way to find a solution using the test source across the terminals a and b?
 

RBR1317

Joined Nov 13, 2010
500
Since the circuit consists both dependent and independent sources, ... can I make the circuit passive
Wouldn't that depend upon whether the presence of a dependent source changed the equivalent resistance? If you analyze the circuit using both methods as you suggest, does each method give the same answer? If not, then which method gives the correct answer?
 

MrAl

Joined Jun 17, 2014
6,604
In order to find Thevenin equivalent of a circuit, I first set open circuit between a and b to find a voltage. Since the circuit consists both dependent and independent sources, in order to find equivalent resistance, can I make the circuit passive - change voltage sources as short and current sources as open? Or is the only way to find a solution using the test source across the terminals a and b?
Hi,

Dependent sources can be tricky. Think of a voltage controlled voltage source in a circuit where there is feedback from the output. With positive feedback we can change the apparent input resistance through a wide range. So in general the answer has to be 'no' because if we kill the source we change the input resistance, but if you have a circuit we can look at that.
 

WBahn

Joined Mar 31, 2012
24,850
In order to find Thevenin equivalent of a circuit, I first set open circuit between a and b to find a voltage. Since the circuit consists both dependent and independent sources, in order to find equivalent resistance, can I make the circuit passive - change voltage sources as short and current sources as open? Or is the only way to find a solution using the test source across the terminals a and b?
Let's consider a trivial example of a circuit consisting of nothing but a series connected resistor, R1, and voltage-controlled voltage source whose output voltage is equal to half the applied voltage between the terminals. What is the Thevenin resistance of this circuit as seen between the terminals? What would you get if you just treated all of the voltage sources as short circuits?
 

MrAl

Joined Jun 17, 2014
6,604
Hi,

I forgot but sometimes it does work if you handle as a multiple source.
For example, in op amp circuits we can short the output to ground and then write an equation, then unshort it and write the second equation as if the output was a second source.
I never looked into if this works on every single circuit we'd ever find though.
Doing it that way might lead to the Thevenin equivalent but that's a little different than just shorting them all out like we do with constant sources. In general we cant just short them all out period.

Yeah we should look at a circuit or two.
 
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