Thévenin's theorem - how to calculate V th?

[GoldDigger]

Occam's razor.

I prefer Occam's electric shaver myself.

 

[Ingenieur]

it can be shown using kcl:
I3 = I5 = I6 = I
I4 = 0
....

 

[GoldDigger]

it can be shown using kcl:
I3 = I5 = I6 = I
I4 = 0
and using kvl:
(R3 + R5 + R6) I = U3 + U5 + U6 or loop = 0

leaves U2 + R2 branch with with a short from top R2 to U2-
I2 = U2/R2

Vab = U2 - I2 R2 = U2 - U2/R2 R2 = U2 - U2 = 0
????

I think you missed on that one. I get I2 = I3 = I6 = (I5-I4)
I do not see I4=0.
The rest does not follow since your first equation was invalid.
I am not going to go any farther than that because I do not want to do the OP's entire job for him.

That I2 = I3 = I6 is a good simplification, and if it were not for that pesky number 4 leg in parallel with the number 5 leg the loop voltage would allow you to easily calculate what I is. Which is why I started with finding the Thevenin equivalent of those two legs in parallel.

 

[Ingenieur]

it can be shown using kcl:
I3 = I5 = I6 = I
I4 = 0
....

Ignore I4 branch since = 0 therefor I2 = I

Sum Ux - I (Sum Rx) = U2 - I R2
I = Sum Ux / Sum Rx
Vab = U2 - (Sum Ux / Sum Rx) R2
???

 

[GoldDigger]

Ignore I4 branch since = 0 therefor I2 = I

Sum Ux - I (Sum Rx) = U2 - I R2
I = Sum Ux / Sum Rx
Vab = U2 - (Sum Ux / Sum Rx) R2
???

I still do not understand why you are so willing to set I4 equal to zero.
If the left hand side of the diagram were not there you would have a closed loop through the 4 and 5 legs.
It seems to me that you are deriving I4 = 0 from the erroneous assumption that I3 = I5.
Are you being confused by the fact that the current arrow on the 4 leg is drawn in the opposite direction? That does not make the current zero.

 

[KamilAussie]

KamilAussie

KamilAussie

Hey guys. I wanna let you know I solved the circuit
Thanks for helping me. :thumbsup:

First task in that exercise was to apply Kirchoff's circuit laws and count current on each resistor. That's how I calculated I₁. Then I used Thévenin's theorem ->I₁=U_th/(R_th+R₁) -> U_th=I_1*(R_th+R₁).

Happy Halloween :happyyes: