Homework Help: Equivalent Inductance

1. Feb 9, 2016

Marcin H

1. The problem statement, all variables and given/known data

2. Relevant equations
Adding inductors in series and parallel (same as resistors)

3. The attempt at a solution
This is the first time I ran into a circuit like this with diagonal wiring and I am not sure how to treat it. Do I add the inductors in the triangle in series? So for example, on the left would it just be 105uH? I don't think that is right. Then that would be in series with the 25uH. How do I do this?

2. Feb 9, 2016

Staff: Mentor

Component arrangement on the page doesn't matter. The only thing that matters is how they connect to each other (the topology). You can feel free to re-draw the circuit with only horizontal and vertical lines and components if you wish, just so long as you maintain all the same connections.

3. Feb 9, 2016

anachin6000

As an indication, at the right end of the circuit 18 an 12 are connected in series and them together are in series with 38. the idea is tha the conductor lines between two connection points are ideal and you can modify their length and form as you wish.

4. Feb 9, 2016

cnh1995

First identify the series and parallel combinations. The circuit collapses into a very simple form then! You will automatically get rid of the diagonal wiring once you spot the series and parallel inductors.

5. Feb 9, 2016

Marcin H

I'm still not sure what would be in series and what would be in parallel. I want to say that the 60uH and 30uH inductors are in parallel, and once I get that would that equivalent be in series with the 25uH and 15uH?

6. Feb 9, 2016

cnh1995

How are they in parallel? Do they have "common end points"?
Start from the rightmost side. What can you say about 18uH and 12uH?

7. Feb 9, 2016

Staff: Mentor

Do they share the same two node connections? Can you draw a closed path that passes only through those two components (where wire doesn't count as a component) ?

8. Feb 9, 2016

Marcin H

Those are in series, so 30uH.

So if they have a common endpoint, they are in series? Like the 18 and 12?

9. Feb 9, 2016

cnh1995

Remember these two points while checking whether two components are in series or parallel:
1)Parallel components have common endpoints.
2) While checking for series components, say R1 and R2, imagine some current flowing through R1. If the same current enters R2 after leaving R1, then R1 and R2 are in series.

10. Feb 9, 2016

Marcin H

Ok, so whenever the current splits into different paths they are in parallel. So going through the 18 and 12 they would have the same current, but then the current would split at the bottom right corner. Current would go towards the 20 and 38. But then what happens?? It looks like current keeps splitting.

11. Feb 9, 2016

cnh1995

Parallel components have common endpoint"s", both the terminals are connected between two points.

Last edited: Feb 9, 2016
12. Feb 9, 2016

cnh1995

18 and 12 will become 30uH. What can you say about this 30uH and the diagonal 20uH?

13. Feb 9, 2016

Marcin H

It looks like the current splits right? So would they be in parallel? (sorry for the crappy drawing)

14. Feb 9, 2016

cnh1995

After splitting at one terminal, if all the split currents reunite at the other terminal, then the components are in parallel.

15. Feb 9, 2016

cnh1995

They are in parallel because their terminals are connected between two common points.

16. Feb 9, 2016

Marcin H

But what happens to the current after it goes through the 38? Wouldn't it split again?

17. Feb 9, 2016

cnh1995

It would, and it will continue to split. But in the end, all the split currents will reunite at the other terminal. Incoming current has to be equal to outgoing current. Isn't this what KCL is all about?

18. Feb 9, 2016

SammyS

Staff Emeritus
This is NOT correct.

Yes, at the right end of the circuit, the 18 and 12 are connected in series, but that combination is in parallel with the 20. The combination of these three is in series with the 38.

etc, etc.

19. Feb 9, 2016

Marcin H

True. Im confused now. You said:
Does this mean that the combined 30 is in parallel with the 20 and 38? Why is sammy saying they are in series?

20. Feb 9, 2016

cnh1995

Combined 30 is in parallel with 20. Their parallel equivalent is in series with 38. This series-parallel stuff goes on and the circuit reduces to a single equivalent inductance between the given terminals.

21. Feb 9, 2016

SammyS

Staff Emeritus
gneill has a very good explanation of determining which circuit elements are in series, and which are in parallel.

I'll take a stab at it.

If exactly two elements share a node, then those two elements are in series. Any current (or charge) passing through such a node, must come from one of the two elements and then pass into the other.

If two nodes are connected to each other by two or more elements, then those elements are in parallel with each other. They all have the same potential across them.

22. Feb 10, 2016

Marcin H

this diagonal stuff is really messing me up. That makes sense once you have the parallel equivalent in series with the 38. But I still don'nt really get why
I still don't understand this. :/ How can I redraw this, so it looks like a standard circuit? I think someone said that I can redraw this into a normal looking circuit, maybe that will help me see what's in series and what's in parallel.

This kinda helped in the sense that I know how to do it now, but I don't really understand why it is like that... :/

23. Feb 10, 2016

SammyS

Staff Emeritus
Do you know what a node is?

24. Feb 10, 2016

Marcin H

That's another thing... My teacher didn't explain that really well. I always thought, from physics, that the dots where nodes, but now I learned that you can combine nodes into essential nodes and stuff like that, but it didn't really make too much sense.

25. Feb 10, 2016

Marcin H

I am currently in an intro to comp E and EE class. first time dealing with circuit analysis.