# Determine resultant resistance in network (Thévenin)

1. Sep 23, 2011

### Twinflower

1. The problem statement, all variables and given/known data
I need to calculate the resultant resistance between points A and B
The |-| parts are short curcuited ideal DC power supplies (illustration only)

This is the circuit:

2. Relevant equations
$$R_P = \frac{1}{\frac{1}{R1}+\frac{1}{R3}+\frac{1}{Rn}}$$
$$R_S = R_1 + R_2 + R_N$$

3. The attempt at a solution
I cant find any pure series or pure parallel resistance, so I really do not know where to start at this.
Simulation in Multisim gives me a resistance at 5.7 Ohm.

2. Sep 23, 2011

### The Electrician

3. Sep 23, 2011

### Twinflower

Thank you.
Also, someone whispered that i had to transfer the two deltas to star-networks, so I am working on that one now.

4. Sep 23, 2011

### uart

It's sufficient to just transform one of the star networks (eg the 4.5, 5, 6 ohms) to delta.

Other ways are to insert a 1 amp current source between A and B and to use nodal analysis to find the input voltage (which is numerically equal to the input resistance) or to insert a 1 volt voltage source between A and B and use mesh analysis to find the input current (which is numerically equal to the input conductance).

5. Sep 23, 2011

### Twinflower

6. Sep 25, 2011

### Staff: Mentor

Nicely done. If you ever find yourself in a situation where you would need to use Y-Delta conversion to simplify a network and can't remember the formulas you can always slap a 1V voltage source on the network (across AB) and use mesh analysis (or your own favorite method) to find the current, I, that this source will be driving into the network. The network resistance is then given by R = V/I.

7. Sep 26, 2011

### Twinflower

Thank you gneill :)

I have a further question about this problem as I am stuck with finding the thévenin voltage of the network, but I havent found the time to create a new post showing my attempts yet.

stay tuned

8. Sep 27, 2011

### Twinflower

Ok, I solved that issue using mesh current analysis. No further assistance needed from my part in this thread :)