Maker Pro
Maker Pro

Need help with voltage divider circuit calculations

yacaph

Jan 11, 2014
9
Joined
Jan 11, 2014
Messages
9
How do I transpose the equation:

"Vout = R2 / (R1 + R2) * Vin" to find the value for an unknown R1 when R2 is known and also to find R2 when R1 is known.

I used to know this stuff, but it's been 40 years since I've used it so any help would be greatly appreciated.

Thanks,
Joseph
 

BobK

Jan 5, 2010
7,682
Joined
Jan 5, 2010
Messages
7,682
Vout = R2 / (R1 + R2) * Vin

Multiply both sides by (R1 + R2):

Vout (R1 + R2) = R2 * Vin

Multiply out the left side:

Vout * R1 + Vout * R2 = R2 * Vin

Subtract Vout * R2 from both sides:

Vout * R1 = R2 * Vin - R2 * Vout

factor out R2 on the right side:

Vout * R1 = R2 * (Vin - Vout)

Divide both sides by Vout:

R1 = (R2 * (Vin - Vout)) / Vout

Example: Vin = 12V Vout = 3V, R2 = 1K

R1 (1000 * (Vin - Vout) / Vout = 1000 * 9 / 3 = 3000

To solve for R2 is left as an exercise for the reader.

Bob
 

Merlin3189

Aug 4, 2011
250
Joined
Aug 4, 2011
Messages
250
One general tip about rearranging formulae:
Put in all the numbers you know first, before you rearrange the formula.

Firstly, some numbers may simplify, so you have fewer things to juggle.
Secondly, most people can rearrange numbers a lot better than symbols.
 

BobK

Jan 5, 2010
7,682
Joined
Jan 5, 2010
Messages
7,682
That is good advice if you only need to solve the equation once, but if you need to do it for multiple values, you need to keep all the variables as I did.

Bob
 
Top