Author Topic: LM317 voltage regulator linearity  (Read 1859 times)

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Offline MandelbrotTopic starter

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LM317 voltage regulator linearity
« on: June 10, 2013, 07:14:34 am »
Hello again!

I'm still working on a linear power supply and I just noticed something that seems strange to me, but I'm probably being really stupid. According to the datasheet http://www.ti.com/lit/ds/symlink/lm117.pdf on page 2, the formula for Vout is 1.25(1 + R1 / R2). If R1 is constant and R2 is a potentiometer, then the equation is very non linear and the regulator would only be useful for a very small range of the potentiometer and far from linear.

I know I'm missing something simple, but I can't figure it out. Do I need a logarithmic potentiometer?

Thanks for the help, I'd be even more lost without this forum.

-Ryan

 

Offline David_AVD

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Re: LM317 voltage regulator linearity
« Reply #1 on: June 10, 2013, 07:23:25 am »
You have R1 and R2 flipped around.  It actually says  1.25(1 + R2 / R1).
 

Offline MandelbrotTopic starter

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Re: LM317 voltage regulator linearity
« Reply #2 on: June 10, 2013, 07:27:15 am »
Yep, I knew I did something stupid, I just wish it wasn't that! I typed it into my calculator wrong the first time and I used what I typed in as a reference from there on. Thanks.

I know how a voltage divider works I swear! Next time I'll try not to blindly follow formulas I guess.
 

Offline TerminalJack505

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Re: LM317 voltage regulator linearity
« Reply #3 on: June 10, 2013, 07:37:29 am »
And, yes, with the correct formula, it is linear.  It is linear also when you factor-in the error term.
 


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