# Designing a circuit (basic electric circuit analysis)

1. Feb 22, 2013

### InvalidID

The problem statement, all variables and given/known data

Design a dc circuit utilizing a 15V voltage battery to provide the following node voltages: +10V, +5V, and -5V w.r.t. a circuit ground node. Select your resistors such that the maximum power demand on the battery does not exceed 1mA.

The attempt at a solution

I've designed the attached circuit, but I'm not sure how to find the resistance for each circuit. I know that the total resistance of all four circuits must add up to 225000Ω using the calculations attached.

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• ###### Calc.png
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2. Feb 22, 2013

### CWatters

Perhaps start by marking the ground node and which nodes you intend being 10V, 5V and -5V. Hint: You only need 3 resistors all the same value.

3. Feb 22, 2013

### Staff: Mentor

mA is not a unit of power; it's a unit of current. Are you sure that the question states that it wants a power limit and not a current limit? Or is it just badly phrased and they really expect a current limit of 1mA? (it would make sense)
Suppose you set a fixed current value. How many resistors do you think you'd need to provide three potential drops? Should the drops be of different or equal magnitude?

4. Feb 23, 2013

### InvalidID

Alright. I'm trying that right now.

Hmm... I doubled checked the question and it seems that they used the wrong units. I guess I'll ask the professor or TA. I think they just wrote the wrong units, because that's an easy mistake to make.

Resistors cause voltage drops, so for 3 voltage drops, I would need three resistors. Since the increments are in equal value (i.e. -5, 0, 5, 10), then I guess that the resistors need to be all the same value.

5. Feb 23, 2013

### InvalidID

I think I did it!

One question: how would I make (1) the ground node and still have the other nodes have voltages of: -5V, +5V, and +10V w.r.t. (1)?

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Last edited: Feb 23, 2013
6. Feb 23, 2013

### Staff: Mentor

Yes, looks fine.
You can't. Making node (1) the reference node would put the other nodes at -5, -10, -15 volts with respect to it.

7. Feb 24, 2013

### CWatters

I think you have the battery the wrong way around.

8. Feb 24, 2013

### CWatters

This is how I would draw it.. Note how this layout has the higher voltage nodes at the top of the page and lower voltage/negative voltages at the bottom.

#### Attached Files:

• ###### voltages.jpg
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Last edited: Feb 24, 2013
9. Feb 24, 2013

### InvalidID

I'm curious. What did you use to draw that circuit?

10. Feb 24, 2013