Solving R_Thevenin in Circuit: Ignoring R_3?

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SUMMARY

In solving for R_Thevenin in a circuit, R_3 is ignored when calculating the short circuit current due to the assumption that the short circuit has zero resistance. This results in R_3 drawing no current, as its resistance becomes negligible compared to the zero ohms of the short circuit. However, R_3 must be considered when determining the open circuit voltage, as it influences the voltage across the terminals in that scenario.

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When solving for [itex]R_{Thevenin}[/itex] in the circuit below, why do you ignore [itex]R_3[/itex]? Is it because the voltage through [itex]R_3[/itex] is zero?

[PLAIN]http://img197.imageshack.us/img197/88/thevenin.png
 
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You would ignore R3 for the short circuit current, but you would need to consider it for the open circuit voltage.

It doesn't draw any current if it is short circuited, because the short circuit is assumed to have zero resistance.
So compared with zero ohms, whatever resistance R3 has is infinitely large and the current it takes is infinitely small, or zero.
 

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