# homework

Discussion in 'Homework Help' started by wafa.chenni, Nov 18, 2014.

1. ### wafa.chenni Thread Starter New Member

Nov 3, 2014
15
0
hey guys, i have an exercise and i can't figure it out :/ the question is to find the equivalent circuit using thevenin theorem. Pls help me :/

Feb 17, 2009
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3. ### WBahn Moderator

Mar 31, 2012
20,075
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What have YOU done to try to figure it out?

When you say that you are asked to find the Thevenin equivalent, you need to specify the perspective. I'm guessing (which is not good, since engineering is not about guessing) that you are being asked to determine the Thevenin equivalent circuit as seen by RL between nodes A and B. Correct?

4. ### wafa.chenni Thread Starter New Member

Nov 3, 2014
15
0
exactly between A and B

5. ### wafa.chenni Thread Starter New Member

Nov 3, 2014
15
0
i disconnected the charge and tried to calculate the Rth (thevenin resistor) disconnected the current source and short-circuited the vsource
i think Rth= (R3+R)//(R1+R2)
and for the Vth i didnt know how to start! i thought may be i can put another vsource which is gv*R2 but i'm not sure

6. ### WBahn Moderator

Mar 31, 2012
20,075
5,666
So, exactly, what have YOU done to attempt to solve YOUR homework?

We need to see your efforts so that we can see where you are going right and where you are going wrong.

7. ### WBahn Moderator

Mar 31, 2012
20,075
5,666
Our posts crossed.

What do you mean that you disconnected the "charge"? What is the "charge" you are referring to?

You need to remove the load and then analyze the circuit that is left to find Vth (the open circuit voltage) and Iss (the short circuit current). You need to approach it this way because you have a dependent source, so you can't just turn it off or remove it.

8. ### wafa.chenni Thread Starter New Member

Nov 3, 2014
15
0
i mean the load RL , i'm sorry but i study electronics in french! Load means charge in french sorry again

9. ### WBahn Moderator

Mar 31, 2012
20,075
5,666
Ah, okay. No need to be sorry, your English is a LOT better than my French!

So just draw two circuits, one with A-B open and find the voltage from A to B and the other with A-B shorted and find the current from A to B. With that, you have what you need to determine the Thevenin equivalent. Show your work if you have trouble.

10. ### wafa.chenni Thread Starter New Member

Nov 3, 2014
15
0
i drew an open circuit to find the Vth

i posed V2 = gv*R2 ( i'm not sure if we're alowed to do so )
and i dont know what to do after this !

Last edited: Nov 18, 2014
11. ### WBahn Moderator

Mar 31, 2012
20,075
5,666
No, you are not allowed to. If nothing else, check the units. The dependent source gain, gv, has units of A/V (it is a voltage dependent current source). If you multiply this by R2, which has units of V/A (i.e., ohms), then you get a dimensionless quantity, which therefore cannot equal V2.

You also now that it is wrong because in the original circuit, the right side was dependent on the left side since the current source was dependent on the voltage between the left side and the right side. But in your circuit, the right side is independent of the left side.

If you want to transform the dependent current source into a dependent voltage source using source transformation, that is fine. But do so using the rules. You take a current source that is in parallel with a resistance and convert that to a voltage source that is in series with a resistance. The resistances in the two cases are the same. What is the relationship between the voltage and the current of the two sources?

Nov 3, 2014
15
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it is V=I*R

13. ### WBahn Moderator

Mar 31, 2012
20,075
5,666
So, in this case, what is 'I' and what is 'R'? What is 'V' then equal to?

14. ### wafa.chenni Thread Starter New Member

Nov 3, 2014
15
0
V is the dependent voltage source R is its intern resistance and I is the dependent current source?

15. ### WBahn Moderator

Mar 31, 2012
20,075
5,666
For THIS PROBLEM, what is 'I' equal to? What is 'R' equal to? What is 'V' equal to?

You have a very specific 'V', 'I', and 'R' for THIS PROBLEM. What are they?

16. ### wafa.chenni Thread Starter New Member

Nov 3, 2014
15
0
no we're not given the values of the components. We're asked to find the relation of Vth and Rth

17. ### WBahn Moderator

Mar 31, 2012
20,075
5,666
I see four resistors in that circuit. One is R, one is R1, one is R2, and one is RL. The value of the first one is R, the value of the second on is R1, the value of the third is R2, and the value of the last one is RL.

When you say that V=IR for this circuit, then the 'R' you MUST be referring to is the left hand vertically-oriented resistor since that's the one you are using.

When you say V=IR for this circuit, then the 'I' is undefined because there IS no 'I' defined anywhere in that circuit.

What is the current of the dependent current source in this circuit? Just look at the schematic! It's right there!

What is the resistance that is in parallel with the dependent current source in this circuit? Just look at the schematic! It's right there!

What do you get when you multiply them together?

Redraw the circuit with the dependent current source and it's parallel resistance replaced with the appropriate voltage source in series with that same resistance.

18. ### wafa.chenni Thread Starter New Member

Nov 3, 2014
15
0
the current of the dependent current source is gv and its parallel resistance is R2
gv*R2=V ( v is the dependent voltage source) but that's what i did earlier !!

19. ### WBahn Moderator

Mar 31, 2012
20,075
5,666
I owe you an apology. In my mind I was thinking g_v (g subscript v), even though it is clearly g * v with 'v' being the voltage between the halves. My bad and sorry to lead you down a rabbit hole.

So, for that part of it, you are correct. But your schematic still needs to show R2 and it also needs to indicate 'v'.

20. ### wafa.chenni Thread Starter New Member

Nov 3, 2014
15
0
R2 is gone now because V2=R2*gv
now i dont know what i should do to calculate Vab :/