# Voltage Divider

#### Steve1992

Joined Apr 7, 2006
100
Vout = Vin x R2 / (R1 + R2)

If Vin is 12V and I want Vout to be 4.5V

What would the equation/math be to find the values for R2 and/or R1 if given.
If R1 is given R2 would be a common factor?

#### Papabravo

Joined Feb 24, 2006
18,405
Originally posted by Steve1992@May 7 2006, 08:33 AM
Vout = Vin x R2 / (R1 + R2)

If Vin is 12V and I want Vout to be 4.5V

What would the equation/math be to find the values for R2 and/or R1 if given.
If R1 is given R2 would be a common factor?
[post=16871]Quoted post[/post]​
There is no unique solution. There must be some other condition. For example R1 + R2 must be 20K Ohms. Now there is a unique solution because you have two equations and two unknowns.

#### Papabravo

Joined Feb 24, 2006
18,405

#### Papabravo

Joined Feb 24, 2006
18,405

#### Papabravo

Joined Feb 24, 2006
18,405

#### Papabravo

Joined Feb 24, 2006
18,405

#### Papabravo

Joined Feb 24, 2006
18,405

#### Papabravo

Joined Feb 24, 2006
18,405

Joined May 30, 2006
17
Originally posted by Steve1992@May 7 2006, 05:33 AM
Vout = Vin x R2 / (R1 + R2)

If Vin is 12V and I want Vout to be 4.5V

What would the equation/math be to find the values for R2 and/or R1 if given.
If R1 is given R2 would be a common factor?
[post=16871]Quoted post[/post]​

Et = 12v

you need to know the current. so use .1 amp arbitrarily.

It = .1amp

find R1 and R2

by using Rt = Et/It or 12v/.1amps
= 120ohms total resistance

if you want 4.5 volts then R1 = 4.5v-12v = 7.5v drop
7.5v / .1amps = 75 ohms = R1
4.5v / .1amps = 45ohms = R2

the same curent flows through both resistors. so that is used to find the total resistance

and ratio is used to find voltage drops

and resistances are in proportion to current.