# Thevenin circuit, basic laws are confusing me

So the question is to find the equivalent Thevenin circuit, I did solve it using Nodal analysis, now the answer in the book is -5V while I got 5V, so it's still the correct answer, right ?

Please correct me if my understanding is wrong about +/- voltage, so we are considering V(ab), so +5V means (a) is 5V above (b) and -5V means (b) is 5V below (a) ? And why in my solution I do get +V ? Is it because of my current direction assumptions ?

• You might have confused yourself by labelling the voltage at an inner node as $V_{TH}$ when the thevenin voltage (what we would normally label as $V_{TH}$) would be the voltage between the nodes labelled 'a' and 'b'. Dec 18, 2017 at 4:10

considering V(ab), so +5V means (a) is 5V above (b)

Yes.

and -5V means (b) is 5V below (a) ?

No, that would be the same as (a) being 5 V above (b). It means b is 5 V above a.

By convention $V_{ab}$ means $V_a - V_b$, so if $V_{ab} > 0$, it means $V_a > V_b$ and if $V_{ab} < 0$, it means $V_b > V_a$.

• Thanks for the answer, so is the answer +5V still correct? And the other thing is what change in my solution would get -5V ? Dec 18, 2017 at 4:31
• $V_{TH}$ as labeled in your diagram is 5 V when the output is open circuit, but that is not the Thevenin equivalent voltage for this circuit. Dec 18, 2017 at 5:58
• Right, I assumed Vth has the same voltage as V(ab) but I think that is not the case here because of the 10V source. So then Nodal analysis won't work in this case ? So I should probably just resort to either KVL or Superposition. Dec 18, 2017 at 18:13
• You should be able to find the open-circuit voltage between a and b by inspection after you've found your $V_{TH}$. Dec 18, 2017 at 18:26
• OK so there is a voltage drop of -10V then there is a voltage rise of 5V giving an overall voltage drop of -5V. I do usually tend to over think things and confuse myself in the process. Now what's bugging me is that for example in Mesh analysis I did always consider - to + terminal as voltage rise and + to - terminal as drop. Now if I do that in here, again I get a +5V as the answer Dec 18, 2017 at 18:50

You have calculated Rth as seen by the red 10V battery. The calculation is ok. You should calculate Vth also as seen by the red 10V battery - its the voltage fom a to b when the red battery is removed. The result will be -5V.