# 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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