# Homework Help: How to find Rth in Thevenin

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1. May 22, 2017

### qwertyuiop

• Member advised to use the homework template for posts in the homework sections of PF.
This is the question

First,I use thevenin theorem,when I close the voltage source, i apply nodal analysis to find rth,is my answer correct?

Last edited by a moderator: May 23, 2017
2. May 22, 2017

### Futurestar33

Just a question but aren't those all resistors if the units are kila-ohms?

3. May 22, 2017

### qwertyuiop

I don't understand what you mean

4. May 23, 2017

### Staff: Mentor

Dependent source 0.003vo should, with your nomenclature, be written 0.003V1

5. May 23, 2017

### Staff: Mentor

6. May 23, 2017

### Staff: Mentor

Moderator's note: I've restructured the original post to make the relevant images visible, and removed the facebook link as it was not accessible. @qwertyuiop is advised to upload relevant content rather than link to a facebook page.

7. May 23, 2017

### The Electrician

Halfway down the page of your calculations you have:

(V1-V2)/22k + 0.003 V2 +1 + V2/30k = 0

This should be:

(V1-V2)/22k + 0.003 V1 +1 - V2/30k = 0

Which should then become:

30 V1 - 30 V2 + 1980 V1 + 660000 - 22 V2 = 0

Which becomes:

2010 V1 - 52 V2 = -660000

8. May 23, 2017

### qwertyuiop

Thank you for you helping,now i know my mistake

9. May 23, 2017

### qwertyuiop

Thank you

10. May 23, 2017

### qwertyuiop

I don't understand why - V2/30k ?

11. May 23, 2017

### The Electrician

Because you have chosen to define currents into a node as positive. The other choice people make is to define current out of a node as positive. But whichever you choose, you must be consistent.

The expression V2/30k gives the current out of the node, so its sign must be negative.

12. May 23, 2017

### qwertyuiop

ok, thank you for your help. In this circuit, I find the rth = -1363ohm. Is possible rth is negative? why?

Last edited: May 23, 2017
13. May 23, 2017

### The Electrician

The dependent source in this circuit is a transconductance source. The value 0.003 is the transconductance. Substitute a variable x for the value 0.003 and solve the circuit for Rth. You will get this expression: Rth = 3.75/(.00025-x). You can see that if x is greater than .00025, Rth will be negative; since .003 is greater than .00025, we get a negative value for Rth.

The circuit of this thread is an example of a circuit with an active element connected in such a way as to generate a negative resistance. There are other well known circuits that can do this, for example: https://en.wikipedia.org/wiki/Negative_impedance_converter

14. May 24, 2017

Thank you