# How do I use the ratio metric method to find resistor values, if input voltage is 24 volts?

Thank you, wgiergus

Hi Curt,

I understand that in your case resistors are connected in series connection, then let's say that we have only two resistors for simplicity and then we can write ratio equation:
Ui / U_R2 = (R1+R2)/R2
where:
Ui = input voltage (voltage on total resistance R1+R2)
U_R2 = voltage measured on R2
R1 and R2 are resistances in series connection.

Ratio equation could be alternatively written this way:
Ui / (R1+R2) = U_R2 / R2
and it still will be the proper equation.

As you can see we have only only one known data and 3 unknown variables, so we will need at least voltage on R1 or voltage on R2.

Then you just need to transform that equation:
R2 = (U_R2 * (R1+R2)) / Ui
and
R1 = (Ui*R2) / U_R2 - R2

I think you are asking about the case called sometimes voltage divider.

@wgiergus I think I overlooked that you will still need to know the total resistance R1+R2 in the first equation to solve first equation (to calculate R1) or current in this circuit, so you can calculate total resistance from Rtotal = Ui / I

Hi
Have you tried using the calculators in the DesignSpark Toolbox app? https://www.rs-online.com/designspark/ds-toolbox
It is available for iOS, Android and Windows devices