I (short circuit) Question

  • #1
63
1

Homework Statement


I'm in a lab and we have this circuit & question as part of the pre-lab
SS 2015-09-20 at 3.53.36 PM.png

SS 2015-09-20 at 3.54.07 PM.png


The Attempt at a Solution


SS 2015-09-20 at 3.54.13 PM.png

I began trying to use nodal analysis to get Va & Vb & to then use Vab to get the voltage and then the current but after doing what you can see above, I stopped because I think there is another way that I'm supposed to do it.
Someone please guide me in the correct direction?
Thank you
H
 

Answers and Replies

  • #2
Hesch
Gold Member
922
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I stopped because I think there is another way that I'm supposed to do it.
Make a Thevenin equivalent for ( R2 and R3 ) and for ( R1 and R4 ).

Vth = Vb = 5V*R3/(R2+R3). ( R2 and R3 is simply a voltage divider ).
Rth = R2 || R3. ( || means in parallel ).

Short circuit the two Thevenin equivalents to find Isc.

REQ = Rth + Rth.
 
  • #3
gneill
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You're looking for the short circuit current, so essentially the current flowing through a wire (replacing RL with a wire). My first instinct would be to use mesh analysis to find that current, but a niftier choice would be to work with what you've already got and proceed to find the Thevenin equivalent (or Norton equivalent), since really all you need to finish is the equivalent resistance of the circuit looking into the open load terminals; The network is not difficult to simplify. Then finding the short circuit current would be trivial.
 
  • #4
63
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okay, using what both of you said, I tried to finish the problem. Tell me what you think,
SS 2015-09-21 at 11.52.40 AM.png
 
  • #5
gneill
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You were doing fine until you went to calculate ISC. It's not clear from your work what values you are using for Vth and Rth. What comprises your final equivalent circuit?
 
  • #6
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I determined Vab by subtracting Vb from Va and then I divided that by my determined REQ
 
  • #7
gneill
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I determined Vab by subtracting Vb from Va and then I divided that by my determined REQ
Okay, can you check (and post) the values?
 

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