# Quick quest. about Thevenin Voc

Discussion in 'Homework Help' started by freemindbmx, May 13, 2014.

1. ### freemindbmx Thread Starter Member

Mar 5, 2014
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0
Im able to write up my equations using mesh analysis for Isc(Current short circuit) and my equations for Voc(voltage open circuit), but cant quite figure out the voltage across A,B for Voc. Would it be the voltage across the 4ohm resistor on top(I3) or the 6ohm resistor in (I1)?

Problems posted below as Thev. Voc & Thev. Isc
My equations for Isc:

I1:
9ΩI1-6ΩI2=15V

Supermesh I2+I3:
-6ΩI1+6ΩI2+4ΩI3=0
AUX Equation: 1.2Io=I3-I2 and Io=I1 so 1.2(I1)=I3-I2

Isc=I2= 0.21A

For Voc im pretty sure I wrote everything down correctly)

I1:
9ΩI1=15
I2=1.2Io and Io=I1 so 1.2(I1)=I2

Any thoughts would be great In the back of the book im able to confirm that Isc is correct but Voc isn't.

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2. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
Voc is the voltage between A and B (you don't indicate polarity, but to be consistent with your direction for Isc it needs to be Vab -- the voltage at A minus the voltage at B).

Is the 4Ω resistor connected between A and B? If not, then the voltage across the 4Ω resistor is not Voc.

Is the 6Ω resistor connected between A and B? If not, then the voltage across the 6Ω resistor is not Voc.

Find the voltage at A (Va) relative to whatever point in the circuit you want to call "ground".

Find the voltage at B (Vb) relative to whatever point in the circuit you want to call "ground".

Subtract Vb from Va to get Vab. That's the voltage at A relative to B, which is also Voc.