Short Circuit Current Calculation

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Discussion Overview

The discussion revolves around calculating the short circuit current in a circuit using Thevenin's equivalent resistance. Participants explore the relationships between various voltages and currents in the circuit, attempting to simplify their equations and clarify the conditions under which certain assumptions hold.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant attempts to derive the short circuit current using Kirchhoff's Voltage Law (KVL) and expresses uncertainty about simplifying the equations, particularly regarding the voltages v2 and v3.
  • Another participant suggests expressing v1 and v3 in terms of current and resistance to simplify the analysis.
  • There is a discussion about the relationships between voltages, with some participants proposing that v1 = IR1, v2 = v1, and v3 = IR2, while others challenge the correctness of these relationships.
  • One participant emphasizes that v3 should not be considered zero and encourages further exploration of its definition in relation to the circuit's nodes.
  • Another participant presents a derived expression for v3 using source transformations and node analysis, suggesting a relationship involving the voltage source and resistances.
  • Participants express confusion and seek clarification on the validity of their assumptions regarding the voltages, particularly v1 and v2 being equal.

Areas of Agreement / Disagreement

Participants generally agree on some relationships between the voltages, specifically that v2 = v1. However, there is disagreement regarding the value of v3 and how it should be defined, indicating that the discussion remains unresolved.

Contextual Notes

Some assumptions about the circuit configuration and the placement of ground are not explicitly stated, which may affect the interpretation of the voltage relationships. The discussion also reflects uncertainty about the simplifications made in the equations.

magnifik
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i am trying to find the short circuit current for the following circuit.
23r4axx.png

the RL is the thevenin equivalent resistance. i am trying to find the current when RL is a wire (short circuit).

by KVL, i have
IR1 + \mu1(v2-v1) + IR2 + \mu2(v3-v1) + IR3 = 0

(the above is not formatted correctly. it should be
IR1 + m1(v2-v1) + IR2 + m2(v3-v1) + IR3 = 0 )

what i am having trouble with is trying to simplify it. specifically, i am trying to get rid of v2 and v3

can i substitute v3 = IR2
and v2 = IR1
??

thanks.
 
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magnifik said:
i am trying to find the short circuit current for the following circuit.
the RL is the thevenin equivalent resistance. i am trying to find the current when RL is a wire (short circuit).

Assuming you are placing your ground at the bottom of the circuit, what can be said about v1 and v2?

If you want to simplify it more, express v1 and v3 in terms of the current times their respective resistance(s).

(HINT: Maybe there is a reason I didn't explicity tell you to solve for v2)
 
jegues said:
Assuming you are placing your ground at the bottom of the circuit, what can be said about v1 and v2?

If you want to simplify it more, express v1 and v3 in terms of the current times their respective resistance(s).

(HINT: Maybe there is a reason I didn't explicity tell you to solve for v2)

Are the following relationships correct...
v1 = IR1
v2 = v1
v3 = IR2
 
magnifik said:
Are the following relationships correct...
v1 = IR1
v2 = v1
v3 = IR2

Give v3 another shot!

Remember it's not simply the voltage across that one resistor, it's the voltage at that node with respect to ground.
 
jegues said:
Give v3 another shot!

Remember it's not simply the voltage across that one resistor, it's the voltage at that node with respect to ground.

is it correct to solve it in this way
v3 + m1(v2-v1) / R2 = 0
v3 = -m1(v2-v1)
 
You told me in your above post that v1=v2, so your implying that v3=0?

V3 isn't 0.
 
i got v3 = -m1(v2-v1), not 0
 
You told me v2=v1, thus v2-v1=0.

Which is in your expression for v3, is it not?
 
oh, i see now. I'm still having trouble trying to figure out v3.
 
  • #10
i tried to do source transforms and analyze viz node method and got
v3 = -v1m2R2/(R3 - m2R2)
 
  • #11
magnifik said:
i tried to do source transforms and analyze viz node method and got
v3 = -v1m2R2/(R3 - m2R2)

Okay now that we know that the voltage source is effectively 0 volts wouldn't v3 be defined as follows,

V3 = I(R1 + R2) ?
 
  • #12
ahh yes, i see that. thanks. were my other assumptions correct?
v1 = v2 ?
 
  • #13
magnifik said:
ahh yes, i see that. thanks. were my other assumptions correct?
v1 = v2 ?

I'm not trying to be rude, but have you read my replies?

I had told that v2-v1 = 0.

So yes, v2 = v1.
 

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