# Resistor parallel to short circuit when finding R_thev ...?

(sorry, I couldn't figure out how to insert an image from my drive) I have a problem where in finding the thevenin resistance I take out my independent sources and as a result I'm left with a resistor parallel to a short circuit. Should I ignore this resistor when calculating equivalent resistance?

I'm confused because we still consider resistors that are on the branches of the +/- terminals of the open circuit voltage.

Thanks!

You're looking at the resistance seen from the terminals. When there's a resistor on the branch of the terminals, the current still must flow through the resistor, so the terminals "see" that resistance. However, if the current has a choice between a branch with a resistor and a short circuit, then you can ignore the resistor completely. In other words, you consider resistors in the terminal branches because the current passes through them. The current does not pass through a resistor that is shorted out.

Naakicibi
You're looking at the resistance seen from the terminals. When there's a resistor on the branch of the terminals, the current still must flow through the resistor, so the terminals "see" that resistance. However, if the current has a choice between a branch with a resistor and a short circuit, then you can ignore the resistor completely. In other words, you consider resistors in the terminal branches because the current passes through them. The current does not pass through a resistor that is shorted out.

Thank you very much, that made it very clear. Maybe you could help me with a similar situation; what should I do when given a voltage source and a resistor in parallel with a short circuit (this time I'm not considering a thevenin equivalent)

It depends on what you're trying to do. Remember an ideal voltage source always maintains the same potential between its terminals, whereas if two terminals are connected by an ideal wire (short circuited), then they are at the same potential, so you can see there's a bit of a conflict there.

It depends on what you're trying to do. Remember an ideal voltage source always maintains the same potential between its terminals, whereas if two terminals are connected by an ideal wire (short circuited), then they are at the same potential, so you can see there's a bit of a conflict there.

http://postimg.org/image/69pyh7uun/

Thanks for your help, I have linked the circuit I am analyzing, and at this point I'm just unsure how to deal with the short circuit all together... I'm asked to find i(0), so I used mesh-current analysis of the middle mesh (when t<0) to get a short circuit current of .1 A, which is the steady state current before t=0. Then when t > 0 and the short circuit is removed, I get i_sc = .1625 A. These values don't seem right because later I'm asked to compute when i = 0, which never occurs with my transient function derived from these currents.

For t<0, does all of the current produced from the voltage and current sources only flow through the short circuit branch, and thus cancel out the branch with the 30 ohm resistor?

thanks again

donpacino
Gold Member
For t<0, does all of the current produced from the voltage and current sources only flow through the short circuit branch, and thus cancel out the branch with the 30 ohm resistor?

no current will still flow through the 30 ohm resistor.

when you say i_sc, do you mean i(t)?
also unless you are finding steady state current I(t) should be a function of t because of the inductor.

donpacino
Gold Member
no current will still flow through the 30 ohm resistor.

when you say i_sc, do you mean i(t)?
also unless you are finding steady state current I(t) should be a function of t because of the inductor.
for t=inf I get i(t)=0.01625A so assuming one of us is simply off by a factor of 10 it looks like your number is correct.

I would not call it I_sc, as sc often stands for short circuit.