# Designing a circuit

1. Jun 11, 2007

### dazedoutpinoy

1. The problem statement, all variables and given/known data
Choosing from the following resistors values (they may be used more than once), set is, R1, R2, to obtain V = 5.5 V

[1 kilo ohm, 3.3 kilo ohm, 4.7 kilo ohm, 10 kilo ohm]

3. The attempt at a solution
I wasn't actually sure how to attempt this problem my textbook didn't really cover any examples of this. I included the figure needed to solve the problem, just any advice on how to start on it would be great.

http://img.photobucket.com/albums/v407/dazedoutpinoy/CircuitHomework001.jpg

2. Jun 19, 2007

### DefaultName

Use current divider. This problem is more of a trial & error.

3. Jun 19, 2007

### Staff: Mentor

What is the value of Is? Once you know Is, then you know what R1 // R2 has to be.

4. Jul 20, 2007

### Shahil

V = IR

lol ... um, seriously, you can't really do anything with the values you did give. You would need to know your Vcc I'm guessing and then use a Voltage Divider...The question is way too inexact.

5. Jul 24, 2007

### EugP

Something is wrong with this problem. Are you sure you didn't get a value for $$I_s$$? Here's why I'm asking:

To find V you need:

$$V = IR$$

Since the circuit you have is parallel, you can rewrite the above equation:

$$V = I_s(\frac{R_1R_2}{R_1 + R_2})$$

Now re-arranging to solve for I:

$$I_s = \frac{V(R_1 + R_2)}{R_1R_2)}$$

You have the value of $$V$$, and plugging in different values of $$R_1$$ and $$R_2$$ will simply give you different values of $$I_s$$, which would all be correct to the values of the resistors.

So either I'm missing something, or this problem is explained wrong.