# find resistance r1 and r2 if only vin and out is given?

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#### Malik Nouman

Mar 17, 2018
1
How to find r1 and r2 values if only vout and vin is given. i have tried all voltage divider formulas but none of them work. I think it cant be found.

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#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,630
3.1415 Ω and 2.7182 Ω

In all honesty: How are we to know which are R1 and R1 without a circuit diagram? Upload a schematic diagram clearly indicating R1, R2, Vin and Vout, then we will be able to help.

#### davenn

Moderator
Sep 5, 2009
14,232
How to find r1 and r2 values if only vout and vin is given. i have tried all voltage divider formulas but none of them work. I think it cant be found.

so do you know how to work out the resistor values ?

this is easily done with Ohms law
Do you know Ohms law ?

#### AnalogKid

Jun 10, 2015
2,860
If the circuit does not have to deliver any significant current (for example, it is driving only a volt meter), then there are an infinite number of resistor value combinations that will do what you want. If you stick to only resistor values you can buy, then there are approximately 331,000 1% tolerance resistor combinations.

ak

#### AnalogKid

Jun 10, 2015
2,860
i have tried all voltage divider formulas but none of them work.
Yes, they do.

ak

#### davenn

Moderator
Sep 5, 2009
14,232
If the circuit does not have to deliver any significant current (for example, it is driving only a volt meter), then there are an infinite number of resistor value combinations that will do what you want. If you stick to only resistor values you can buy, then there are approximately 331,000 1% tolerance resistor combinations.

ak

That's seriously overcomplicating the discussion

#### duke37

Jan 9, 2011
5,364
12V is reduced to 9V so one third of the voltage is across R1 and two thirds across R2.
Chose a resistor (any value) for R1 and then R2 will be double this.
[mod edit: solution removed]

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#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,630
Chose a resistor (any value) for R1 and then R2 will be double this.
This is probably the direction the original assignment was directed at. Give the required ratio of R1/R2 or an equation for R1=f(R2) (or vice versa).

We have here one equation (voltage divider) but 2 degrees of freedom (R1, R2). This cannot be evaluated without making assumptions (e.g. about the value of either R1 or R2) that are not warranted by the task description (as far as we know it).

#### AnalogKid

Jun 10, 2015
2,860
Chose a resistor (any value) for R1 and then R2 will be double this.
?? ?

ak

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,630
Not only ???
Even if it were correct, it is our policy not to reveal solutions for homework. We'll be happy to guide the op in finding the correct solution himself.

#### AnalogKid

Jun 10, 2015
2,860
I understand how a homework forum functions. My point is that the guidance in post #8, which I quoted in post #9, is incorrect.

ak

#### hevans1944

##### Hop - AC8NS
Jun 21, 2012
4,875
I like @davenn's post #3, to which the OP has not bothered to respond. All remaining comments in this thread (including mine, which was deleted for revealing too much) are just noise IMHO until @Malik Nouman responds to Dave's questions. Unfortunately, Malik appears to be just another "drive-by poster" looking for a quick answer to a homework problem, rather than a participant in a learning experience. What a FWOT!

#### davenn

Moderator
Sep 5, 2009
14,232
I'm going to close this thread

if @Malik Nouman ever decides to return and wants to give some answers
he can PM me and I will reopen the thread

Dave

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