Thevenin $V_{th}$ calculation problem

I'm back with my basic questions.

I'm trying to answer some questions, using Thevenin, for this circuit:

simulate this circuit – Schematic created using CircuitLab

• $R_{th}$
• $V_{th}$
• Find short circuit current on A-B

For $R_{th}$ it is quite simple ( I hope...), and can be done calculating the total resistance of the following :

simulate this circuit

Which results in 0,75 ohm.

Now my problems comes. To calculate $V_{th}$, I started using superposition calculating the voltages for $A_1$ and $E_1$.

For A1 I calculate:

simulate this circuit

Considering that the current in A-B is the $A_1$ current source, the voltage is: $$V_{ab}'=A_1\times R_{th}=2A\times 0.75\,\Omega = 1.5V$$

To calculate voltage using $E_1$ source. My problem is that the point $A$, from the $E_1$ point of view i in between of a $\Delta$ resistors connection ($R_1$, $R_2$, $R_3$).

I tried to do $\Delta$ to $Y$ like:

simulate this circuit

Which is most probably wrong, 'cause it means that voltage on A-B is the same as $E_1$, that gives $$V_{ab}=-E_1+V_{ab}'=-16V+1,5V= 14,5V$$

That is wrong, 'cause I know the result is $-12.5V$

I did something from or need I a different approach?

• The circuit have used to calculate $R_{th}$ is wrong.. Commented Oct 26, 2017 at 8:52
• @nidhin Well,... good. Any hint? Because of the result is correct.....
– LPs
Commented Oct 26, 2017 at 8:58
• @nidhin I corrected the first circuit, that was the wrong one
– LPs
Commented Oct 26, 2017 at 10:07
• @nidhin Yes, sorry. Edited
– LPs
Commented Oct 26, 2017 at 10:10
• are you sure of your answer because a simulation from proteus yields +10V as the thevinins voltage. try obtaining the nortons short circuit current first, and then multiply it with the thevinins resistance to obtain the thevinins voltage. Commented Oct 26, 2017 at 11:42

The two $400\, m\Omega$ resistors in your last schematics are connected in parallel. This can happen only if the current source is replaced with a short circuit. Which is wrong.

See the schematic below. (a) is the delta version and (b) is the Wye.

simulate this circuit – Schematic created using CircuitLab

• Ok, so point A is floating with $Y$ connection. I thought that but sounded wrong to me.
– LPs
Commented Oct 26, 2017 at 10:29
• @LPs A will be floating. And hence voltage at node A will be same as that at node A1. Commented Oct 26, 2017 at 10:36
• Good. Thank you. I'll re-do my calculation
– LPs
Commented Oct 26, 2017 at 10:38
• Well, all done and crystal now. Thank you very much.
– LPs
Commented Oct 26, 2017 at 11:24