Calculation for Voltage Divider

In previous post we had gone through the formula and the circuit used as voltage divider using two resistors.

Example Circuit :

A resistive divider is the case where both the resistors R1 and R2, are purely resistive (Image 1).

Substituting R1 and R2 into the previous expression gives:

Given : Vin = 9V , Vout = 3V

If R1 = R2 then

If Vout = 6V and Vin = 9V (these are widely used voltages), so:

and from above value of R2 must be twice value of R1.

Here, both the resistors are unknown. So we have to assume one resistor R1/R2, keeping above ration in mind.

Let R2 = 10 KΩ 

To solve for R1 (if R2 is assumed ):

By using : Vout = 3V , Vin = 9V and R2 = 10 KΩ  in above equation

We get R1 = 5 KΩ 


To solve for R2 (if R1 is assumed):

NOTE: This Vout/Vin ratio being >1 is not possible. By using these resistors alone it is not possible to either increase Vout above Vin or invert the voltage.

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