[Circuits] Designing a simple voltage-divider

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Discussion Overview

The discussion revolves around designing a voltage divider to achieve a specific output voltage of approximately 5V across a variable load, with considerations for load current and supply voltage. The context includes homework-related problem-solving and circuit design principles.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant presents a circuit design and establishes a relationship between resistors, stating that ##R_1=2R_2## under the assumption of no load.
  • Another participant suggests calculating the maximum load current to determine the resistor values, indicating that the lowest acceptable output voltage corresponds to the maximum load current.
  • There is a question about whether the lowest acceptable voltage is 4.5V and how this relates to the maximum current, with a participant wondering if the highest voltage should have been considered for initial calculations.
  • Further clarification is provided that smaller load resistances lead to lower output voltage and higher current, implying a relationship between load resistance, current, and voltage output.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the relationship between load current and output voltage, with some agreeing on the lowest acceptable voltage being 4.5V, while others question the implications of this relationship on initial calculations. The discussion remains unresolved with multiple competing views on the calculations and assumptions.

Contextual Notes

There are unresolved assumptions regarding the load conditions and the specific calculations for resistor values, as well as the implications of varying load resistance on output voltage and current.

ainster31
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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):

h_1390930293_9756129_f0b4069220.png


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.
 
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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.
 
mfb said:
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?
 
For no load set Vout at 5.5V, Now with 5mA load set Vout at 4.5V.
 
Jony130 said:
For no load set Vout at 5.5V, Now with 5mA load set Vout at 4.5V.
That is even better.

ainster31 said:
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.
 

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