That is Mesh-Current Method.hi. this is what im done... im very confused at this time ... im use voltage test... sorry 4 my bad english
You found Iu when you have a 1V test source applied and the independent sources turned off. So what is the next step in finding Rth?hi. this is what im done... im very confused at this time ... im use voltage test... sorry 4 my bad english
He's using Mesh-Current in order to solve for the Thevenin resistance, which is perfectly acceptable.That is Mesh-Current Method.
You said that you wanted to use Thevenin Equivalent Circuit. You have not done so. Is the assignment to use Thevenin?
You found Iu when you have a 1V test source applied and the independent sources turned off. So what is the next step in finding Rth?
Don't worry about your bad English -- we'll muddle through. I assure you that your English is much better than my [insert your native language here].
You want to break most problems into three distinct pieces. The first is to clearly layout the problem with a properly annotated schematic (which you have done nicely, thank you). The second and most important part is setting up the equations. This is where ALL of the EE stuff is. Everything after that (the second piece) is just math. So put all of the EE stuff in one place so that you can check it easily because any mistakes here usually can't be caught and corrected later on because "later on" is just math and if your setup is wrong then it is merely describing some other circuit instead of the one you are really working with and the math doesn't care.
When setting up your equations you always want to verify them after you have written them down to ensure that they are right before you proceed because if they are wrong, then all of the math you do after that is wasted time and effort because the answer is virtually guaranteed to be wrong.
In your case, your very first equation (the Loop 1 initial equation) is wrong. Everything after that is wasted time and effort.
After writing down
\(
5I_u \, + \, 1 \; = \; 80I_1 \, - \, 80I_3
\)
You should have asked whether the two voltages on the left-hand side add as you go around the loop. Do they?
Another part of the first part is to see if you can reorganize the layout and perhaps simplify it to make the next parts easier to do. In this case there is a LOT of simplification that can be done.
Finally, you should always, always, ALWAYS track your units. Nearly all authors and instructors (most of whom have never worked in the real world and seen someone die because they couldn't be bothered to track their units) are inexcusably sloppy about units. But the simple fact is that you WILL make stupid math mistakes -- we all do -- and that most of those mistakes will screw up the units, provided they are there to be screwed up. If the units don't work out, the answer is wrong. Period. Tracking units is very probably the single most effective error detection tool available to the engineer. NEVER just tack the units that WANT the answer to have onto the end result. Doing so, in my opinion, is criminally negligent and, at times, courts have agreed when deciding and awarding wrongful death and injury cases.
So let's look at that equation again:
\(
5I_u \, + \, 1 \; = \; 80I_1 \, - \, 80I_3
\)
The first term on the left is a current. The second term is just a number. You can't add a current and a number. Both terms on the right are currents. But is that really what they are? Remember that Mesh Current Analysis is nothing more than a systematic application of KVL in which you are summing up voltages. That '80' is not a number, it is supposed to be a resistance and a resistance times a current yields a voltage. Similarly, the '1' on the left hand side is your test source voltage, so it is 1V.
Also, that '5' in the first term is not just a number. We have a current controlled voltage source which means that we take a current and multiply it by a gain to get a voltage, so that gain MUST have units of voltage per current. Most authors will leave this out which, again, is really pretty inexcusable. But they do and so we have to patch up their work by assuming (something that engineering should NOT be about!) what the author meant, namely that it is 5V/A.
So your equation should have been
\(
\(5\frac{V}{A}\)I_u \, + \, 1V \; = \; \(80\Omega\)I_1 \, - \, \(80\Omega\)I_3
\)
That doesn't count as "showing your work".the Rth is 40 ohm ? that right ? 1v/0.025A = 40 ohm
Yep, that's a documentation typo that didn't get caught. I knew I was calculating Va and in my mind I was thinking Vb - IR and I just wrote Vb on the left of the equals sign since I was planning to use the numbers immediately. Notice that in the actual work I almost always start with a symbolic statement of the expression and plug the numbers in separately. I tend to relax the degree of formalism in the checks. Good catch. That's one of the reasons why I annotate the diagram with the voltages and currents and then use the diagram as my source of information for calculations.If only tracking units would tip you off that you said VB = -60V + 6.18A*20Ω = 63.6V, when it's actually VA that equals 63.6V.
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