# Why do we not consider Thevenin Voltage source while dealing with Dependent sources in RLC circuits?

"If no controlling current or voltage associated with the dependent source is of interest, we may simply find the Thévenin equivalent connected to the inductor and capacitor."

This is what's written in Engineering Circuit Analysis- Hyatt, Kimberly.

After doing a question this way, I found it a little strange: We're finding the Thevenin Equivalent which includes an independent voltage(Thevenin) source and a Thevenin Resistance, and the total circuit consists of the capacitor and inductor as well. Since it includes an independent voltage source the total response should be the sum of the natural and the forced response, but we're not including the forced response in it. Why so?

• Reading your question it looks like you are referring to a specific schematic - can you include it? Mar 20, 2019 at 12:55
• The image you included talks about finding only the equivalent resistance, $R_{eq}$, not a Thevenin equivalent circuit or the Thevenin voltage source. Mar 20, 2019 at 15:52
• Exactly, why we're concerned about only the Thevenin Resistance and not the Thevenin Voltage, it's as if we're ignoring it without any reason. I'm asking why are we doing so? Mar 21, 2019 at 11:27

I believe what they are saying is the dependent source and the two resistors can be boiled down into an R-equivalent resistor (with the excitation source removed). This would be advantageous because one could then analyze the circuit as an RLC which simplifies the math.

So what they do is remove the L and the C and subsitute a current source, and then say "Hey, all these components are linear and function like a resistor"

If you go around the loop and find the equivalent to Vtest you get a resistor, this allows you to ignore

What they are trying to show is that for some circuits it is easier to simplify the circuit before trying to solve the circuit.

We are not concerned with the forced response of the circuit because there is no zero state response, the circuit has to be excited for there to be a response.

In addition 1A current through the loop helps you find Vtest for an initial condition, which will help you find Vc, because when the circuit starts up, the inductor is shorted.

• I understand that, what I'm asking is if I make the Thevenin Equivalent of the above circuit then it includes a Thevenin Independent source and a Thevenin Resistance, what you're suggesting(and perhaps the book suggests the same) is that you ignore the Thevenin Independent source, then even the Thevenin Circuit simplifies to only consisting of a Resistor, a Capacitor and an Inductor, and then simply find the natural response, but my doubt is how can we just neglect the Thevenin Independent Voltage source in the first place? Mar 21, 2019 at 11:25