# Manipulating formulas - LM723 Regulator

#### Dieverse

Joined Feb 22, 2016
10
Hey there, well as the title states when it comes to manipulation of formulas to solve for a certain variable I have to say, I really am not the best. I absolutely love electronics, and being hands on. But my math could be stronger....

Anyways I need to solve for R1 using the following formula:

Vout = Vref * (R1 + R2) / R2

Vref is given on the data sheet of the Lm723 and I am also supplied with Vout and R2

12V = 7.15V *(R1+10K)/10K

From here I have problems manipulating the formula properly to solve for R1.
Any help is greatly appreciated

#### ericgibbs

Joined Jan 29, 2010
13,267
hi D,
Show us your calculation attempt, we could then see where you are going wrong.
E
Note: Would recommend you use the correct parenthesis [brackets] in the formula's.

I am assuming you are using this formula from the d/s?

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#### Dieverse

Joined Feb 22, 2016
10
I duno, I think I have it....just need to watch units

12V = 7.15V * (R1+10K)/10K

12V = (715 * 10^-6)(R1) + 7.15

R1 = 12 - 7.15 / (715 * 10^-6)

R1 = 6.78kohm

#### ericgibbs

Joined Jan 29, 2010
13,267
hi D,
Looking good.
What you should do now is to put those values in the formula and see if you get 12V. OK.?
E

EDIT:

Your method works, but it looks a little messy.

Vo = (Vref * (R1 + R2)/R2)

Vo/Vref = (R1 + R2)/R2

R2(Vo/Vref) = R1 + R2

So R1 = R2(Vo/Vref) - R2

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#### Dieverse

Joined Feb 22, 2016
10
Yes I get 12V, well 11.99...The way I typed it out was more just laziness , I apologize. Its about 5:30am where I am and I have been working on assignments for various classes all night...

The way you laid yours out is how I tried to originally do it, but then would just confuse myself with the second R2

Thank You for your help, really appreciate it.

#### ericgibbs

Joined Jan 29, 2010
13,267
You are welcome.
When you post a Homework problem always post your attempt at solving it.
E