# Basic circuit solution needed ?

Discussion in 'General Electronics Chat' started by curry87, Sep 2, 2010.

1. ### curry87 Thread Starter Member

May 30, 2010
101
0
How do i solve this circuit for total current usage so far I calculated that the equivalent parallel resistance is 166 ohms but got stuck on whether to calculate their current from the first series resistor voltage or 10v?

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2. ### Potato Pudding Well-Known Member

Jun 11, 2010
684
92
Simplest way is to set up the circuit in a Sim like LTSpice and test for DC operating point.

--- Operating Point ---

V(n001): 10 voltage
V(n002): 2.11009 voltage
V(n004): 1.55963 voltage
V(n003): 2.01835 voltage
I(R7): -0.000458716 device_current
I(R6): 0.00155963 device_current
I(R5): -0.000458716 device_current
I(R4): -0.00110092 device_current
I(R3): -0.00211009 device_current
I(R2): -0.00422018 device_current
I(R1): -0.00788991 device_current
I(V1): -0.00788991 device_current

3. ### Potato Pudding Well-Known Member

Jun 11, 2010
684
92
Sorry, because that didn't help you to actually do the math - but it will let you check your math which is often just as good.

For example test your calculation for the parallel resistance against what the sim gave for total current. I think you will see that the total load resistance across the voltage source is 1267Ω which after taking out the first series resistor of 1k means that the remaining resistors are an equivalent series resistor or 267Ω.

4. ### curry87 Thread Starter Member

May 30, 2010
101
0
Its like all the simulators it tells you the figures of a given circuit but not how to get those figures so your left there going over and over wondering how the hell they got that answer until finally given up.

5. ### Ghar Active Member

Mar 8, 2010
655
72
How did you calculate that resistance?

Start from right to left...

200 + 1000 = 1200 (in series)
1200 || 500 = 352.9 (in parallel)
1000 + 352.9 = 1352.9
1352.9 || 1000 = 575
575 || 500 = 267
1000 + 267 = 1267

1267 ohms is the total resistance... so total current = 10 V / 1267 Ohms = 7.9 mA

6. ### Potato Pudding Well-Known Member

Jun 11, 2010
684
92
I suggest trying to set up resistor maps in a simulator and work out by hand what the Sim will tell you.

Start with 3 Resistors. Make up three of those circuits and solve them.

Then use 4 Resistors. Make 4 different circuits and solve them.

Then use 5 Resistors. x5

6 Resistors x6

7 Resistors x 7

That should be enough but you can keep going if you think it is still helping enough to compensate for the time used.

The idea is to start easy and get them done quickly and keep trying to work quickly,

More practice is the only thing that will teach you.

Soon it will be basic to you.

Everything builds on the stuff you already know so it is worth taking the time to beat stuff to death with your brains.

If you settle for some shaky - pass the test and forget it - learning then everything you learn that relates to that knowledge is going to be shaky unless you go back later to reinforce it.

The same goes for the losing interest in stuff. Sometimes you naturally take a real hate for subject topics and try to brush them off. But those topics, especially the complicated ones can have so many threads going through them that not only do they turn out to be really important but they turn out to be interesting and fun.

The worst problems with learning.

Don't Expect it to be easy. It can be but only if you stick with it, try stuff, and ask questions until you find a way to make it click.

Don't tell yourself you can't do it. You can do anything. The world is full of examples that show just about anybody can manage to do anything if they want to or have enough motivation, or in the desparate cases do it on their own (at least for a while) so that they don't have other people around that make it look easy - which can make\ it seem harder for you when you need to catch up On the other hand you might not want to to expect to be the best at everything. Sometimes you aim for just gaining useful knowledge and don't think of it as what you know compared to the rest of the class - just what you know compared to what you knew before learning that subject. Education is not a competition. If everybody gets educated then everybody wins. Tell yourself the truth. It is not that you can't do it, or that you don't want to do it. You just don't like working too hard at it. Who does? Very few people die from hard work. A lot more people die from "I didn't know that." Many jobs turn out to have life or death elements to them so if you are desparate then convince yourself that learning whatever, is of life or death, hero or horrors importance. If you are desparate enough it will be easy to convince yourself and a little adrenalin can help boost your brain cells and your will to keep trying. For electronics just think of all the dangerous stuff that could happen in some circuits if you don't learn enough to handle them without errors. "I swear judge that it was just supposed to be a cute brightly flashing toy. I never meant for all those kids to end up tasering themselves!"

Don't expect it to always be fun. At least not normal fun. Sometimes a topic has to change your ideas of what fun is before you get over the hill and start really rolling. Most of learning is actually fun for anybody that wants to learn. But anybody that says their learning was all fun has a case of selective memory. Thats fine because you will tend to forget all the crap moments. You are building something in your brain like a construction worker building a house. As the house gets finished all the debris and crap gets dumpstered, mistakes get fixed, wounds are healed and it is all forgotten, until only the house remains. We tend to forget for ourselves all the problems we have had with other things we learned. As long as we learn it is just sweeter if we got so frustrated and angry while we were learning that we put a hole in the wall for no reason at all. So next time we learn it is the old drama brand new again. "I have never had this much trouble learning anything before! I don't think I am getting this at all. I should be able to do this already and starting on something else." We had all these wrong ideas the last time and the time before that but then realized we had it wrong and promptly forgot about all the grief because it turned out it was wasting our energy. We forget and then it becomes the same thing all over again next time.

Don't relax either too much or not enough. It is a miracle that anybody learns anything, because so many people do not give themselves the best chance to learn. Their learning efficiency, speed, and retention can be pretty low when they are at one end more interested in chasing girls and parties, and at the other end stressing out and numbing their brain with meaningless study habits like rote reading. (Ability to use knowledge from any massive volume of reading done only 24 hours before can be so low that it can be unmeasureable in some cases when the subject matter is technical and unfamilar to the reader.) Just learn. Whatever is pumping up your brain, give it a chance to work and you will get there. No stress required. Some time and interest is required.

Sorry to get all "Let me explain!" That is just how I am sometimes and now more than ever because I am really trying to psych myself up for school. Back to school at 40 years old feels weird but it is what I wanted to do.

Last edited: Sep 3, 2010
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7. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
201
It's all simple math if you work from the right back to the left where the source is.

Remember - the teacher may want to now how you arrived at your answer.

8. ### Tomax New Member

Sep 3, 2010
2
0
ghar is correct,follow him only.baki sab bakwas hai

9. ### curry87 Thread Starter Member

May 30, 2010
101
0
What does the || operator mean ?

I keep getting a different total resistance.

Im going 200ohm + 1000ohm = 1/1200
500ohm+1k = 1500 = 1/1500
rt = 1/ 1/1500 + 1/1000 + 1/1500 + 1/1200 =317ohm

10. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
201
With our limited keyboards we can still form an almost perfect approximation of the symbol that means

"in parallel with"

||

Just as with the other buttons they tend to move it around on keyboards but it's usually above the backslash \ character.

---

If you want to have some fun try computing the equivalent resistance from any corner to any corner of a "cube" built from 1K ohm resistors.

11. ### eblc1388 Senior Member

Nov 28, 2008
1,542
102
Ω
Another method is to use Superposition theorem to work out the answer.

A 22.2V voltage source will give the same voltage and current reading in the drawing. Since you have 10V instead, every voltage or current in the drawing will be correct if you multiply them with the scaling factor 10/22.2.

So 17.6mA becomes 7.9mA as your answer. This method will give you all the corresponding voltage and current of each branch.

This method is especially good if the question is: "What is the difference in current magnitude between the two 500Ω resistors?"

The answer will be: (9.2-2.4)*10/22.2mA.

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Last edited: Sep 4, 2010
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12. ### Potato Pudding Well-Known Member

Jun 11, 2010
684
92
Yes I would say that deserves thanks because that is a great example of how to answer this type of question.