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I'm back with my basic questions.

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

schematic

simulate this circuit – Schematic created using CircuitLab

I must answer:

  • \$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 :

schematic

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:

schematic

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:

schematic

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?

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  • \$\begingroup\$ The circuit have used to calculate \$R_{th}\$ is wrong.. \$\endgroup\$
    – nidhin
    Oct 26, 2017 at 8:52
  • \$\begingroup\$ @nidhin Well,... good. Any hint? Because of the result is correct..... \$\endgroup\$
    – LPs
    Oct 26, 2017 at 8:58
  • \$\begingroup\$ @nidhin I corrected the first circuit, that was the wrong one \$\endgroup\$
    – LPs
    Oct 26, 2017 at 10:07
  • \$\begingroup\$ @nidhin Yes, sorry. Edited \$\endgroup\$
    – LPs
    Oct 26, 2017 at 10:10
  • \$\begingroup\$ 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. \$\endgroup\$ Oct 26, 2017 at 11:42

1 Answer 1

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Your Delta-Wye transformation is wrong.
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.

schematic

simulate this circuit – Schematic created using CircuitLab

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

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