Help! Mesh analysis, I'm totally lost (engineering student level) - Page 1
Forum section: electronics / beginners

author: victoriak
author_rank: Newbie
timestamp: February 21, 2019, 09:12:04 pm
content: Hello, it's my first post hereI'm getting totally lost with my circuit analysis class's assignments, I still struggle to grasp the basics but I'm here in the lab trying to figure it out.During yesterday's class, our professor left us this mesh analysis circuit, he told us it had a trick on how to implement it. I have been trying to draw the circuit in Multisim (have Proteus as well) just to begin, but couldn't even figure out even the way to redraw it with pencil/paper.The task involves "implementing, modifying (redrawing) and calculating the currents in the circuit ( we just covered the supermesh analysis, how to eliminate the source and to generate an auxiliary equation of the supermesh, and then solving the equation system), with the only caveat/warning/tip to use different grounds for each power source. With 4 mesh and its own current.[Here its my attempt on multisim:https://i.imgur.com/feakKLf.pngI will try to resolve the resistors in parallel first to get a more simple circuit.I have the impression this may be easier than I think, but I'm clueless. I couldn't start calculations cause I cant be sure how to redraw it. Them I have to set it up on the breadboard. Guy,s I'm getting desperate here, if you guys can pinpoint me on the right direction or what I'm doing wrong let me know. I appreciate your time and help in advance.

author: rstofer
author_rank: Super Contributor
timestamp: February 21, 2019, 10:15:00 pm
content: You can replace the 1k the 2.7k and the 500 Ohm in the lower part with a single equivalent resistorYou can replace the 1k and 100k in the upper left with a single equivalent resistorNow that horizontal rail in the middle of the diagram looks a lot like 'ground'.Now that there is so little circuit left, the supermesh approach will be fairly easy.Just set the 6V source to 0V and follow along with the discussion.

author: dmills
author_rank: Super Contributor
timestamp: February 21, 2019, 10:22:21 pm
content: Firstly, step away from the breadboard and put that simulator downThis is a pencil and paper (and maybe a calculator if feeling lazy) problem, not something that needs spice.I would observe that the bottom and right hand sides are a common node and that the 100k and 1k around the upper current source are in parallel and can be collapsed to a single resistor of ~1k without touching any loop you care about.Not sure if this counts as mesh current analysis, but I would do it in two stages, set the 5mA source to open circuit and calculate the 4 currents, then open the 10mA source and turn the 5mA one back on, recalculate the currents, then just add em up.Regards, Dan.

author: rstofer
author_rank: Super Contributor
timestamp: February 21, 2019, 10:43:20 pm
content: Collapse all the parallel resistors I mentioned above (and any I forgot), solve for the mesh currents and then calculate the node voltages. Use these voltages with the original circuit to find the various component voltages and currents.

author: victoriak
author_rank: Newbie
timestamp: February 22, 2019, 10:26:07 pm
content: Thank you so much for your help! I can't be more thankful really, I'm doing really bad so far and this kind of circuits are confusing to me, I will collapse all the resistors and first then, I think I now have way more of a grasp how to solve it with your help.

author: victoriak
author_rank: Newbie
timestamp: February 22, 2019, 10:28:33 pm
content: Thank you Dan, you are correct, I should start with the basics of the analysis with just a pen and paper, your insight allows me to see a path to how to solve it, I can't thank you enough for that, I'm completely clueless and that's why I go to other stuff like the sim and proto to figure it out.

author: dmills
author_rank: Super Contributor
timestamp: February 22, 2019, 10:53:50 pm
content: Beginners are often way too quick to reach for a simulator, that is not the way to develop skills (In skilled hands they are useful for telling us we screwed up, but that is about all).The truth is that as a working stiff you will have to solve networks like that approximately never (I think I designed ONE or TWO gnarly resistor network around opamps in the last 5 years!), the exist in school because they teach methods and practice the maths, and those things are important.This is very early lecture stuff, so if you are having trouble get with your lecturer or TA and get whatever you are misunderstanding sorted quick like, it only gets harder from here on out and if you don't get the basics then you are going to be in real trouble.Regards, Dan.

author: rstofer
author_rank: Super Contributor
timestamp: February 23, 2019, 12:16:43 am
content: At some point, you are going to wish for a math solver. It isn't long until the equations get to be 6 variables, 6 unknowns and it just takes too long to solve the matrix equations. And it's error prone!I would highly recommend MATLAB but there is a cost, even for students. So, maybe look into Octave which is a MATLAB workalike. I don't use it much so I really don't know a lot about it; I use MATLAB. Node and mesh equations are a lot easier to solve by machine. If nothing else, to check your work.The attached .txt file should be renamed as a .m file and it will run as a script in Octave and MATLAB. It is a little mesh problem with 8 resistors and 3 voltage sources. Not that the code is useful without a schematic but I added it so you can see the flavor of the solution to a 4x4 set of equations.There are other solvers including some that are web based like Wolfram Alpha. I don't know anything about them.I don't view this in the same way as using a simulator. You have to do every bit of the equation writing to get the values to put in the matrices. The only thing the computer does is grind through the numbers. Something I had to do with a sliderule way back when.As an aside, if you aren't doing function graphing at desmos.com and calculus problems at symbolab.com, you are missing out on some great tools. These I do use!

author: The Electrician
author_rank: Frequent Contributor
timestamp: February 23, 2019, 09:26:41 am
content: victoriak, just for a start, can you write the mesh equations for I1 and I2?

author: dmills
author_rank: Super Contributor
timestamp: February 23, 2019, 05:22:31 pm
content: Yep, at some point a computer algebra system or matrix solver takes the tedium and sign errors out of it, and simulation is wonderful for checking that you didn't screw up, but for learning the approach, pencil and paper is still the way to play.In fact Davecad is still where most of my design time is spent, at least for time spent doing real design not doing 'datasheet copy and paste'.Sim is a great tool for verifying that you have not done something silly, but it is mostly a tool that tells you 'Wont work', not one that reliably says "Yes"....Regards, Dan.

author: IanB
author_rank: Super Contributor
timestamp: February 23, 2019, 05:45:42 pm
content: Quote from: victoriak on February 22, 2019, 10:28:33 pmYou must resist doing that. It's like saying, "I'm not very fit, that's why I go everywhere by car instead of doing exercise."These exercises are designed to build mental fitness. You have to figure them out with pencil and paper to improve your skill level, because without building that skill you will get completely stuck on more advanced material that comes later.Stick at it and don't give up.Try to apply some basic concepts like this one: If two points are joined by a line, you can remove the line and combine the two points.If you learn to do things like this you will start to see where two separate resistors are actually in parallel with each other and can be replaced by one resistor.