# 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
20,772
2,540
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

4. ### bertus Administrator

Apr 5, 2008
15,806
2,389
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
20,772
2,540
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

7. ### bertus Administrator

Apr 5, 2008
15,806
2,389
Hello,

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

Bertus

8. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Your answers are the same as those that I calculated.

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

Apr 5, 2008
15,806
2,389