Question : Thevenins Equivalent

[gatsbycollege]

Homework Statement

http://img716.imageshack.us/img716/8021/69925791.jpg

Find The Thevenins Equivalent of the circuit shown .
at the terminal a and b i think

Homework Equations

Vth = Rth(Ith)
V= IR

The Attempt at a Solution

For Vth
i tried using Nodal Analysis Having 4 Node Voltages

but i don't know if the 5 ohms resistor and 8 ohms resistor is having flow of current in there?

For Rth

the Current source is change into open ckt.
then solve by having 40 and 20 in series then parallel in 10 ohms
therefor [(40+20)x10]/70 + 45 + 5 + 8 = 78 ohms = RthSo I am wondering how should i get the Vth now,,
And is Vth = Va + 35V and Vth = Vb +15 V ? or I = (Va + 35 - Vth) / 5 ohms

 

[lewando]

For Vth
i tried using Nodal Analysis Having 4 Node Voltages

It might be easier to find the short circuit current, I_SC, using mesh analysis: 3 currents (1 of them you already know-->2 equations). Then V_TH = I_SCR_TH.

For Rth

the Current source is change into open ckt.
then solve by having 40 and 20 in series then parallel in 10 ohms
therefor [(40+20)x10]/70 + 45 + 5 + 8 = 78 ohms = Rth

Not sure what you are doing here. Like, where's the 10 ohms coming from? The 70? Somehow, your answer is correct.

 

[gatsbycollege]

It might be easier to find the short circuit current, I_SC, using mesh analysis: 3 currents (1 of them you already know-->2 equations). Then V_TH = I_SCR_TH.


Not sure what you are doing here. Like, where's the 10 ohms coming from? The 70? Somehow, your answer is correct.

oh sorry that 10 should be 30 ,,

thx for the answer,, i got my Isc = 1.84 and Vth = 143.52

tried to use that mesh analysis

i got equations

60V - 90 Ia - 40V +30 Isc = 0
35V - 15V - Isc (5 +30 +45 +8) + 30 Ia +90 =0
then by elimination
igot Isc , correct me if I am wrong thanks