# Thevenin's Theorem

#### energizedlow

Joined Nov 22, 2023
3
Hello, I am new to solving things using Thevenin's Theorem and I am getting very stuck on a problem. I completed 5 other ones in no time but this one just seems to stump me.

I have a program where I can check my answers after drawing out the circuit and I have no idea how to get the numbers I'm looking for.

The Load Resistor for this problem is R3.

I think my Vco/VTh is off but shouldn't it be 20?

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#### WBahn

Joined Mar 31, 2012
29,857
Why should it be 20? 20 what? 20 V? Units matter.

You need to show your work. There's no way we can do anything but guess at what you are doing right and what you are doing wrong unless you show us what you are doing.

#### energizedlow

Joined Nov 22, 2023
3
Why should it be 20? 20 what? 20 V? Units matter.

You need to show your work. There's no way we can do anything but guess at what you are doing right and what you are doing wrong unless you show us what you are doing.
Yes, 20 volts...

I can't really show progress reports if I get stuck on the very first thing.

Wouldn't the 220v and the 240v subtract once you remove the load because of the polarities? From there, what would be the steps to get the Voc/Vth.

Using KVL, I get a current of 2.9 mA. Then I look for Vd and that's where I get confused.

#### WBahn

Joined Mar 31, 2012
29,857
Yes, 20 volts...

I can't really show progress reports if I get stuck on the very first thing.
You say that you solved five others and yet can't even make an attempt at getting started on this one? This makes me think that you are relying to much on lucky guessing and too little on applying the fundamentals.

What steps are you supposed to apply to find a Thevenin equivalent circuit? There are a few common approaches, but at the end of the day they all come down to applying the same analysis techniques that you should already be comfortable with, such as mesh analysis, nodal analysis, and superposition.

Wouldn't the 220v and the 240v subtract once you remove the load because of the polarities?
So if you have a 220 ft tall building next to a 240 ft tall building and you string a wire between the tops of two, are you thinking that the midpoint of the wire is only 20 ft off the ground?

From there, what would be the steps to get the Voc/Vth.
You need to find the voltage that appears across where the load resistor was.

Very carefully identify where in the circuit your 20 V appears across. Is it the load resistor? If not, then that's not the open circuit voltage.

Using KVL, I get a current of 2.9 mA. Then I look for Vd and that's where I get confused.
A current of 2.9 mA where? In what direction? What is Vd? There is no mention of Vd in your diagram. Don't make use dust off our crystal balls to figure out what you are referring to.

#### energizedlow

Joined Nov 22, 2023
3
You say that you solved five others and yet can't even make an attempt at getting started on this one? This makes me think that you are relying to much on lucky guessing and too little on applying the fundamentals.

What steps are you supposed to apply to find a Thevenin equivalent circuit? There are a few common approaches, but at the end of the day they all come down to applying the same analysis techniques that you should already be comfortable with, such as mesh analysis, nodal analysis, and superposition.

So if you have a 220 ft tall building next to a 240 ft tall building and you string a wire between the tops of two, are you thinking that the midpoint of the wire is only 20 ft off the ground?

You need to find the voltage that appears across where the load resistor was.

Very carefully identify where in the circuit your 20 V appears across. Is it the load resistor? If not, then that's not the open circuit voltage.

A current of 2.9 mA where? In what direction? What is Vd? There is no mention of Vd in your diagram. Don't make use dust off our crystal balls to figure out what you are referring to.
Okay so basically no helping. K cool, thanks for wasting time.

This is a different issue than what I was doing with the other 5, similar to other practice problems I find. I do node voltage for most of my circuits and that usually does the trick since we wanted to talk about other methods, which isn't the topic. I follow the steps, I do the dos, end up with wildly different things.

And I think it's pretty obvious that Vd is voltage drop, if you thought for more than 2 seconds instead of just writing down everything that wouldn't help.

Please remember that I'm a beginner, as the tags say. You can talk about "fundamentals", but there is not a consideration that I'm still learning those and implementing them.

#### WBahn

Joined Mar 31, 2012
29,857
Vd is normally used to mean "the voltage at Node 'd' relative to the circuit common".

Even if I guess that YOU meant it to be voltage drop, that still leaves the question of WHERE this voltage drop is.

I've given you a HUGE pointer at where to look. You are focusing on a voltage difference of 20 V and trying to blindly think that that somehow related directly to your Thevenin voltage. But the Thevenin voltage is the voltage difference between two specific points in the circuit, while the 20 V you are so focused on is the voltage between two completely different points.

Redraw your circuit with the load resistor removed. Clearly label the Thevenin voltage on this diagram (including it's polarity -- it's arbitrary, so you get to choose, but it's a choice you have to make).

Now analyze the circuit using whatever techniques you know to find the voltages at those two nodes. Vthev is then the voltage at the node you decided is the positive node minus the voltage at the node you decided is the negative node.

If you like to do node voltage analysis, then do node voltage analysis. Remember that you get to pick one node -- any node, but you need to pick one -- as your 0 V reference node (often called "ground"). Be sure to indicate which node that is.

Then show your best attempt to apply node voltage analysis to the circuit.

If nothing else, at least show a sketch of the circuit with the load removed, your definition of Vthev, your choice of ground, and labels for all of the other nodes.