# at a total loss with mesh analysis and need help

Discussion in 'Homework Help' started by hunterage2000, Mar 18, 2011.

1. ### hunterage2000 Thread Starter Senior Member

May 2, 2010
470
1
At a bit of a loss with mesh current analysis and cant see what is going on with it. I can grasp Node analysis but dont really understand mesh. Can someone look at the attached jpeg and talk me from start to finish how this is solved?

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2. ### hunterage2000 Thread Starter Senior Member

May 2, 2010
470
1
no takers? No one can spend 5 minutes to show me how to answer this?

3. ### justtrying Active Member

Mar 9, 2011
330
786
have you tried setting up the equations? there are three loops, therefore three equations to solve a matrix

4. ### hunterage2000 Thread Starter Senior Member

May 2, 2010
470
1
I've tried with what I think is right but LTspice says its wrong. But to be honest I dont fully understand what Im meant to be doing.

5. ### justtrying Active Member

Mar 9, 2011
330
786
I understand, still try writing the three loop equations - see attachment.
I1R11+I2R12+I3R13 = V1
I1R21+I2R22+I3R23 = V2
I1R31+I2R32+I3R33 = V3

where R11, R22 and R33 are resistances within the loops and R12, R13 etc are resistances shared by the currents. When you assign the currents, the direction is arbitrary, as you work through the equations, when you do the resistances shared by the loops, if two currents go in opposite direction, the sign is negative, if they go in the same direction the sign is positive.

The other danger is sign for the voltage - again see how it is with respect to your assigned current.

I got I1 = -15.14A (that means it is actually opposite to my assigned direction)
I2 = 3.396A and I2=0.2539A. I would imagine the problem actually asks about currents through resistors, so you would then have to add your mesh currents to find actual circuit currents. Checked this on multisim, got exact values.

• ###### loops.jpg
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Last edited: Mar 18, 2011
6. ### hunterage2000 Thread Starter Senior Member

May 2, 2010
470
1
I have tried again and got the wrong answers. Can you show me what you got for the 3 equations

7. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
789
You should to write out the three equations as suggested by justtrying.

For instance for loop I1

7*I1-4*I3=-107

8. ### justtrying Active Member

Mar 9, 2011
330
786
you should post your equations so that we can help you figure out what is wrong, most likely source of error is signs. Right now al info you need to write the equations is there.