# Finding Req for thevnin theorem

Discussion in 'Homework Help' started by Muqaddas Jamil, Dec 25, 2008.

Dec 23, 2008
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Hi !
we are currently doing thevnin and norton theorems in class and therez some problem with me in finding req.I am posting a problem from sergio Franco book....Plz see attached file.

Plz i need step by step explanation 4 every step.

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2. ### silvrstring Active Member

Mar 27, 2008
159
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It's hard to tell without drawings of how you manipulated the circuit before you decided to combine R5 and R3. R5 and R3 are the first ones I found the equivalent to, but it was 4 ohms--not 10; they are in parallel.

If this is a class assignment, don't count on simulation--it's result will be off by at least 30 mΩ.

Tryto redraw the circuit in a way that A and B are at one end before you start making calculations. Sorry, but I don't have scanner to send help.

3. ### vvkannan Active Member

Aug 9, 2008
138
11
hello jamil,
I think you can convert the star connected R9,R10 and R7 into delta and then it would become easier.
Or you can consider it like this. The current through the parallel combination of R5 and R3 can reach B through R1 or R4 and hence they are in parallel.
these two parallel combinations are in series and this combination is again parallel with R2 .

Dec 23, 2008
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i really cant get it....can u plz give me sum demonstration through diagrams?

5. ### silvrstring Active Member

Mar 27, 2008
159
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Jamil,

look at this walkthrough. Hope it helps you understand.

Take care

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6. ### mik3 Senior Member

Feb 4, 2008
4,846
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You have to use a delta to star resistor transformation for resistors R8, R9 and R10. After that, the rest will be parallel and series resistor combinations.

7. ### Ratch New Member

Mar 20, 2007
1,068
4

You almost solved it in your third diagram. Put R9 and R7 in parallel for a resistance of 5 ohms. Then you have 10 ohms in parallel with 9 ohms for a Req of 4.74 ohms. For Voc, a three loop mesh gives a voltage of 35.2 volts. For Isc a four loop mesh gives 7.2 amps. Using the resistance formula, 35.2/7.2 gives 4.89 ohms, which is with the limits of error for 3 significant digits.

Ratch

8. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,630
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I get 4.73684 ohms for Rth, obviously the same as you get.

But, I get 23.6842 volts for Vth, and 5 amps for Isc.

9. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,630
468
Did you take the voltage of the floating battery to be 100.3 volts? It looks to me like it's 100/3 volts.

10. ### Ratch New Member

Mar 20, 2007
1,068
4
The Electrician,

That voltage figure is rather murky, but I think you are right. It should be 100/3. I calculated it for 100, so lets go with your answer. Of course no matter what the voltage, Req will remain the same. At least we appear to have that right.

Ratch