# Vth voltage in thevenin theorem

Discussion in 'Homework Help' started by PG1995, May 25, 2011.

1. ### PG1995 Thread Starter Well-Known Member

Apr 15, 2011
813
6
Hi

Please have a look on Example 4.8. There are two scans.

1: http://img585.imageshack.us/img585/6673/understandingthevenin1.jpg
2: http://img818.imageshack.us/img818/3301/understandingthevenin2.jpg

First we find the equivalent resistance of the circuit on the left of terminals "a" and "b". The equivalent resistance comes to be 4Ω. In other words, we can replace the circuit part lying on left of terminals "a" and "b" with only one resistor of 4Ω. I hope I have it right.

Then we find Vth voltage which is appearing around terminals "a" and "b". This is the voltage which can be detected by a voltmeter around the terminals a-b. Vth is 30V.

Now have a look on Figure 4.29 in scan #2. We have 4Ω resistor (Rth, or equivalent resistance) in series with 30V DC source and RL (load resistor) which is connected to terminals a-b. It is obvious that some of the Vth, 30V, is going to drop around 4Ω resistor which means now the voltage which will appear around the terminals "a" and "b" will be less than 30V. Originally we found the Vth as the voltage which appears around the terminals a-b but now voltage which is appearing on terminals a-b is not Vth. Why is so? I hope you understand my question. Could you please tell me? Thanks.

2. ### jegues Well-Known Member

Sep 13, 2010
735
45
You determined Vth by finding the open circuit voltage across the a-b terminals.

You've also found Thevenin Resistance Rth.

Your Thevenin equivalent should be an equivalent model of the previous circuit.

When your Thevenin equivalent circuit has no load across the a-b terminals, the open circuit voltage will appear across the a-b terminals. (Write a KVL, no current, Vth = Voc)

When it has a load across the a-b terminals, you shouldn't expect to see the open circuit voltage across the a-b terminals right? After all, it is no longer and open circuit!

I hope this clears things up.

By the way, I didn't bother to check whether the values for Vth and Rth are correct, I assumed they are.

Last edited: May 25, 2011
3. ### PG1995 Thread Starter Well-Known Member

Apr 15, 2011
813
6
jegues, thanks a lot. You have helped me with many problems, much grateful.