Hey all.

Question

I have to find Thevenin’s UTHEV and RTHEV, which can replace the circuit found here:

img24.imageshack.us/img24/1723/thevenin.jpg

I can do it for small circuits, but this one confuses me a bit. Hope you can get me goig.

Thank you very much.

Answer 1

I don't see any shortcuts to find them. I think I would go ahead and write net equations, and solve as a system.

Answer 2

First of all R8 has nothing to do with it, no current

I put i1 through R1
i2 through R2
then
(i1-i2) through R3
i3 through R4
then
(i1-i2-i3) through R10
then
i2+i1-i2-i3 = (i1-i3) through R9
i4 through R6
then
(i3-i4) through R5
and also
(i3-i4) through R7
Voltage out = (i3-i4) R7
Now start voltages along the paths from +10 to ground
All the resistances are the same, call it r
10 = r i1 + r i2 + r (i1-i3)
10 = r i1 + r(i1-i2) + r (i1-i2-i3) +r (i1-i3)
10 = r i1 + r(i1-i2) +r i3 + r i4
10 = r i1 + r(i1-i2) +r i3 + 2r(i3-i4)
combining like terms
10/r = 2 i1 + i2 - i3
10/r = 4 i1 -2 i2 -2 i3
10/r = 2 i1 -i2 + i3 + i4
10/r = 2 i1 - i2 + 3 i3 - 2 i4

four linear equations, four unknowns
remember output voltage = r(i3-i4)
Once you solve it, you can do what you want with it. You have an equivalent resistance or conductance etc.

Answer 3

Thanks! I understand it all except why RTHEV is easily calculated by now. Hope you can help me.

Answer 4

To find the Thevinin equivalent you need the open circuit voltage and the short circuit current.
Get the open circuit voltage at the output using what you have so far from the linear system I gave you.
Then short circuit the output terminals and resolve with R8 back in the circuit and find the output current which is the current through R8
Then you have open circuit voltage and closed circuit current. Req = V/i