# Calculating Resistors in a Voltage Divider

Discussion in 'Homework Help' started by pelorus32, May 2, 2010.

1. ### pelorus32 Thread Starter New Member

May 2, 2010
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0
This is probably very simple but my brain won't get around it.

How do I calculate resistor values in a voltage divider circuit as follows:

Vin = 255V
R1 = unknown
Vout after R1 = 195V
R2 = unknown
Vout after R2 = 150V
R3 = 10K ohms
Vout after R3 = 0V

What are the steps for calculating R1 and R2?

Many thanks
Mike

2. ### Wendy Moderator

Mar 24, 2008
20,735
2,499
It is all a matter of proportion, involving fractions.

Show a schematic to work with would help, otherwise too many assumptions creep in.

3. ### pelorus32 Thread Starter New Member

May 2, 2010
4
0
Hi Bill,

thanks for the reply. I don't have the tools to draw a schematic. However it is a series circuit and the order of the items as set out in my original post is the order that they appear in the circuit.

Regards
Mike

Apr 5, 2008
15,539
2,307
Hello,

Here is a drawing made in paint:

The current can be calculated using the voltage drop of R3 and the given value.
With this current you can calculate the other resistors.

Bertus

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5. ### Wendy Moderator

Mar 24, 2008
20,735
2,499
Take a look at my blog. I offer a set of templates under the name of PaintCAD that will work for any graphics program (I use MS Paint), along with a fairly extensive cookbook of drawings made with this package.

Bill's Index

I am one of this sites graphic artists.

6. ### pelorus32 Thread Starter New Member

May 2, 2010
4
0
Thanks to both of you for your assistance. My answers so far from that are:

Circuit Current: 15mA
R2: 3 K ohms
R1: 4 K ohms

Does that sound right?

Many thanks
Mike

Apr 5, 2008
15,539
2,307
Hello,

You are correct.
By making use of Ohms law the answers where calculated correctly.

Bertus

Jan 28, 2005
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214

hgmjr

9. ### pelorus32 Thread Starter New Member

May 2, 2010
4
0
Many thanks to all of the people who assisted.

No doubt I'll have other questions -

Mike