Question about a 'basic' equivalent resistive circuit problem

[ajovcu]

Hello all, I'm a new member. I've decided banging my forehead until its red will not make me any more able to solve my practice problems. I hope I haven't posted this in the wrong forum but here is the question in concern:

Select R in the circuit so that V_L= 5 V

(see circuit in attachment below)



Hopefully you will be able to read this, I wrote it in OneNote- here is my attempt at a solution:

 

[berkeman]

Hello all, I'm a new member. I've decided banging my forehead until its red will not make me any more able to solve my practice problems. I hope I haven't posted this in the wrong forum but here is the question in concern:

Select R in the circuit so that V_L= 5 V

(see circuit in attachment below)



Hopefully you will be able to read this, I wrote it in OneNote- here is my attempt at a solution:

Welcome to the PF.

I would try using the KCL on the original circuit. I usually find that KCL works best for me, and is more intuitive than using source transformations, etc.

Can you give the KCL approach a try?

 

[ajovcu]

ay carumba, sorry for not posting the units in that last KVL expression... should've read something like:

-50/(7+R) V + (17 + 3R)/(7 + R)kOhm * ImA + 5 V = 0

 

[ajovcu]

Welcome to the PF.

I would try using the KCL on the original circuit. I usually find that KCL works best for me, and is more intuitive than using source transformations, etc.

Can you give the KCL approach a try?

absolutely!, I appreciate the suggestion. I'll post back my results

 

[gneill]

If you take another look at your second diagram (after the first source transformation) you might notice that there is a clear example of a simple voltage divider. The 1K resistor will not affect the output voltage, as no current will flow through it.

25V source -- voltage divider -- 5V out. Should be able to solve for R with little pain

 

[ajovcu]

wow thanks a bunch gneil, I assumed that those open nodes were there to represent some device or external circuit that absorbs 5V and could be even represented by a voltage source of 5V. Thanks a lot, I'd literally been racking my brains over the last day and a half on this problem.