What Is the Correct Way to Calculate Resistance in Complex Resistor Networks?

[Drao92]

Homework Statement

Hi guys,
Im new here and I am working for the first time with with these applications.
So, i have this resistor network https://www.physicsforums.com/attachment.php?attachmentid=44965&stc=1&d=1331497208 where R1=220 ohm and R2=330 ohm.
I must find the electrical resistence between pins 1-8, 1-2 and 2-4. They must give me around 92 ohm, 168 ohm and 265 ohm.

The Attempt at a Solution

For 1-8 i made (R1+R2)/6=91.66 ohm but for the others i don't know how to solve them. Ill be very glad if you`ll explain me 1-2 or 2-4 to understand how to solve these kind of problems.
PS: The resistors are not isolated, i also don't know what this means.

Well, for 1-2 i made ((R1+R2)/5+R1)*R2/(((R1+R2)/5+R1)+R2=165 ohm

 

[gneill]

Homework Statement

Hi guys,
Im new here and I am working for the first time with with these applications.
So, i have this resistor network https://www.physicsforums.com/attachment.php?attachmentid=44965&stc=1&d=1331497208 where R1=220 ohm and R2=330 ohm.
I must find the electrical resistence between pins 1-8, 1-2 and 2-4. They must give me around 92 ohm, 168 ohm and 265 ohm.

The Attempt at a Solution

For 1-8 i made (R1+R2)/6=91.66 ohm but for the others i don't know how to solve them. Ill be very glad if you`ll explain me 1-2 or 2-4 to understand how to solve these kind of problems.
PS: The resistors are not isolated, i also don't know what this means.

Well, for 1-2 i made ((R1+R2)/5+R1)*R2/(((R1+R2)/5+R1)+R2=165 ohm

Hello Drao92, Welcome to Physics Forums.

As a start, after picking two of the terminals that you want to find the resistance between, redraw the circuit having removed all of the extraneous leads and terminals that are not connected to anything. This will leave a less cluttered picture to contemplate.

Next, realize that when resistors are in parallel it doesn't matter what order they are in so long as all of them are accounted for. So you can "shuffle" the order of the transistor pairs (R1 and R2 series units) at will along the top and bottom rails. A handy choice might be to move the pair that are externally connected to be next to each other, say to the left end of the string. This might give you some ideas about simplification

 

[Drao92]

Hi gneill
Im doing 2-3 atm and i found out from google its an irreductible network and it can be solved with
node-voltage puting a 1 Amp test source at pins and find the voltage at pins, in fact i must find only V1, because i make V0=0, then R=(Vpin2(v1 in photo)-Vpin3(v0 in photo))/1amp.
This is how i transformed the circuit for 2-3 case.
Later edit: The first photo was wrong, this one is good.
https://www.physicsforums.com/attachment.php?attachmentid=44967&stc=1&d=1331502295

 

[gneill]

Actually it's not quite irreducible; One could apply a Δ-Y transformation to continue reducing the circuit. Of course, whether or not that takes less effort than doing a little mesh or nodal analysis is a personal choice

 

[Drao92]

Man, you are awsome, thanks u very much, this is more easy :).
I learned just the theory of these theorems, never made application and yes, those "square" loops are triangles :).