Designing a circuit (basic electric circuit analysis)

[InvalidID]

Homework Statement

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.

 

[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.

 

[gneill]

Homework Statement

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.

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)

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.

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?

 

[InvalidID]

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.

Alright. I'm trying that right now.

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)

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.

How many resistors do you think you'd need to provide three potential drops? Should the drops be of different or equal magnitude?

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.

 

[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)?

 

[gneill]

I think I did it!

Yes, looks fine.

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)?

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

 

[CWatters]

I think I did it!

I think you have the battery the wrong way around.

 

[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.

 

[InvalidID]

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

 

[CWatters]

Nothing special, just a simple free graphics program.

http://www.getpaint.net/index.html

You could just as easily use MS Paint.