How to find circuit equivalent resistance as seen by a terminal

Discussion in 'Homework Help' started by mjakov, Sep 3, 2014.

1. mjakov Thread Starter New Member

Feb 13, 2014
20
5
Hi there!

How would you go about finding the short circuit (s.c.) current $I_{ab}$ and the open circuit (o.c.) voltage $V_{ab}$ in order to calculate the equivalent resistance as seen from the terminal ab?

$R_{ab} = \frac{V_{o.c.}} {I_{s.c.}} = \frac{V_{ab}} {I_{ab}}$

I have been trying to use the node voltage method for the s.c. case with node c as the principal node and node d as reference. This does not give the correct result.

Is it correct to assume then that the voltage at point c is always 20 V?

Is it correct to assume that the voltage at point a is always 12 V?

Would it be better to assume that there are 2 principal nodes c and d and the reference node e when using the node voltage method?

2. vvkannan Active Member

Aug 9, 2008
138
11
You can just omit the voltage sources (short circuit) and calculate the equivalent resistance. Here, (5||2 ) +2 is the equivalent resistance Rab.

If you are using the node voltage method with node d as reference you cannot assume voltage at c as 20v as voltage at c is 20 more than the point 'e'and not 'd'. To know the voltage at c with d as reference you can use KCL at point c or KVL for the loop with voltage source 20v and 5 and 2 ohm resistors.

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3. dalam Member

Aug 9, 2014
58
6
As the branch C-D is open circuited so no current flows.
Vcd= I*2ohm
Vab=Vcd-12V using KVL.
I=20/7A

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