Solving a Circuit with Nodal Analysis

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Discussion Overview

The discussion revolves around solving a circuit problem using nodal analysis and Thevenin's theorem as part of a pre-lab assignment. Participants explore different methods to find voltages and currents in the circuit.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses uncertainty about the approach to use for solving the circuit and seeks guidance on nodal analysis.
  • Another participant suggests creating Thevenin equivalents for specific resistors and provides a formula for calculating the Thevenin voltage (Vth) and resistance (Rth).
  • A different participant proposes using mesh analysis to find the short circuit current but also supports the idea of finding the Thevenin equivalent for simplification.
  • One participant attempts to apply the suggestions but receives feedback questioning the clarity of their calculations for the short circuit current (Isc) and the equivalent circuit.
  • Another participant asks for the specific values used in the calculations of Vab and REQ to verify the work done.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the best approach to solve the problem, with multiple methods being proposed and discussed. Uncertainty remains regarding the calculations and the final equivalent circuit.

Contextual Notes

There are unresolved aspects regarding the values used for Vth and Rth, as well as the final equivalent circuit, which may affect the calculations of the short circuit current.

hdp12
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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
 
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hdp12 said:
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.
 
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.
 
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
 
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?
 
I determined Vab by subtracting Vb from Va and then I divided that by my determined REQ
 
hdp12 said:
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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