# Finding Vth from thevenin theorem

#### gbox

Joined Dec 29, 2015
42

I have manage to find the Rth=200/3

Now to find Vth I just need to find the voltage on the branch where the 200Omh resistor and the 6V battery are.

The answer is Vth=2/3Vin-2 and I can find out why

#### JoeJester

Joined Apr 26, 2005
4,390
Now to find Vth I just need to find the voltage on the branch where the 200Omh resistor and the 6V battery are.
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The answer is Vth=2/3Vin-2 and I can find out why
What is the source of that answer?

You can't find out why because it's NOT true or you typed it incorrectly.

Show you work ... otherwise the members here can only speculate how you achieved such an answer.

Last edited:

#### MrAl

Joined Jun 17, 2014
8,386
Hi,

What is the question here?

#### JoeJester

Joined Apr 26, 2005
4,390
I believe the title is the question ... and the TS has yet to account for the diode in the circuit. From the diagram, I suspect the real question is What is Vo?

We shall see if the TS returns.

#### MrAl

Joined Jun 17, 2014
8,386
Hi Joe,

Yes i see that diode as bring another problem that has to be addressed if Vin can go high enough. I would like to see more information about what Vin really is.

#### JoeJester

Joined Apr 26, 2005
4,390
I'll give the TS a few more days before I mention my thoughts on Vin.

#### ErnieM

Joined Apr 24, 2011
8,171
I am wondering how long a linear analysis will be applied to this non-linear circuit.

#### WBahn

Joined Mar 31, 2012
26,398
As near as I can tell, the diode is being treated as the load and the rest of the circuit is what the Thevenin equivalent is modeling. If so, that's perfectly valid.

#### WBahn

Joined Mar 31, 2012
26,398
View attachment 106669

I have manage to find the Rth=200/3

Now to find Vth I just need to find the voltage on the branch where the 200Omh resistor and the 6V battery are.

The answer is Vth=2/3Vin-2 and I can find out why
There are several ways to find Vth. In this case, you might use superposition.

#### WBahn

Joined Mar 31, 2012
26,398
What is the source of that answer?

You can't find out why because it's NOT true or you typed it incorrectly.
Vth=2/3Vin-2

Assuming this is

$$V_{th} \; = \; \frac{2}{3}V_{in} \, - \, 2 V$$

What's wrong with it?

#### JoeJester

Joined Apr 26, 2005
4,390
What's wrong with it?
Nothing is wrong with it as I just re-examined the schematic.

#### WBahn

Joined Mar 31, 2012
26,398
Nothing is wrong with it as I just re-examined the schematic.
I was wondering if perhaps you might have been looking at a different port than I was -- the TS wasn't too clear on what he was doing.

#### JoeJester

Joined Apr 26, 2005
4,390
It looks like the TS is not coming back.

The answer is Vth=2/3Vin-2 and I can find out why
I suspect the TS mean to imply he can't find out why the answer was correct. In my first view, I had the batter polarity reversed when I said the answer was incorrect, but WBahn's posting reminded me to pay closer attention.

I used superposition theorem to identify the Vth voltage which confirmed the correct answer.

I am wondering how long a linear analysis will be applied to this non-linear circuit.
It goes as far as a perfect diode and not one step further. But for a first "cut" as what to expect, who knows. What we don't know is if the TS had drawn the diode characteristic curve in their labs. We don't know a lot about the TS's studies and labs. All we have is the diagram and a member interpreted question.

#### WBahn

Joined Mar 31, 2012
26,398
It goes as far as a perfect diode and not one step further. But for a first "cut" as what to expect, who knows. What we don't know is if the TS had drawn the diode characteristic curve in their labs. We don't know a lot about the TS's studies and labs. All we have is the diagram and a member interpreted question.
As long as the diode is not part of the circuit that the Thevenin equivalent is modeling, it is every bit as valid as not using a Thevenin equivalent at all. We do this all the time. For instance, reducing a biasing network to it's Thevenin equivalent and using that to determine the operating point of a transistor circuit.

#### JoeJester

Joined Apr 26, 2005
4,390
I agree.

Here is an "excel" graph

Then the simulation:

And of course the Vth without the diode: