Using source transformation for dependent sources

[Saeed.z]

Hi all ,

is it possible to use the source transformation for dependent source ..?

i guess yes because i came across a problem which asks to use source transformation to find Vx as shown :

http://img571.imageshack.us/img571/2389/50111057.jpg

and my solution was :

http://img36.imageshack.us/img36/7417/ansu.jpg

thanks ..

 

[Saeed.z]

BTW, i checked my answer by using Voltage node analysis and found that Vx = 7.5 V !

why the 2 answers are different ? thanks

 

[gneill]

You "transformed away" the component that the controlled source referred to. When you make a transformation there's no guarantee that the "new" components will have the same voltage or current as the "old" components they replace, only that the overall behavior at the output of the transformed block of components will be the same.

 

[sandmanumd]

Hi I was also having trouble with a similar problem I was working on and solved it in a similar manner to the poster with the same issues arising of getting a different answer than when using nodal analysis. I found a similar problem online that gets the correct solution:
The solution is online under example 2 at http://waleedeid.tripod.com/Lecture7_cir_analysis.pdf
(sorry my browser won't let me copy the image into the post).

My question is why did he make the voltage drop vx across the the 3V source and the 1Ohms resistor? When I did the problem I only put the voltage drop across the 1 Ohm resistor because I though R(Norton) = R(thevnin).

 

[gneill]

Hi I was also having trouble with a similar problem I was working on and solved it in a similar manner to the poster with the same issues arising of getting a different answer than when using nodal analysis. I found a similar problem online that gets the correct solution:
The solution is online under example 2 at http://waleedeid.tripod.com/Lecture7_cir_analysis.pdf
(sorry my browser won't let me copy the image into the post).

My question is why did he make the voltage drop vx across the the 3V source and the 1Ohms resistor? When I did the problem I only put the voltage drop across the 1 Ohm resistor because I though R(Norton) = R(thevnin).

See my post immediately above yours. If you transform away a specific component then you can no longer use that component as a reference. Doesn't matter if a "replacement" component has the same numerical value; it's a different component in a different circuit configuration.

The only thing guaranteed is that the OUTPUT TERMINALS of the transformed circuit block will behave the same as the original.

In your referenced example, Vx is not so much associated with a particular 2Ω resistor but the NODE where all the resistors come together. That node remains despite the source transformations that take place, so it remains a "safe" reference point.

 

[sandmanumd]

o ok, I see that now and understand. Thanks so much for all your help!

 

[Saeed.z]

Thanks all of you for your help , i appreciate that !

BTW i got the same answer here :

http://img715.imageshack.us/img715/6691/ee1w.png

http://img198.imageshack.us/img198/2976/ee2u.png

thanks again :)