# Circuit in neither series or parallel (is wye-delta transformation necessary?)

I have been trying out this one problem and I just want to know if I am on the right track, because it feels like a large amount of work based on such a simple circuit and it is getting quite messy. I have attached the picture of the problem along with my work thus far.

This is not listed under the wye-delta transformation under the problems section, which is in fact after this section, so that is another reason for my uncertainty. I feel like I may be misunderstanding some concept. Anyway, I appreciate the help.

I feel like ground is indicated by the bottom node, such that 10 + 4 + R are all in series, but that still confuses me as to where to go from there.

Using the method suggested: R = 20 = 60 || 14 + R = (60(14+R))/(60+14+R); R ends up being 16 Ω, which is correct. Are there any other methods for conceptual understanding?

Diagram:

• The star delta transform is making more work for yourself I have looked at what I could read of your working and felt that you were on the right track with just the series and parallel resistance equations Commented Jun 11, 2015 at 1:30
• This is a simple series/parallel circuit problem - no wyes or deltas involved at all. See my answer. Commented Jun 11, 2015 at 1:40
• Like on your other question, please do not delete the circuit diagram from the question. It's impossible to answer a question if you don't show the circuit you're asking about. I've reverted your edit. Commented Oct 6, 2022 at 3:49

Most of these sorts of problems are drawn in a way to confuse the student.

I would re-draw your circuit like so:

simulate this circuit – Schematic created using CircuitLab

R 5, 6, and 7 are in parallel, and all 12 Ohms, so the equivalent resistance of that group is 4 Ohms.

R5,6,7 plus R3 is 14 Ohms, which is in series with the unknown R4, so the circuit simplifies to

simulate this circuit

Since we want an equivlaent resistance for the whole circuit of 50 Ohms, R2 (60 Ohms) in parallel with R3 (14 Ohms) and R4 (unknown) in series must equal 20 Ohms

The remainder of the solution is, as they say, left as an exercise for the student.

• Nice. I agree that most of these problems are drawn in the most confusing manner possible -- maybe that's why so many electronics newbies are happy drawing chaotic, haphazard circuits... Commented Jun 11, 2015 at 1:40

Try working from the other end as well. If Req = 50 Ohms and Req = 30 Ohms plus some other stuff, the other stuff must combine to give X Ohms. That value (X) is what you get with 60 Ohms in parallel with Y Ohms, so Y must be ...

• You pretty much play it by ear, based on the problem you face. As Peter noted, problems like this typically arise on homework problem sets and exams, and often are arranged to take some thought. That's rather the point, really, of seeing if you can apply the principles you should know. Commented Jun 12, 2015 at 17:30
• As a general rule, perhaps you could say "simplify the problem, starting with the information you know, working towards what you don't know." While your original approach would have given you the right answer eventually, that the equations were getting messy should have been a sign. For your amusement, a pathological example: xkcd.com/730 Commented Jun 12, 2015 at 17:36