Register to reply 
Vth voltage in thevenin theorem 
Share this thread: 
#1
May2511, 04:57 PM

P: 317

Hi,
Please have a look on Example 4.8. There are two scans. 1: http://img585.imageshack.us/img585/6...gthevenin1.jpg 2: http://img818.imageshack.us/img818/3...gthevenin2.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 ab. 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 ab. 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 ab but now voltage which is appearing on terminals ab is not Vth. Why is so? I hope you understand my question. Could you please tell me? Thanks. Cheers 


#2
May2511, 05:57 PM

PF Gold
P: 322

The Thevinin Equivalent Circuit transforms a messy circuit into a simple voltage source + resistor series combination that is equivalent (I'll leave it for you to investigate the proof behind this long standing claim).
As a side exercise, assume that Vab is not an open circuit, but an unknown voltage, and calculate what it comes out as. In the end, what appears on the terminals ab will be exactly the same as if you had either the original circuit or the Thevinin Equivalent circuit. A different way of calculating the Thevinin Resistance is using Ohms law, with Voc / Isc, where Voc is the Open Circuit Voltage of ab as calculated above, and Isc is similar, but calculating the current with terminals ab Short Circuited instead. 


Register to reply 
Related Discussions  
Thevenin's Theorem and Norton's Theorem  Engineering, Comp Sci, & Technology Homework  1  
Thevenin voltage and resistance  Engineering, Comp Sci, & Technology Homework  1  
Determining node voltage using Mesh Analysis and Thevenin's Theorem  Engineering, Comp Sci, & Technology Homework  3  
Thevenin voltage  General Physics  2  
Thevenin voltage help  Introductory Physics Homework  4 