# Thevenin Equivalent

Discussion in 'Homework Help' started by hugoleon, Jun 16, 2011.

1. ### hugoleon Thread Starter New Member

Jun 15, 2011
13
0
derive the thevenin equivalent of the circuit, I understand the equation but i don't know were to start on this equation.

......................r1 2.2kΩ r2 3kΩ
-----------------------------------------1kΩ
..................... r3 1.2kΩ r4 300ohm
.........9v.........................................................RL

how do i find the rth for this equation?

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
Are you able to post a schematic drawing? I don't understand what you have entered.

3. ### hugoleon Thread Starter New Member

Jun 15, 2011
13
0
thanks i hope this helps a little better

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4. ### jegues Well-Known Member

Sep 13, 2010
735
43
Start by determining the open circuit voltage.

To do this, remove the load and calculate the voltage across the terminals of the load.

Note that when you remove the load, the other end of the circuit is no longer connected to ground. What effect will this have on the circuit? (Think current)

After determining the open circuit voltage, you need to find the equivalent Thevenin resistance (Rth) as seen by the load.

To do so, short out the voltage source and determine the equivalent resistance as seen by the load terminals.

If you are still confused, post your work and an attempt at the solution and if need be we can further assist you.

Last edited: Jun 16, 2011
5. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,403
1,230
From your notes, it looks like your having difficulties determining series and parallel circuits.

6. ### hugoleon Thread Starter New Member

Jun 15, 2011
13
0
my question is how do i start? i know that i have to make r1 to r4 into 1 resistor but do i divide by one or add?? what do i do to the r5?

Dec 26, 2010
2,147
300
You would start by reducing R1 and R2 to a single resistance: R1 is in series with R2, although you seem to have written R1 // R2.

Similarly, get the total value for R3 and R4. Now work out the parallel combination of the two branch totals.

You now have a single resistance equivalent to R1...R4, in series with R5. Work out the total.

You must get clear in your mind the difference between series and parallel connections, and how to calculate their equivalent values. http://en.wikipedia.org/wiki/Series_and_parallel_circuits

8. ### hugoleon Thread Starter New Member

Jun 15, 2011
13
0
I combined r1//r2 r3//r4// and i get 201kohm and now and 1kohm?

9. ### hugoleon Thread Starter New Member

Jun 15, 2011
13
0
i get total of 201.83ohms from r1 to r4

Dec 26, 2010
2,147
300
I cannot see how you got the values in your last two posts. 201KΩ is very much too big, while 201.83Ω is too small.

Try again: you are looking for an answer in the region of a few KΩ.

11. ### hugoleon Thread Starter New Member

Jun 15, 2011
13
0
now i got 1.164kOhm from r1 to r4

12. ### hugoleon Thread Starter New Member

Jun 15, 2011
13
0
now i got 1.164kOhm from r1 to r4

13. ### jegues Well-Known Member

Sep 13, 2010
735
43
That is correct.

14. ### hugoleon Thread Starter New Member

Jun 15, 2011
13
0
is this right (9*1kohm)/(1.164kohm+1kOhm)=4.1v?

15. ### jegues Well-Known Member

Sep 13, 2010
735
43
What exactly are you trying to calculate?

16. ### hugoleon Thread Starter New Member

Jun 15, 2011
13
0
DERIVE THE THENVENIN EQUIVALENT OF THE CIRCUIT SHOWN IN FIGURE 7.47a THIS IS THE QUESTION

17. ### jegues Well-Known Member

Sep 13, 2010
735
43
Yes I'm aware of that.

I'm asking you, what were you trying to calculate?

Was that an attempt at calculating the Equivalent Thevenin voltage (Vth)? (i.e. the open circuit voltage)

As I mentioned in my previous post,

Last edited: Jun 16, 2011
18. ### hugoleon Thread Starter New Member

Jun 15, 2011
13
0
Yes that was for (vth)

19. ### jegues Well-Known Member

Sep 13, 2010
735
43
It is not correct.

When you remove the load to find the open circuit voltage, the circuit is no longer complete.

What does this mean in terms of the current flowing throughout the circuit?

What can you conclude about Vth from this? (Hint: Write a KVL)

Jun 15, 2011
13
0
Ok so its 9v