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

• Naakicibi
In summary: I would call it i_short. 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_short would be a good name for it. i_sc would be more accurate but might be confusing.

#### Naakicibi

(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
axmls said:
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.

axmls said:
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

Naakicibi said:
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 said:
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.

## 1. What is the purpose of finding Rthev in a parallel resistor and short circuit circuit?

Finding Rthev helps determine the equivalent resistance of a circuit, which is an important parameter in understanding how the current flows in a parallel resistor and short circuit circuit.

## 2. How is Rthev calculated in a parallel resistor and short circuit circuit?

Rthev is calculated by taking the reciprocal of the sum of the reciprocals of all the resistances in the circuit. In a parallel circuit, this means adding the inverse of each resistance and then taking the inverse of the total.

## 3. What happens to Rthev when a short circuit occurs in a parallel resistor circuit?

When a short circuit occurs in a parallel resistor circuit, the equivalent resistance Rthev becomes 0, as the short circuit provides a path of no resistance for the current to flow.

## 4. Why is Rthev important in a parallel resistor and short circuit circuit?

Rthev helps in understanding the total resistance of a circuit and how it affects the current. It also helps in calculating the total power dissipated in the circuit.

## 5. Can Rthev ever be greater than the largest resistance in a parallel resistor and short circuit circuit?

No, Rthev can never be greater than the largest resistance in a parallel resistor and short circuit circuit. This is because, in a parallel circuit, the total resistance is always less than the smallest individual resistance.