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
    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
    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.
    mjakov likes this.
  3. dalam


    Aug 9, 2014
    As the branch C-D is open circuited so no current flows.
    Vcd= I*2ohm
    Vab=Vcd-12V using KVL.
    mjakov likes this.