# Neither parallel nor series

Discussion in 'Electronic Basics' started by Wong, Oct 20, 2004.

1. ### WongGuest

What theorem or method can be applied to calculate the total
resistance at point A to B?

----- A -----
| |
| |
R1 R2
| |
| |
------R3------
| |
| |
R4 R5
| |
| |
----- B -----

Thanks.

3. ### Jack// aniGuest

Star to delta conversion will work here.

Your above star connection can be converted into equivalent delta
connection, like this

RB
___
|--------------|---|___|----------------|
| | | |
| | | |
| | | |
.-. .-.
B RA | | | | RC A
| | | |
| '-' '-' |
| | | |
| ___ | | ___ |
|----|___|-----|-----------|-----|___|--|

R5 R2

created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

RA = ( R4*R1 + R1*R3 + R3*R4 ) / R1

RB = ( R4*R1 + R1*R3 + R3*R4 ) / R3

RC = ( R4*R1 + R1*R3 + R3*R4 ) / R4

Now it is easy enough, for you to further simply it.

4. ### KMGuest

Use Delta to STAR conversion,

the result became, and it is series with parallel series.

A > | | |

Ra = (R1+R2)/(R1+R2+R3)
Rb = (R1+R3)/(R1+R2+R3)
Ra = (R2+R3)/(R1+R2+R3)

it is in Electrical technology by Edward Hughes

KM

5. ### WongGuest

I think there is a correction here, Ra should equal to
(R1*R2)/(R1+R2+R3) and Rb, Rc as well.
So the total resistance is (Ra + ((R4+Rb) || (R5+Rc)))
Let say, R1=1k, R2=2k, R3=3k, R4=4k and R5=5k. I will get the answer
of 2.9k and it is same with my simulation. Thanks !!

6. ### Jack// aniGuest

whoops, what an ugly mistake i did; feeling very shameful.
should be delta to star. really frustrating isn't it……should go and die somewhere.

7. ### John PopelishGuest

If that is the biggest mistake you make this year, you are doing
better than most of us.

8. ### Jack// aniGuest

well i hope so.
thanks

9. ### William J. BeatyGuest

You forgot to say "I don't like doing my own homework, please
do it for me."

And the correlary: if this ISN'T a homework problem, you
should say so. It also would help if you mentioned what
you've tried so far, and whether you're an absolute beginner
or not.

10. ### Bill BowdenGuest

I'm not following these formulas.
If all the resistors R1-R5 are 1 ohm, then the
total resistance from a to b will be 1 ohm.
But, according to the formulas, Ra, Rb ,Rc will
be 2/3 or 0.67 ohms. So, we have 1.67 in parallel
with 1.67 which is 0.83 which is in series with
Ra (0.67). That all adds up to 1.5 ohms, but it
should be 1 ohm.

What am I missing?

-Bill

11. ### Don KellyGuest

---------
Nothing:

The formula is wrong for delta to star conversion- try Ra=(R1*R2)/(R1
+R2+R3) etc
This will give you 1 ohm.

In the case where R1/R4 =R2/R5, you can simply remove R3 as it won't have
any effect. (figure out why).

12. ### Rich GriseGuest

Because the current's flowing both ways simultaneously, and so, from the
POV of our perceptive systems, cancel each other out and so don't exist.

Cheers!
Rich

13. ### John FieldsGuest

---
Puh-leeze...

If R1/R4 = R2/R5, then the voltages at the junctions of R1R4 and R2R5
will be equal, and with no potential difference across R3 current
can't flow through it, whether it's an open, a short, or anything in
between.

As a matter of fact that's why, at null, the detector across a
Wheatstone bridge vanishes.