[Circuits] Designing a simple voltage-divider

[ainster31]

Homework Statement

Design a voltage divider to provide a voltage of about 5V (±10%) across the variable load. The load-current demand varies in the range of 0 to 5 mA and the available dc-supply voltage is 15V.

Homework Equations

The Attempt at a Solution

Here is the circuit given (I've added ground, 15V, and 5V):

First, I assumed that there was no load. This gave me the following relationship: $R_1=2R_2$ but I'm not sure how to proceed.

 

[mfb]

The maximal current for the load will correspond to the lowest acceptable voltage (can you calculate that?). This allows to determine R1 and R2 together with the equation you found.

 

[ainster31]

The maximal current for the load will correspond to the lowest acceptable voltage (can you calculate that?). This allows to determine R1 and R2 together with the equation you found.

The lowest acceptable voltage would be 4.5V, right?

How do you know that maximum current would correspond to lowest voltage? Does that mean lowest current would correspond to highest voltage? Should I have used 5.5V instead of 5V in my first calculation then?

 

[Jony130]

For no load set Vout at 5.5V, Now with 5mA load set Vout at 4.5V.

 

[mfb]

For no load set Vout at 5.5V, Now with 5mA load set Vout at 4.5V.

That is even better.

The lowest acceptable voltage would be 4.5V, right?

Right.

How do you know that maximum current would correspond to lowest voltage? Does that mean lowest current would correspond to highest voltage? Should I have used 5.5V instead of 5V in my first calculation then?

Smaller load resistances lower the voltage and increase the current, so the smallest current corresponds to the largest voltage.