# noobish question

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#### rom7

##### New Member
How do i calculate the following for a voltage divider the missing resistor value R1 ?

Vin = 9v
vout = 6v across R2

R1 = to find
R2 = 60 ohm

#### Hero999

##### Banned
$V_\mathrm{out} = \frac{R_2}{R_1+R_2} \cdot V_\mathrm{in}\\ R_1 = \frac{R_2 \cdot V_\mathrm{in}}{V_\mathrm{out}} - R_2$
Voltage divider

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#### rom7

##### New Member
Thanks also what if both R1 and R2 values are unknown ?

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#### ericgibbs

##### Well-Known Member
Thanks also what if both R1 and R2 values are unknown ?

Vin = 9v
vout = 6v across R2

For that specific question, make the bottom resistor twice the value of the top.!

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#### Hero999

##### Banned
Thanks also what if both R1 and R2 values are unknown ?
Then use values that won't load the source too much or give an output impedance that will be loaded by what's connected to the output.

If the source and load impedances are significant then you should account for them in your calculations. If the load impedance is too low or varies significantly then you'll need a buffer amplifier.

#### colin55

##### Well-Known Member
If the value of both voltage-dividing resistors are unknown or you want to produce a voltage at the "take-off" point, you need to know the current to be delivered and the allowable voltage-drop. With these two values you can start to work out the values for the two voltage-dropper resistors.

#### Hero999

##### Banned
Look at the formulae on Wikipedia.

It's quite easy to work out the ration of R1 to R2, when given the input and output voltages.

Hint: set the value of R1 or R2 to 1.

#### rom7

##### New Member
Another question using a voltage divider circuit how would one work out the following ?

atm
Vin =9v
vout = 2v
R1 = 525 ohm
R2 150 ohm

a.I want to apply a load across R2 which requires 2v and 20 ma to function how do I compensate for the load on voltage drop and ensure that it has can draw 20 ma ?

b.When designing a voltage divider how can one be sure to get the desired output that uses resistors in a range that are available ?

#### colin55

##### Well-Known Member
How many mV drop will you accept when 20mA flows

#### rom7

##### New Member
The load is a led the vout drops to 1.85v but doesnt allow enough current <10ma using this configuration.

#### colin55

##### Well-Known Member
You don't want a voltage divider configuration. Just a voltage dropper or current limiter configuration.

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