Confused about Equiv. Resistance for this Circuit

Click For Summary
SUMMARY

The discussion focuses on calculating the equivalent resistance (R-equivalent) in a circuit involving resistors of 10 ohms, 20 ohms, 40 ohms, and 50 ohms. The user initially miscalculated the equivalent resistance by incorrectly combining resistors in parallel and series. The correct approach involves first finding the Thevenin equivalent by removing the voltage source and combining the 20 ohm and 40 ohm resistors in series before calculating the overall equivalent resistance. The final R-equivalent is determined to be 37.27 ohms, with the Thevenin resistance at the 50 ohm resistor calculated as 8.57 ohms.

PREREQUISITES
  • Understanding of series and parallel resistor combinations
  • Familiarity with Thevenin's theorem
  • Basic circuit analysis techniques
  • Knowledge of voltage sources and their effects on circuit behavior
NEXT STEPS
  • Study Thevenin's theorem in detail, focusing on practical applications
  • Learn how to analyze circuits with multiple resistors using equivalent resistance techniques
  • Practice problems involving series and parallel resistor combinations
  • Explore advanced circuit analysis tools like SPICE for simulating circuit behavior
USEFUL FOR

Electrical engineering students, circuit designers, and anyone involved in circuit analysis and design will benefit from this discussion.

stanners
Messages
11
Reaction score
0

Homework Statement


(this wasn't the original problem, but a part of it, but I got this part wrong)
Find the R equivalent of the circuithttp://img508.imageshack.us/img508/2191/circuitam6.jpg

Homework Equations


(parallel) 1/Req = 1/R1 + 1/R2 + 1/R3 + ...
(series) Req = R1 + R2 + R3 + ...

The Attempt at a Solution


My teacher gave back the paper, and I got it wrong, but I still don't understand why mine is wrong and his is the right way to do it.

1. I took the 10-ohm and 50-ohm in parallel, and got 8.33-ohm as equivalent.
2. (this is where the mistake occurred) I took the 20 ohm and 40ohm in series, so it'll be 60-ohm equivalent.
3. Then I took the 60-ohm and 8.33-ohm and put them in parallel, but that's wrong, but I don't understand why that's wrong. How would I know that I would have to put the 20ohm and 8.33 ohm in series, then parallel with the 40ohm INSTEAD of doing the way I did it?

Please help, thanks
 
Last edited by a moderator:
Physics news on Phys.org
It seems from the diagram you gave that your first mistake was in step 1. Why did you take the 10 and 50 ohm resisters in parallel?
 
Maybe I should write out the whole problem, I was trying to find the R-thevenin. So I took out the voltage source.

The problem was to find the V-thevenin with terminals at the 50-ohm resistor
 
in finding v thevanin, break the circuit at the point at which you want the voltage. in this case you would want to remove the 50 ohm resistor and find the voltage across that break.

also in finding r-thevanin, you would remove the 50 ohm resistor, turn off the voltage source, and combine the resistors using the revalent equations you posted. then put them into the circuit that includes the v-thev and r-thev.
 
The 20 ohms and the 40 ohms resistances share the same current, therefore you should combine them in series. Then go from there, shouldn't be hard.
 
Last edited:
R-eqv = 10+ (50||(20+40)) ohm
very simple.
ans: 37.27 ohm (check it)

now if u want to find out the R-thv at 50 ohm resistor,
step-1: short the volt source(88 v)
step-2: uproot the 50 ohm resistor from the ckt.
step-3: then look throgh the ckt from the uprooted resistor, can u feel that (20 n 40 ohm in series) which is parallel to 10 ohm?
so R-thv= 10 || (20+40) =8.57 ohm .. ans
 
Last edited:

Similar threads

How to find the equivalent resistance of this electric circuit?
  • · Replies 5 ·
Replies
5
Views
4K
Engineering Calculating the voltage at a certain Resistor
  • · Replies 3 ·
Replies
3
Views
2K
Engineering Find the equivalent Resistance of the circuit
  • · Replies 14 ·
Replies
14
Views
5K
Engineering Solve Superposition Theorem for R1, R2 Circuit
  • · Replies 18 ·
Replies
18
Views
3K
Engineering How Do You Solve a Parallel Circuit Problem with Unequal Currents?
  • · Replies 8 ·
Replies
8
Views
2K
Engineering Is my method of solving this correct? Superposition theorem
  • · Replies 15 ·
Replies
15
Views
2K
How to find the relative error in the voltage reading
  • · Replies 9 ·
Replies
9
Views
7K
Calculating Total Resistance in a Parallel Circuit
  • · Replies 2 ·
Replies
2
Views
2K
Engineering Question on basic circuit analysis
  • · Replies 4 ·
Replies
4
Views
2K
Engineering Use Circuit reduction, voltage and current divider to find i
  • · Replies 11 ·
Replies
11
Views
2K