Find Rth in Thevenen Equivalent Circuit: R1,R2,R3,R4,R5,R6,Vth

[kliker]

[PLAIN]http://img695.imageshack.us/img695/2420/physicsm.jpg

i need to find thevenen equivalent circuit using R5 as load

Ε1=20V, Ε2=10V, R1=6Ω, R2=12Ω, R3= 25Ω, R4=20Ω,
R5=10Ω, R6=5Ω, Ι=0,2Α.

Vth = -5V


but how much is Rth?

i can see that R4 is in series with (R2+R1)||R3 and R6

is this correct?

so the circuit will be

[PLAIN]http://img37.imageshack.us/img37/9126/62188572.jpg

please tell me if I am correct or not i need to know for the exams :(

 

[vela]

Your calculation for Rth is correct, but I got a different value for Vth.

 

[kliker]

Your calculation for Rth is correct, but I got a different value for Vth.

i replaced the current source with a voltage source

we have no current on teh second loop

so the voltage source has 5V

hence-E2 + 5V = -5V will be the Vth

am i doing anything wrong?

oh but we have current going through the voltage source

so i have to find the current in the first loop

E1 - E3 - I*(R1+R2+R3) = 0

I = 15/43 = 0.348837209 A

so Vth = -5 + (0.348837209*R3) = -5 + 8.72093023 = 3.72093023 V

is this correct?

so in the shape the + will be on top and - on bottom

 

[vela]

Yes, that matches what I calculated.

 

[kliker]

thanks a lot for your help