Equivalent resitance?Thevenin equivalent problem

[Learnphysics]

Homework Statement

http://img825.imageshack.us/img825/2306/theveninequiv.png

I'm trying to find the Thevenin equivalent resistance for this circuit, but it's proving difficult.

I'v tried to Zero all sources, (assuming that Zero'd current sources are open circuits), and i drew the resulting circuit.

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Homework Equations

I Guess my biggest problem is that R3 resistor that's just floating there, and has one end hooked up to nothing.

The Attempt at a Solution

I know that R4 and R1 are in series. And that R4+R1 are || with R2. But i don't know how this R3 fits into all of this.

Thank you.

 

[Maru]

Hi Learnphysics,

You have done the transformation correctly. Since R3 is dangling, it is not part of the Thevenin equivalent resistance, and so you ignore it. No current is flowing through it since it is an open, so it cannot contribute to the equivalent resistance.
So You'll just have: Zth = (R1+R4) || R2

However, since you turn the sources back on afterword to compute the Thevenin voltage, it becomes part of the circuit again since it is no longer dangling (an open).