Math Question

[oex]

Hi,

I was asked this math/electrics question and I hate math :lol

Vout = 1.25*(1+R2/R1))+iadj*R2

.... All are known (vout, R1 and iadj) except R2 however how to put in terms of R2?

ie R2=Whatever for a spreadsheet formula

Maybe it is simple but I have coded too much the last few weeks and it just looks like garbage to me.... :lol

Here are some sample values....
50.6435 = 1.25*(1+12870/330)+.00005*12870

Vout = 50.6435
R1 = 330
R2 = 12870
Iadj = .00005

Apparently it is a formula needed for or used by a Voltage Regulator :lol

I know it's not your regular ASM forum question but might be an obvious solution/idiotic question for a maths boffin :lol.... I wish I'd listened harder in maths class.... I've been making up for it my entire life :lol

[anunitu]

It appears to resemble a polynomial equation, though that would require two unknowns. That type of equation(polynomial ) would yield a range of answers.

It might resolve to R2=Vout/1.25/(1+R2/R1))+iadj but my math skills are a bit rusty

This page explains the formula used

http://www.circuit-innovations.co.uk/LM317.html

[jj2007]

   Vout = 1.25*(1+R2/R1))+iadj*R2
   Vout = 1.25+1.25*R2/R1+iadj*R2
   Vout-1.25=1.25*R2/R1+iadj*R2
   Vout-1.25=R2*(1.25/R1+iadj)
   R2=(Vout-1.25)/(1.25/R1+iadj)
   R2=(50.6435-1.25)/(1.25/330+0.00005)=12870

[oex]

ty guys.... question was for my Dad, I have passed on the answer and am having an early night so will check I have asked the question right in the morning :U

[oex]

Cheers JJ, works perfectly :bg

[dedndave]

it helps if you have a drawing
even though the LM317 won't handle 50 V, there are ways to make it happen
and - other regulators use a similar configuration

[oex]

:lol cheers Dave this means nothing to me but I will pass on to my Dad :U

[dedndave]

well - there is a lot of info on the web about voltage regulators
if you put 50 or more volts on a LM317, it may get warm - for a minute
my guess is it won't last long enough to get hot
i have never tried it - lol

there are a lot of ways to make the LM317 handle higher voltages
one way is to use two of them @ 25 V each

probably a better way is to use a full wave bridge rectifier with a center-tapped transformer
you can get + and - complimentary voltages
then use a LM317 to regulate the + side to +25V
and use a LM337 to regulate the - side to -25V

it depends on the situation, particularly the grounding requirements

there are high-voltage regulators, and methods of using high-voltage transistors to extend the range, as well
these methods are a little more costly, though
LM317's and LM337's are cheap - that's why i suggest the work-around

[oex]

Aparently there is a HV version :wink <---- Like he almost knows what he's talking about :lol

[dedndave]

yah - wasn't sure
it has been a few years since i had to design power supplies - and then, they were usually switchers