# Circuit design problem

1. Feb 2, 2005

### painfive

Hi. I'm having trouble on this problem, and hopefully someone here can help me. It seems really simple, but every way I try to do it either leads to a dead end, or in one case, a very complicated answer that's probably wrong.

Here's the problem (the picture's attatched). RS is the internal resistance of the battery, and is 35 Ohms when it's new and gets up to 200 after it wears down. R1 and R2 must be chosen to fit the following specifications: 1) Vout must be between 4 and 5 volts when no load is attatched. 2) Vout cannot go down by more than 5% when a load RL is attatched. The answers may or may not involve RL.

After trying a few things that went nowhere, I finally found a way that seemed to work. I was able to find the region in the R1-R2 plane where the Vout will be between 4 and 5 (a wedge with the corner at about (258,367) and slopes 4/5 and 5/4). So any (R1,R2) in this region that satisfies the 5% requirement would be an answer. The problem is that the region which satisfied the 5% inequality was underneath a hyperbola, and it didn't always intersect the other region, depending on RL (after doing a lot of work, I'm pretty sure RL must be greater than R1, which can't be less than 258). Not only did the problem not give any restrictions on RL, but it was much more work than I thought it would be. Both of these lead me to believe I'm wrong and/or doing it a much harder way than necessary. Thanks in advance.

#### Attached Files:

• ###### circuit.bmp
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33.4 KB
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Last edited: Feb 2, 2005
2. Feb 2, 2005

### chroot

Staff Emeritus
Welcome to PF, painfive!

I'm afraid your picture didn't make it into your last post -- would you like to try posting the picture again?

- Warren

3. Feb 2, 2005

### painfive

ok, its fixed now

Last edited: Feb 3, 2005
4. Feb 3, 2005

### painfive

Does anyone have any ideas? I can post some more of my work if you want, but it's probably wrong anyway.